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!standard A.17          04-08-31 AI95-00302-03/06
!class amendment 04-01-14
!status work item 04-01-14
!status received 04-01-14
!priority Medium
!difficulty Hard
!subject Container library
!summary
The following API describes a standard container library for Ada. The library comprises sequence containers (vectors and lists), for inserting elements at specified positions, and associative containers (sets and maps), which position elements in order by key. The library is general, flexible, and efficient, and its design has been guided by the philosophy that a library should stay out of the programmer's way.
!problem
If is often the case that the solution to a programming problem requires a collection of elements. The Ada language of course provides a built-in array type, but typical problems often require a data structure with better time behavior or more sophisticated semantics. Even if you can get by with using an array, any non-trivial array manipulation quickly becomes unwieldy, and hence error-prone.
An array is also not general enough in that it only provides a mapping from a discrete index to the element type. The developer typically needs a container that can map an element to any arbitrary type. The key type often needs to be a string, but of course this cannot be used as an array index subtype.
With no general-purpose standard container library, a developer is left to craft his own container type. The data structure invariably chosen is some kind of linked-list, because it's relatively simple. However, manipulation of raw access types tends to be error-prone, and is a source of memory leaks. A linked-list also does not perform well as a general-purpose set or map when the number of elements is large.
One argument for having a standard library is that it's nice to be able to use a language right out-of-the-box. If a developer has to leave the language to solve common problems then perhaps that is a sign the language doesn't provide enough support for the developer. Other languages with which Ada competes, such as C++ or Java, come with rich standard container libraries.
Many existing container libraries are either badly designed or difficult to use. One issue is that it should be simple and easy to instantiate a component. If it requires more than a single instantiation to create a useable container, then it's too much pain and few developers will bother. If he does bother his experience will be unpleasant. Programming is work, but it should be fun work, not drudgery.
Libraries often have the problem that they are not sufficiently general, or simply not intended as industrial-strength libraries. The library designer can't possibly predict all the myriad uses to which his library will be put, and so the library must be general and flexible enough to suit unimagined uses.
Another problem is that these libraries often don't scale well to large numbers of elements. Their behavior is neither predictable nor is it even specified. They are often both time and space inefficient for various other reasons. In real-time programs especially, it's important to have predictable run-time behavior. An industrial-strength, general-purpose standard container library must provide containers that perform better than linear time complexity. Searches in particular need to be fast.
!proposal
This library API proposal is modeled on the Standard Template Library (STL), an algorithms and data structure library popularized by Alexander Stepanov, and included in the C++ standard library.
We can broadly characterize containers as being either "sequence containers" or "associative containers." All sequence containers allow insertion and deletion at any position in the container, but each one optimizes differently for insertions at certain positions. Associative containers associate elements with a key, which defines how elements are ordered in the container.
A vector is a sequence container optimized for insertion at the back end. It of course allows insertion at any position, but as you move toward the front the cost of vector insertion approaches linear time complexity. A vector supports constant-time random access of elements.
A linked-list is a sequence container that provides constant-time insertion and deletion at any position, but does not provide random access. The list container is doubly-linked, having nodes with next and previous pointers, and thus supports both forward and reverse iteration.
All of the containers specify positions using a cursor, which is similar to an access type in that it designates an element. It is not (necessarily) implemented as an access type, so access to the element is through a selector function. Many cursor operations are common to all containers. Containers also support operations specific to that container; for instance, vectors also specify positions via an integer index subtype.
The associative containers order their elements by key. A map has an explicit key type, but with a set the key is implicit in the element itself.
The hashed map associative containers scatter keys in the container according to a hash function. The size of the hash table is automatically increased when the length of the container equals its capacity (which specifies the length before which it is guaranteed that no automatic hash table resizing occurs), thus preserving constant time complexity for insertions.
The set associative container maintains elements in sort order. Insertions and searches have O(log N) time complexity even in the worst case.
All of the containers have alternate forms that accept an element type that is indefinite. The indefinite hashed maps also accept an indefinite key type, allowing (for example) type String to be used as the generic actual key type.
There are also library-level subprograms for returning the hash value of strings, and generic procedures for sorting an array.
The design of these packages follow the principle that they can be implemented in Ada without any special implementation-specific magic. This is important, as we do not want to burden implementers with special purpose requirements. We also want to follow the principle that any capability used for the implementation of the language is made available to users.
This API is based on an existing container library called Charles. The source code itself and a couple of papers about the design of Charles can be found at:
<http://home.earthlink.net/~matthewjheaney/charles/index.html>
Following the API proper, this proposal concludes with an examples section that illustrates the kinds of problems the library solves, and the most effective way to use the library to solve them.
!wording
Add Ada.Strings.Hash and Ada.Strings.Unbounded.Hash to A.4.7(29).
[This gives us Wide_ versions of these functions.]
A.4.8 String Hashing
Static Semantics
The following library-level subprograms are defined:
with Ada.Containers; function Ada.Strings.Hash (Key : String) return Containers.Hash_Type; pragma Pure (Ada.Strings.Hash);
Return an implementation-defined value which is a function of the value of Key. If A and B are strings such that A equals B, Hash(A) equals Hash(B).
with Ada.Containers; function Ada.Strings.Unbounded.Hash (Key : Unbounded_String) return Containers.Hash_Type; pragma Pure (Ada.Strings.Unbounded.Hash);
Return an implementation-defined value which is a function of the value of Key. If A and B are unbounded strings such that A equals B, Hash(A) equals Hash(B).
Implementation Advice
The various Hash functions should be good hash functions, returning a wide spread of values for different string values. It should be unlikely for similar strings to return the same value.
A.17 Containers
This clause presents the specifications of the package Containers and several child packages, which provide facilities for storing collections of elements.
Within this clause we provide Implementation Advice for the desired average or worst case time complexity of certain operations on a container. This advice is expressed using a big-O notation. A complexity of O(f(N)), presuming f is some function of a length parameter N and t(N) is the time the operation takes (on average or worst case, as specified) for the length N, means that there exists a finite A such that for any N, t(N)/f(N) < A.
AARM Note: Of course, an implementation can do better than a specified O(f(N)): for example, O(1) meets the requirements for O(log N).
If the advice suggests that the complexity should be less than O(f(N)), then for any arbitrarily small positive real D, there should exist a positive integer M such that for all N > M, t(N)/f(N) < D.
AARM Text Language Design Principles
This clause provides a number of useful containers for Ada. Only the most useful containers are provided. Ones that are relatively easy to code, redundant, or rarely used are omitted from this set, even if they are generally included in containers libraries.
The containers packages are modeled on the Standard Template Library (STL), an algorithms and data structure library popularized by Alexander Stepanov, and included in the C++ standard library. The structure and terminology differ from the STL where that better maps to common Ada usage. For instance, what the STL calls "iterators" are called "cursors" here.
The following major non-limited containers are provided:
* (Expandable) Vectors of any non-limited type;
* Doubly-linked Lists of any non-limited type;
* Hashed Maps keyed by any non-limited type containing any
non-limited type;
* Ordered Sets of any non-limited type.
Separate versions for definite and indefinite element types are provided, as those for definite types can be implemented more efficiently.
Each container includes a cursor, which is a reference to an element within a container. Cursors generally remain valid as long as the container exists and the element referenced is not deleted. Many operations on cursors are common to all of the containers. This makes it possible to write generic algorithms that work on any kind of container.
The containers packages are structured so that additional packages can be added in the future. Indeed, we hope that these packages provide the basis for a more extensive secondary standard for containers.
If containers with similar functionality (but different performance characteristics) are provided, we suggest that a prefix be used to identify the class of the functionality: "Ada.Containers.Bounded_Sets" (for a set with a maximum number of elements); "Ada.Containers.Protected_Maps" (for a map which can be accessed by multiple tasks at one time); "Ada.Containers.Persistent_Vectors" (for a persistent vector which continues to exist between executions of a program) and so on.
Note that the language already includes several requirements that are important to the use of containers. First, library packages must be reentrant - multiple tasks can use the packages as long as they operate on separate containers. Thus, it is only necessary for a user to protect a container if a single container needs to be used by multiple tasks.
Second, the language requires that language-defined types stream "properly". That means that the stream attributes can be used to implement persistence of containers when necessary, and containers can be passed between partitions of a program.
Finally, the language requires that equality of language-defined types compose "properly". This means that the version of "=" directly used by users is the same one that will be used in generics and in predefined equality operators of types with components of the containers and/or cursors. This prevents the abstraction from breaking unexpectedly.
If a container's element type is controlled, the point at which the element is finalized will depend on the implementation of the container. We do not specify precisely where this will happen (it will happen no later than the finalization of the container, of course) in order to give implementation's flexibility to cache, block, or split the nodes of the container. In particular, Delete does not necessarily finalize the element; the implementation may (or may not) hold the space for reuse. (The reference implementations show this well, as Delete for a Vector does not finalize the element, while Delete for an Ordered_Set does.)
This is not likely to be a hardship, as the element type has to be non-limited. Types used to manage scarce resources generally need to be limited. Otherwise, the amount of resources needed is hard to control, as the language allows a lot of variation in the number or order of adjusts/finalizations. For common uses of non-limited controlled types such as managing storage, the types already have to manage arbitrary copies.
The use of controlled type also brings up the possibility of failure of finalization (and thus deallocation) of an element. This is a "serious bug", as AI-179 puts it, so we don't try to specify what happens in that case. The implementation should propagate the exception, and should not damage the container (so that it can continue to be used without further error).
End AARM Text
A.17.1 The Package Containers
The package Containers is the root of the containers subsystem.
Static Semantics
The library package Containers has the following declaration:
package Ada.Containers is pragma Pure;
type Hash_Type is mod <Implementation-Defined>;
type Count_Type is range 0 .. <implementation-defined>;
end Ada.Containers;
Hash_Type represents the range of the result of a hash function. Count_Type represents the (potential or actual) number of elements of a container.
Implementation Advice
Hash_Type'Modulus should be at least 2**32. Count_Type'Last should be at least 2**31-1.
AARM Note: This is not a requirement so that these types can be declared properly on machines with native sizes that are not 32 bits. For instance, a 24-bit target could use 2**24 for Hash_Type'Modulus.
A.17.2 The Package Containers.Vectors
The language-defined package Containers.Vectors provides a private type Vector and a set of operations. A vector container allows insertion and deletion at any position, but it is specifically optimized for insertion and deletion at the high end (the end with the higher index) of the container. A vector container also provides random access to its elements.
A vector container object manages an unconstrained internal array, which expands as necessary as items are inserted. The length of a vector is the number of elements that the vector contains. The capacity of a vector corresponds to the number of elements that can be stored in the internal array, and will always be no less than the length of the vector.
A vector container may contain empty elements. Empty elements do not have a specified value.
AARM Notes:
Vectors are not intended to be sparse (that is, there are elements at all defined positions). Users are expected to use other containers (like a Map) when they need sparse structures (there is a Note to this effect at the end of this subclause).
The internal array is a conceptual model of a vector. There is no requirement for an implementation to be a single contiguous array. End AARM Notes.
Static Semantics
The library package Containers.Vectors has the following declaration:
generic
type Index_Type is range <>;
type Element_Type is private;
with function "=" (Left, Right : Element_Type) return Boolean is <>;
package Ada.Containers.Vectors is pragma Preelaborate (Vectors);
pragma Assert (Index_Type'Base'First < Index_Type'First);
AARM Note: This pragma insures that Last_Index is always able to return a valid value, even for an empty vector. It would not be able to this if Index_Type'Base'First = Index_Type'First, so this pragma serves as documentation of the requirement on Index_Type. End AARM Note.
subtype Index_Subtype is Index_Type;
type Vector is tagged private;
type Cursor is private;
Empty_Vector : constant Vector;
No_Element : constant Cursor;
function To_Vector (Length : Count_Type) return Vector;
function To_Vector
(New_Item : Element_Type;
Length : Count_Type) return Vector;
function "&" (Left, Right : Vector) return Vector;
function "&" (Left : Vector;
Right : Element_Type) return Vector;
function "&" (Left : Element_Type;
Right : Vector) return Vector;
function "&" (Left, Right : Element_Type) return Vector;
function "=" (Left, Right : Vector) return Boolean;
function Capacity (Container : Vector) return Count_Type;
procedure Set_Capacity (Container : in out Vector;
Capacity : in Count_Type);
function Length (Container : Vector) return Count_Type;
function Is_Empty (Container : Vector) return Boolean;
procedure Clear (Container : in out Vector);
function To_Cursor (Container : Vector; Index : Index_Type'Base) return Cursor;
function To_Index (Position : Cursor) return Index_Type'Base;
function Element (Container : Vector; Index : Index_Type'Base) return Element_Type;
function Element (Position : Cursor) return Element_Type;
procedure Query_Element (Container : in Vector; Index : in Index_Type'Base; Process : not null access procedure (Element : in Element_Type));
procedure Query_Element (Position : in Cursor; Process : not null access procedure (Element : in Element_Type));
procedure Update_Element (Container : in Vector; Index : in Index_Type'Base; Process : not null access procedure (Element : in out Element_Type));
procedure Update_Element (Position : in Cursor; Process : not null access procedure (Element : in out Element_Type));
procedure Replace_Element (Container : in Vector; Index : in Index_Type'Base; By : in Element_Type);
procedure Replace_Element (Position : in Cursor; By : in Element_Type);
procedure Assign (Target : in out Vector; Source : in Vector);
procedure Move (Target : in out Vector; Source : in out Vector);
procedure Insert (Container : in out Vector; Before : in Index_Type'Base; New_Item : in Vector);
procedure Insert (Container : in out Vector; Before : in Cursor; New_Item : in Vector);
procedure Insert (Container : in out Vector; Before : in Cursor; New_Item : in Vector; Position : out Cursor);
procedure Insert (Container : in out Vector; Before : in Index_Type'Base; New_Item : in Element_Type; Count : in Count_Type := 1);
procedure Insert (Container : in out Vector; Before : in Cursor; New_Item : in Element_Type; Count : in Count_Type := 1);
procedure Insert (Container : in out Vector; Before : in Cursor; New_Item : in Element_Type; Position : out Cursor; Count : in Count_Type := 1);
procedure Prepend (Container : in out Vector; New_Item : in Vector);
procedure Prepend (Container : in out Vector; New_Item : in Element_Type; Count : in Count_Type := 1);
procedure Append (Container : in out Vector; New_Item : in Vector);
procedure Append (Container : in out Vector; New_Item : in Element_Type; Count : in Count_Type := 1);
procedure Insert_Space (Container : in out Vector; Before : in Index_Type'Base; Count : in Count_Type := 1);
procedure Insert_Space (Container : in out Vector; Before : in Cursor; Position : out Cursor; Count : in Count_Type := 1);
procedure Set_Length (Container : in out Vector; Length : in Count_Type);
procedure Delete (Container : in out Vector; Index : in Index_Type'Base; Count : in Count_Type := 1);
procedure Delete (Container : in out Vector; Position : in out Cursor; Count : in Count_Type := 1);
procedure Delete_First (Container : in out Vector; Count : in Count_Type := 1);
procedure Delete_Last (Container : in out Vector; Count : in Count_Type := 1);
function First_Index (Container : Vector) return Index_Type;
function First (Container : Vector) return Cursor;
function First_Element (Container : Vector) return Element_Type;
function Last_Index (Container : Vector) return Index_Type'Base;
function Last (Container : Vector) return Cursor;
function Last_Element (Container : Vector) return Element_Type;
procedure Swap_Elements (Container : in Vector; I, J : in Index_Type'Base);
procedure Swap_Elements (I, J : in Cursor);
generic with function "<" (Left, Right : Element_Type) return Boolean is <>; procedure Generic_Sort (Container : in Vector'Class);
function Find_Index (Container : Vector; Item : Element_Type; Index : Index_Type'Base := Index_Type'First) return Index_Type'Base;
function Find (Container : Vector; Item : Element_Type; Position : Cursor := No_Element) return Cursor;
function Reverse_Find_Index (Container : Vector; Item : Element_Type; Index : Index_Type'Base := Index_Type'Last) return Index_Type'Base;
function Reverse_Find (Container : Vector; Item : Element_Type; Position : Cursor := No_Element) return Cursor;
function Contains (Container : Vector; Item : Element_Type) return Boolean;
function Next (Position : Cursor) return Cursor;
function Previous (Position : Cursor) return Cursor;
procedure Next (Position : in out Cursor);
procedure Previous (Position : in out Cursor);
function Has_Element (Position : Cursor) return Boolean;
procedure Iterate (Container : in Vector; Process : not null access procedure (Position : in Cursor));
procedure Reverse_Iterate (Container : in Vector; Process : not null access procedure (Position : in Cursor));
private
... -- not specified by the language
end Ada.Containers.Vectors;
The type Vector needs finalization (see 7.6).
Empty_Vector represents the empty vector container. It has a length of 0. If an object of type Vector is not otherwise initialized, it will be initialized to the same value as Empty_Vector.
No_Element represents a cursor that designates no element. If an object of type Cursor is not otherwise initialized, it will be initialized to the same value as No_Element.
function To_Vector (Length : Count_Type) return Vector;
Returns a vector with a length of Length. The vector is filled with empty elements.
function To_Vector
(New_Item : Element_Type;
Length : Count_Type) return Vector;
Returns a vector with a length of Length, and elements initialized to the value New_Item.
function "&" (Left, Right : Vector) return Vector;
Returns a vector comprising the elements of Right appended in their original order to the vector Left.
function "&" (Left : Vector;
Right : Element_Type) return Vector;
Returns a vector comprising the element Right appended to the vector Left.
function "&" (Left : Element_Type;
Right : Vector) return Vector;
Returns a vector comprising the element Left prepended to the vector Right.
function "&" (Left, Right : Element_Type) return Vector;
Returns a vector comprising the element Left followed by the element Right.
function "=" (Left, Right : Vector) return Boolean;
If Left denotes the same object as Right, then this function returns True. Otherwise if Left and Right have different lengths, then this function returns False. Otherwise, it compares each element in Left to the corresponding element in Right using the generic formal equality operator. If element equality returns False, then this function returns False; otherwise, this function returns True. Any exception raised during evaluation of element equality is propagated.
function Capacity (Container : Vector) return Count_Type;
Returns the length of the internal array.
procedure Set_Capacity (Container : in out Vector; Capacity : in Count_Type);
If Capacity < Length (Container), Constraint_Error is raised. Otherwise, Set_Capacity sets the capacity of Container to a value which is at least the value Capacity, expanding (or contracting) the internal array to hold at least Capacity elements. Expansion or contraction will require allocation, and possibly copying and deallocation of elements. Any exceptions raised by these operations are propagated, leaving the container with at least the original Length and elements.
AARM Notes
Expanding the internal array can be done by allocating a new, longer array, copying the elements, and deallocating the original array. This may raise Storage_Error, or cause an exception from a controlled subprogram. We require that a failed Set_Capacity does not lose any elements if an exception occurs, but we do not require a specific order of evaluations or copying.
This routine is used to preallocate the internal array to the specified capacity such that future Inserts do not require memory allocation overhead. Therefore, the implementation should allocate the needed memory to make that true at this point, even though the visible semantics could be preserved by waiting until the memory is needed. This doesn't apply to the indefinite element container, because elements will have to be allocated individually.
The implementation does not have to contract the internal array if the capacity is reduced, as any capacity greater than or equal to the specified capacity is allowed.
End AARM Notes
function Length (Container : Vector) return Count_Type;
The Length function returns the number of active elements in Container.
function Is_Empty (Container : Vector) return Boolean;
Equivalent to Length (Container) = 0.
procedure Clear (Container : in out Vector);
Sets the length of Vector to 0. Its capacity does not change.
function To_Cursor (Container : Vector; Index : Index_Type'Base) return Cursor;
If the value of Index is not in the range First_Index (Container) .. Last_Index (Container), then No_Element is returned. Otherwise, a cursor designating the element currently at Index in Container is returned.
function To_Index (Position : Cursor) return Index_Type'Base;
If Position is No_Element, Constraint_Error is propagated. Otherwise, the index (within its containing vector) of the element designated by Cursor is returned.
AARM Note: This implies that the index is determinable from a bare cursor alone. The basic model is that a vector cursor is implemented as a record containing an access to the vector container and an index value. This does constrain implementations, but it also allows all of the cursor operations to be defined in terms of the corresponding index operation (which should be primary for a vector).
function Element (Container : Vector; Index : Index_Type'Base) return Element_Type;
If Index is not in the range First_Index (Container) .. Last_Index (Container), then Constraint_Error is propagated. Otherwise this function returns the element at position Index.
function Element (Position : Cursor) return Element_Type;
Returns the element designated by Position. If Position equals No_Element, then Constraint_Error is propagated.
procedure Query_Element
(Container : in Vector;
Index : in Index_Type'Base; Process : not null access procedure (Element : in Element_Type));
If Index is not in the range First_Index (Container) .. Last_Index (Container), then Constraint_Error is propagated. Otherwise, it calls Process.all with the element at position Index as the parameter. Any exception raised by Process is propagated.
procedure Query_Element
(Position : in Cursor;
Process : not null access procedure (Element : in Element_Type));
Calls Process.all with the element designated by Position as the parameter. If Position equals No_Element, then Constraint_Error is propagated. Any exceptions raised by Process are propagated.
procedure Update_Element
(Container : in Vector;
Index : in Index_Type'Base; Process : not null access procedure (Element : in out Element_Type));
If Index is not in the range First_Index (Container) .. Last_Index (Container), then Constraint_Error is propagated. Otherwise, it calls Process.all with the element at position Index as the parameter. Any exception raised by Process is propagated.
If Element_Type is unconstrained and definite, then the Element parameter of Process.all shall be unconstrained.
AARM Note: This means that the elements cannot be aliased nor directly allocated from the heap; it must be possible to change the discriminants of the element in place.
The element at position Index is not an empty element after successful completion of this operation.
AARM Note: Since reading an empty element is a bounded error, attempting to use this procedure to replace empty elements may fail. Use Replace_Element to do that reliably.
procedure Update_Element
(Position : in Cursor;
Process : not null access procedure (Element : in out Element_Type));
Calls Process.all with the element designated by Position as the parameter. If Position equals No_Element, then Constraint_Error is propagated. Any exceptions raised by Process are propagated.
If Element_Type is unconstrained and definite, then the Element parameter of Process.all shall be unconstrained.
The element designated by Position is not an empty element after successful completion of this operation.
procedure Replace_Element (Container : in Vector; Index : in Index_Type'Base; By : in Element_Type);
If Index does not specify a value in the range First_Index (Container) .. Last_Index (Container), then Constraint_Error is propagated. Otherwise this function assigns the value By to the element at position Index. Any exceptions raised during the assignment are propagated. The element at position Index is not an empty element after successful completion of this operation.
procedure Replace_Element (Position : in Cursor; By : in Element_Type);
This function assigns the value By to the element designated by Position. If Position equals No_Element, then Constraint_Error is propagated. Any exception raised during the assignment is propagated. The element designated by Position is not an empty element after successful completion of this operation.
AARM Note: Replace_Element and Update_Element are the only ways that an element can change from empty to non-empty.
procedure Assign (Target : in out Vector; Source : in Vector);
If Target denotes the same object as Source, then the operation has no effect. Otherwise, Assign first calls Clear (Target), then Set_Capacity (Target, Length (Source)). It then assigns the active elements of Source to the corresponding positions in Target, and then sets the length of Target to the length of Source. Any exception raised during element assignment is propagated.
procedure Move (Target : in out Vector; Source : in out Vector);
If Target denotes the same object as Source, then the operation has no effect. If Length (Target) is greater than 0, then it raises Constraint_Error. Otherwise, the internal array is removed from Source (making its capacity 0) and moved to Target (making its capacity the original capacity of Source). The length of Target is set to the length of Source, and the length of Source is set to 0.
AARM Note:
If Capacity (Target) /= 0, the previous internal array may need to be deallocated. We don't mention this explicitly, because it is covered by the "no memory loss" Implementation Requirement. If the deallocation raises an exception, the implementation should leave both Vectors in a consistent state (usually empty for Target and the original state for Source).
procedure Insert (Container : in out Vector; Before : in Index_Type'Base; New_Item : in Vector);
If Length(New_Item) is 0, then Insert does nothing. Otherwise, it calculates the new length NL as the sum of the current length and Length (New_Item); if the value of Last appropriate for length NL would be greater than Index_Type'Last then Constraint_Error is propagated. If the value of Before is not in the range First_Index (Container) .. Last_Index (Container) + 1, then Constraint_Error is propagated.
If the current vector capacity is less than or equal to the new length, Set_Capacity (Container, NL) is called to increase the vector capacity. Then Insert slides the elements in the range Before .. Last_Index (Container) up by Length(New_Item) positions, and then copies the elements of New_Item to the positions starting at Before. Any exception raised during the copying is propagated.
AARM Note
Moving the elements does not necessarily involve copying. Similarly, since Set_Capacity does not require the copying of elements, it does not need to be explicitly called (the implementation can combine the operations if it wishes to). [Note to reviewers: I didn't want to duplicate the messy wording and notes about exceptions not losing anything.]
procedure Insert (Container : in out Vector; Before : in Cursor; New_Item : in Vector);
If Length(New_Item) is 0, then Insert does nothing. If Before is No_Element, then is equivalent to Insert (Container, Last_Index (Container) + 1), New_Item); otherwise is equivalent to Insert (Container, To_Index (Before), New_Item);
procedure Insert (Container : in out Vector; Before : in Cursor; New_Item : in Vector; Position : out Cursor);
If Length(New_Item) is 0, then Insert does nothing. Otherwise, create a temporary (call it Temp_Index) and set it to Last_Index (Container) + 1 if Before equals No_Element, and To_Index (Before) otherwise. Then Insert (Container, Before, New_Item) is called, and finally Position is set to To_Cursor (Container, Temp_Index).
AARM Note: The messy wording because Before is invalidated by Insert, and we don't want Position to be invalid after this call. An implementation probably only needs to copy Before to Position.
procedure Insert (Container : in out Vector; Before : in Index_Type'Base; New_Item : in Element_Type; Count : in Count_Type := 1);
Equivalent to Insert (Container, Before, To_Vector (New_Item, Count));
procedure Insert (Container : in out Vector; Before : in Cursor; New_Item : in Element_Type; Count : in Count_Type := 1);
Equivalent to Insert (Container, Before, To_Vector (New_Item, Count));
procedure Insert (Container : in out Vector; Before : in Cursor; New_Item : in Element_Type; Position : out Cursor; Count : in Count_Type := 1);
Equivalent to Insert (Container, Before, To_Vector (New_Item, Count), Position);
procedure Prepend (Container : in out Vector; New_Item : in Vector; Count : in Count_Type := 1);
Equivalent to Insert (Container, Index_Type'First, New_Item).
procedure Prepend (Container : in out Vector; New_Item : in Element_Type; Count : in Count_Type := 1);
Equivalent to Insert (Container, Index_Type'First, New_Item, Count).
procedure Append (Container : in out Vector; New_Item : in Vector);
Equivalent to Insert (Container, Last_Index (Container) + 1, New_Item).
procedure Append (Container : in out Vector; New_Item : in Element_Type; Count : in Count_Type := 1);
Equivalent to Insert (Container, Last_Index (Container) + 1, New_Item, Count).
procedure Insert_Space (Container : in out Vector; Before : in Index_Type'Base; Count : in Count_Type := 1);
Equivalent to Insert (Container, Before, New_Item, Count), with the difference that the inserted elements are empty elements.
procedure Insert_Space (Container : in out Vector; Before : in Cursor; Position : out Cursor; Count : in Count_Type := 1);
If Length(New_Item) is 0, then Insert does nothing. Otherwise, create a temporary (call it Temp_Index) and set it to Last_Index (Container) + 1 if Before equals No_Element, and To_Index (Before) otherwise. Then Insert_Space (Container, Temp_Index, Count) is called, and finally Position is set to To_Cursor (Container, Temp_Index).
procedure Set_Length (Container : in out Vector; Length : in Count_Type);
Calls Set_Capacity (Container, Length), then sets the length of the Container to Length. If Length is greater than the original length of Container, the added elements are empty elements.
procedure Delete (Container : in out Vector; Index : in Index_Type'Base; Count : in Count_Type := 1);
If Count is 0, the operation has no effect. If Index does not specify a value in the range First_Index (Container) .. Last_Index (Container), then Constraint_Error is propagated. Otherwise Delete slides the active elements (if any) starting Index plus Count down to Index. Any exceptions raised during element assignment are propagated.
AARM Note: If Index + Count >= Last_Index(Container), this effectively truncates the vector (setting Last_Index to Index - 1 and consequently setting Length to Index - Index_Type'First).
procedure Delete (Container : in out Vector; Position : in out Cursor; Count : in Count_Type := 1);
If Count is 0, the operation has no effect. Otherwise is equivalent to Delete (Container, To_Index (Position), Count).
procedure Delete_First (Container : in out Vector; Count : in Count_Type := 1);
Equivalent to Delete (Container, Index_Type'First, Count).
procedure Delete_Last (Container : in out Vector; Count : in Count_Type := 1);
If Length (Container) <= Count then is equivalent to Clear (Container); otherwise is equivalent to Delete (Container, Index_Type'Val(Index_Type'Pos(Last_Index(Container)) - Count + 1), Count).
function First_Index (Container : Vector) return Index_Type'Base;
Returns the value Index_Type'First.
AARM Note: We'd rather call this "First", but then calling most routines in here with First (Some_Vect) would be ambiguous.
function First (Container : Vector) return Cursor;
If Container is empty, First returns the value No_Element. Otherwise, returns a cursor that designates the first element in Container.
function First_Element (Container : Vector) return Element_Type;
Equivalent to Element (Container, First_Index (Container)).
function Last_Index (Container : Vector) return Index_Type'Base;
If Container is empty, returns Index_Type'First - 1; otherwise, returns the position of the last element in Container.
function Last (Container : Vector) return Cursor;
If Container is empty, Last returns the value No_Element. Otherwise, returns a cursor that designates the last element in Container.
function Last_Element (Container : Vector) return Element_Type;
Equivalent to Element (Container, Last_Index (Container)).
procedure Swap_Elements (Container : in Vector; I, J : in Index_Type'Base);
If I or J does not specify a value in the range First_Index (Container) .. Last_Index (Container), then Constraint_Error is propagated. Otherwise Swap_Elements exchanges the values of the elements at positions I and J.
AARM Notes: To Be Honest: The implementation is not required to actually copy the elements if it can do the swap some other way. But it is allowed to copy the elements if needed.
procedure Swap_Elements (I, J : in Cursor);
If either I or J is No_Element, then Constraint_Error is propagated. Otherwise Swap_Elements exchanges the values of the elements designated by I and J.
AARM Notes: After a call to Swap_Elements, I designates the element value previously designated by J, and J designates the element value previously designated by I. The cursors do not become ambiguous from this operation.
To Be Honest: The implementation is not required to actually copy the elements if it can do the swap some other way. But it is allowed to copy the elements if needed. End AARM Notes.
generic with function "<" (Left, Right : Element_Type) return Boolean is <>; procedure Generic_Sort (Container : in Vector'Class);
Reorders the elements of Container such that the elements are sorted smallest first as determined by the generic formal "<" operator provided. Any exception raised during evalution of "<" is propagated.
AARM Notes This implies swapping the elements, usually including an intermediate copy. This means that the elements will usually be copied. (As with Swap_Elements, if the implementation can do this some other way, it is allowed to.) Since the elements are non-limited, this usually will not be a problem. Note that there is Implementation Advice below that the implementation should use a sort that minimizes copying of elements.
The sort is not required to be stable (and the fast algorithm required will not be stable). If a stable sort is needed, the user can include the original location of the element as an extra "sort key". We considered requiring the implementation to do that, but it is mostly extra overhead -- usually there is something already in the element that provides the needed stability.
If this operation calls primitive operations of Vector, it should take care that they are not dispatching - overridden routines will not necessarily have the required semantics. (Note that this routine is not defined in terms of any primitive routines.) End AARM Notes
function Find_Index (Container : Vector; Item : Element_Type; Index : Index_Type'Base := Index_Type'First) return Index_Type'Base;
Searches the elements of Container for an element equal to Item, starting at position Index. If Index is less than Index_Type'First, then Constraint_Error is propagated. If there are no elements in the range Index .. Last_Index (Container) equal to Item, then Find_Index returns Last_Index (Container) + 1. Otherwise, it returns the index of the matching element.
function Find (Container : Vector; Item : Element_Type; Position : Cursor := No_Element) return Cursor;
Searches the elements of Container for an element equal to Item, starting at the first element if Cursor equals No_Element, and at the element designated by Cursor otherwise, and searching to the last element in Container. If an item equal to Item is found, Find returns a cursor designating the first element found equal to Item. If no such item is found, it returns No_Element.
function Reverse_Find_Index (Container : Vector; Item : Element_Type; Index : Index_Type'Base := Index_Type'Last) return Index_Type'Base;
Searches the elements of Container in reverse for an element equal to Item, starting at position Index. If Index is greater than Last_Index (Container), then the search begins at position Last_Index (Container). If there are no elements in the range Index_Type'First .. Index equal to Item, then Reverse_Find_Index returns Last_Index (Container) + 1. Otherwise, it returns the index of the matching element.
function Reverse_Find (Container : Vector; Item : Element_Type; Position : Cursor := No_Element) return Cursor;
Searches the elements of Container for an element equal to Item, starting at the last element if Cursor equals No_Element, and at the element designated by Cursor otherwise, and searching backwards to the first element in Container. If an item equal to Item is found, Find returns a cursor designating the first element found equal to Item. If no such item is found, it returns No_Element.
function Contains (Container : Vector; Item : Element_Type return Boolean;
Equivalent to Has_Element (Find (Container, Item)).
function Next (Position : Cursor) return Cursor;
If Position equals No_Element or designates the last element of the container, then Next returns the value No_Element. Otherwise, returns a cursor that designates the element with index To_Index (Position) + 1 in the same vector as Position.
function Previous (Position : Cursor) return Cursor;
If Position equals No_Element or designates the first element of the container, then Previous returns the value No_Element. Otherwise, returns a cursor that designates the element with index (To_Index (Position) - 1) in the same vector as Position.
The procedure Next is equivalent to Position := Next (Position).
The procedure Previous is equivalent to Position := Previous (Position).
function Has_Element (Position : Cursor) return Boolean;
Returns True if Position designates an element, and returns False otherwise.
AARM Note: To Be Honest: This function may not detect cursors that designate deleted elements; such cursors are invalid (see below) and any use of them (including in this routine) is erroneous.
procedure Iterate
(Container : in Vector;
Process : not null access procedure (Position : in Cursor));
Invokes Process.all with a cursor that designates each element in Container, in index order. Any exception raised by Process is propagated.
procedure Reverse_Iterate
(Container : in Vector;
Process : not null access procedure (Position : in Cursor));
Iterates over the nodes in Container as per Iterate, except that elements are traversed in reverse order.
Legality Rules
An instantiation of Containers.Vectors shall be at the library level.
AARM Note A implementation will typically need to use controlled types to insure that the Implementation Requirements are met. These would require all instantiations to occur at the library level. We certainly do not want to require magic for nested container instantiations, while not giving similar capabilities to users. We've made this a legality rule to enhance portability. This rule will be dropped if AI-344 or some other solution to nested controlled types is adopted.
Bounded (Run-Time) Errors
Reading the value of an empty element by calling Element, Query_Element, Update_Element, Generic_Sort, "=", Find, or Reverse_Find is a bounded error. The implementation may treat the element as having any valid value of the element type, or raise Constraint_Error or Program_Error before modifying the vector.
AARM Notes: For instance, a default initialized element could be returned. Or some previous value of an element. But returning random junk is not allowed if the type has default initial value(s).
Assignment and streaming of empty elements are NOT bounded errors. This is consistent with regular composite types, for which assignment and streaming of uninitialized components do not cause a bounded error, but reading the uninitialized component does cause a bounded error.
There are other operations which are defined in terms of the operations listed above. End AARM Notes.
A Cursor value is ambiguous if any of the following have occurred since it was created:
* Insert or Delete has been called on the vector that contains the element the cursor designates with an index value (or a cursor designating an element at such an index value) less than or equal to the index value of the element designated by the cursor;
* The vector that contains the element it designates has been passed to an instance of Generic_Sort.
It is a bounded error to call any subprogram other than "=" or Has_Element declared in Containers.Vectors with an ambiguous (but not invalid, see below) cursor parameter. Possible results are:
* The cursor may be treated as if it was No_Element;
* The cursor may designate some element in the vector (but not necessarily the element that it originally designated);
* Constraint_Error may be raised; or
* Program_Error may be raised.
AARM Note: Cursors are made ambiguous if an Insert or Delete occurs that moves the elements in the internal array including the designated ones. After such an operation, the cursor probably still designates an element (although it might not after a deletion), but it is a different element. That violates the definition of cursor -- it designates a particular element.
For "=" or Has_Element, the cursor works normally (it would not be No_Element). We don't want to trigger an exception simply for comparing a bad cursor.
While it is possible to check for or prevent these cases, in many cases the overhead necessary to make the check (or prevent the problems) is substantial in time or space. End AARM Notes.
Erroneous Execution
A Cursor value is invalid if any of the following have occurred since it was created:
* The vector that contains the element it designates has been finalized;
* The vector that contains the element it designates has been used as the Source or Target of a call to Move;
* The element it designates has been deleted.
The result of "=" or Has_Element is unspecified if it is called with an invalid cursor parameter. Execution is erroneous if any other subprogram declared in Containers.Vectors is called with an invalid cursor parameter, or if the cursor designates an element in a different vector object than the appropriate one specified in the call.
AARM Notes: The list above (combined with the bounded error cases) is intended to be exhaustive. In other cases, a cursor value continues to designate its original element. For instance, cursor values survive the appending of new elements. End AARM Notes.
Implementation Requirements
No storage associated with a vector object shall be lost upon assignment or scope exit.
Implementation Advice
Containers.Vectors should be implemented similarly to an array. In particular, if the length of a vector is N, then
* the worst-case time complexity of Append with Count=1 and Element should be O(log N);
* the worst-case time complexity of Prepend with Count=1 or Delete_First with Count=1 of the vector should be O(N log N).
AARM Note We do not mean to overly constrain implementation strategies here. However, it is important for portability that the performance of large containers has roughly the same factors on different implementations. If a program is moved to an implementation that takes O(N) time to access elements, that program could be unusable when the vectors are large. We allow O(log N) access because the proportionality constant and caching effects are likely to be larger than the log factor, and we don't want to discourage innovative implementations.
The worst-case time complexity of a call on an instantiation of Containers.Vectors.Generic_Sort should be O(N**2), and the average time complexity should be better than O(N**2).
AARM Note In other words, we're requiring the use of a better than O(N**2) sorting algorithm, such as Quicksort. No Bubble sorts allowed!
Containers.Vectors.Generic_Sort should minimize copying of elements.
AARM Note - To Be Honest We do not mean "absolutely minimize" here; we're not intending to require a single copy for each element. Rather, we want to suggest that the sorting algorithm chosen is one that does not copy items unnecessarily. Bubble sort would not meet this advice, for instance.
NOTE: All elements of a vector occupy locations in the internal array. If a sparse container is required, a Hashed_Map should be used rather than a vector.
A.17.3 The Package Containers.Doubly_Linked_Lists
The language-defined package Containers.Doubly_Linked_Lists provides private types List and Cursor, and a set of operations for each type. A list container is optimized for insertion and deletion at any position.
A doubly-linked list container object manages a linked list of internal storage nodes, each of which contains an element and pointers to the next (successor) and previous (predecessor) internal nodes. A cursor is used to specify positions of particular storage nodes within a list.
Static Semantics
The library package Containers.Doubly_Linked_Lists has the following declaration:
generic
type Element_Type is private;
with function "=" (Left, Right : Element_Type) return Boolean is <>;
package Ada.Containers.Doubly_Linked_Lists is pragma Preelaborate (Doubly_Linked_Lists);
type List is tagged private;
type Cursor is private;
Empty_List : constant List; --NOTE: function or deferred constant?
No_Element : constant Cursor;
function "=" (Left, Right : List) return Boolean;
function Length (Container : List) return Natural;
function Is_Empty (Container : List) return Boolean;
procedure Clear (Container : in out List);
function Element (Position : Cursor) return Element_Type;
procedure Query_Element (Position : in Cursor; Process : not null access procedure (Element : in Element_Type));
procedure Update_Element (Position : in Cursor; Process : not null access procedure (Element : in out Element_Type));
procedure Replace_Element (Position : in Cursor; By : in Element_Type);
procedure Move (Target : in out List; Source : in out List);
procedure Prepend (Container : in out List; New_Item : in Element_Type; Count : in Count_Type := 1);
procedure Append (Container : in out List; New_Item : in Element_Type; Count : in Count_Type := 1);
procedure Insert (Container : in out List; Before : in Cursor; New_Item : in Element_Type; Count : in Count_Type := 1);
procedure Insert (Container : in out List; Before : in Cursor; New_Item : in Element_Type; Position : out Cursor; Count : in Count_Type := 1);
procedure Insert (Container : in out List; Before : in Cursor; Position : out Cursor; Count : in Count_Type := 1);
procedure Delete (Container : in out List; Position : in out Cursor; Count : in Count_Type := 1);
procedure Delete_First (Container : in out List; Count : in Count_Type := 1);
procedure Delete_Last (Container : in out List; Count : in Count_Type := 1);
generic with function "<" (Left, Right : Element_Type) return Boolean is <>; procedure Generic_Sort (Container : in out List'Class);
generic with function "<" (Left, Right : Element_Type) return Boolean is <>; procedure Generic_Merge (Target : in out List'Class; Source : in out List'Class);
procedure Reverse_List (Container : in out List);
procedure Swap (Container : in out List; I, J : in Cursor);
procedure Splice (Target : in out List; Before : in Cursor; Source : in out List);
procedure Splice (Target : in out List; Before : in Cursor; Position : in Cursor);
procedure Splice (Target : in out List; Before : in Cursor; Source : in out List; Position : in Cursor);
function First (Container : List) return Cursor;
function First_Element (Container : List) return Element_Type;
function Last (Container : List) return Cursor;
function Last_Element (Container : List) return Element_Type;
function Contains (Container : List; Item : Element_Type) return Boolean;
function Find (Container : List; Item : Element_Type; Position : Cursor := No_Element) return Cursor;
function Reverse_Find (Container : List; Item : Element_Type; Position : Cursor := No_Element) return Cursor;
function Next (Position : Cursor) return Cursor;
function Previous (Position : Cursor) return Cursor;
procedure Next (Position : in out Cursor);
procedure Previous (Position : in out Cursor);
function Has_Element (Position : Cursor) return Boolean;
procedure Iterate (Container : in List; Process : not null access procedure (Position : in Cursor));
procedure Reverse_Iterate (Container : in List; Process : not null access procedure (Position : in Cursor));
private
... -- not specified by the language
end Ada.Containers.Doubly_Linked_Lists;
The type List needs finalization (see 7.6).
Empty_List represents the empty list. If an object of type List is not otherwise initialized, it will be initialized to the same value as Empty_List.
No_Element represents a cursor that designates no element. If an object of type Cursor is not otherwise initialized, it will be initialized to the same value as No_Element.
function "=" (Left, Right : List) return Boolean;
If Left denotes the same object as Right, then this function returns True. Otherwise if Left and Right have different lengths, then this function returns False. Otherwise, it compares each element in Left to the corresponding element in Right using the generic formal equality operator. If element equality returns False, then this function returns False; otherwise, this function returns True. Any exception raised during evaluation of element equality is propagated.
The function Length returns the number of elements in Container.
The function Is_Empty is equivalent to Length (Container) = 0.
Clear removes all the nodes in Container, and sets the length to 0. Any exceptions raised during deallocation of storage are propagated.
function Element (Position : Cursor) return Element_Type;
Returns the element stored on the node designated by Position. If Position equals No_Element, then Constraint_Error is propagated.
procedure Query_Element
(Position : in Cursor;
Process : not null access procedure (Element : in Element_Type));
If Position equals No_Element, then Constraint_Error is propagated. Otherwise, Query_Element invokes Process.all with the element on node designated by Position as the argument.
procedure Update_Element
(Position : in Cursor;
Process : not null access procedure (Element : in out Element_Type));
If Position equals No_Element, then Constraint_Error is propagated. Otherwise, Update_Element invokes Process.all with the element on node designated by Position as the argument.
If Element_Type is unconstrained and definite, then the Element parameter of Process.all shall be unconstrained.
AARM Note: This means that the elements cannot be aliased nor directly allocated from the heap; it must be possible to change the discriminants of the element in place.
procedure Replace_Element (Position : Cursor; By : Element_Type);
Assigns the value By to the element stored on the node designated by Position. If Position equals No_Element, then Constraint_Error is propagated.
procedure Move (Target : in out List; Source : in out List);
If Target denotes the same object as Source, then the operation has no effect. If Length (Target) is greater than 0, then it raises Constraint_Error. Otherwise, the nodes in Source are moved to Target. The length of Target is set to the length of Source, and the length of Source is set to 0.
Prepend is equivalent to Insert (Container, First (Container), New_Item, Count).
Append is equivalent to Insert (Container, No_Element, New_Item, Count).
procedure Insert (Container : in out List; Before : in Cursor; New_Item : in Element_Type; Count : in Count_Type := 1);
Insert allocates Count new nodes whose element is initialized to the value New_Item, and inserts them prior to the node designated by Before. If Before equals No_Element, the new nodes are inserted immediately following the last node (if any). Any exception raised during allocation of internal storage is propagated, and Container is not modified.
procedure Insert (Container : in out List; Before : in Cursor; New_Item : in Element_Type; Position : out Cursor; Count : in Count_Type := 1);
Insert allocates Count new nodes whose element is initialized to the value New_Item, and inserts them prior to the node designated by Before. If Before equals No_Element, the new nodes are inserted immediately following the last node (if any). Position designates the first newly-inserted node. Any exception raised during allocation of internal storage is propagated, and Container is not modified.
procedure Insert (Container : in out List; Before : in Cursor; Position : out Cursor; Count : in Count_Type := 1);
Allocates Count new nodes with elements initialized with any implicit initial values for any part (as for an object_declaration with no initialization expression - see 3.3.1), and inserts them prior to the node designated by Before. If Before equals No_Element, the new node is inserted immediately following the last node (if any). Position designates the first newly-inserted node. Any exception raised during allocation of internal storage is propagated, and Container is not modified.
procedure Delete (Container : in out List; Position : in out Cursor; Count : in Count_Type := 1);
If Position equals No_Element, the operation has no effect. Otherwise Delete removes Count nodes starting at the node designated by Position from Container (or all of the nodes if there are less than Count nodes starting at Position). Any exception raised during deallocation of internal storage is propagated.
procedure Delete_First (Container : in out List; Count : in Count_Type := 1);
If Length(Container) >= Count, Delete_First removes the first Count nodes from Container; otherwise, all of the nodes in Container are removed. Any exceptions raised during deallocation of storage are propagated.
procedure Delete_Last (Container : in out List; Count : in Count_Type := 1);
If Length(Container) >= Count, Delete_Last removes the last Count nodes from Container; otherwise, all of the nodes in Container are removed. Any exceptions raised during deallocation of storage are propagated.
generic with function "<" (Left, Right : Element_Type) return Boolean is <>; procedure Generic_Sort (Container : in out List'Class);
Reorders the nodes of Container such that the elements are sorted smallest first as determined by the generic formal "<" operator provided. The sort must be stable. Any exception raised during evaluation of "<" is propagated.
AARM Notes Unlike array sorts, we do require stable sorts here. That's because algorithms in the merge sort family (as described by Knuth) can be both fast and stable. Such sorts use the extra memory as offered by the links to provide better performance.
Note that list sorts never copy elements; it is the nodes, not the elements, that are reordered. End AARM Notes
generic with function "<" (Left, Right : Element_Type) return Boolean is <>; procedure Generic_Merge (Target : in out List; Source : in out List);
If Source denotes the same object as Target, the operation has no effect. Otherwise this operation reorders nodes such that they are removed from Source and moved to Target. Target and Source are assumed to be sorted smallest first as determined by the generic formal "<" operator. The nodes in Source containing elements that are less than elements in Target are spliced in before the elements already in Target. The nodes in Source container elements that are equal to or greater than elements in Target are spliced in after the elements already in Target. Any exception raised during evaluation of "<" is propagated, immediately terminating the merge operation.
procedure Reverse_List (Container : in out List);
Reorders the nodes of Container in reverse order.
procedure Swap (Container : in out List; I, J : in Cursor);
Swap exchanges the nodes designated by I and J.
AARM Note: Unlike Swap_Elements for vectors, this exchanges the nodes, not the elements. No copying is performed. I and J designate the same elements after this call as they did before it. This is important, as this operation is provided as it can provide better performance than a straight copying swap. The programmer can writing a copying swap if they need one. This difference in semantics is the reason that this operations have different names in the List and Vector containers.
procedure Splice (Target : in out List; Before : in Cursor; Source : in out List);
If Source denotes the same object as Target, the operation has no effect. Otherwise, Splice reorders nodes such that they are removed from Source and moved to Target, just prior to Before. If Before equals No_Element, the nodes of Source are spliced after the last node of Target. The length of Target is incremented by the number of nodes in Source, and the length of Source is set to 0.
procedure Splice (Target : in out List; Before : in Cursor; Position : in Cursor);
If Position equals No_Element, or if Position equals Before, or if the successor of Position equals Before, the operation has no effect. Otherwise the node designated by Position becomes the predecessor of Before. If Before equals No_Element, then Position is moved after the last node.
procedure Splice (Target : in out List; Before : in Cursor; Source : in out List; Position : in Cursor);
If Source denotes the same object as Target, then Splice is equivalent to the Splice operation sans Source parameter. If Position equals No_Element, the operation has no effect. Otherwise the node designated by Position is removed from Source and moved to Target, immediately prior to Before. If Before equals No_Element, then Position is moved after the last node of Target. The length of Target is incremented, and the length of Source is decremented.
function First (Container : List) return Cursor;
Returns a cursor that designates the first node. If Container is empty, First returns the value No_Element.
function First_Element (Container : List) return Element_Type;
If Container is empty, then Constraint_Error is propagated. Otherwise, it returns Element (First (Container)).
function Last (Container : List) return Cursor;
Returns a cursor that designates the last node. If Container is empty, Last returns the value No_Element.
function Last_Element (Container : List) return Element_Type;
If Container is empty, then Constraint_Error is propagated. Otherwise, it returns Element (Last (Container)).
function Contains (Container : List; Item : Element_Type) return Boolean;
Equivalent to Find (Container, Item) /= No_Element.
function Find (Container : List; Item : Element_Type; Position : Cursor := No_Element) return Cursor;
Searches the nodes of Container for an element equal to Item. The search starts at the node designated by Position. If Position equals No_Element, then the search begins at the first node. If no element is found that matches Item, then Find returns the value No_Element.
function Reverse_Find (Container : List; Item : Element_Type; Position : Cursor := No_Element) return Cursor;
Searches the nodes of Container in reverse for an element equal to Item. The search starts at the node designated by Position. If Position equals No_Element, then the search begins at the last node. If no element is found that matches Item, then Find returns the value No_Element.
function Next (Position : Cursor) return Cursor;
If Position equals No_Element or designates the last node of the container, then Next returns the value No_Element. Otherwise, returns a cursor that designates the successor of the node designated by Position.
function Previous (Position : Cursor) return Cursor;
If Position equals No_Element or designates the first node of the container, then Previous returns the value No_Element. Otherwise, returns a cursor that designates the predecessor of the node designated by Position.
The procedure Next is equivalent to Position := Next (Position).
The procedure Previous is equivalent to Position := Previous (Position).
function Has_Element (Position : Cursor) return Boolean;
Returns True if Position designates an element, and returns False otherwise.
AARM Note: To Be Honest: This function may not detect cursors that designate deleted elements; such cursors are invalid (see below) and any use of them (including in this routine) is erroneous.
procedure Iterate
(Container : in List;
Process : not null access procedure (Position : in Cursor));
Invokes Process.all with a cursor that designates each node in Container. Any exceptions raised during Process are propagated.
procedure Reverse_Iterate
(Container : in List;
Process : not null access procedure (Position : in Cursor));
Iterates over the nodes in Container as per Iterate, except that elements are traversed in reverse order.
Erroneous Execution
A Cursor value is invalid if any of the following have occurred since it was created:
* The list that contains the element it designates has been finalized;
* The list that contains the element it designates has been used as the Source or Target of a call to Move;
* The element it designates has been deleted.
The result of "=" or Has_Element is unspecified if it is called with an invalid cursor parameter. Execution is erroneous if any other subprogram declared in Containers.Doubly_Linked_Lists is called with an invalid cursor parameter, or if the cursor designates an element in a different list object than the appropriate one specified in the call.
AARM Notes: The list above is intended to be exhaustive. In other cases, a cursor value continues to designate its original element. For instance, cursor values survive the insertion and deletion of other nodes.
While it is possible to check for these cases, in many cases the overhead necessary to make the check is substantial in time or space. Implementations are encouraged to check for as many of these cases as possible and raise Constraint_Error if detected. End AARM Notes.
Implementation Requirements
No storage associated with a doubly-linked List object shall be lost upon assignment or scope exit.
Implementation Advice
Containers.Doubly_Linked_Lists should be implemented similarly to a linked list. In particular, if N is the length of a list, then the worst-case time complexity of Element, Insert with Count=1, and Delete with Count=1 should be O(log N).
AARM Note We do not mean to overly constrain implementation strategies here. However, it is important for portability that the performance of large containers has roughly the same factors on different implementations. If a program is moved to an implementation that takes O(N) time to access elements, that program could be unusable when the lists are large. We allow O(log N) access because the proportionality constant and caching effects are likely to be larger than the log factor, and we don't want to discourage innovative implementations.
The worst-case time complexity of a call on an instantiation of Containers.Doubly_Linked_Lists.Generic_Sort should be O(N**2), and the average time complexity should be better than O(N**2).
AARM Note In other words, we're requiring the use of a better than O(N**2) sorting algorithm, such as Quicksort. No Bubble sorts allowed!
NOTE Sorting a list never copies elements, and is a stable sort (equal elements remain in the original order). This is different than sorting an array or vector, which may need to copy elements, and is probably not a stable sort.
A.17.4 The Package Containers.Hashed_Maps
The language-defined package Containers.Hashed_Maps provides private types Map and Cursor, and a set of operations for each type. A hashed map container allows an arbitrary type to be used as a key to find the element associated with that key.
AARM Note: The name is "Hashed_Maps" to allow for a secondary standard to include "Ordered_Maps".
Static Semantics
The library package Containers.Hashed_Maps has the following declaration:
generic
type Key_Type is private;
type Element_Type is private;
with function Hash (Key : Key_Type) return Hash_Type;
with function "=" (Left, Right : Key_Type) return Boolean is <>;
with function Is_Equal_Key (Left, Right : Key_Type) return Boolean is "=";
with function "=" (Left, Right : Element_Type) return Boolean is <>;
package Ada.Containers.Hashed_Maps is pragma Preelaborate (Hashed_Maps);
type Map is tagged private;
type Cursor is private;
Empty_Map : constant Map;
No_Element : constant Cursor;
function "=" (Left, Right : Map) return Boolean;
function Length (Container : Map) return Count_Type;
function Is_Empty (Container : Map) return Boolean;
procedure Clear (Container : in out Map);
function Element (Position : Cursor) return Element_Type;
procedure Query_Element (Position : in Cursor; Process : not null access procedure (Element : in Element_Type));
procedure Update_Element (Position : in Cursor; Process : not null access procedure (Element : in out Element_Type));
procedure Replace_Element (Position : in Cursor; By : in Element_Type);
procedure Move (Target : in out Map; Source : in out Map);
procedure Insert (Container : in out Map; Key : in Key_Type; New_Item : in Element_Type; Position : out Cursor; Inserted : out Boolean);
procedure Insert_Or_Replace (Container : in out Map; Key : in Key_Type; New_Item : in Element_Type);
procedure Insert (Container : in out Map; Key : in Key_Type; Position : out Cursor; Inserted : out Boolean);
procedure Delete (Container : in out Map; Key : in Key_Type);
procedure Delete (Container : in out Map; Position : in out Cursor);
function Contains (Container : Map; Key : Key_Type) return Boolean;
function Find (Container : Map; Key : Key_Type) return Cursor;
function Element (Container : Map; Key : Key_Type) return Element_Type;
function Capacity (Container : Map) return Count_Type;
procedure Set_Capacity (Container : in out Map; Capacity : in Count_Type);
function First (Container : Map) return Cursor;
function Next (Position : Cursor) return Cursor;
procedure Next (Position : in out Cursor);
function Has_Element (Position : Cursor) return Boolean;
function Key (Position : Cursor) return Key_Type;
function Is_Equal_Key (Left, Right : Cursor) return Boolean;
function Is_Equal_Key (Left : Cursor; Right : Key_Type) return Boolean;
function Is_Equal_Key (Left : Key_Type; Right : Cursor) return Boolean;
procedure Iterate (Container : in Map; Process : not null access procedure (Position : in Cursor));
private
... -- not specified by the language
end Ada.Containers.Hashed_Maps;
An object of type Map contains an expandable hash table, which is used to provide direct access to elements. The capacity of an object of type Map is the maximum number of elements that can be inserted into the hash table prior to it being automatically expanded. The length of an object of type Map object is the number of elements it contains. If an object of type Map is not otherwise initialized, it will be initialized with a length of 0.
A map contains pairs of keys and elements, called a node. Map cursors designate nodes, but also can be thought of as designating an element (the element contained in the node) for consistency with the other containers.
The type Map needs finalization (see 7.6).
AARM Notes The expected implementation for a Map uses a hash table which is grown when it is too small, with linked lists hanging off of each bucket. Note that in that implementation a cursor needs a back pointer to the Map object to implement iteration; that could either be in the nodes, or in the cursor object. To provide an average O(1) access time, capacity would typically equal the number of buckets in such an implementation, so that the average bucket linked list length would be no more than 1.0.
There is no defined relationship between elements in a map. Typically, iteration will return elements in the order that they are hashed in. End AARM Notes
Function Hash is expected to return the same result value each time it is called with a particular key value. For any two key values for which Is_Equal_Key returns True, Hash should return the same result value. If Hash behaves in some other manner, the behavior of this package is unspecified. Which subprograms of this package call Hash, and how many times they call Hash, is unspecified.
AARM Notes The implementation is not required to protect against Hash raising an exception, or returning random numbers, or any other "bad" behavior. It's not practical to do so, and a broken Hash function makes the container unusable.
The implementation can call Hash whenever it is needed; we don't want to specify how often that happens. The result must remain the same (this is logically a pure function), or the behavior is unspecified. End AARM Notes
Function Is_Equal_Key and the generic formal equality operator for keys are expected to return the same result value each time either is called with a particular pair of key values. If the generic formal equality operator returns True for a particular pair of key values, then the formal function Is_Equal_Key is expected to return True for the same pair of key values. If Is_Equal_Key or the generic formal equality operator for keys behaves in some other manner, the behavior of this package is unspecified. Which subprograms of this package call Is_Equal_Key or the generic formal equality operator for keys, and how many times they call Is_Equal_Key or the generic formal equality operator for keys, is unspecified.
AARM Notes As with Hash, the implementation is not required to protect against Is_Equal_Key or the generic formal equality operator for keys raising an exception or returning random results. Similarly, the implementation can call either of these operations whenever they are needed. The result must remain the same (these are logically pure functions), or the behavior is unspecified.
The generic equality operation for keys is not expected to be explicitly specified in instantiations; the default equality is good enough. Key equality for a map is named Is_Equal_Key in order that named notation can be used in an instantiation. Otherwise, there would be two parameters named "=", and named notation could not be used. This generic has too many formal parameters to allow confortable use of positional notation; named notation is required. Since key equality is likely to be overridden and needs to be frequently referenced in the wording, it is given the explicit name. End AARM Notes
If the value of a key stored in a node of a map is changed other than by an operation in this package such that at least one of Hash, Is_Equal_Key, or the generic formal equality operator give different results, the behavior of this package is unspecified.
AARM Notes The implementation is not required to protect against changes to key values other than via the operations declared in the Hashed_Maps package.
To see how this could happen, imagine an instantiation of Hashed_Maps where the key type is an access-to-variable type and Hash returns a value derived from the components of the designated object. Then, any operation that has a key value could modify those components and change the hash value:
Key (Map).Some_Component := New_Value;
This is really a design error on the part of the user of the map; it shouldn't be possible to modify keys stored in a map. But we can't prevent this error anymore than we can prevent someone passing as Hash a random number generator. End AARM Notes
Empty_Map represents the empty Map object. It has a length of 0. If an object of type Map is not otherwise initialized, it will be initialized to the same value as Empty_Map.
No_Element represents a cursor that designates no element. If an object of type Cursor is not otherwise initialized, it will be initialized to the same value as No_Element.
function "=" (Left, Right : Map) return Boolean;
If Left and Right denote the same map Map object, then the function returns immediately the value True. If Left and Right have different lengths, then the function returns the value False. Otherwise, for each key K in Left, the function returns False if:
* K is not present in Right (using the generic formal equality operator for keys); or
* the element associated with K in Left is not equivalent to the element associated with K in Right (using the generic formal equality operator for elements).
If the function has not returned a result after checking all of the keys, the function returns True. Any exception raised during evaluation of key or element equality is propagated.
The function Length returns the number of key/element pairs in Map.
The function Is_Empty is equivalent to Length (Container) = 0.
Clear removes all the elements from Map. The capacity of Map is not affected. Any exception raised during deallocation of storage is propagated.
function Element (Position : Cursor) return Element_Type;
If Position equals No_Element, then Constraint_Error is propagated. Otherwise, this operation returns the element designated by Position.
procedure Query_Element
(Position : in Cursor;
Process : not null access procedure (Element : in Element_Type));
If Position equals No_Element, then Constraint_Error is propagated. Otherwise, Query_Element invokes Process.all with the element on node designated by Position as the argument.
procedure Update_Element
(Position : in Cursor;
Process : not null access procedure (Element : in out Element_Type));
If Position equals No_Element, then Constraint_Error is propagated. Otherwise, Update_Element invokes Process.all with the element designated by Position as the argument.
If Element_Type is unconstrained and definite, then the Element parameter of Process.all shall be unconstrained.
AARM Note: This means that the elements cannot be aliased nor directly allocated from the heap; it must be possible to change the discriminants of the element in place.
procedure Replace_Element (Position : in Cursor; By : in Element_Type);
If Position equals No_Element, then Constraint_Error is propagated. Otherwise this operation assigns By to the element of the node designated by Position.
procedure Move (Target : in out Map; Source : in out Map);
If Target denotes the same object as Source, then this operation has no effect. If the length of Target is greater than 0, then Move raises Constraint_Error. Otherwise, the internal hash table of Target is deallocated; then the internal hash table is removed from Source and moved to Target. Source has capacity 0 after a successful call to Move.
procedure Insert (Container : in out Map; Key : in Key_Type; New_Item : in Element_Type; Position : out Cursor; Inserted : out Boolean);
If Length (Container) equals Capacity (Container), then Insert calls Set_Capacity to increase the capacity of Container to some larger value. Insert then uses Hash and Is_Equal_Key to check if Key is already present in Container. If a key matches, Inserted returns False and Position designates the element with the matching key. Otherwise, Insert allocates a new node, initializes it to Key and New_Item, and adds it to Container. Inserted returns True and Position designates the newly-inserted node. Any exception raised during allocation is propagated and Container is not modified.
AARM Notes: Insert should only compare keys that hash to the same bucket in the hash table.
We specify when Set_Capacity is called to bound the overhead of capacity expansion operations (which are potentially expensive). Moreover, expansion can be predicted by comparing Capacity(Map) to Length(Map). Since we don't specify by how much the hash table is expanded, this only can be used to predict the next expansion, not later ones. End AARM Notes.
procedure Insert_Or_Replace (Container : in out Map; Key : in Key_Type; New_Item : in Element_Type);
Insert_Or_Replace inserts Key and New_Item as per Insert, with the difference that if Key is already in the map, then this operation assigns New_Item to the element associated with Key. Any exception raised during assignment is propagated.
AARM Note: This is equivalent to:
declare Inserted : Boolean; C : Cursor; begin Insert (Container, Key, New_Item, Inserted => Inserted, Position => C); if not Inserted then Replace_Element (Container, C, New_Item); end if; end;
but is much more convinient, as there are no out parameters to deal with.
procedure Insert (Container : in out Map; Key : in Key_Type; Position : out Cursor; Inserted : out Boolean);
Inserts Key into Container as per Insert (Container, Key, New_Item, Position, Inserted), with the difference that an object initialized with any implicit initial values for any part (as for an object_declaration with no initialization expression - see 3.3.1) is inserted.
procedure Delete (Container : in out Map; Key : in Key_Type);
It uses Hash and Is_Equal_Key to check if Key is present in Container. If Key matches the key of a node, Delete removes the node from the map and then deallocates the node.
AARM Notes: Delete should only compare keys that hash to the same bucket in the hash table. Delete should work on an empty map; nothing happens in that case.
procedure Delete (Container : in out Map; Position : in out Cursor);
If Position equals No_Element, this operation has no effect. Otherwise, Delete removes the node from the map and deallocates the node. Position is set to No_Element on return.
function Find (Container : Map; Key : Key_Type) return Cursor;
If Length (Container) equals 0, then this function returns No_Element. Otherwise, it uses Hash and Is_Equal_Key to check if Key is present in Container. If Key is present in Container, it returns a cursor designating the node with the matching key; otherwise, it returns No_Element.
AARM Note: Find should only compare keys that hash to the same bucket in the hash table.
The function Contains is equivalent to Find (Container, Key) /= No_Element.
The function Element is equivalent to Element (Find (Container, Key)).
The function Capacity returns the capacity of Container.
procedure Set_Capacity (Container : in out Map; Capacity : in Count_Type);
If Capacity < Length (Container), Constraint_Error is raised. Otherwise, Set_Capacity allocates a new hash table such that the length of the resulting map can become at least the value Capacity without requiring an additional Set_Capacity operation. If the allocation fails, the exception is propagated and Container is not modified. It then rehashes the nodes in Container onto the new hash table. It replaces the old hash table with the new hash table, and then deallocates the old hash table.
AARM Notes: This routine is used to preallocate the internal hash table to the specified capacity such that future Inserts do not require expansion of the hash table. Therefore, the implementation should allocate the needed memory to make that true at this point, even though the visible semantics could be preserved by waiting until enough elements are inserted.
While Set_Capacity can be used to reduce the capacity of a map, we do not specify whether an implementation actually supports reduction of the capacity. Since the actual capacity can be anything greater than or equal to Count, an implementation never has to reduce the capacity. End AARM Notes
function First (Container : Map) return Cursor;
If Length (Container) = 0, then First returns No_Element. Otherwise, First returns a cursor that designates the first hashed node in Container.
AARM Note: In a typical implementation, this will be the first node in the lowest numbered hash bucket that contains a node.
function Next (Position : Cursor) return Cursor;
Returns a cursor that designates the node that immediately follows Position. If there are no more nodes in the map identified by Position, then No_Element is returned. If Position equals No_Element, then No_Element is returned.
If there are no other intervening operations, calling Next in a loop starting with First (Container) shall return a cursor designating each node in the Container (other than First) exactly once, then return No_Element.
AARM Notes In a typical implementation, this will return the next node in a bucket; if Position is the last node in a bucket, this will return the first node in the next non-empty bucket.
A typical implementation will need to a keep a pointer at the map container in the cursor in order to implement this function.
The procedure Next is equivalent to Position := Next (Position).
function Has_Element (Position : Cursor) return Boolean;
Returns True if Position designates an element, and returns False otherwise.
AARM Note: To Be Honest: This function may not detect cursors that designate deleted elements; such cursors are invalid (see below) and any use of them (including in this routine) is erroneous.
function Key (Position : Cursor) return Key_Type;
If Position equals No_Element, then Constraint_Error is propagated. Otherwise, this operation returns the key component of the node designated by Position.
The function Is_Equal_Key (Left, Right : Cursor) is equivalent to Is_Equal_Key (Key (Left), Key (Right)).
The function Is_Equal_Key (Left : Cursor; Right : Key_Type) is equivalent to Is_Equal_Key (Key (Left), Right).
The function Is_Equal_Key (Left : Key_Type; Right : Cursor) is equivalent to Is_Equal_Key (Left, Key (Right)).
procedure Iterate
(Container : in Map;
Process : not null access procedure (Position : in Cursor));
Iterate calls Process.all with a cursor that designates each node in the Container. Any exception raised by Process is propagated.
Legality Rules
An instantiation of Containers.Hashed_Maps shall be at the library level.
AARM Note A implementation will typically need to use controlled types to insure that the Implementation Requirement is met. These would require all instantiations to occur at the library level. We certainly do not want to require magic for nested container instantiations, while not giving similar capabilities to users. We've made this a legality rule to enhance portability. This rule will be dropped if AI-344 or some other solution to nested controlled types is adopted.
Erroneous Execution
A Cursor value is invalid if any of the following have occurred since it was created:
* The map that contains the element it designates has been finalized;
* The map that contains the element it designates has been used as the Source or Target of a call to Move;
* The element it designates has been deleted from the map.
The result of "=" or Has_Element is unspecified if it is called with an invalid cursor parameter. Execution is erroneous if any other subprogram declared in Containers.Hashed_Maps is called with an invalid cursor parameter, or if the cursor designates an element in a different map object than the appropriate one specified in the call.
AARM Notes: The list above is intended to be exhaustive. In other cases, a cursor value continues to designate its original element. For instance, cursor values survive the insertion and deletion of other nodes.
While it is possible to check for these cases, in many cases the overhead necessary to make the check is substantial in time or space. Implementations are encouraged to check for as many of these cases as possible and raise Constraint_Error if detected. End AARM Notes.
Implementation Requirements
No storage associated with a Map object shall be lost upon assignment or scope exit.
Implementation Advice
If N is the length of a map, the average time complexity of Element, Insert, and Find should be O(log N).
AARM Note We do not mean to overly constrain implementation strategies here. However, it is important for portability that the performance of large containers has roughly the same factors on different implementations. If a program is moved to an implementation for which Find is O(N), that program could be unusable when the maps are large. We allow O(log N) access because the proportionality constant and caching effects are likely to be larger than the log factor, and we don't want to discourage innovative implementations.
A.17.5 The Package Containers.Ordered_Sets
The language-defined package Containers.Ordered_Sets provides private types Set and Cursor, and a set of operations for each type. An ordered set container orders its elements per a specified relation.
AARM Note: This is called "Ordered_Sets" so as to allow a possible future enhancement to include unsorted sets (which would be called "Sets") or hashed sets (which would be called "Hashed_Sets").
Static Semantics
The package Containers.Ordered_Sets has the following declaration:
generic
type Element_Type is private;
with function "<" (Left, Right : Element_Type) return Boolean is <>;
with function "=" (Left, Right : Element_Type) return Boolean is <>;
package Ada.Containers.Ordered_Sets is pragma Preelaborate (Ordered_Sets);
type Set is tagged private;
type Cursor is private;
Empty_Set : constant Set;
No_Element : constant Cursor;
function "=" (Left, Right : Set) return Boolean;
function Length (Container : Set) return Count_Type;
function Is_Empty (Container : Set) return Boolean;
procedure Clear (Container : in out Set);
function Element (Position : Cursor) return Element_Type;
procedure Query_Element (Position : in Cursor; Process : not null access procedure (Element : in Element_Type));
procedure Move (Target : in out Set; Source : in out Set);
procedure Insert (Container : in out Set; New_Item : in Element_Type; Position : out Cursor; Inserted : out Boolean);
procedure Insert (Container : in out Set; New_Item : in Element_Type);
procedure Delete (Container : in out Set; Item : in Element_Type);
procedure Delete (Container : in out Set; Position : in out Cursor);
procedure Delete_First (Container : in out Set);
procedure Delete_Last (Container : in out Set);
procedure Union (Target : in out Set; Source : in Set);
function Union (Left, Right : Set) return Set;
function "or" (Left, Right : Set) return Set renames Union;
procedure Intersection (Target : in out Set; Source : in Set);
function Intersection (Left, Right : Set) return Set;
function "and" (Left, Right : Set) return Set renames Intersection;
procedure Difference (Target : in out Set; Source : in Set);
function Difference (Left, Right : Set) return Set;
function "-" (Left, Right : Set) return Set renames Difference;
procedure Symmetric_Difference (Target : in out Set; Source : in Set);
function Symmetric_Difference (Left, Right : Set) return Set;
function "xor" (Left, Right : Set) return Set renames Symmetric_Difference;
function Is_Disjoint (Left, Right : Set) return Boolean;
function Is_Subset (Subset : Set; Of_Set : Set) return Boolean;
function Contains (Container : Set; Item : Element_Type) return Boolean;
function Find (Container : Set; Item : Element_Type) return Cursor;
function Floor (Container : Set; Item : Element_Type) return Cursor;
function Ceiling (Container : Set; Item : Element_Type) return Cursor;
function First (Container : Set) return Cursor;
function First_Element (Container : Set) return Element_Type;
function Last (Container : Set) return Cursor;
function Last_Element (Container : Set) return Element_Type;
function Next (Position : Cursor) return Cursor;
function Previous (Position : Cursor) return Cursor;
procedure Next (Position : in out Cursor);
procedure Previous (Position : in out Cursor);
function Has_Element (Position : Cursor) return Boolean;
function "<" (Left, Right : Cursor) return Boolean;
function ">" (Left, Right : Cursor) return Boolean;
function "<" (Left : Cursor; Right : Element_Type) return Boolean;
function ">" (Left : Cursor; Right : Element_Type) return Boolean;
function "<" (Left : Element_Type; Right : Cursor) return Boolean;
function ">" (Left : Element_Type; Right : Cursor) return Boolean;
procedure Iterate (Container : in Set; Process : not null access procedure (Position : in Cursor));
procedure Reverse_Iterate (Container : in Set; Process : not null access procedure (Position : in Cursor));
generic
type Key_Type (<>) is limited private;
with function Key (Element : Element_Type) return Key_Type;
with function "<" (Left : Key_Type; Right : Element_Type) return Boolean is <>;
with function ">" (Left : Key_Type; Right : Element_Type) return Boolean is <>;
package Generic_Keys is
function Contains (Container : Set; Key : Key_Type) return Boolean;
function Find (Container : Set; Key : Key_Type) return Cursor;
function Floor (Container : Set; Item : Key_Type) return Cursor;
function Ceiling (Container : Set; Item : Key_Type) return Cursor;
function Key (Position : Cursor) return Key_Type;
function Element (Container : Set; Key : Key_Type) return Element_Type;
procedure Delete (Container : in out Set; Key : in Key_Type);
function "<" (Left : Cursor; Right : Key_Type) return Boolean;
function ">" (Left : Cursor; Right : Key_Type) return Boolean;
function "<" (Left : Key_Type; Right : Cursor) return Boolean;
function ">" (Left : Key_Type; Right : Cursor) return Boolean;
procedure Checked_Update_Element (Container : in out Set; Position : in Cursor; Process : not null access procedure (Element : in out Element_Type));
end Generic_Keys;
private
... -- not specified by the language
end Ada.Containers.Ordered_Sets;
The type Set needs finalization (see 7.6).
Functions "<" and "=" on Element_Type values are expected to return the same result value each time they are called with a particular pair of element values. If A = B returns True, then B = A should also return True. If A < B returns True, then B < A should return False. If "<" or "=" behaves in some other manner, the behavior of this package is unspecified. Which subprograms of this package call "<" and "=", and how many times the functions are called, is unspecified.
Two elements E1 and E2 are equivalent if not (E1 < E2) and not (E2 < E1) is true for the elements, using the generic formal less-than operator for elements.
AARM Notes The implementation is not required to protect against "<" or "=" raising an exception, or returning random results, or any other "bad" behavior. It's not practical to do so, and a broken "<" or "=" function makes the container unusable.
The implementation can call "<" and "=" whenever it is needed; we don't want to specify how often that happens. The result must remain the same (these are logically pure functions), or the behavior is unspecified. End AARM Notes
If the value of an element stored in a set is changed other than by an operation in this package such that at least one of "<" or "=" give different results, the behavior of this package is unspecified.
AARM Notes The implementation is not required to protect against changes to element values other than via the operations declared in the Ordered_Sets package.
To see how this could happen, imagine an instantiation of Ordered_Sets package where the element type is an access-to-variable type and "<" returns a value derived from comparing the components of the designated objects. Then, any operation that has a element value (even if the element value is constant) could modify those components and change the result of "<":
Element (Set).Some_Component := New_Value;
This is really a design error on the part of the user of the set; it shouldn't be possible to modify elements stored in a set such that "<" changes. But we can't prevent this error anymore than we can prevent someone passing as "<" a routine that produces random answers. End AARM Notes
Empty_Set represents the empty ordered set. If an object of type Set is not otherwise initialized, it will be initialized to the same value as Empty_Set.
No_Element represents a cursor that designates no element. If an object of type Cursor is not otherwise initialized, it will be initialized to the same value as No_Element.
function "=" (Left, Right : Set) return Boolean;
If Left and Right denote the same object, then the function returns True. If Left and Right have different lengths, then the function returns False. Otherwise, it compares elements in sequential order using the generic formal equality operator for elements. Any exception raised during evaluation of element equality is propagated. If a corresponding pair of elements compare False, the function returns False. Otherwise if all corresponding elements compare True, the function returns True.
The function Length returns the number of elements in Container.
The function Is_Empty is equivalent to Length (Container) = 0.
Clear removes all the elements in Set, and sets the length to 0.
function Element (Position : Cursor) return Element_Type;
If Position equals No_Element, then Constraint_Error is propagated. Otherwise, it returns the element designated by Position.
procedure Query_Element
(Position : in Cursor;
Process : not null access procedure (Element : in Element_Type));
If Position equals No_Element, then Constraint_Error is propagated. Otherwise, Query_Element invokes Process.all with the element on node designated by Position as the argument.
procedure Move (Target : in out Set; Source : in out Set);
If the length of Target is greater than 0, then Move raises Constraint_Error. Otherwise, the elements are removed from Source and moved to Target. The length of Source is 0 after a successful call to Move.
procedure Insert (Container : in out Set; New_Item : in Element_Type; Position : out Cursor; Inserted : out Boolean);
Insert compares New_Item to the elements in Container using the generic formal less-than operator for elements. If an element equivalent to New_Item is already in Container, Inserted is False and Position designates the element. Otherwise, it allocates a new element which is initialized to New_Item. Inserted returns True and Position designates the newly-inserted element. Any exception raised during allocation and initialization is propagated and Container is not modified.
procedure Insert (Container : in out Set; New_Item : in Element_Type);
Insert compares New_Item to the elements in Container using the generic formal less-than operator for elements. If an element equivalent to New_Item is already in Container, Insert does nothing. Otherwise, it allocates a new element which is initialized to New_Item. Any exception raised during allocation and initialization is propagated and Container is not modified.
procedure Delete (Container : in out Set; Item : in Element_Type);
Delete searches Container for an element equivalent to Item. If there is an element in Container equivalent to Item, the element is removed from Container.
AARM Note: The internal node containing the element may be deallocated now, or it may be saved and reused later.
procedure Delete (Container : in out Set; Position : in out Cursor);
If Position equals No_Element, the operation has no effect. Otherwise, Delete removes the element designated by Position from Container. Position is set to No_Element on return.
procedure Delete_First (Container : in out Set);
If Container is empty, the operation has no effect. Otherwise the element designated by First (Container) is removed from Container.
procedure Delete_Last (Container : in out Set);
If Container is empty, the operation has no effect. Otherwise the element designated by Last (Container) is removed from Container.
procedure Union (Target : in out Set; Source : in Set);
The elements of Source that are not equivalent to items already in Target are inserted into Target.
AARM Note: If the objects are the same, the result is the same as the original object. The implementation needs to take care so that aliasing effects do not make the result trash; Union (S, S); must work.
function Union (Left, Right : Set) return Set;
Returns a set comprising all of the elements of Left, and the elements of Right that are not equivalent to elements of Left.
procedure Intersection (Target : in out Set; Source : in Set);
All the elements of Target that are not equivalent to the corresponding elements of Source are deleted from Target.
AARM Note: If the objects are the same, the result is the same as the original object. The implementation needs to take care so that aliasing effects do not make the result trash; Intersection (S, S); must work.
function Intersection (Left, Right : Set) return Set;
Returns a set comprising all the elements of Left that are equivalent to the corresponding elements of Right.
procedure Difference (Target : in out Set; Source : in Set);
If Target denotes the same object as Source, then the operation clears Target. Otherwise, it deletes the elements of Target that are equivalent to elements of Source.
function Difference (Left, Right : Set) return Set;
Returns a set comprising the elements of Left that are not equivalent to the corresponding elements of Right.
procedure Symmetric_Difference (Target : in out Set; Source : in Set);
If Target denotes the same object as Source, then the operation clears Target. Otherwise, it deletes the elements of Target that are equivalent to elements of Source, and inserts the elements are Source that are not equivalent to the corresponding elements of Target.
function Symmetric_Difference (Left, Right : Set) return Set;
Returns a set comprising the elements of Left that are not equivalent to the corresponding elements of Right, and the elements of Right that are not equivalent to the corresponding elements of Left.
function Is_Disjoint (Left, Right : Set) return Boolean;
If an element of Left is equivalent to some element of Right, then Is_Disjoint returns False. Otherwise it returns True.
AARM Note: This operation is communitive.
function Is_Subset (Subset : Set; Of_Set : Set) return Boolean;
If an element of Subset is not equivalent to some element of Of_Set, the function returns False. Otherwise it returns True.
AARM Note: This operation is not communitive, so we use parameter names that make it clear in named notation which set is which.
function Find (Container : Set; Item : Element_Type) return Cursor;
The Find operation searches for the element equivalent to Item. If it finds an equivalent element, it returns a cursor designating the element. Otherwise it returns No_Element.
The function Contains is equivalent to Find (Set, Item) /= No_Element.
function Floor (Container : Set; Item : Element_Type) return Cursor;
The Floor operation searches for the largest element not greater than Item, using the generic formal less-than operator for elements. If there is an element in Set that is not greater than Item, it returns a cursor designating the node containing the element. Otherwise it returns No_Element.
function Ceiling (Container : Set; Item : Element_Type) return Cursor;
The Ceiling operation searches for the smallest element not less than Item, using the generic formal less-than operator for elements. If there is an element in Set that is not less than Item, it returns a cursor designating the node containing the element. Otherwise it returns No_Element.
function First (Container : Set) return Cursor;
Returns a cursor that designates the smallest element. If Container is empty, it returns No_Element.
function First_Element (Container : Set) return Element_Type;
If Container is empty, then Constraint_Error is propagated. Otherwise, it returns the element designated by First (Container).
function Last (Container : Set) return Cursor;
Returns a cursor that designates the greatest element. If Container is empty, it returns No_Element.
function Last_Element (Container : Set) return Element_Type;
If Container is empty, then Constraint_Error is propagated. Otherwise, it returns the element designated by Last (Container).
function Next (Position : Cursor) return Cursor;
If Position equals No_Element, then No_Element is returned. Otherwise it returns the successor of the element designated by Position.
function Previous (Position : Cursor) return Cursor;
If Position equals No_Element, then No_Element is returned. Otherwise it returns the predecessor of the element designated by Position.
The procedure Next is equivalent to Position := Next (Position).
The procedure Previous is equivalent to Position := Previous (Position).
function Has_Element (Position : Cursor) return Boolean;
Returns True if Position designates an element, and returns False otherwise.
AARM Note: To Be Honest: This function might not detect cursors that designate deleted elements; such cursors are invalid (see below) and any use of them (including in this routine) is erroneous.
The function "<" (Left, Right : Cursor) is equivalent to Element (Left) < Element (Right).
The function ">" (Left, Right : Cursor) is equivalent to Element (Right) < Element (Left).
The function "<" (Left : Cursor; Right : Element_Type) is equivalent to Element (Left) < Right.
The function ">" (Left : Cursor; Right : Element_Type) is equivalent to Right < Element (Left).
The function "<" (Left : Element_Type; Right : Cursor) is equivalent to Left < Element (Right).
The function ">" (Left : Element_Type; Right : Cursor) is equivalent to Element (Right) < Left.
procedure Iterate
(Container : in Set;
Process : not null access procedure (Position : in Cursor));
Invokes Process.all with a cursor that designates each element in Container.
procedure Reverse_Iterate
(Container : in Set;
Process : not null access procedure (Position : in Cursor));
Iterates over the elements in Container as per Iterate, with the difference that the nodes are traversed in reverse order.
The package Generic_Keys provides operations that allow set manipulation in terms of a key (typically, a portion of an element) instead of a complete element.
The formal function Key is intended to extract a key value from an element. For any two elements E1 and E2, it is expected that (E1 < E2) = (Key(E1) < E2) = (Key(E2) > E1). If Key, "<", or ">" behaves in some other manner, the behavior of this package is unspecified. Which subprograms of this package call Key, "<" and ">", and how many times the functions are called, is unspecified.
AARM Note: As for the main package, if the formal subprograms don't make sense, the package is not required to do anything that makes sense.
The operations in package Generic_Keys named Contains, Find, Floor, Ceiling, Element, Delete, and operators designated "<" and ">", are equivalent to the corresponding operations in the parent package, with the difference that the Key subprogram parameter is compared to elements in the container using the Generic_Keys generic formal relational operators.
function Key (Position : Cursor) return Key_Type;
If Position equals No_Element, then Constraint_Error is propagated. Otherwise, this operation returns the key obtained by calling the generic formal Key operation on the element designated by Position.
procedure Checked_Update_Element
(Container : in out Set;
Position : in Cursor; Process : not null access procedure (Element : in out Element_Type));
If Position equals No_Element, then Constraint_Error is propagated. Otherwise, Checked_Update_Element uses Key to save the key value (KB) of the element. Checked_Update_Element then invokes Process.all with the element designated by Position as the argument. The key KB is compared to the new element value using the generic formal less-than and greater-than operators; if either returns True, the element is removed from the set, then is inserted back into the set. If the insertion fails, Constraint_Error is raised.
AARM Note: The key check insures that the invariants of the set are preserved by the modification. If not, we try to re-insert the element at the correct place. If that fails, we have no choice other than to drop the element completely, because saving a value to roll back to would be expensive and defeat the purpose of this routine (which is cheap, in-place modifications).
If Element_Type is unconstrained and definite, then the Element parameter of Process.all shall be unconstrained.
AARM Note: This means that the elements cannot be aliased nor directly allocated from the heap; it must be possible to change the discriminants of the element in place.
Legality Rules
An instantiation of Containers.Ordered_Sets shall be at the library level.
AARM Note A implementation will typically need to use controlled types to insure that the Implementation Requirement is met. These would require all instantiations to occur at the library level. We certainly do not want to require magic for nested container instantiations, while not giving similar capabilities to users. We've made this a legality rule to enhance portability. This rule will be dropped if AI-344 or some other solution to nested controlled types is adopted.
Erroneous Execution
A Cursor value is invalid if any of the following have occurred since it was created:
* The set that contains the element it designates has been finalized;
* The set that contains the element it designates has been used as the Source or Target of a call to Move;
* The element it designates has been deleted from the set
The result of "=" or Has_Element is unspecified if it is called with an invalid cursor parameter. Execution is erroneous if any other subprogram declared in Containers.Ordered_Sets is called with an invalid cursor parameter, or if the cursor designates an element in a different set object than the appropriate one specified in the call.
AARM Notes: The list above is intended to be exhaustive. In other cases, a cursor value continues to designate its original element. For instance, cursor values survive the insertion and deletion of other elements.
While it is possible to check for these cases, in many cases the overhead necessary to make the check is substantial in time or space. Implementations are encouraged to check for as many of these cases as possible and raise Constraint_Error if detected. End AARM Notes.
Implementation Requirements
No storage associated with an ordered set object shall be lost upon assignment or scope exit.
Implementation Advice
If N is the length of a set, then:
* The worst-case time complexity of Insert and Find should be O((log N)**2) or better;
* the worst-case time complexity of Element should be O(log N) or better.
AARM Note A balanced (red-black) tree for keys has O(log N) worst-case performance. Note that a O(N) worst-case implementation (like a list) would be wrong. We do not mean to overly constrain implementation strategies here. However, it is important for portability that the performance of large containers has roughly the same factors on different implementations. If a program is moved to an implementation that takes O(N) to find elements, that program could be unusable when the sets are large. We allow the extra log N factors because the proportionality constant and caching effects are likely to be larger than the log factor, and we don't want to discourage innovative implementations.
A.17.6 The Package Containers.Indefinite_Vectors
The language-defined package Containers.Indefinite_Vectors provides a private type Vector and a set of operations. It provides the same operations as the package Vectors does, with the difference that the generic formal Element_Type is indefinite.
Static Semantics
The declaration of the library package Containers.Indefinite_Vectors has the same contents as Containers.Vectors except:
* The generic formal Element_Type is indefinite.
* The Element parameter of access subprogram Process of Update_Element may be constrained even if Element_Type is unconstrained.
A.17.7 The Package Containers.Indefinite_Doubly_Linked_Lists
The language-defined package Containers.Indefinite_Doubly_Linked_Lists provides private types List and Cursor, and a set of operations for each type. It provides the same operations as the package Doubly_Linked_Lists does, with the difference that the generic formal Element_Type is indefinite.
Static Semantics
The declaration of the library package Containers.Indefinite_Doubly_Linked_Lists has the same contents as Containers.Doubly_Linked_Lists except:
* The generic formal Element_Type is indefinite.
* The procedure with the profile:
procedure Insert (Container : in out List; Before : in Cursor; Position : out Cursor; Count : in Count_Type := 1);
is omitted.
AARM Note: This procedure is omitted because there is no way to create a default-initialized object of an indefinite type. We considered having this routine insert an empty element similar to the empty elements of a vector, but rejected this possibility because the semantics are fairly complex and very different from the existing case. That would make it more error-prone to convert a container from a definite type to an indefinite type; by omitting the routine completely, any problems will be diagnosed by the compiler.
* The Element parameter of access subprogram Process of Update_Element may be constrained even if Element_Type is unconstrained.
A.17.8 The Package Containers.Indefinite_Hashed_Maps
The language-defined package Containers.Indefinite_Hashed_Maps provides private types Map and Cursor, and a set of operations for each type. It provides the same operations as the package Hashed_Maps does, with the difference that the generic formal types Key_Type and Element_Type are indefinite.
Static Semantics
The declaration of the library package Containers.Indefinite_Hashed_Maps has the same contents as Containers.Hashed_Maps except:
* The generic formal Key_Type is indefinite.
* The generic formal Element_Type is indefinite.
* The procedure with the profile:
procedure Insert (Container : in out Map; Key : in Key_Type; Position : out Cursor; Inserted : out Boolean);
is omitted.
AARM Note: This procedure is omitted because there is no way to create a default-initialized object of an indefinite type. We considered having this routine insert an empty element similar to the empty elements of a vector, but rejected this possibility because the semantics are fairly complex and very different from the existing case. That would make it more error-prone to convert a container from a definite type to an indefinite type; by omitting the routine completely, any problems will be diagnosed by the compiler.
* The Element parameter of access subprogram Process of Update_Element may be constrained even if Element_Type is unconstrained.
A.17.9 The Package Containers.Indefinite_Ordered_Sets
The language-defined package Containers.Indefinite_Ordered_Sets provides private types Set and Cursor, and a set of operations for each type. It provides the same operations as the package Ordered_Sets does, with the difference that the generic formal type Element_Type is indefinite.
Static Semantics
The declaration of the library package Containers.Indefinite_Ordered_Sets has the same contents as Containers.Ordered_Sets except:
* The generic formal Element_Type is indefinite.
* The Element parameter of access subprogram Process of Checked_Update_Element may be constrained even if Element_Type is unconstrained.
A.17.10 Array Sorting
The language-defined procedures Containers.Generic_Array_Sort and Containers.Generic_Constrained_Array_Sort provide sorting on arbitrary array types.
Static Semantics
The procedure Containers.Generic_Array_Sort has the following declaration:
generic type Index_Type is (<>); type Element_Type is private; type Array_Type is array (Index_Type range <>) of Element_Type; with function "<" (Left, Right : Element_Type) return Boolean is <>; procedure Ada.Containers.Generic_Array_Sort (Container : in out Array_Type); pragma Pure (Ada.Containers.Generic_Array_Sort);
Reorders the elements of Container such that the elements are sorted smallest first as determined by the generic formal less-than operator provided. Any exception raised during evaluation of less-than is propagated.
AARM Notes This implies swapping the elements, usually including an intermediate copy. This of course means that the elements will be copied. Since the elements are non-limited, this usually will not be a problem. Note that there is Implementation Advice below that the implementation should use a sort that minimizes copying of elements.
The sort is not required to be stable (and the fast algorithm required will not be stable). If a stable sort is needed, the user can include the original location of the element as an extra "sort key". We considered requiring the implementation to do that, but it is mostly extra overhead -- usually there is something already in the element that provides the needed stability.
The procedure Containers.Generic_Constrained_Array_Sort has the following declaration:
generic type Index_Type is (<>); type Element_Type is private; type Array_Type is array (Index_Type) of Element_Type; with function "<" (Left, Right : Element_Type) return Boolean is <>; procedure Ada.Containers.Generic_Constrained_Array_Sort (Container : in out Array_Type); pragma Pure (Ada.Containers.Generic_Constrained_Array_Sort);
Reorders the elements of Container such that the elements are sorted smallest first as determined by the generic formal less-than operator provided. Any exception raised during evaluation of less-than is propagated.
Implementation Advice
The worst-case time complexity of a call on an instantiation of Containers.Generic_Array_Sort or Containers.Generic_Constrained_Array_Sort should be O(N**2) or better, and the average time complexity should be better than O(N**2).
AARM Note In other words, we're requiring the use of a sorting algorithm better than O(N**2), such as Quicksort. No Bubble sorts allowed!
Containers.Generic_Array_Sort and Containers.Generic_Constrained_Array_Sort should minimize copying of elements.
AARM Note - To Be Honest We do not mean "absolutely minimize" here; we're not intending to require a single copy for each element. Rather, we want to suggest that the sorting algorithm chosen is one that does not copy items unnecessarily. Bubble sort would not meet this advice, for instance.
!example
A.17.2 The Package Containers.Vectors
Append is the canonical method for inserting items into a vector container:
procedure Copy (A : Array_Subtype) is V : Vector; begin Set_Capacity (V, Capacity => A'Length);
for I in A'range loop Append (V, New_Item => A (I)); end loop; ... end Copy;
The Set_Capacity operation tells the vector object to preallocate an internal array having at least the capacity specified. If you need to perform many repeated insertions, then if you know the ultimate length apriori you should always call Set_Capacity beforehand. This is more efficient because it allocates the internal array once, and therefore avoids the repeated reallocation, copying, and deallocation cycles that might be necessary otherwise as the array is expanded.
Instead appending new items to the back of the vector, another technique is to declare the vector object with the requisite length immediately:
procedure Copy (A : Array_Subtype) is V : Vector := To_Vector (Count => A'Length); J : Index_Subtype'Base := Index_Subtype'First; begin for I in A'range loop Replace_Element (V, Index => J, By => A (I)); J := J + 1; end loop; ... end Copy;
Here the elements of the vector are initialized to an "empty" value. When the element value is replaced using Replace_Element, the state of the element changes from empty to non-empty.
The procedure Set_Length can be used to change the length of a vector outside of a declarative region. (One could use To_Vector too, of course, but it's more efficient to use Set_Length, since that operation allocates a new array only if the capacity of the existing array is too small.)
You can use a vector to implement a stack in the traditional way:
package Stacks is new Ada.Containers.Vectors (ET); use Stacks;
Stack : Stacks.Vector;
procedure Push (E : in ET) is begin Append (Stack, New_Item => E); end;
function Top return ET is begin return Last_Element (Stack); end;
procedure Pop is begin Delete_Last (Stack); end;
The Insert_Space operation essentially opens up a "hole" in the middle of the internal array. It's more efficient to do it this way than inserting items one-at-a-time, because the sliding is done only once. For example, we can copy an array (or any other container) into a vector at some arbitrary position like this:
procedure Copy
(A : in Array_Subtype;
V : in out Vector; I : in Index_Type'Base) is
J : Index_Type'Base := I;
begin Insert_Space (V, Before => I, Count => A'Length); -- dig the hole
for Index in A'range loop Replace_Element (V, J, By => A (Index)); -- fill the hole J := J + 1; end loop; ... end Copy;
You can think of Clear as "removing" the elements in the container, but of course it does not really remove them. The elements that were formerly active simply now become inactive. In particular, the internal array is not altered, and no "finalization" of the active elements occurs. (Of course, the elements are finalized when the master of the vector object is left.) If this is required, then then user must effect this himself prior to clearing the vector. Here is one way to do that:
procedure Finalize (Element : in out My_Element_Type) is ...;
procedure My_Clear (V : in out Vector) is begin for I in First_Index (V) .. Last_Index (V) loop Update_Element (V, I, Finalize'access); end loop;
Clear (V); end My_Clear;
Here we use the Update_Element modifier, and pass Finalize as the Process parameter.
The internal array is managed by the implementation, and there is no requirement that it has any particular capacity; the only requirement is that the actual capacity is the same or larger than the most recent call to Set_Capacity. Thus Set_Capacity (V, 0) might, but might not, deallocate the internal array. If you want to clear the vector and also deallocate the internal array, you can use Move:
procedure Clear_And_Deallocate (V : in out Vector) is
V2 : Vector; -- length is 0; assume capacity is 0 begin Clear (V); -- sets length to 0, but capacity > 0 Move (Target => V, Source => V2); -- deallocate V's array end;
The internal array that belonged to V is deallocated, and the null (or otherwise small) array of V2 is moved into V.
These, however, are exceptional cases. Usually it is best to ignore the existence of the internal array altogther, and just let the container implementation manage memory (other than possibly setting an initial capacity).
If some sort of finalization of the last element is necessary prior to its "removal" by a deletion operation, the programmer is responsible for effecting this action prior to calling the operation. As an example, suppose we have a vector whose elements are access objects, and we need to deallocate the element when it is "popped" from the vector using Delete_Last. We can do that like this:
procedure Pop (V : in out Vector) is
procedure Free is -- Convention Intrinsic. new Ada.Unchecked_Deallocation (T, T_Access);
procedure Process (V : in out Vector) is -- Convention Ada. begin Free (V); end Process;
begin Update_Element (V, Index => Last_Index (V), Process => Process'access); Delete_Last (V); end Pop;
The First_Index and Last_Index selectors allow iteration over a vector analogous to iteration over an array, using the loop machinery provided by the language:
procedure Op (V : in Vector) is procedure Process (E : in Element_Subtype) is ...; begin for I in First_Index (V) .. Last_Index (V) loop Process (E => Element (V, I)); end loop; end Op;
We could also use the cursor-based operations to do the same thing.
procedure Op (V : in Vector) is procedure Process (E : in Element_Subtype) is ...;
I : Cursor := First (V); begin while Has_Element (I) loop Process (E => Element (I)); Next (I); end loop; end Op;
Here we iterate over all of the vector elements. Alternatively we could use a passive iterator:
procedure Op (V : in Vector) is procedure Process (E : in Element_Subtype) is ...;
procedure Process (I : in Cursor) is begin Process (E => Element (I)); end;
begin Iterate (V, Process'access); end Op;
The Update_Element operation is very important, as it allows in-place modification of elements. For example, suppose we have a vector whose elements are lists, and we want to append an item to the list element at a specified vector position. We can do that as follows:
procedure Append
(V : Vector_of_Lists.Vector;
I : Index_Type'Base; E : Element_Subtype) is
procedure Process (L : in out List) is begin Append (L, New_Item => E); end;
begin Update_Element (V, Index => I, Process => Process'access); end;
If we have a container whose elements are vectors, we can use Update_Element in combination with Move to insert a vector onto the container without actually copying the vector. (Actually, this is true for all containers -- not just vectors.) Suppose that we have a list of vectors:
procedure Op (L : in List_of_Vectors.List) is V : Vector;
procedure Move_V (E : in out Vector) is begin Clear (E); -- technically E should already be empty Move (Target => E, Source => V); end;
begin Append (V, New_Item => E); ... -- populate vector some more
Append (L, New_Item => Empty_Vector);
-- Move (don't copy) vector V onto list L: Update_Element (Last (L), Move_V'access); end;
A new, default-initialized vector element is appended to L by Insert, and then immediately replaced by moving the internal array of vector V into that new, empty element, without any copying.
If ordinary assignment of elements is acceptable, then Replace_Element allows array-like modification of vector elements:
procedure Op (V : in out Vector) is I : Index_Subtype := ...; E : Element_Subtype := ...; begin Insert (V, Before => I, New_Item => E); ... -- modify E some more Replace_Element (V, Index => I, By => E); -- aka V(I) := E; end;
All containers are non-limited, and hence allow ordinary assignment. In the unique case of a vector, there is a separate assignment procedure:
Assign (Target => V1, Source => V2);
The reason is that the model for a vector is that it's implemented using an unconstrained array. During ordinary assignment, the internal array is deallocated (during controlled finalization), and then a new internal array is allocated (during controlled adjustment) to store a copy of the elements from the source vector. However, assignment would probably be more efficient if it were able to hold on to the existing array, and simply copy the elements of the source onto that array, instead of having to allocate a new one. This is exactly how Assign works.
A.17.3 The Package Containers.Doubly_Linked_Lists
You can use a doubly-linked list to implement a queue in the traditional way:
package Queues is new Ada.Containers.Doubly_Linked_Lists (ET); use Queues;
Queue : Queues.List;
procedure Push (E : in ET) is begin Append (Queue, New_Item => E); end;
function Top return ET is begin return First_Element (Queue); end;
procedure Pop is begin Delete_First (Queue); end;
The doubly-linked list container allows iteration in both directions. To iterate forward you start at first and increment the cursor:
procedure Op (L : in List) is C : Cursor := First (L); begin while Has_Element (C) loop Process (C); Next (C); end loop; end;
To iterate in reverse you start at last and decrement the cursor:
procedure Op (L : in List) is C : Cursor := Last (C); begin while Has_Element (C) loop Process (C); Previous (C); end loop; end;
Note that in both cases the iteration terminates by falling off the end of the list, at which point the cursor assumes the distinguished value No_Element, and Has_Element returns False.
All of the containers have an operation to swap a pair of elements in the container:
procedure Swap
(V : in out Vector_Of_Lists.Vector;
I, J : in Index_Type'Base) is
begin Vector_Of_Lists.Swap (V, I, J); -- vector operation end;
For the definite form (the case here), this will make a copy of the element in order to perform the swap. However, we would prefer not to make a copy of this element, which is a list and hence potentially large (or otherwise expensive to copy). To avoid making a copy, we can use Move and nested processing routines:
procedure Swap
(V : in out Vector_Of_Lists.Vector;
I, J : in Index_Type'Base) is
procedure Process_I (IE : in out List) is
procedure Process_J (JE : in out List) is IE_Temp : List; begin Move (Target => IE_Temp, Source => IE); Move (Target => IE, Source => JE); Move (Target => JE, Source => IE_Temp); end;
begin Update_Element (V, Index => J, Process => Process_J'access); end;
begin Update_Element (V, Index => I, Process => Process_I'access); end Swap;
To do the swap, we need direct visibility to both objects, so nested process procedures are required. We first move the list element at index I of the vector into a temporary. This allows another vector to by copied into that index position. We do that, moving the list element at index J into (empty) list element at index I. We then move the temporary (holding the list that was originally at index I) into the vector at index position J.
It's often the case that during an insertion you don't have an item value to assign to the new element, and instead you want simply to insert a new element (initialized to the default value for its type, if applicable) and then modify it directly. For example:
procedure Op (L : in out List) is
procedure Process (E : in out ET) is begin ... -- manipulate E as appropriate end;
C : Cursor; begin Insert -- Allocate new element (Container => L, Before => No_Element, -- Insert at back end Position => C); -- Return value
-- Modify default-initialized value: Update_Element (C, Process'access); end Op;
A.17.4 The Package Containers.Hashed_Maps
It's often the case that you know apriori the total number of elements you intend to insert into the map. Under these circumstances you should always Set_Capacity the map first (similar to a vector container), and then perform the insertions. This preallocates a hash table that has the proper capacity, and thus avoids the automatic rehashing that occurs during normal insertion to preserve the load factor. For example:
procedure Op (N : Count_Type) is M : Map_Types.Map; -- Capacity = 0 (or small)
Position : Map_Types.Cursor; Inserted : Boolean; begin Set_Capacity (M, Capacity => N); -- Capacity >= N
for I in 1 .. N loop Insert -- no expansion and rehashing will occur (Container => Map, Key => New_Key (I), New_Item => New_Element (I), Position => Position, Inserted => Inserted); end loop; ... end Op;
Note that Clear doesn't delete the internal hash table -- it just deletes the nodes hanging off the hash table.
The simplest and fastest way to iterate over all the elements in the map is to use a passive iterator:
procedure Op (M : in Map_Types.Map) is
procedure Process (C : in Map_Types.Cursor) is K : Key_Subtype := Key (C); E : Element_Subtype := Element (C); begin ... -- do whatever end;
begin Iterate (M, Process'access); end;
You could of course implement this function yourself, by iterating over the items in the map:
procedure Op (M : in Map_Types.Map) is
procedure Process (C : in Map_Types.Cursor) is ...;
C : Map_Types.Cursor := First (M);
begin
while Has_Element (C) loop Process (C); Next (C); end loop;
end Op;
However, a manual loop probably isn't as efficient as a passive iterator (especially for a hashed map), because the passive iterator can store implementation-specific context in order to find elements in sequence more quickly.
generic algorithms are typically written to work with iterators this way:
generic type Cursor is private; with function Next (C : Cursor) return Cursor is <>; with procedure Process (C : Cursor) is <>; with function Has_Element (C : Cursor) return Boolean is <>; procedure Generic_Algorithm (First : in Cursor);
The implementation would look something like this:
procedure Generic_Algorithm (First : in Cursor) is C : Cursor := First; begin while Has_Element (C) loop ... Process (C); ... C := Next (C); end loop; end Generic_Algorithm;
The benefit is that this algorithm will work with any "sequence of items," which just means any container with a cursor having the requisite properties, as specified in the generic formal region. The virtue of this approach is that it abstracts-away the container. The generic algorithm above (and others like it) works with all the containers in the library -- it even works for built-in array types.
To make this work with a map, we can just instantiate with an appropriate Process operation:
procedure Op (M : in Map_Types.Map) is
procedure Process (C : Cursor) is ...;
procedure Algorithm is new Generic_Algorithm (Map_Types.Cursor); -- accept defaults begin Algorithm (First (Map)); end;
In a POSIX OS that supports real-time signals, the OS will deliver a payload-carrying signal to the app. In the case of a socket, when I/O completes asynchronously, the OS delivers an RT signal that specifies the file descriptor of the socket whose I/O completed.
The problem is that I typically declare the socket as part the representation of some abstraction that gets allocated dynamically, and therefore I have no idea which object the socket belonged to, so I have no idea how to act on the information the OS is providing me.
The abstraction I have in mind looks like this:
package Sessions is
type Session_Type (<>) is limited private;
function Session_Access is access all Session_Type;
function Setup_Session return Session_Type; -- ctor
procedure Play (Session : access Session_Type; Stream : in String); ... procedure IO_Completion (Session : access Session_Type);
private
type Session_Type is limited record Socket : Socket_Type; ...; end Session_Type;
end Sessions;
What I need to do is correlate the file descriptor reported in the RT signal to a session object. With a map it's almost trivial. In the body I can instantiate the map as follows. First we make a hash function for socket descriptors:
function Hash_FD (fd : in C.int) return Ada.Containers.Hash_Type is begin return Ada.Containers.Hash_Type (fd); -- fd > 0 end;
Next we instantiate the hashed map package using our hash function:
package FD_Map_Types is new Ada.Containers.Hashed_Maps (Key_Type => C.int, Element_Type => Session_Access, Hash => Hash_FD);
Now I can declare a map object in the body:
package Sessions is ... FD_Map : FD_Map_Types.Map;
When I allocate a new session object, this opens the socket. A socket object has a selector function to return its file descriptor. I use this as the key to insert the session object into the map:
function Setup_Session return Session_Access is Session : constant Session_Access := new Session_Type;
Position : FD_Map_Types.Cursor; Inserted : Boolean; begin Open (Session.Socket, ...);
Insert (Container => FD_Map, Key => FD (Session.Socket), New_Item => Session, Position => Position, Inserted => Inserted); ... return Session; end;
Now that the session object has inserted itself into the map, I can use map lookup to find that session when I receive a signal. Something like:
procedure Handle_Signal (Siginfo : in Siginfo_Type) FD : constant C.int := Siginfo.FD; C : constant Cursor := Find (FD_Map, Key => FD); begin if C /= No_Element then -- if search was successful IO_Completion (Element (C)); end if; end Handle_Signal;
and then the session object reacts to the I/O completion accordingly.
Hashed maps with type String as the key are nearly ubiquitous. The canonical example is of course the word-frequency problem, in which "words" (using some suitable definition of delimiter) are counted as they are scanned in the input stream.
We can solve this problem easily using the indefinite form of the hashed map, with string as the key and subtype Natural as the element:
with Ada.Strings.Hash_String;
package Wordcount_Maps is new Ada.Containers.Indefinite_Hashed_Maps (Key_Type => String, Element_Type => Natural, Hash => Ada.Strings.Hash); -- case-sensitive
Wordcount_Map : Wordcount_Maps.Map;
Here we've specified the hash function for strings provided by the library. The scanning phase looks like this:
Open (File, In_File, Name);
Scan_Lines: while not End_Of_File (File) loop
Get_Line (File, Line, Line_Last);
Line_First := Line'First;
Scan_Line: loop
Find_Token (Line (Line_First .. Line_Last), ...);
exit when Word_Last = 0;
-- the interesting part happens here: Insert (Word => Line (Word_First .. Word_Last));
Line_First := Word_Last + 1;
end loop Scan_Line;
end loop Scan_Lines;
Now all we have to do is implement Insert. That function looks like this:
procedure Insert (Word : String) is
procedure Increment (Count : in out Natural) is begin Count := Count + 1; end;
Position : Wordcount_Maps.Cursor; Inserted : Boolean;
begin -- Insert
Insert (Container => Wordcount_Map, Key => To_Lower (Word), New_Item => 0, -- yes Position => Position, Inserted => Inserted);
Update_Element (Position, Increment'access);
end Insert;
Map (and set) insertion works conditionally. It searches the container to determine whether there is an equal key already in the map.
Note that in the example above, the New_Item parameter has the value 0. This is deliberate. What happens is that if the word is already in the map, then the insertion "fails" in the sense that no new node is allocated. Rather, Insert reports the fact that the key was already in the map (by returning the value False for Inserted), and a cursor that designates the node with the matching key.
But not inserting a new node is exactly the behavior we want. In the case of a word already in the map, the cursor returned designates an existing word/count pair, whose count is non-zero. When we update the count object, we simply increment its value.
However, the word might not be in the map, in which case the insertion "succeeds," which means a new node is inserted whose element is initialized to the value of New_Item, which here is 0. Position designates the newly-inserted element (really, it designates the node containing that key/element pair). When we update the element, the count has the value 0, and so by incrementing it the count gets set to the correct value 1.
Conditional insertion is a necessary feature of any efficient map abstraction. It makes no sense to search for the element (via Find, say) to determine whether it's in the map, and if it's not in the map call a separate operation to insert it. This would be horribly inefficient because the lookup done by insert would only duplicate the lookup just done by the search.
To dump the contents of the map, you can use the passive iterator:
declare procedure Process (C : in Wordcount_Maps.Cursor) is begin Put (Key (C)); Put (':'); Put (Element (C)); New_Line; end; begin Iterate (Map, Process'access); end;
This would display the words in their order in the hashed map. That's probably not what you want (especially for a well-performing hash table, which would scatter keys everywhere), which is to display them in order by frequency. We can do that easily enough by populating an array with map cursors, and then sorting the array according to element value:
procedure Print_Results (Histogram : in Wordcount_Map) is
type Cursor_Array is array (Count_Type range <>) of Wordcount_Maps.Cursor;
Cursors : Cursor_Array (1 .. Length (Histogram));
I : Count_Type := Cursors'First;
procedure Process (C : in Wordcount_Maps.Cursor) is begin Cursors (I) := C; I := Count_Type'Succ (I); end;
begin -- Print_Results
Iterate (Histogram, Process'access); -- Populate array
... -- see below
end Print_Results;
Here we use the passive iterator for maps to populate the array. As with all containers, it's usually simpler and more efficient to use a passive iterator if you're going to traverse all the elements in the container.
We now need to sort the array of cursors, and to do that we need an order relation for cursors. We want to sort the elements in reverse order, so that largest histogram count is listed first in the output. We can define the order relation like this:
Sort_Array: declare function "<" (L, R : Wordcount_Maps.Cursor) return Boolean is begin return Element (L) > Element (R); -- yes end;
procedure Sort is new Generic_Array_Sort (Index_Type => Count_Type, Element_Type => Word_Count_Maps.Cursor, Array_Type => Cursor_Array); -- accept "<" default begin Sort (Cursors); end Sort_Array;
We can do better though: suppose that for counts that are equal, we want break the tie by listing the items in alphabetic order of the words. We only have fix our order relation to compare keys, too:
Sort_Array: declare function "<" (L, R : Wordcount_Maps.Cursor) return Boolean is begin if Element (L) = Element (R) then return Key (L) < Key (R); -- compare String else return Element (L) > Element (R); -- compare Integer end if; end;
procedure Sort is new Generic_Array_Sort (Index_Type => Count_Type, Element_Type => Word_Count_Maps.Cursor, Array_Type => Cursor_Array); -- accept "<" default begin Sort (Cursors); end Sort_Array;
To display the results, we iterate through the sorted cursor array:
Print_Array: for Index in Cursors'range loop declare C : constant Wordcount_Maps.Cursor := Cursors (Index); begin Put (Element (C), Width => 0); -- the count Put (':'); Put (Key (C)); --the word New_Line; end; end loop Print_Array;
As another example, consider a client that connects to a streaming media server to play a video. The server opens the file and then transmits frames to the client. Ultra-efficient file I/O is usually done by memory-mapping the sections on disk. Typically, a server maintains its own file cache, so that many clients streaming the same file can share the memory-mapped sections.
When the client request comes in, the server must look up the file by name in order to see if it's in cache. If it's already in cache, then we increment a reference count. If it's not, we create some context for the file and create a new entry for it in the cache.
Suppose the context type looks like this:
type Context_Type is limited record File : File_Type; Ref_Count : Natural; ...; end record;
type Context_Access is access Context_Type;
Our cache is just a map, indexed by file name:
package File_Cache_Types is new Ada.Containers.Indefinite_Hashed_Maps (Key_Type => String, Element_Type => Context_Access, Hash => Hash_String_Case_Insensitive, Is_Equal_Key => Equal_String_Case_Insensitive);
File_Cache : File_Cache_Types.Map;
The naive way to implement the lookup is:
procedure Setup (Name : in String) is
Position : File_Cache_Types.Cursor := Find (File_Cache, Key => Name);
Inserted : Boolean; Context : Context_Access;
begin -- Setup
if Position = No_Element then
Context := new Context_Type; Context.Ref_Count := 0; ... -- init Context
Insert (Container => File_Cache, Key => Name, New_Item => Context, Position => Cursor, Inserted => Inserted);
else
Context := Element (Position);
end if;
Context.Ref_Count := Context.Ref_Count + 1; ... -- use Context
end Setup;
The problem is that we're duplicating work: we first searched for the file context in the cache, and if it wasn't found we insert a new entry, which just searches again. The correct way to do this is as follows:
procedure Setup (Name : in String) is
Position : File_Cache_Types.Cursor; Not_In_Cache : Boolean; Context : Context_Access;
begin
Insert (Container => File_Cache, Key => Name, New_Item => null, -- yes Position => Position, Inserted => Not_In_Cache);
if Not_In_Cache then
pragma Assert (Element (Position) = null);
Context := new Context_Type; Context.Ref_Count := 0; ... -- init context
Replace_Element (Position, By => Context);
else
Context := Element (Position);
end if;
Context.Ref_Count := Context.Ref_Count + 1; ... -- use context
end Setup;
Here we make an insertion attempt, by trying to insert a null context into the map. If it's already in the map, then the insertion fails, but that's just what we want to happen, because we wish to share the file already in cache. If it's not in the map, the insertion succeeds, by creating a slot for this file (the context is null), which we just fill in with a newly-allocated context object.
In the RTSP protocol, requests are sent to a server in (clear) text. To create a session, a client connects to the server and sends it a SETUP request, e.g.
SETUP rtsp://mheaney/thematrix.avi RTSP/1.0 Transport: ...
Each RTSP session has a "session id," which is a random string of characters at least 8 characters long. When a SETUP request doesn't specify a session id, this is interpreted to mean that the client wishes to create a new session.
On the server side, it allocates a session object (described above), and generates a session id for it. The representation of the session object looks like this:
type Session_Type is limited record Id : String (1 .. 8); ...; end record;
And the session constructor looks like:
function Setup_Session return Session_Access is Session : constant Session_Access := new Session_Type; begin Generate_Id (Session.Id); ... -- see below end;
The server responds by including the session id in the response:
RTSP/1.0 200 OK Session: HAiye8-r
And thereafter, a client sends requests to a session by specifying a session id header:
PLAY rtsp://mheaney/thematrix.avi RTSP/1.0 Range: npt=101.42- Session: HAiye8-r
The server-side problem is this. When the server receives the request from the client, it parses the request and looks for a session id. The problem then becomes finding the session object that is associated with that unique id. There might very well be hundreds of session objects, so whatever method we use has to run fast. (The is a real-time streaming server, and RTSP request/response overhead must be kept to a minimum.)
What we do is declare a string-key map object that uses the session id as the key, and a Session_Access as the element, like this:
package Id_Maps is new Ada.Containers.Indefinite_Hashed_Maps (Key_Type => String, Element_Type => Session_Access, Hash => Ada.Strings.Hash); -- case-sensitive
Id_Map : Id_Maps.Map; use Id_Maps;
When the session is allocated, we insert the id/session pair into the map like this:
function Setup_Session return Session_Access is Session : constant Session_Access := new Session_Type;
Position : Id_Map_Types.Cursor; Inserted : Boolean; begin Generate_Id (Session.Id);
Insert (Container => Id_Map, Key => Session.Id, New_Item => Session, Position => Position, Inserted => Inserted); ... return Session; end;
When a client issues a request, we parse out the session id, and then forward the command to the session object associated with that session id key:
procedure Play
(Session_Id : in String;
NPT_Range : in NPT_Range_Type; RTSP_Status : out RTSP_Status_Type) is
Position : constant Cursor := Find (Id_Map, Key => Session_Id);
Session : Session_Access;
begin if Position = No_Element then RTSP_Status := RTSP.Session_Not_Found; return; end if;
Session := Element (Position);
Play (Session, NPT_Range, RTSP_Status); end;
When we create a session object, we insert a pointer to the session object in the Id_Map. The complementary problem is how to handle deletion of the session object. Suppose we have a function like this:
procedure Free (X : in out Session_Access);
What's the best way to remove the object from the map? Since the session knows its own Id, it can use the key-form of Delete:
procedure Free (X : in out Session_Access) is begin if X /= null then Delete (Id_Map, Key => X.Id); Deallocate (X); end if; end;
Another option is to use the cursor-form of Delete. What we can do is cache the cursor returned by Insert as part of the state of the session object:
function Setup_Session return Session_Access is Session : constant Session_Access := new Session_Type; Inserted : Boolean; begin Generate_Id (Session.Id);
Insert (Container => Id_Map, Key => Session.Id, New_Item => Session, Position => Session.Id_Map_Position, Inserted => Inserted);
pragma Assert (Inserted); pragma Assert (Key (Session.Id_Map_Position) = Session.Id); pragma Assert (Element (Session.Id_Map_Position) = Session); ...
return Session; end Setup_Session;
Now we can implement Free this way:
procedure Free (X : in out Session_Access) is begin if X /= null then Delete (Id_Map, Position => X.Id_Map_Position); Deallocate (X); end if; end;
This turns out to be a very common idiom. In the body of a package that declares a type, you declare a set or map container to keep track of instances of the type. As part of its representation, the type includes a cursor that designates the node that contains this instance. When the object is finalized, it deletes itself from the container.
In the examples we've seen, the session object inserts itself into the map during the constructor function Setup_Session, and deletes itself during the destructor operation Free. If the type is controlled, another possibility is to do the insertion during Initialize, and the deletion during Finalize. This technique would work even for stack-allocated (controlled) objects.
Let's pretend Free doesn't remove the designated session object from the map, but rather has its traditional semantics of merely deallocating the object. To shutdown all the sessions, we could do this:
Id_Map : Id_Map_Types.Map; ...
procedure Shutdown_Sessions is
procedure Process (C : in Id_Map_Types.Cursor) is begin Update_Element (C, Free'access); -- Free a session. end;
begin -- Shutdown_Sessions
Iterate (Id_Map, Process'access); -- Free all sessions.
Clear (Id_Map);
end Shutdown_Sessions;
The passive iterator visits all the sessions in the map and Free's them; this also sets all of the map elements to null. We then Clear the map, which sets its length to 0.
In other examples, we have been silent about the operation of Generate_Id, which makes a session id for us comprising a sequence of 8 random characters. One of our requirements for a session id is that it must be unique among all the sessions currently being streamed by this server.
Even if we use a random number generator to synthesize characters, we still must check to ensure that this session id is unique. The obvious way is to use Find to see if it's in the map already:
procedure Generate_Id (Id : out Id_Subtype) is Position : Id_Maps.Cursor; begin loop Synthesize_Random_String (Id); Position := Find (Id_Map, Key => Id); exit when Position = No_Element; -- good: not found end loop; end Generate_Id;
The constructor for session objects generates a unique session id, and then uses the id as the key when inserting the new session object:
function Setup_Session return Session_Access is
I : Id_Maps.Cursor; B : Boolean;
Session : Session_Access := new Session_Type; begin Generate_Id (Session.Id); -- Id is guaranteed to be unique
Insert (Container => Id_Map, Key => Session.Id, Element => Session, Position => I, Inserted => B); ... end Setup_Session;
One issue with this algorithm is that Insert duplicates the work done just earlier by Find, when checking the newly-synthesized id for uniqueness. A simpler way to efficiently generate a unique session id and insert it into the map is to just check whether the insertion succeeded:
function Setup_Session return Session_Access is
Position : Id_Maps.Cursor; Inserted : Boolean;
Id : Id_Subtype;
begin -- Setup_Session
loop
Generate_Id (Id);
Insert (Container => Id_Map, Key => Id, Position => Position, Inserted => Inserted);
exit when Inserted;
end loop; ... end Setup_Session;
Here we don't need a separate call to Find, because the regular Insert operation does a search anyway.
A.17.5 The Package Containers.Ordered_Sets
Suppose we have a set and want to copy the elements from the set into an array. Here's one way to do it:
procedure Op (S : in Integer_Sets.Set) is
A : Integer_Array (1 .. Length (S)); I : Integer_Sets.Cursor := First (S); J : Positive := A'First; begin while Has_Element (I) loop A (J) := Element (I); Next (I); J := J + 1; end loop; ... end Op;
Here we're incrementing both the array index and the set cursor manually. However, when you're iterating over two containers simultaneously, you can often let one or the other drive the iteration, so that only one position needs to be incremented manually.
We can change the example use a built-in for loop:
procedure Op (S : in Integer_Sets.Set) is
A : Integer_Array (1 .. Length (S)); I : Integer_Sets.Cursor := First (S); begin for J in A'range loop A (J) := Element (I); Next (I); end loop; ... end Op;
This lets the array drive the iteration. In general, however, you should use a passive iterator in preference to an explicit loop. The reason is that if a container knows that its elements are being traversed sequentially, then it can use a traversal algorithm that takes advantage of that container's unique representation. The algorithm above might run faster if we do this:
procedure Op (S : in Integer_Sets.Set) is
A : Integer_Array (1 .. Length (S)); J : Positive := A'First;
procedure Fill_Element_of_A (I : Integer_Sets.Cursor) is begin A (J) := Element (I); J := J + 1; end;
begin Iterate (S, Fill_Element_of_A'access); ... end Op;
This lets the set drive the iteration.
Let's continue the streaming server examples from earlier. In TCP, a client "connects" to a server, who is "listening" on a port known to the client. When the server "accepts" the connection, this establishes a dedicated connection with that client. We can reify this in code as follows:
package Connections is
type Connection_Type (<>) is limited private;
type Connection_Access is access Connection_Type;
function Accept_Connection (Socket : Socket_Type) return Connection_Access; -- the ctor for connection objects ... end Connections;
We have a need to keep track of all of our current connections. So when we create a new connection object, what we do is insert it in a set:
package body Connections is
function "<" (L, R : Connection_Access) return Boolean is begin return L.all'Address < R.all'Address; end;
package Connection_Sets is new Ada.Containers.Ordered_Sets (Connection_Access);
Connection_Set : Connection_Sets.Set;
function Accept_Connection (Socket : Socket_Type) return Connection_Access is
Connection : constant Connection_Access := new Connect_Type;
Position : Connection_Sets.Cursor; Inserted : Boolean; begin Insert (Container => Connection_Set, New_Item => Connection, Position => Position, Inserted => Inserted); ... return Connection; end; ... end Connections;
When a new connection object is allocated, it is inserted into the Connection_Set. Here insertions will always succeed because each allocated object has a unique access value.
Now let's suppose we want to shutdown a specific connection. We can do it like this:
procedure Shutdown (X : in out Connection_Access) is begin if X /= null then Delete (Connection_Set, Item => X); Free (X); end if; end Shutdown;
Here we use the item-form of Delete to simply remove the item from the set, and then deallocate it.
Now let's suppose we want to shutdown the entire system, and so we need to clear out all of the connection objects. We could do it like this:
procedure Shutdown_Connections is X : Connection_Access; begin while not Is_Empty (Connection_Set) loop X := First_Element (Connection_Set); Shutdown (X); --removes X from Connection_Set (see above) end loop; end Shutdown_Connections;
Another technique would be to use an active iterator, like this:
procedure Shutdown_Connections is I : Cursor; X : Connection_Access; begin while not Is_Empty (Connection_Set) loop I := First (Connect_Set); X := Element (I); Delete (Connection_Set, Position => I); Free (X); end loop; end Shutdown_Connections;
Here we use the cursor-form of Delete. This is probably more efficient than using the item-form of Delete, since the cursor-form doesn't have to search for the item.
One of the canonical container examples is the set-of-employees. Suppose we have an employee type defined this way:
type Employee_Type is record SSN : SSN_Type; -- social security no. Name : Name_Type; Home_Address : Home_Address_Type; ...; end record;
To make a set, we need to establish an order relationship for its elements. Since each social security number is presumably unique (unless your identity has been stolen), we can use that to define an order for Employee_Type:
function "<" (L, R : Employee_Type) return Boolean is begin
return L.SSN < R.SSN;
end;
This allows us to instantiate the set in the normal way:
package Employee_Sets is new Ada.Containers.Ordered_Sets (Employee_Type);
Employees : Employee_Sets.Set;
When someone gets a job at our firm, we add them to our little database as follows:
procedure Hire (Name : Name_Type; SSN : SSN_Type; ...) is
Employee : Employee_Type;
Position : Employee_Sets.Cursor; Inserted : Boolean; begin Employee.SSN := SSN; Employee.Name := Name; ... Insert (Employees, Employee, Position, Inserted); end;
Now let's suppose that we need to modify some information for an employee. Like a map, a set orders its elements in key order, except that for a set the element is its own key. In the example here, the key is really the SSN part of the employee object.
Suppose we only know the employee's social security number. How do we find that employee? Remember that the "key" of a set is just the element itself. One way is to synthesize a dummy employee object, and then look it up by element type:
procedure Change_Address
(SSN : SSN_Type;
New_Home : Home_Address_Type) is
Position : Cursor;
begin declare Dummy : Employee_Type; begin Dummy.SSN := SSN; Position := Find (Employees, Item => Dummy); end;
if Has_Element (Position) then ...; end;
But this is kind of a hack. I don't really want to make a dummy element just to look up the real element. For many types synthesizing a dummy object this way might not even be possible (say, because the type is private and you only have visibility to the partial view of the type).
A much more elegant technique is to use the nested generic package Generic_Keys, which allows you to explicitly name the key-part of the element. We can instantiate that package like this:
function "<"
(Employee : Employee_Type;
SSN : SSN_Type) return Boolean is
begin return Employee.SSN < SSN; end;
function ">" (Employee : Employee_Type; SSN : SSN_Type) return Boolean is begin return Employee.SSN > SSN; end;
function SSN (Employee : Employee_Type) return SSN_Type is begin return Employee.SSN; end;
package SSN_Keys is new Employee_Sets.Generic_Keys (SSN_Type, Key => SSN); -- Use defaults for other parameters
With this new package we can now look up the employee by his SSN directly:
procedure Change_Address
(SSN : SSN_Type;
New_Home : Home_Address_Type) is
Position : Cursor := Find (Employees, Key => SSN);
begin if Has_Element (Position) then ...; end;
To actually change the employee's address in the example above, we use the special element modifier operation:
procedure Change_Address
(SSN : SSN_Type;
New_Home : Home_Address_Type) is
procedure Set_Home (Employee : in out Employee_Type) is begin Employee.Home := New_Home; end;
Position : Cursor := Find (Employees, Key => SSN);
begin if Has_Element (Position) then SSN_Keys.Checked_Update_Element (Position => Position, Process => Set_Home'Address); ... end if; end Change_Address;
Modifying a set element requires special handling, since a set orders its elements by key. Indiscriminate changes to element values are therefore not allowed, as this would break the set abstraction if the order relation were changed. The operation Checked_Update_Element allows the element to be modified, but it checks to determine whether the key has changed, and if so it moves the element to a new position that preserves the order relation among existing elements. In this example we only modify the Home component of the Element_Type record, so Checked_Update_Element would not need to take any further action.
Now let's say that the employee's wallet was stolen, which contained his social security card. In order to prevent identity theft, he needs to apply for a new social security number, and then change his entry in the database.
One way to do this would be to first copy the employee object and remove it from the set, then change the value of its SSN field, and finally (re)insert the element:
procedure Change_SSN
(Old_SSN : SSN_Type;
New_SSN : SSN_Type) is
Old_Position, New_Position : Cursor; Inserted : Boolean;
begin if New_SSN = Old_SSN then return; end if;
Old_Position := Find (Employees, Key => Old_SSN);
if not Has_Element (Old_Position) then return; end if;
New_Position := Find (Employees, Key => New_SSN);
if Has_Element (New_Position) then raise Duplicate_SSN; end if;
declare Employee : Employee_Type := Element (Old_Position); begin Delete (Employees, Old_Position);
Employee.SSN := New_SSN;
Insert (Employees, Employee, New_Position, Inserted); pragma Assert (Inserted); end; end Change_SSN;
Another technique is to use Checked_Update_Element, which allows the element's key to be modified, and then moves then element to its new relative location in the set:
procedure Change_SSN
(Old_SSN : SSN_Type;
New_SSN : SSN_Type) is
Old_Position, New_Position : Cursor; Inserted : Boolean;
begin if New_SSN = Old_SSN then return; end if;
Old_Position := Find (Employees, Key => Old_SSN);
if not Has_Element (Old_Position) then return; end if;
New_Position := Find (Employees, Key => New_SSN);
if Has_Element (New_Position) then raise Duplicate_SSN; end if;
declare procedure Set_SSN (Employee : in out Employee_Type) is begin Employee.SSN := New_SSN; end; begin SSN_Keys.Checked_Update_Element (Position => Old_Position, Process => Set_SSN'access); end; end Change_SSN;
Changing the key this way is probably more efficient than deleting and then immediately re-inserting the element, since the element doesn't need to be copied, and there is no deallocation and re-allocation of the element's internal node. When it detects that the element's key has been changed, Checked_Update_Element works by first removing the node from the tree and then inserting the same node back into the tree.
Suppose now we want a list all the employees in the firm. One way to do it is like this:
procedure Display is
procedure Print (I : in Employee_Sets.Cursor) is
procedure Do_Print (E : in out Employee_Type) is begin Put ("Name: "); Put (E.Name); Put ("SSN: "); Put (E.SSN); ...; end;
begin Query_Element (Position => I, Process => Do_Print'access); end;
begin Iterate (Employees, Print'access); end;
However, this will list employees in order of their social security number. This is probably not what we want, which is to print employees in order of name.
One way would be to copy the elements into some other container, which is sorted according to the criterion we desire. However, if elements are large or otherwise not easily copied, then this is not not really an option.
A much better way is not to copy elements directly but rather to copy cursors that designate those elements:
procedure Display_Employees_In_Name_Order is
function "<" (L, R : Employee_Sets.Cursor) return Boolean is
Result : Boolean;
procedure Process_LE (LE : in out Employee_Type) is
procedure Process_RE (RE : in out Employee_Type) is begin Result := LE.Name < RE.Name; end;
begin Query_Element (R, Process_RE'access); end Process_LE;
begin Query_Element (L, Process_LE'access); return Result; end;
type Cursor_Array is array (Count_Type range <>) of Employee_Sets.Cursor;
procedure Sort is new Generic_Array_Sort (Count_Type, Cursor, Cursor_Array);
procedure Do_Print (E : in out Employee_Type) is begin Put ("Name: "); Put (E.Name); Put ("SSN: "); Put (E.SSN); ...; end;
C : Employee_Sets.Cursor := First (Employee_Sets);
function Get_Cursor return Employee_Sets.Cursor is Result : Cursor := C; begin Next (C); return Result; end;
Cursors : Cursor_Array (1 .. Length (Employee_Set)) := (others => Get_Cursor);
begin
Sort (Cursors);
for Index in Cursors'range loop C := Cursors (Index); Query_Element (Position => C, Process => Do_Print'access); end loop;
end Display_Employees_In_Name_Order;
First we use an active iterator for sets to insert a cursor designating every employee into the array. Next we define an order for relation for the array elements, which here are just set cursors. We wish to print employees in order of name, so the order relation for cursors is defined in terms of the names of the employees designated by the cursors.
Implementing the sort order relation turns out to slightly tricky, because we don't want to make a copy of the employee just do get its name. We use nested process routines for Query_Element to create a context in which both employee objects are directly visible, and then compare employee names by querying the employee elements directly.
Now that the employees (really, cursors that designate the employees) have been sorted, we loop to traverse all the set cursors, and print each employee (in name order) in turn.
In you were paying very careful attention to the Id_Map hashed-map example, you might have realized that since the session object (which was the element of the map) had an Id field, we were in fact duplicating the Id object, since it's also stored as the key-part of the map entry.
In turns out we didn't really need to use a map. We could have used a set, and instantiated the generic package Generic_Keys using type String as the formal Key_Type.
function "<" (L, R : Session_Access) return Boolean is begin
return L.Id < R.Id;
end;
package Session_Set_Types is new Ada.Containers.Ordered_Sets (Session_Access);
-- instead of Id_Map, use a set to store sessions: Session_Set : Session_Set_Types.Set;
function "<" (Session : Session_Access; Id : String) return Boolean is begin return Session.Id < Id; end;
function ">" (Session : Session_Access; Id : String) return Boolean is begin return Session.Id > Id; end;
function Id (Session : Session_Access) return String is begin return Session.Id; end;
package Id_Keys is new Session_Set_Types.Generic_Keys (String, Key => Id);
This lets us perform session lookups based on the session identifier:
procedure Play
(Session_Id : in String;
NPT_Range : in NPT_Range_Type; RTSP_Status : out RTSP_Status_Type) is
Position : constant Session_Set_Types.Cursor := Find (Session_Set, Key => Session_Id);
Session : Session_Access;
begin if Position = No_Element then RTSP_Status := RTSP.Session_Not_Found; return; end if;
Session := Element (Position);
Play (Session, NPT_Range, RTSP_Status); end;
We can insert a session object into the set in the normal way, using the item-form of insertion:
function Setup_Session return Session_Access is Session : constant Session_Access := new Session_Type; -- allocate
Position : Session_Set_Types.Cursor; Inserted : Boolean; begin Generate_Id (Session.Id);
Insert (Container => Sessions_Set, New_Item => Session, -- element, has its own key Position => Position, Inserted => Inserted); ... return Session; end;
This example also illustrates that sets and maps are essentially the same. The only real difference is where the key lives.
--!corrigendum A.17
!ACATS Test
ACATS tests will be needed for this library.
!appendix

Report of the ARG Select Committee on Containers
February 3, 2004


Executive Summary

The committee selected the second proposal as a starting point for a
standard containers library, with a number of simple changes. The
changes were simple enough that we produced a version of the library with
the changes made (AI-00302-3/01).

The resulting proposal is not much larger than the Vector and Matrix
libraries already adopted for the standard. It also should be a good seed
for a more encompassing secondary standard.

Therefore, we recommend that the ARG adopt this alternative for the standard.

     By the ARG Select Committee on Containers:

           Randy Brukardt
           Bob Duff
           Tucker Taft

Full Report

Goals

A core library of containers is an important addition to Ada. Other competitive
programming languages include standard sets of containers, and these are
widely used. Users often note that a standard set of containers is a missing
piece of Ada. In addition, adding such containers to the standard is not
a large burden on implementers.

However, the resources available for work on the standard preclude adding a
large container library to the standard. If the library is too large, it will
be insufficiently reviewed, and that has the danger of providing something
useless.

Therefore, the committee settled on a limited set of goals:
   (1) To provide a number of the most useful containers to Ada users in
       a standard fashion;
   (2) To provide a framework for future work in this area (hopefully leading
       to a secondary or de-facto standard).

We considered other goals as well. Performance issues were deemed of secondary
importance. Most uses of containers (indeed, most software) do not have
critical performance requirements. To provide a library with the variety of
components needed to meet critical requirements (bounded and unbounded forms,
array and list implementations, etc.) would be beyond the resources available
to work on the standard. Moreover, the existence of many components actually
makes construction of simple applications harder: the programmer has to choose
a component based on performance considerations that are simply irrelevant for
the application.

Evaluation of existing proposals

We determined that the most important containers are the following:
    * extensible "vectors" (like an array, indexed by any discrete type);
    * (hashed) "maps" (or "hash table", with arbitrary keys);
    * (sorted) "sets" (set of arbitrary items).
The names "map", "set", and "vector" are those used in the Java containers.

We evaluated the two proposals for their support of these components.

Alternative 1 (AI-302-1/07) contains a number of low level data structure
components such as Lists, Bags, Queues, etc. These can be used to create
"vector", "map", and "set" containers, but the containers themselves are
absent. Moreover, most of these components are relatively easy to create
when needed.

Alternative 2 (AI-302-2/02) contains mainly five containers: vector, list,
map, set, and multiset. These include the abstractions mentioned above. We
also determined that the basic design was consistent and sound.

Therefore, we discarded alternative 1, and concentrated on improving and
simplifying alternative 2.

We decided the sorts of changes that we would consider. The great value to
having containers in the standard is that they are standard: everybody has
them and can use them. Perfection is not required of the standard components.
Moreover, what is one person's "improvement" is another's "mistake". In
addition, we run the risk of introducing real errors by further fiddling.

Therefore, we decided to simplify the interfaces by deleting unnecessary
capabilities, by systematic substitutions, and by introducing missing
capabilities (along with general wordsmithing). In particular, we avoided
changing existing interfaces unless there was a clear error.

The specific improvements and simplifications are detailed in the Appendix.

Performance issues

For the purposes of components in the standard, the precise performance of them
is not important. Whatever the performance is will be good enough for the vast
majority of uses - in prototyping, quick and dirty programs, and the majority
of programs that aren't performance critical. Therefore, we provide only a
single version of each component. We don't, for instance, provide both Vectors
and Lists, which are really the same abstraction with the different performance
characteristics.

However, it is important that the performance characteristics of the components
be specified. That is, if searches are expected to be no worse than O(N), we
need to say that. That's because we want programs using the components to be
portable. That wouldn't be true for programs using components with large
numbers of items if the performance characteristics vary widely between
implementations. Consider a Vector component. It could in theory be implemented
with an array or with a linked list. The cost of an arbitrary insertion is O(N)
for the array implementation and O(1) for the list implementation. If a program
using a large vector is moved from a list implementation to an array
implementation, the performance change could be so large as to make the program
non-functional. That is unacceptable, so we specify minimum performance
characteristics. But those characteristics are not intended to specify a
particular implementation, only to insure that some characteristics can be
relied upon.

Therefore, the containers library needs to suggest some performance
characteristics. We believe Implementation Advice is best for this purpose,
as we don't have to be as precise in the language defining the characteristics,
and implementations are required to document deviations from the given
advice.

Appendix

Detailed changes made to the Alternative 2 proposal

The Unchecked_Modification packages were dropped. These are just a hack to
avoid copying keys - a solely performance-based concern. Other stuff does
not logically belong in keys, and modifying the key value itself it a disaster
waiting to happen.

The Vector, List, and Multiset abstractions are essentially the same
abstraction with differences in performance and details. When performance
is not critical, only one is needed.

The package structure has many levels of empty packages for organization.
These are unnecessary when there are only a few packages. Moreover, related
packages can be given similar names (i.e. "Bounded_Set", "Protected_Vector"),
which provides all of the organization needed. The extra empty packages
were eliminated. Similarly, "Unbounded" was dropped; these are the most
general forms, and should be the ones used for general-purpose programming.
Other forms (in a secondary standard) would be more specialized.

We discussed dropping the special string maps. We eventually decided to keep
them, because string maps are common, and a Map cannot be instantiated with
"String" (the key type must be definite).

We also discussed whether Sets should be sorted. We concluded that the extra
cost of sorted insertions is fairly small, and thus there is little advantage
to using unsorted sets other than when performance is critical (which again
is not the purpose of the standard). We did, however, name the package
"Sorted_Sets" so that a basic unsorted set could be provided in a secondary
standard without contorted naming.

We added a modular Hash_Type to Ada.Containers. The choice of Integer'Base is
a horrible one on compilers that use 16-bit Integer (as allowed by the
standard), and in any case, the hashing type should be modular.

The string hash and comparison functions were moved to be part of the
Ada.Strings hierarchy. It would be a bad idea to have the hash functions
of all types gathered in one place (in the containers library).

Unbounded string hash and comparison functions were added.

We changed the type names to the more descriptive "Vector_Type", "Map_Type",
and "Set_Type". These are much better for users who use Use clauses. The
argument that having a common name makes it easier to change between containers
is mostly irrelevant: changing between the provided containers is going to be
rare. Moreover, qualifying "Container_Type" (that is, "Vector.Container_Type")
would be necessary in any unit with more than one container -- eliminating any
advantage for using the same name.

The proposal confused the meaning of "iterator", using it both for the
code that visits each element of a container (the conventional meaning) and
the "handle" or "cursor" used to access an element of a container. We decided
to use "cursor" for the second meaning to make the interfaces clearer.

We added a sort routine to Vector. For some reason, this was only present
in the (removed) List abstraction. Having a simple sort available can
simplify many programming problems.

We added legality rules that these packages must be instantiated at the
library level. The requirement that these packages do not
leak memory (like Ada.Strings.Unbounded) imply that they are implemented with
controlled types (or something like controlled types). We do not want to
implicitly require implementers to support nested controlled types without
making that support available to users. (If AI-344 or AC-50 were adopted, we
could drop these rules.)

The proposal was completely missing definitions for the string hash and
compare functions.

Performance requirements were moved from the !proposal into Implementation
Advice. As much as possible, mention of specific implementation strategies
was moved into AARM notes following that Advice. (We're not going to specify
red-black trees!).

An Assert pragma was added to the Vector package to prevent instantiation with
a type for which Index_Type'Base'First = Index_Type'First. For such a type,
the initial value of Last and the value of Front must necessarily raise
Constraint_Error. It's better to fail an assertion immediately, rather than
during some later operation.

The Map container in the proposal is far too specific to a particular
implementation. It exposes that implementation in the interface, and as a
result makes iteration operations harder. That seems like a bad choice for
a simple abstraction; it's fine to suggest an implementation, but bad to
make it part of the interface. We therefore simplified the interface and the
description. (We consulted with the author of the proposal on this and other
changes.)

****************************************************************

From: Marius Amado Alves
Sent: Wednesday, February 4, 2004  5:13 AM

>Report of the ARG Select Committee on Containers
>February 3, 2004
>...

Sorry for my poor knowledge of ARG procedure.
Does this step mean the library is secured for Ada 2005?
Thanks.

****************************************************************

From: Martin Dowie
Sent: Wednesday, February 4, 2004  5:30 AM

> Sorry for my poor knowledge of ARG procedure.
> Does this step mean the library is secured for Ada 2005?
> Thanks.

Nope - it's still a "Work Item", see:

http://www.ada-auth.org/cgi-bin/cvsweb.cgi/AIs/AI-20302.TXT?rev=1.1



Also, in the text of the AI :-

with Ada.Containers;
package Ada.Strings.Case_Insensitive is
    pragma Pure (Case_Insensitive);

    function "=" (Left, Right : String) return Boolean;
    function "/=" (Left, Right : String) return Boolean;
             ^^^^
Guess this wasn't really meant.

****************************************************************

From: Martin Dowie
Sent: Wednesday, February 4, 2004  8:11 AM

1) Couple of typos in package Ada.Containers.Maps

     generic
        with procedure Process (Cursor : in Cursor_Type) is <>;
     procedure Generic_Iteration (Map : in Map_Type);

     - description refers to 'Generic_Cursor'

     function Length (Map : Map_Type) return Natural;

     - description refers to 'Container' when it should be 'Map'

2) For routines like 'Generic_Iteration' shouldn't the 'Process'
   generic subprogram parameter not have a 'Stop : out Boolean'
   parameter? To allow early exit of the iteration, without
   having to raise exceptions?

****************************************************************

From: Martin Dowie
Sent: Wednesday, February 4, 2004  8:21 AM

Is package Ada.Containers.Maps.Strings[ACMS] really what is
intended, as Ada.Containers.Maps[ACM] is generic this means
to use ACMS a user must first instantiate ACM and then
instantiate ACMS.

Charles didn't suffer from this problem as Unbounded maps (~ACM)
and String Maps (~ACMS) were siblings not parent/child.

****************************************************************

From: Matthew Heaney
Sent: Wednesday, February 4, 2004  8:57 AM

>2) For routines like 'Generic_Iteration' shouldn't the 'Process'
>   generic subprogram parameter not have a 'Stop : out Boolean'
>   parameter? To allow early exit of the iteration, without
>   having to raise exceptions?

Just use an active iterator.

****************************************************************

From: Matthew Heaney
Sent: Wednesday, February 4, 2004  9:52 AM

Note that that's not really the correct mode anyway: it should be inout,
not just out, like this:

   generic
      with procedure Process
         (Cursor : in     Cursor_Type;
           Done   : in out Boolean) is <>;
   procedure Generic_Iteration (Map : in out Map_Type);

The problem with just out-mode is that you always have to give the
parameter a value.  But this is wrong, since you shouldn't be compelled
to say anything if you merely want to continue.  You should only have
say something when you want to stop.

If you only want to visit some of the items, then just use an active
iterator, and exit the loop when you need to:

   declare
      I : Cursor_Type := First (M);
      J : constant Cursor_Type := Back (M);
   begin
      while I /= J loop
         declare
            E : Element_Type := Element (I);
         begin
            --do something with E
            exit when Predicate (E);
         end;

          Increment (I);
      end loop;
   end;

****************************************************************

From: Martin Dowie
Sent: Wednesday, February 4, 2004  10:06 AM


[snip]
> If you only want to visit some of the items, then just use an active
> iterator, and exit the loop when you need to:
[snip]

I could but wasn't part of the purpose of the library to allow us to
do common things more easily? And I'd have to say I'd use a 'Quit'
version a _lot_ more than the current process everything,
every time one.

I'd be delighted if both versions could be included! :-)

****************************************************************

From: Matthew Heaney
Sent: Wednesday, February 4, 2004  11:16 AM

Dowie, Martin (UK) wrote:
> I could but wasn't part of the purpose of the library to allow us to
> do common things more easily? And I'd have to say I'd use a 'Quit'
> version a _lot_ more than the current process everything,
> every time one.

It would be helpful if you could be specific about what kind of
container you were using.

The vector has neither active nor passive iterators, which means that
for a vector you have to use a loop anyway.

For the hashed map, I would find it very odd if you needed to traverse
only some of its elements, since elements are stored in hash order.
What would be the nature of the predicate?

The sorted set is the borderline case.

****************************************************************

From: Peter Hermann
Sent: Wednesday, February 4, 2004  5:57 AM

> package Ada.Strings.Case_Insensitive is

indeed useful.
expected to be overloaded for fixed and (un)bounded strings.

****************************************************************

From: Matthew Heaney
Sent: Wednesday, February 4, 2004  9:02 AM

>Is package Ada.Containers.Maps.Strings[ACMS] really what is
>intended, as Ada.Containers.Maps[ACM] is generic this means
>to use ACMS a user must first instantiate ACM and then
>instantiate ACMS.

That's definitely a bug in the report.  The string-key map is not a
child of a generic.  Maybe we should do this:

package Ada.Containers.Maps
package Ada.Containers.String_Maps

****************************************************************

From: Marius Amado Alves
Sent: Wednesday, February 4, 2004  9:07 AM

Yes, please change that. There is a steady requirement that a single
instantiation must be enough to get a container.

****************************************************************

From: Pascal Obry
Sent: Wednesday, February 4, 2004 10:10 AM

> The problem with just out-mode is that you always have to give the
> parameter a value.  But this is wrong, since you shouldn't be compelled
> to say anything if you merely want to continue.  You should only have
> say something when you want to stop.

Agreed, this is the way iterators are designed in the POSIX 1003.5 standard
for example.

****************************************************************

From: Marius Amado Alves
Sent: Wednesday, February 4, 2004  9:02 AM

> 2) For routines like 'Generic_Iteration' shouldn't the 'Process'
>    generic subprogram parameter not have a 'Stop : out Boolean'
>    parameter? To allow early exit of the iteration, without
>    having to raise exceptions?

Indeed some people ban the use of exceptions for control flow. I guess they
are not a majority in the committee. Fortunately ;-)

/* However to take the exception route the exception should be defined.
(Exit/Terminate_Immediately, _Now, _Prematurely?) Or a specification be made
of what exceptions the iterator is guaranteed to propagate. Simply "all"
would do. Maybe this is already there. I'm sorry, I didn't had time to read
the AI fully yet. */

****************************************************************

From: Randy Brukardt
Sent: Wednesday, February 4, 2004  8:55 PM

Marius Amado Alves wrote:

> Sorry for my poor knowledge of ARG procedure.
> Does this step mean the library is secured for Ada 2005?

What it means is that the study committee has issued a report. No more, and
no less. I would hope that we know more after the March ARG meeting, but
there is no guarantee that we'll work on it (we never seem to get to
everything on the agenda - we didn't work on AI-351, Time Ops in San Diego,
for instance).

Primarily, we just "cleaned up" Matt Heaney's proposal. We didn't change (as
opposed to remove) functionality, with the exception of the Map container
(where we reverted to a design more like the one Charles actually uses -
with Matt's input). So the vast majority of design decisions are Matt's --
we'd prefer to avoid design-by-committee.

Martin Dowie wrote:

> Is package Ada.Containers.Maps.Strings[ACMS] really what is
> intended, as Ada.Containers.Maps[ACM] is generic this means
> to use ACMS a user must first instantiate ACM and then
> instantiate ACMS.

Nope, that's clearly a bug. String_Maps ought to be usable by itself (it
doesn't depend on the other package at all). (And this one is my fault, for
not noticing the effect of the change.)

And later, replying to Matt:

>[snip]
>> If you only want to visit some of the items, then just use an active
>> iterator, and exit the loop when you need to:
>[snip]

>I could but wasn't part of the purpose of the library to allow us to
>do common things more easily? And I'd have to say I'd use a 'Quit'
>version a _lot_ more than the current process everything,
>every time one.

My understanding of Matt's design is that you use the passive iterator when
you want to process everything (which is by far the most common), and you
use an active iterator when you want to process part of the items. You might
use an exception to terminate iteration in an error case, but not if you
intended only to process part of the items. (Of course, there is no law
requiring that, so YMMV!)

I hadn't noticed that there is no passive iterator for vectors until Matt
pointed it out last night (about 20 minutes before we released the report!).
Consistency would suggest that there should be one, but note that it is
easier to write an active iterator for a vector than it is to  write a
passive one:

    for I in First(Vect) .. Last(Vect) loop
        -- Do whatever.
    end loop;

versus

    declare
       procedure Process (I : in Index_Subtype) is
       begin
           -- Do whatever.
       end Process;
       procedure Do_It_All is new Generic_Iterator (Process);
    begin
       Do_It_All (Vect);
    end;

Besides being longer and harder to read, you have to know or look up the
index subtype for the vector in order to write this. So we reached no
conclusion about that in the 20 minutes we had to think about it.

Marius Amado Alves wrote:

> /* However to take the exception route the exception should be defined.
> (Exit/Terminate_Immediately, _Now, _Prematurely?) Or a specification be
made
> of what exceptions the iterator is guaranteed to propagate. Simply "all"
> would do. Maybe this is already there. I'm sorry, I didn't had time to
read
> the AI fully yet. */

The wording for Generic_Iteration for a Map says:

Generic_Iteration calls Process with a cursor that designates each
node in the Map. Any exceptions raised during Process are propagated.

So it's covered. This is important, because it means that the implementation
must be able to clean itself up (if any is needed) when an exception
propagates - it can't leave the Map in an unstable state.

****************************************************************

From: Jeffrey Carter
Sent: Wednesday, February 4, 2004  8:53 PM

AI-302-03 asks

> Anybody got better wording [for the quality of the String hashing
> function]? Matt was nice enough to ignore these definitions
> completely!

See

P. K. Pearson, "Fast Hashing of Variable-Length Text Strings," Comm.
ACM, 1990 Jun

It describes a "hashing function specifically tailored to
variable-length text strings." It says that "similar strings are not
likely to collide." (An implementation can be found in
PragmARC.Hash_Fast_Variable_Length.) Perhaps you might think this last
quote is "better wording".

The actual algorithm produces 8-bit hash values, which may no longer be
considered adequate, given

> Hash_Type'Modulus shall be at least as large as the smaller of
> System.Max_Binary_Modulus and 2**32.

I have some comments on the proposal:

The proposal has a structure called a "Vector" which is actually a list,
which is a sequence that allows insertions and deletions at any point.
"Vector" refers to a mathematical concept related to matrices to most
software engineers. It may be that the STL refers to lists as vectors,
but I hope we do not have to follow C++'s mistakes.

Further, the proposal requires an inefficient array implementation, and
several of the operations refer to this implementation. I think this is
a mistake. Specify an general, unbounded list and let the implementor
choose the implementation (which could be an array). As the proposal
points out, correctly implementing a general list is not trivial, so it
makes sense for a standard library to provide a list.

Maps and sets also specify a specific implementation.

If the intention is to have an extensible array structure, then I
suggest that they be called Extensible_Arrays.

Vector should have an iterator, in addition to allowing the user to
explicitly iterate over the structure.

> Open issue: This function returns a value that doesn't depend on it's
>  parameter. It possibility could be removed in favor of just saying
> Index_Type'Pred(Index_Type'First) appropriately. Committee discussion
>  with the original proposal's author was inconclusive.

I'd say that it should be a constant, not a function. The same seems to
hold for First.

Given that Back is defined as Index_Type'Succ (Last (Vector) ), and Last
(Vector) could be Index_Type'Last, there seems to be a problem. There
should be an assertion that Index_Type'Base'Last > Index_Type'Last.

All the problems with Index_Type disappear with a general list, which
would use a cursor.

I would propose that the sort algorithm be made available to users for
normal array types as well as for vectors. That would involve putting it
in its own library unit and refering to that unit in Vectors.

The Map structure is required to be implemented with a hash table. If
we're going to have such a requirement, it should at least be named
Hashed_Maps.

An important thing about maps is that they provide fast searching,
typically based on a lower-level structure such as a hash table or
balanced tree. Such structures have uses of their own in addition to
creating maps, and independent of the key/value concept of a map. For
example, an application may collect a number of values and then need to
quickly determine if a value is in that collection, and a searchable
structure with a Get_First operation can be used for a priority queue.
None of these applications use key/value pairs. Therefore, I think it's
important to provide the underlying searchable structure to users.
(Indeed, given the ease with which a user can wrap a key/value pair in a
record, define comparison operations for that record that only use the
key part, and create a map structure, given the existence of a
searchable structure, it could be argued, since the proposal states that
easily implemented structures should not be part of the library, that
the library should only supply searchable structures, and not maps.)

Do we really need Maps.[Wide_]Strings, given that an Unbounded_String
can be used for the key type, and that this library should not be used
for applications in which the use of Unbounded_Strings is not acceptable?

The Sets package is mostly incomprehensible. Sets deal with elements,
and operations must include testing if an element is in a set, creating
a set from a list of elements (set "literals"), and set union,
intersection, difference, and symmetric difference. Except for the
membership test, these are missing from the package, so I don't see what
it has to do with sets. It appears to be a searchable structure, not a
set. This is corroborated by the package Generic_Keys, which allows the
structure to be used as a map.

The discussion of the package begins by talking about nodes, which is an
undefined term. The reader has no idea what it has to do with the
package, which is not specified in terms of nodes.

"Sans" is a French word. Since the ARM is in English, we should use the
English "without" instead. "No" might also be acceptable.

I'd like to thank the select committee for their work. No library will
completely please everyone. I will welcome any standard container
library in Ada 0X.

****************************************************************

From: Tucker Taft
Sent: Wednesday, February 4, 2004  9:24 PM

The term "vector" for extensible array is used in Java
as well.  I think we should strive to use terminology
that has become widely used in the programming community.

I personally consider an extensible array (i.e. a vector) a useful and
important standard container.  I don't feel the same way about a linked
list, because it is so easy to implement what you want, and there
are so many options when it comes to how to link the objects
together that having a standard container for that hardly
seems worthwhile (IMHO).

So we settled on Vector, Map, and Set as three basic yet
important abstractions that will help lift the level of
programming above arrays and records.  In my experience
with using languages that have large container libraries,
it is these three that are used more widely than all
the others combined.

****************************************************************

From: Randy Brukardt
Sent: Wednesday, February 4, 2004  9:29 PM

I agree with one caveat: we're already adding something else called "Vector"
to the standard (see AI-296), and two might just be too confusing.

But, the container vector is more useful than the list container (because of
the calculated O(1) access to elements). And they're too similar to support
both when we're trying to support something managable.

****************************************************************

From: Randy Brukardt
Sent: Wednesday, February 4, 2004  9:39 PM

Jeffrey Carter said:
...
> Further, the proposal requires an inefficient array implementation, and
> several of the operations refer to this implementation. I think this is
> a mistake. Specify an general, unbounded list and let the implementor
> choose the implementation (which could be an array). As the proposal
> points out, correctly implementing a general list is not trivial, so it
> makes sense for a standard library to provide a list.
>
> Maps and sets also specify a specific implementation.

No, an implementation is suggested (in AARM notes), as are performance
characteristics. That was one of the larger changes to Matt's original
proposal. If we made that change incompletely somewhere, that needs to be
fixed.

That said, the most important thing is that all implementations have
consistent performance characteristics (so that porting a program from GNAT
to ObjectAda doesn't fail for performance reasons). If GNAT used an array
implementation and ObjectAda used a list implementation for a Vector, access
to elements (which would be O(N) on the imagined OA implementation) could be
too slow for the port to be viable. That needs to be avoided. OTOH,
specifying too much about the implementation would prevent using a better
one -- in that case, we might as well just specify the source code of the
entire library (including the bodies!), and we don't need all of this
wording!

> I would propose that the sort algorithm be made available to users for
> normal array types as well as for vectors. That would involve putting it
> in its own library unit and refering to that unit in Vectors.

Bad idea. To do that, you'd need provide generic formal accessor functions;
that would have a huge overhead of function calls for both Vectors and
Arrays. On a code shared implementation like Janus/Ada, it probably would
run ten times slower than the specified one.

If we want an array sort, we should declare one:

    generic
       type Index_Type is (<>);
       type Element_Type is private;
       function "<" (Left, Right : Element_Type) return Boolean is <>;
       type Array_Type is array (Index_Type) of Element_Type;
    procedure Ada.Generic_Sort (Arr : in out Array_Type);

(We'd need an unconstrained version, too.) But keep it separate from the
Vector one (or any List one, for that matter).

****************************************************************

From: Matthew Heaney
Sent: Thursday, February 5, 2004  9:31 AM

I have hosted a reference implementation at my Earthlink home page:

<http://home.earthlink.net/~matthewjheaney/charles/ai302-20040205.zip>

For now it only includes the vector.  There's a test_sort program in
there too, so you have something you can run.

I'll have the set and maps done in a few days.

****************************************************************

From: Robart A. Duff
Sent: Thursday, February 5, 2004  10:13 AM

Thanks, Matt!

****************************************************************

From: Jeffery Carter
Sent: Thursday, February 5, 2004  10:58 AM

Randy Brukardt wrote:

> No, an implementation is suggested (in AARM notes), as are performance
> characteristics. That was one of the larger changes to Matt's original
> proposal. If we made that change incompletely somewhere, that needs to be
> fixed.

The normative text for vectors says

"A vector container object manages an unconstrained internal array"

That specifies an array implementation.

> Bad idea. To do that, you'd need provide generic formal accessor functions;
> that would have a huge overhead of function calls for both Vectors and
> Arrays. On a code shared implementation like Janus/Ada, it probably would
> run ten times slower than the specified one.

Given that an array implementation is specified, there is no need for
formal accessor functions. The vector can simply call an instantiation
of the sort with the appropriate slice of its internal array. Since we
require such an algorithm to exist, and it is useful to many users, it
makes sense for it to be available outside the vector package.

> If we want an array sort, we should declare one:
>
>     generic
>        type Index_Type is (<>);
>        type Element_Type is private;
>        function "<" (Left, Right : Element_Type) return Boolean is <>;
>        type Array_Type is array (Index_Type) of Element_Type;
>     procedure Ada.Generic_Sort (Arr : in out Array_Type);
>
> (We'd need an unconstrained version, too.) But keep it separate from the
> Vector one (or any List one, for that matter).

If we only have one, I'd prefer it to be unconstrained. That allows
operations such as the vector sort discussed above, where the size of
the slice may change from call to call, without repeated instantiations.

Sort for a list is a different creature. Merge sort is a good choice
there, since a list already has the O(N) additional space that merge
sort requires for array sorting (in the links), provided you have access
to the list internals. Thus you get O(N log N) time in all cases and
O(1) space.

****************************************************************

From: Randy Brukardt
Sent: Thursday, February 5, 2004  3:23 PM

Jeff Carter wrote:

> The normative text for vectors says
>
> "A vector container object manages an unconstrained internal array"
>
> That specifies an array implementation.

Precisely my point. That is intended to say that there is a logical array in
the container, but not necessarly an actual one. Matt's descriptions were
too implementation-specific, and we moved most of that. But I'm not
surprised that some was missed.

...
> Given that an array implementation is specified, there is no need for
> formal accessor functions. The vector can simply call an instantiation
> of the sort with the appropriate slice of its internal array. Since we
> require such an algorithm to exist, and it is useful to many users, it
> makes sense for it to be available outside the vector package.

There is no intent that an array implementation is specified (it certainly
won't be implemented that way on Janus/Ada); only that the performance
characteristics are similar (or better) than that of an array
implementation.

In any case, I have no idea how an external generic would be able to mess
around with the internal array - it certainly can't see it! You'd have to
put the sort into the spec in order to do that -- and that's whats proposed
and what you're objecting to.

****************************************************************

From: Matthew Heaney
Sent: Thursday, February 5, 2004  3:40 PM

Randy Brukardt wrote:

> Precisely my point. That is intended to say that there is a logical array in
> the container, but not necessarly an actual one.

Yes, exactly.  This allows the implementor to leave the type system in
order choose the optimal implementation for the vector container.

An implementor can use any implementation that satisfies the property
that insertion at the back end is (amortized) constant time, and the
property that random access is constant time.

****************************************************************

From: Jeffrey Carter
Sent: Thursday, February 5, 2004  6:52 PM

Randy Brukardt wrote:

>>The normative text for vectors says
>>
>>"A vector container object manages an unconstrained internal array"
>>
>>That specifies an array implementation.
>
> Precisely my point. That is intended to say that there is a logical array in
> the container, but not necessarly an actual one. Matt's descriptions were
> too implementation-specific, and we moved most of that. But I'm not
> surprised that some was missed.

I read it as specifying an implementation. I suggest the wording be
revised to make it clear that the discussion is of a logical array, not
a requirement for an actual array.

> In any case, I have no idea how an external generic would be able to mess
> around with the internal array - it certainly can't see it! You'd have to
> put the sort into the spec in order to do that -- and that's whats proposed
> and what you're objecting to.

I guess I wasn't clear. You would provide the external sort, and also
specify the sort in the spec, with wording that the sort has the same
characteristics as the external sort. This is based on the assumption
that an array implementation is specified, so the sort algorithm, useful
on arrays, must exist anyway.

I'm reminded of my surprise that Ada-83 compilers had to support
inifinte-precision arithmetic, but the language did not require that it
made available to users. If the compiler writers have to implement the
functionality, why not make it available to users? Case-insensitive
string comparison is a similar thing: compilers have to recognize that
frog, Frog, and FROG are the same identifier, but are (were) not
required to make such comparisons available to users.

****************************************************************

From: Jeffrey Carter
Sent: Thursday, February 5, 2004  11:38 AM

Tucker Taft wrote:

> The term "vector" for extensible array is used in Java
> as well.  I think we should strive to use terminology
> that has become widely used in the programming community.

I disagree, even though I know that's dangerous when discussing Ada with
STT. An application that uses both extensible arrays and mathematical
vectors will be very confusing if both are called vectors. Since an
explicit design goal of Ada is to emphasize ease of reading, calling an
extensible array a vector seems inappropriate.

> I personally consider an extensible array (i.e. a vector) a useful and
> important standard container.  I don't feel the same way about a linked
> list, because it is so easy to implement what you want, and there
> are so many options when it comes to how to link the objects
> together that having a standard container for that hardly
> seems worthwhile (IMHO).

I have no objections to an extensible array, provided it's clearly
identified as such. I think it should look different from the proposal,
but that's mainly a taste issue. I'd want direct analogs to indexing,
both LHS and RHS (Put and Get?); slices, both LHS and RHS (Replace_Slice
and Slice?); and 'First, 'Last, and 'Length (though 'First is a constant
for an EA). An equivalent to 'range would be nice, but impossible. The
only difference to a normal array would be that Put and Replace_Slice
can accept indices not in First .. Last. I haven't given it a great deal
of thought, so I'm sure I'm missing some subtleties, but I don't see a
need for Front, Back, Insert, Delete, and so on.

The proposal says that containers "that are relatively easy to code,
redundant, or rarely used are omitted". It also says that lists are
difficult to implement correctly. Given a list, structures such as
deques, stacks, and especially queues are easy to implement. Since
queues are common structures and not redundant (none of the proposed
containers provides an efficient implementation of a queue), the
proposal itself seems to argue that lists should be provided, since they
are not easy to code correctly, and provide a basis for the user to
easily code queues.

> So we settled on Vector, Map, and Set as three basic yet
> important abstractions that will help lift the level of
> programming above arrays and records.  In my experience
> with using languages that have large container libraries,
> it is these three that are used more widely than all
> the others combined.

There was an article by Mills [Harlan D. Mills, Richard C. Linger: Data
Structured Programming: Program Design without Arrays and Pointers. IEEE
Trans. Software Eng. 12(2): 192-197 (1986)] that proposed that
applications only use queues, stacks, and sets (real sets, with union,
intersection, and such operations). It's an interesting concept, and I
agree with the aim of programs using appropriate abstractions and hiding
lower level implementation details, especially use of pointers.

****************************************************************

From: Alexandre E. Kopilovitch
Sent: Thursday, February 5, 2004  9:04 AM

Tucker Taft wrote:

> The term "vector" for extensible array is used in Java
> as well.  I think we should strive to use terminology
> that has become widely used in the programming community.

So call it Java_Vector - that will be at least consistent.

Do you think that Java meaning for "vector" is more significant for Ada than
mathematical meaning of this term (which never implied extensibility) ?

Why not call that thing Flexible_Array (after Algol-68, I think) - this name
will directly reflect the essense.

****************************************************************

From: Robert A. Duff
Sent: Thursday, February 5, 2004  1:37 PM

Bill Wulf and other professors at CMU circa late 1970's were using the
term "vector" to mean "array" (not necessarily extensible); that's the
first time *I* heard it.  So it's not a Java-ism.

I think this meaning of "vector" derives from the maths meaning,
even if it's not precisely the same thing.

****************************************************************

From: Stephen Leake
Sent: Thursday, February 5, 2004  2:18 PM

Jeffrey Carter <jrcarter@acm.org> writes:

> Tucker Taft wrote:
>
> > The term "vector" for extensible array is used in Java
> > as well.  I think we should strive to use terminology
> > that has become widely used in the programming community.
>
> I disagree, even though I know that's dangerous when discussing Ada
> with STT. An application that uses both extensible arrays and
> mathematical vectors will be very confusing if both are called
> vectors. Since an explicit design goal of Ada is to emphasize ease of
> reading, calling an extensible array a vector seems inappropriate.

I agree with Tucker. I have code that uses both Cartesian vectors and
extensible arrays. One is SAL.Math_Double.DOF_3.Cart_Vector_Type, the
other is SAL.Poly.Unbounded_Arrays. Obviously, I have different names
for them, as Carter wants. But if I called them
SAL.Math_Double.DOF_3.Vector and SAL.Poly.Vector, I would have no
chance of confusion. That's what package hierarchies are for.

Since both Java and C++ use the term "vector" for an extensible array,
I think Ada should also. Part of the point of the OY revision is to
make the language more attractive to current users of other languages.
This is an easy way to do that.

> (Replace_Slice and Slice?); and 'First, 'Last, and 'Length (though
> 'First is a constant for an EA).

'First is not constant for SAL.Poly.Unbounded_Arrays; I provide both
Append and Prepend operations. I don't think I've ever used Prepend,
though; it was really just an exercise in what was possible.

> .. I don't see
> a need for Front, Back, Insert, Delete, and so on.

I use Insert and Delete in real applications.

> The proposal says that containers "that are relatively easy to code,
> redundant, or rarely used are omitted". It also says that lists are
> difficult to implement correctly. Given a list, structures such as
> deques, stacks, and especially queues are easy to implement. Since
> queues are common structures and not redundant (none of the proposed
> containers provides an efficient implementation of a queue), the
> proposal itself seems to argue that lists should be provided, since
> they are not easy to code correctly, and provide a basis for the
> user to easily code queues.

I agree. A lists package would be nice.

But I also agree with Tucker, that it is difficult to come up with one
list package that really meets a wide range of needs.

Perhaps one list package, that meets a narrow range of needs, would
still be useful. It would set a style standard for other list packages.

****************************************************************

From: Matthew Heaney
Sent: Thursday, February 5, 2004  2:48 PM

Alexandre E. Kopilovitch wrote:

> So call it Java_Vector - that will be at least consistent.
>
> Do you think that Java meaning for "vector" is more significant for Ada than
> mathematical meaning of this term (which never implied extensibility) ?
>
> Why not call that thing Flexible_Array (after Algol-68, I think) - this name
> will directly reflect the essense.

Tucker T. and Bob D. are both correct: the container is a "vector."

Alexandre K. and Jeff C. are both incorrect.  The container is not a
list, not a Java_Vector, not an Extensible_Array, and not a Flexible_Array.

It is a vector.  It has the same semantics as the identically-named
container in the STL.  The one named "vector."

The container whose name is vector does not have array semantics.  There
is no slicing for example.

The container whose name is vector has the following important properties:

o inserting at the back end is amortized constant time
o supports random access of elements, in constant time

Yes, internally a vector is implemented as an array.  The Size function
returns the length of this internal array, and Resize can be used to
expand its length.

But it is not an array.  It is a container.  Whose name is "vector".
Just like the one in the STL.

****************************************************************

From: Alexandre E. Kopilovitch
Sent: Thursday, February 5, 2004  3:46 PM

No problem with all that if another term was chosen. Now, with "vector", this
is name squatting (well, participation in name squatting in Ada case), which
is fully appropriate for Java, somehow understandable for C++, but seems
(still) inappropriate for Ada, especially taking into account that the involved
term belongs to some Ada-friendly domain.

****************************************************************

From: Robert A. Duff
Sent: Thursday, February 5, 2004  3:38 PM

I wrote:

> Bill Wulf and other professors at CMU circa late 1970's were using the
> term "vector" to mean "array" (not necessarily extensible); that's the
> first time *I* heard it.  So it's not a Java-ism.

Actually, the meaning was "one-dimensional array".  But there was no
implication that they could grow.

> I think this meaning of "vector" derives from the maths meaning,
> even if it's not precisely the same thing.

I mean, what's a vector in 3-space?  Basically, a one-dimensional array
of 3 real numbers -- the X, Y, and Z coordinates.

Matt wrote:

> It is a vector.  It has the same semantics as the identically-named
> container in the STL.  The one named "vector."

This stuff comes from the C++ STL.  I think gratuitous differences from
that are unhelpful.  (But I admit that I was one of the folks pushing
for "cursor" instead of "iterator".)

> The container whose name is vector does not have array semantics.  There
> is no slicing for example.

Well, "no slicing" is hardly fundamental.  It could be added, or
programmed by the client.

> The container whose name is vector has the following important properties:
>
> o inserting at the back end is amortized constant time
> o supports random access of elements, in constant time

I think "random access" is the essence of array semantics.  After all,
anything you can do with an array you can do with a linked list, and
vice versa -- the only fundamental difference is the efficiency
properties.

****************************************************************

From: Matthew Heaney
Sent: Thursday, February 5, 2004  9:31 AM

Robert A Duff wrote:

> This stuff comes from the C++ STL.  I think gratuitous differences from
> that are unhelpful.  (But I admit that I was one of the folks pushing
> for "cursor" instead of "iterator".)

Yes.  The world has settled on the name "vector."  Let's use the terms
everyone else is using, unless we have a good reason not to.

(BTW, that's also why I used the name "Iterator_Type".  But I have no
issues with the name "Cursor_Type".)


> I think "random access" is the essence of array semantics.  After all,
> anything you can do with an array you can do with a linked list, and
> vice versa -- the only fundamental difference is the efficiency
> properties.

But that's the essence of the argument!

Yes, it's *possible* to seek to specific elements in a linked list, but
I would hardly call that "random access."

If you need fast random access to the elements in a container, and the
number of elements in the container is large, then you can effectively
rule out using a linked list as the container.

Of course you could make the argument the other way.  If you need
constant-time insertion of elements at any position, then that
effectively rules out a vector, in favor of a list.

****************************************************************

From: Alexandre E. Kopilovitch
Sent: Thursday, February 5, 2004  3:21 PM

Robert A Duff wrote:

> Bill Wulf and other professors at CMU circa late 1970's were using the
> term "vector" to mean "array" (not necessarily extensible); that's the
> first time *I* heard it.

Yes, CMU always was (as far as I know) primarily engineering educational
facility, and I know well that engineers (not software engineers, but rather
general kind of engineers) often called "vector" any column or row of numbers.
(not bothering themselves with the question how the components of that "vector"
transform with a change of coordinate system). But apparently they never used
this term for arrays of any other objects, and I almost never seen a case
(even in engineering) where "vector" was used for extensible array - except
Java and perhaps some C++ libraries.

A notable exception is APL, in which "vector" is the basic term, and that
"vector" is extensible. But in APL that "vector" is equipped with vast
nomenclature of functions, many of them associated with genuine mathematical
vectors, so the entire balance for the term was acceptable.

> So it's not a Java-ism.

Yes, not exactly - there were other precedents of sloppy usage of this term.
But nevertheless a strong impression remains that it is exactly Java, which
is a real reason, ground and reference for proposing this term for extensible
arrays *now and for Ada0Y*.

> I think this meaning of "vector" derives from the maths meaning,
> even if it's not precisely the same thing.

No, not at all - it lacks the primary mathematical meaning of it, and adds
the primary feature, which meaning is totally non-mathematical (that is, there
is no attempt to bring any mathematical meaning to it... and it will not be
simple, if attempted).

****************************************************************

From: Matthew Heaney
Sent: Thursday, February 5, 2004  5:11 PM



Jeffrey Carter wrote:

> The actual algorithm produces 8-bit hash values, which may no longer be
> considered adequate, given
>
>> Hash_Type'Modulus shall be at least as large as the smaller of
>> System.Max_Binary_Modulus and 2**32.

In Charles I copied the hash function from GNAT.  I figure if it's good
enough for Robert Dewar it's good enough for me...



> Vector should have an iterator, in addition to allowing the user to
> explicitly iterate over the structure.

No.  Vector iterators are fragile, and hence very error prone.

They are fragile because the (logical) internal array gets thrown away
during expansion, which invalidates the iterator.  It's too hard to keep
track of whether a vector iterator is still valid, and most of the time
you end up with a dangling reference.

The STL has vector iterators in order to provide the infrastructure
necessary to support generic algorithms.

In Ada they are not necessary, because you can use locally-declared
subprograms to fit within such a framework.




>> Open issue: This function returns a value that doesn't depend on it's
>>  parameter. It possibility could be removed in favor of just saying
>> Index_Type'Pred(Index_Type'First) appropriately. Committee discussion
>>  with the original proposal's author was inconclusive.
>
>
> I'd say that it should be a constant, not a function. The same seems to
> hold for First.

Front can probably go away.  First is there for consistency with other
containers.



> Given that Back is defined as Index_Type'Succ (Last (Vector) ), and Last
> (Vector) could be Index_Type'Last, there seems to be a problem. There
> should be an assertion that Index_Type'Base'Last > Index_Type'Last.

That's not really possible for generic actual index types such as
Natural or Positive.

We could get rid of the assertion, but this would impact implementors.
That's why it's still an open issue.

In my reference implementation, I don't think the generic actual type
has to have IT'Base'First < IT'First, since internally I use Integer
subtypes for everything.

http://home.earthlink.net/~matthewjheaney/charles/ai302-20040205.zip



> All the problems with Index_Type disappear with a general list, which
> would use a cursor.

The original proposal included list containers, but they were not
included in the subcommittee report, in order to keep the size of the
report more manageable.



> An important thing about maps is that they provide fast searching,
> typically based on a lower-level structure such as a hash table or
> balanced tree.

My original proposal had both sorted and hashed maps, but in order to
keep the subcommittee report small support for sorted maps was removed.


> Such structures have uses of their own in addition to
> creating maps, and independent of the key/value concept of a map. For
> example, an application may collect a number of values and then need to
> quickly determine if a value is in that collection, and a searchable
 > structure with a Get_First operation can be used for a priority queue.

That's what the sorted set is for.


> None of these applications use key/value pairs.

So use the sorted set.


> Therefore, I think it's
> important to provide the underlying searchable structure to users.

Just use the sorted set container.  If guarantees that searches only
take O (log N) even in the worst case.


> (Indeed, given the ease with which a user can wrap a key/value pair in a
> record, define comparison operations for that record that only use the
> key part, and create a map structure, given the existence of a
> searchable structure, it could be argued, since the proposal states that
> easily implemented structures should not be part of the library, that
> the library should only supply searchable structures, and not maps.)

The (hashed) map stores the key and element as separate components of
the internal node of storage.

If you have a record like that, containing a key-part component, then
use the sorted set, and instantiate the nested generic package Generic_Keys.


> Do we really need Maps.[Wide_]Strings, given that an Unbounded_String
> can be used for the key type, and that this library should not be used
> for applications in which the use of Unbounded_Strings is not acceptable?

Yes, we really need string-key maps.


> The Sets package is mostly incomprehensible. Sets deal with elements,
> and operations must include testing if an element is in a set, creating
> a set from a list of elements (set "literals"), and set union,
> intersection, difference, and symmetric difference. Except for the
> membership test, these are missing from the package, so I don't see what
> it has to do with sets. It appears to be a searchable structure, not a
> set. This is corroborated by the package Generic_Keys, which allows the
> structure to be used as a map.

A "set" is really any sorted sequence of items.  If you want set
intersection, symmetric difference, etc, then just use a generic
algorithm.  See the Charles library for such algorithms.

Of course, if you want target of a set union operation to be the set
itself, then just use Insert to insert the items.

The subcommittee report has several examples of how sets are used, and
there's at least one example showing how to use the nested generic package.

See the last two slides in my AE-2003 paper presentation for an example
of how to take the union of a set and a (sorted) list:

http://home.earthlink.net/~matthewjheaney/charles/charlesppt.htm

My original proposal has the same example at the very end:

http://home.earthlink.net/~matthewjheaney/charles/ai302.txt



> "Sans" is a French word. Since the ARM is in English, we should use the
> English "without" instead. "No" might also be acceptable.

Je crois que non.  C'est une bonne idea.

The name for Delete_Sans_Increment comes from Emacs lisp, which has the
functions file-name-sans-extension and file-name-sans-versions.

It was also in homage to Ada's French history, given that her original
designer was French, and worked for a French company.

Why do you think "rendevous" was named that way?



> I'd like to thank the select committee for their work. No library will
> completely please everyone. I will welcome any standard container
> library in Ada 0X.

If you don't immediately grok how vectors and sets and maps work, then I
suggest familiarizing yourself with the STL. There are lots of tutorials
on the WWW.

I also recommend Stanley Lippman's little book Essential C++.  That was
my introduction to the STL, and what originally convinced me that
Stepanov's approach was the correct one.

You might also like Accelerated C++ by Andrew Koenig and Barbara Moo,
which uses the STL as a basis for teaching C++.

****************************************************************

From: Randy Brukardt
Sent: Thursday, February 5, 2004  5:49 PM

Matt's too modest. The tutorial that makes up the !example section is
actually quite good. I learned a lot about how the packages work (and how to
use them) from reading it carefully, and I recommend that everyone do that
to better understand Matt's work.

****************************************************************

From: Randy Brukardt
Sent: Thursday, February 5, 2004  3:48 PM

Jeffrey Carter wrote:

...
> > I personally consider an extensible array (i.e. a vector) a useful and
> > important standard container.  I don't feel the same way about a linked
> > list, because it is so easy to implement what you want, and there
> > are so many options when it comes to how to link the objects
> > together that having a standard container for that hardly
> > seems worthwhile (IMHO).
>
> I have no objections to an extensible array, provided it's clearly
> identified as such. I think it should look different from the proposal,
> but that's mainly a taste issue. I'd want direct analogs to indexing,
> both LHS and RHS (Put and Get?); slices, both LHS and RHS (Replace_Slice
> and Slice?); and 'First, 'Last, and 'Length (though 'First is a constant
> for an EA). An equivalent to 'range would be nice, but impossible. The
> only difference to a normal array would be that Put and Replace_Slice
> can accept indices not in First .. Last. I haven't given it a great deal
> of thought, so I'm sure I'm missing some subtleties, but I don't see a
> need for Front, Back, Insert, Delete, and so on.

Let's see:
- direct analogs to indexing, both LHS and RHS (Element, Replace_Element);
- slices (nope);
- 'First (First), 'Last (Last), 'Length (Length);

Looks like pretty much everything is in there. And slicing will be expensive
if the implementation is not a straight array, so it's somewhat dubious.
Insert and Delete provide easier ways of adding or removing items than
slices - and how often do you use a slice of a non-string type for something
other than inserting or deleting elements anyway??

Ada doesn't (and isn't) going to support user-defined indexing or
user-defined attributes, so this is about the best you can do. So what's the
complaint (other than the name)??

> The proposal says that containers "that are relatively easy to code,
> redundant, or rarely used are omitted". It also says that lists are
> difficult to implement correctly.

I think that's a mistake; only very rare operations are difficult to code.
We didn't update every piece of the original text, and that one is
misleading.

> Given a list, structures such as
> deques, stacks, and especially queues are easy to implement. Since
> queues are common structures and not redundant (none of the proposed
> containers provides an efficient implementation of a queue), the
> proposal itself seems to argue that lists should be provided, since they
> are not easy to code correctly, and provide a basis for the user to
> easily code queues.

The user can easily code a queue in terms of a Vector (that's one of the
uses of Insert!). We dropped the list component because it had an identical
interface to the Vector component, but was less flexible (no computed O(1)
access).

In any case efficiency is not a goal of the standard containers. It would be
incorrect for the standard to specify performance to the point that only a
single implementation would be possible. Moreover, we anticipate a secondary
standard that *does* try to provide more control over performance (by adding
lists, bounded forms, etc.)

In my view, it is a mistake for projects to depend on standard containers
where there are critical performance requirements (not just time, but also
space as well). In that case, you really have to have control of the
implementation -- you really need *all* of the source code. You can't trust
something provided by the standard (or your compiler vendor) in those cases.

In any case, the purpose of these containers is to provide a seed and a
standard direction. I would hope that they would reduce the tower of babel
that Ada containers are nowdays - by providing a style for other containers
to follow. No one is suggesting that these are sufficient to solve all
programming problems - just 80% of them, especially in prototypes and in Q&D
programs.

****************************************************************

From: Martin Dowie
Sent: Thursday, February 5, 2004  5:50 PM

> Dowie, Martin (UK) wrote:
> > I could but wasn't part of the purpose of the library to allow us to
> > do common things more easily? And I'd have to say I'd use a 'Quit'
> > version a _lot_ more than the current process everything,
> > every time one.
>
> It would be helpful if you could be specific about what kind of
> container you were using.

I was thinking, primarily, of a project that used single (bounded) lists to
hold commands (a basic, domain-specific, scripting language I guess),
one of which was 'stop this sequence of commands'.

This pattern has since shown itself to be quite common in embedded
systems - for either domain-specific scripting languages or graphics.

There is the other idiom where one is processing an iteration of items
and an external event occurs that stops the processing - e.g. the 'stop'
button is pushed on a GUI-search window, but it could equally be a
50Hz message over a 1553.

****************************************************************

From: Randy Brukardt
Sent: Thursday, February 5, 2004  6:14 PM

> I was thinking, primarily, of a project that used single (bounded) lists to
> hold commands (a basic, domain-specific, scripting language I guess),
> one of which was 'stop this sequence of commands'.

My understanding of the model is that passive iterators are only for cases
where you want to iterate over the entire container. Thus, this is clearly a
use for an active iterator. Indeed, given the iteration model of packages,
there's hardly any reason to use a passive iterator. They're harder to write
(a subprogram and instantiation are required), and (especially if a Quit
parameter is provided), harder to understand.

We dropped the passive iterator from the Ada.Directories package precisely
because even ARG members were confused about how it worked. Even though it
was a classic passive iterator with a Quit parameter. Perhaps the confusion
really was the Quit parameter (I thought it was the whole idea), but in any
case, you've got to keep them simple.

> This pattern has since shown itself to be quite common in embedded
> systems - for either domain-specific scripting languages or graphics.
>
> There is the other idiom where one is processing an iteration of items
> and an external event occurs that stops the processing - e.g. the 'stop'
> button is pushed on a GUI-search window, but it could equally be a
> 50Hz message over a 1553.

It seems to me that an abort situation is best handled by propagating an
exception. Otherwise, you end up distributing termination code/flags
everywhere in the application. But YMMV.

****************************************************************

From: Jeffery Carter
Sent: Thursday, February 5, 2004  6:39 PM

Matthew Heaney wrote:

> Alexandre K. and Jeff C. are both incorrect.  The container is not a
>  list, not a Java_Vector, not an Extensible_Array, and not a
> Flexible_Array.

Matthew H. is incorrect. The data structure is not a vector.

I am at least as qualified as Matthew H. to make such pronouncements.

****************************************************************

From: Jeffery Carter
Sent: Friday, February 6, 2004  1:05 PM

A comment on type names.

Ada 83, with the unfortunate* exception of File_Type, did not use
"_Type" on the end of predefined type names. We have Address and Count,
not Address_Type and Count_Type. Ada 95 adhered to this principle, so we
have Storage_Element and Unbounded_String, not Storage_Element_Type and
Unbounded_String_Type.

For consistency, I think the Ada-0X process should also adhere to this
principle. The use of "_Type" on type names in the proposal should be
eliminated. This takes some time and thought to do well; I am willing to
volunteer for the effort if the Committee cannot spare the time and
cannot find anyone preferable.

This is a matter of consistently. While it is not my style, and not
recommended by the Quality and Style Guide, I have used libraries that
use the "_Type" convention without problem. I am concerned that the ARM
be consistent far more than I am about what convention the ARM uses.

*"Unfortunate" because it is inconsistent.

****************************************************************

From: Matthew Heaney
Sent: Friday, February 6, 2004  9:33 AM

I have updated the reference implementation, which now has the sorted
set container, too.

There's also a test_sets.adb, so you have something to run.  You can
pass a seed on the command line.

<http://home.earthlink.net/~matthewjheaney/charles/ai302-20040206.zip>

I'll take care of the hashed map containers this weekend, and post Mon AM.

****************************************************************

From: Matthew Heaney
Sent: Friday, February 6, 2004  3:36 PM

Martin Dowie wrote:

> I was thinking, primarily, of a project that used single (bounded) lists to
> hold commands (a basic, domain-specific, scripting language I guess),
> one of which was 'stop this sequence of commands'.

It sounds like you have a sequence container, that you traverse from
front to back.

The only sequence container in the proposal is a vector, which doesn't
have a passive iterator.  Again, I recommend just using a loop:

    for Index in First (V) .. Last (V) loop
       declare
          Command : Command_Type := Element (V, Index);
       begin
          exit when Is_Stop (Command);
          -- process command
       end;
    end loop;


If these are commands that have an order (say, each command has a
timestamp, and commands are executed in timestamp order), then you can
use the sorted set.  Again, an explicit loop is appropriate:

declare
    I : Cursor_Type := First (S);
    J : constant Cursor_Type := Back (S);
begin
    while I /= J loop
       declare
          Command : Command_Type := Element (I);
       begin
          exit when Is_Stop (Command);
          -- process command
       end;

       Increment (I);
    end loop;
end;

****************************************************************

From: Alexandre E. Kopilovitch
Sent: Friday, February 6, 2004  4:24 PM

> The only sequence container in the proposal is a vector,

Ah, yes, it's Sequence - quite right name for that container (and not Vector).

****************************************************************

From: Jeffrey Carter
Sent: Friday, February 6, 2004  7:17 PM

Randy Brukardt wrote:

> Let's see:
> - direct analogs to indexing, both LHS and RHS (Element, Replace_Element);
> - slices (nope);
> - 'First (First), 'Last (Last), 'Length (Length);
>
> Looks like pretty much everything is in there. And slicing will be expensive
> if the implementation is not a straight array, so it's somewhat dubious.
> Insert and Delete provide easier ways of adding or removing items than
> slices - and how often do you use a slice of a non-string type for something
> other than inserting or deleting elements anyway??

Slicing isn't included because C++ doesn't have slices, so it's a
foreign concept to its library and users. If we want to attract users of
inferior languages to Ada, it should be because Ada is better. Ada's
slices are a way that Ada is better; Ada's standard extensible array
component should be better than its competition by also offering them. I
do not see mimicking C++'s shortcomings as advisable.

Insertion and deletion are basic operations of lists, but not of arrays.
That's why the list and vector components had the same set of
operations: they both specify lists with different implementations.

Since String is an array, and [Un]Bounded_String is an extensible array,
and we're now told the correct name is Vector, shouldn't these be
renamed to something like Character_Vector?

> Ada doesn't (and isn't) going to support user-defined indexing or
> user-defined attributes, so this is about the best you can do. So what's the
> complaint (other than the name)??

I don't expect user-defined indexing, slices, or attributes, which is
why I talked about "analogs" to them. Missing slices is one complaint.
And, yes, the name is unarguably wrong.

In the C family of languages, users are accustomed to having to look at
implementations in order to understand how to use something. Subprogram
"prototypes" (yet another misused term to add to the collection) are
generally insufficient, and appropriate comments are often lacking. So
it comes as no surprise to me that C++ expects newcomers to its library,
looking for an extensible array, and not finding anothing with an
appropriate name, to have to look at the operations of the components to
find that the inappropriately named "vector" is really an extensible array.

However, this is not the Ada way, and I think it completely
inappropriate to mimick this mistake. Looking at other languages'
library to select useful components is fine; insisting that an Ada
version must be identical to that of another language, including
mistakes, is not.

> The user can easily code a queue in terms of a Vector (that's one of the
> uses of Insert!). We dropped the list component because it had an identical
> interface to the Vector component, but was less flexible (no computed O(1)
> access).

The perfomance of a queue based on an extensible array is likely to be
just as objectionable as extracting an element from an extensible array
based on a list. That the vector and list components both had the same
interface is further evidence that mimicking the STL is a bad idea.
Insert and delete are as foreign to an extensible array as indexing and
slicing should be to a list.

> In my view, it is a mistake for projects to depend on standard containers
> where there are critical performance requirements (not just time, but also
> space as well). In that case, you really have to have control of the
> implementation -- you really need *all* of the source code. You can't trust
> something provided by the standard (or your compiler vendor) in those cases.

I agree. That doesn't mean that the standard shouldn't provide a basis
for queues with performance characteristics suitable for performance
non-critical applications, which an extensible array does not provide.

****************************************************************

From: Randy Brukardt
Sent: Friday, February 6, 2004  8:24 PM

Jeff Carter wrote:

...
> I agree. That doesn't mean that the standard shouldn't provide a basis
> for queues with performance characteristics suitable for performance
> non-critical applications, which an extensible array does not provide.

Huh? You've said, in effect, that the performance isn't good enough for
applications where the performance doesn't matter. That's a pretty goofy
statement!

My opinion has not changed: if you care about performance *at all*, you
*cannot* depend on *any* standard containers. But usually the performance
does not matter at all (or so little as to be equivalent to not at all): the
number of elements in the container is small (which would be true for
virtually all queues), and/or it is used infrequently, and/or the
application is a throw-away.

Otherwise, if you are writing portable code, you shouldn't use a predefined
container library at all -- the performance is likely to vary much more
across implementations than code you write yourself. For instance, on
Janus/Ada, any generic list container is going run 2-5 times slower than the
same list created yourself -- that's just the effect of the extra call
overhead and the shared body (which means the elements will be dynamically
allocated - separately - in any case - at least doubling the allocation
overhead). I'd expeect that effect to be much less on GNAT, for example,
because they don't share generic bodies and thus don't have the double
allocation overhead.

If your application doesn't care about the component being 5 times slower,
then it is highly unlikely that it is going to care about whether the
Vector/Sequence/List component is implemented as an array, as a list, as a
tree, as a hash table, or as something else.

My preference with these components would be to say absolutely nothing about
performance or implementation (because anything said is as meaningless as
real-time metrics are). But others believe that that would cause real
portability problems, and I'm willing to go along with that.

The problem I see is a lot of people are looking far too closely at tiny
pieces of abstractions.  You might have a queue or a list as part of a large
abstraction, but they're pretty much useless by themselves. And given that
creating a queue or stack (both of which have only two operations, both
trivial!) would take 3 minutes max, it makes no sense to use a complex (and
necessarily slow) container library for just that -- indeed, it probably
would be more work to use a container than the 3 minutes.

I much prefer the vision of this containers library, where the only
containers included are those that are large, complex, multi-purpose, and
have a clear abstraction.

****************************************************************

From: Jeffrey Carter
Sent: Friday, February 6, 2004  7:39 PM

Matthew Heaney wrote:

> No.  Vector iterators are fragile, and hence very error prone.

Modifying a structure from an iterator should be a bounded error.

> They are fragile because the (logical) internal array gets thrown away
> during expansion, which invalidates the iterator.  It's too hard to keep
> track of whether a vector iterator is still valid, and most of the time
> you end up with a dangling reference.

You can only talk about what happens internally during an operation if a
specific implementation is required, which Randy assures us is not the case.

> A "set" is really any sorted sequence of items.  If you want set
> intersection, symmetric difference, etc, then just use a generic
> algorithm.  See the Charles library for such algorithms.

I've used sets for decades, in discrete math, in specification languages
such as Z, and in programming. A set is an unordered collection of
elements from a universe that provides operations such as membership,
union, intersection, and the like, represented by mathematical symbols
that I can't reliably represent in an e-mail.

An implementation of a set may be sorted to speed up operations, but
that's a feature of the implementation, not of the concept implemented.
That's a distinction that many users of C-family languages seem unable
to make, but that I expect from those who embrace Ada.

> The name for Delete_Sans_Increment comes from Emacs lisp, which has the
> functions file-name-sans-extension and file-name-sans-versions.

Yet another case of mimicking others' errors.

> It was also in homage to Ada's French history, given that her original
> designer was French, and worked for a French company.
>
> Why do you think "rendevous" was named that way?

"Rendezvous" is not a predefined indentifier in the ARM. It was chosen
because no English word has the precise meaning intended, and Ada's
designers understood the importance of precise terminology.

> If you don't immediately grok how vectors and sets and maps work, then I
> suggest familiarizing yourself with the STL. There are lots of tutorials
> on the WWW.

I've been using arrays, including extensible arrays, sets, and maps for
decades. I've also been using vectors for decades, having done a lot of
scientific programming that required matrix math. I doubt that a study
of C++ mistakes would have any effect besides raising my blood pressure.

****************************************************************

From: Jeffrey Carter
Sent: Friday, February 6, 2004  7:22 PM

Randy Brukardt wrote:

> Precisely my point. That is intended to say that there is a logical array in
> the container, but not necessarly an actual one. Matt's descriptions were
> too implementation-specific, and we moved most of that. But I'm not
> surprised that some was missed.

On closer inspection, the Size and Resize operations certainly imply an
array implementation; they are meaningless otherwise.

****************************************************************

From: Randy Brukardt
Sent: Friday, February 6, 2004  9:09 PM

Huh? Resize tells the container a reasonable size to use; what the container
does with that information is up to it. Size simply returns that
information.

That's no different than many of the attributes in Ada, which (if set),
always return the values that they were set to. But what the compiler does
with those values is (almost) completely implementation-defined.

The only real requirement here is O(1) element access (which prevents the
use of a straight linked list).

Janus/Ada will probably use an array of pointers (or possibly array of
arrays of pointers); we're going to be (implicitly) allocating the elements
anyway, we might as well do it explicitly and take advantage of that to make
Insert/Delete/Sort (and any expansions) much cheaper (presuming the elements
are bigger than scalar types). An array of arrays of pointers is even
better, because insertion cost is bounded by the maximum size of an array
chunk -- but there is more overhead and complexity, so I'd like to see some
real uses before deciding on an implementation.

Note that a pure list component has no real opportunity for "better"
implementations, and indeed, any implementation on Janus/Ada would suffer
from "double" allocation.

****************************************************************

From: Martin Dowie
Sent: Saturday, February 7, 2004  4:02 AM

> We dropped the passive iterator from the Ada.Directories package precisely
> because even ARG members were confused about how it worked. Even though it
> was a classic passive iterator with a Quit parameter. Perhaps the confusion
> really was the Quit parameter (I thought it was the whole idea), but in any
> case, you've got to keep them simple.

I didn't find it confusing so I provided an extra child
Ada.Directories.Iterate - and I've used it repeatedly!

> > This pattern has since shown itself to be quite common in embedded
> > systems - for either domain-specific scripting languages or graphics.
> >
> > There is the other idiom where one is processing an iteration of items
> > and an external event occurs that stops the processing - e.g. the 'stop'
> > button is pushed on a GUI-search window, but it could equally be a
> > 50Hz message over a 1553.
>
> It seems to me that an abort situation is best handled by propagating an
> exception. Otherwise, you end up distributing termination code/flags
> everywhere in the application. But YMMV.

I have tended to work in deeply enbedded systems, where exceptions (in
any language!) are at best frowned upon and quite often forbidden! :-(

****************************************************************

From: Martin Dowie
Sent: Saturday, February 7, 2004  4:25 AM

> > I was thinking, primarily, of a project that used single (bounded) lists to
> > hold commands (a basic, domain-specific, scripting language I guess),
> > one of which was 'stop this sequence of commands'.
>
> It sounds like you have a sequence container, that you traverse from
> front to back.

Pretty much, although we also read in where each 'First' is as the whole
contained many 'subroutines'.


> The only sequence container in the proposal is a vector, which doesn't
> have a passive iterator.  Again, I recommend just using a loop:

I suspect the first thing I will do is add an extra child generic subprogram
Ada.Containers.Vectors.Iterate! :-)

****************************************************************

From: Martin Krischik
Sent: Saturday, February 7, 2004  6:16 AM

> I suspect the first thing I will do is add an extra child generic
> subprogram Ada.Containers.Vectors.Iterate! :-)

Well, guess don't use GNAT. GNAT gets quite upset if you try to add something
to the Ada packages.

****************************************************************

From: Marius Amado Alves
Sent: Saturday, February 7, 2004  7:45 PM

I'd expect *any* compiler to get really upset with this ;-)

****************************************************************

From: Martin Dowie
Sent: Sunday, February 8, 2004  2:08 AM

"gcc -gnatg" or "gnatmake -a" will stop any warnings :-)

****************************************************************

From: Martin Krischik
Sent: Saturday, February 7, 2004  5:09 AM

> Jeffrey Carter wrote:

> > Given a list, structures such as
> > deques, stacks, and especially queues are easy to implement. Since
> > queues are common structures and not redundant (none of the proposed
> > containers provides an efficient implementation of a queue), the
> > proposal itself seems to argue that lists should be provided, since they
> > are not easy to code correctly, and provide a basis for the user to
> > easily code queues.

> The user can easily code a queue in terms of a Vector (that's one of the
> uses of Insert!). We dropped the list component because it had an identical
> interface to the Vector component, but was less flexible (no computed O(1)
> access).

True enough. But if you wanted a build generic queue on top of the vector the
tag should not be hidden from view. Otherwise one need to repeat all the
access methods instead of just renaming the one provided from the parent
package.

In fact the hidden tag is the one feature which I realey dislike in charles.

****************************************************************

From: Stephen Leake
Sent: Saturday, February 7, 2004  8:40 AM

"Randy Brukardt" <randy@rrsoftware.com> writes:

> Report of the ARG Select Committee on Containers
> February 3, 2004

Thanks for the committee's hard work on this.

What is the rationale for making the Map Key_Type definite, as opposed
to indefinite? Since an indefinite Key_Type is required for
Containers.Maps.Strings, why not make that capability available to the
users?

I don't see a discussion of this in AI-302-03/01.


Another point: Containers.Vectors.Size should return Index_Type'Base,
and the Size parameter in Resize should also be Index_Type'Base. It's
confusing to have different types for Size and Index.

There's also a problem if Natural'Last < Index_Type'Last; you
can't have a vector that contains every index!

****************************************************************

From: Randy Brukardt
Sent: Saturday, February 7, 2004  6:03 PM

> What is the rationale for making the Map Key_Type definite, as opposed
> to indefinite?

The 'committee' primarily adopted the existing proposal submitted by Matt
Heaney. We decided not to change any of the major design decisions of that
proposal - because no package will suit everyone or every need, and we felt
it was more important to standardize something coherently designed for most
needs than to fiddle endlessly with it and risk introducing serious bugs.

Which is to say, I don't know. :-)

> Since an indefinite Key_Type is required for
> Containers.Maps.Strings, why not make that capability available to the
> users?

We definitely expect that the strings container will use a purpose-built
data structure for storing strings, not some general indefinite item
capability. Ways to compactly and efficiently store sets of varying size
strings are well known and commonly used.

Such algorithms could be extended to a general "unconstrained array of
elementary", but that hardly seems to be a worthwhile definition for keys.

...
> Another point: Containers.Vectors.Size should return Index_Type'Base,
> and the Size parameter in Resize should also be Index_Type'Base. It's
> confusing to have different types for Size and Index.
>
> There's also a problem if Natural'Last < Index_Type'Last; you
> can't have a vector that contains every index!

Yes, that's a serious problem on Janus/Ada (Integer is 16-bit). However, you
want the Size and Resize operations to take a numeric type that contains
zero -- and certainly Index_Type is not that. Index_Type could be a subtype
of an enumeration type or a subtype of a modular type (neither of which can
contain zero) or a subtype of an integer type not containing zero.

We had a short, inconclusive discussion about whether the index type ought
to be range <> rather than (<>) (because enumeration and modular types fail
the assertion and thus aren't directly usable), but that still doesn't
guarantee a zero. Moreover, if the integer type has negative numbers, then
the Length of the vector could be larger than Index_Type'Last.

So I don't see a great solution. I wondered about using "Hash_Type" here (it
has the correct properties), but that seems like a misuse of the type (and a
bad idea in a library that most Ada programmers will read - you want to show
them good style in standard libraries).

****************************************************************

From: Martin Krischik
Sent: Saturday, February 7, 2004  5:15 AM

> The perfomance of a queue based on an extensible array is likely to be
> just as objectionable as extracting an element from an extensible array
> based on a list. That the vector and list components both had the same
> interface is further evidence that mimicking the STL is a bad idea.
> Insert and delete are as foreign to an extensible array as indexing and
> slicing should be to a list.

Well, depends. Most queues are not supposed to grow indefinetly so an using a
vector with an modular type as index will give you good perfomace. Every Ada
tutorial contains a expample on how to do it.

****************************************************************

From: Martin Krischik
Sent: Saturday, February 7, 2004  6:14 AM

> The committee selected the second proposal as a starting point for a
> standard containers library, with a number of simple changes. The
> changes were simple enough that we produced a version of the library with
> the changes made (AI-00302-3/01).

Any place where I can actualy read the draft?

Anyway, looking at the reference impementation vom Matthew Heaney (thanks for
the quick responce) I have an improvements to suggest:

type Element_Type is private;

I said this bevore that is too limiting. With that signature you can't even
store strings. And more important you cant store Element'Class. In fact I
predict that with that signature 80% of all data stored will be "access to
something".

I have often heard Ada does not need garbage collection since a good container
library should take care of memory management - and now I ready to follow
that point. But taking that argument, vector is not a good container.

Since vector will need heap storrage anyway and performace is only a minor
issue I suggest:

type Element_Type (<>) is private;

****************************************************************

From: Randy Brukardt
Sent: Saturday, February 7, 2004  6:05 PM

> Any place where I can actualy read the draft?

The same place that you can read any other AI: www.ada-auth.org.

****************************************************************

From: Martin Krischik
Sent: Sunday, February 8, 2004  4:58 AM

I looked there but I only found a very long discussion but not the
actual concluding decision.

****************************************************************

From: Randy Brukardt
Sent: Monday, February 9, 2004  6:03 PM

Don't know what you're looking for, but certainly the entire AI is posted
there. As with all AIs, the !wording section is what goes into the standard.

****************************************************************

From: Martin Krischik
Sent: Saturday, February 7, 2004  6:24 AM

> > The only sequence container in the proposal is a vector,
>
> Ah, yes, it's Sequence - quite right name for that container (and not
> Vector).

No, in my book elements in a Sequence have only a relative positions, or at
least the relative position is the primary position and absolut position is
only the secondary.

That is: Get_Next (V); is faster or as fast as Get (V, 5);

****************************************************************

From: Martin Krischik
Sent: Saturday, February 7, 2004  6:32 AM

> My understanding of the model is that passive iterators are only for cases
> where you want to iterate over the entire container.

Yes.

> Indeed, given the iteration model of packages,
> there's hardly any reason to use a passive iterator.

Passive Iterators should allways provide the fastes mean to iterate over the
hole container. They should do so by knowing the internals of the container.

Of course it only matters in advanced container with B-Trees or AVL-Trees as
as internal structure. But I have only seen those in IBM's Open Class Library
(which is far better the the STL).

But there are no advanced containers in AI 302.

****************************************************************

From: Randy Brukardt
Sent: Saturday, February 7, 2004  6:21 PM

> Passive Iterators should allways provide the fastes mean to iterate over the
> hole container. They should do so by knowing the internals of the
> container.

That might be true in a language with a built-in iterator construct, but it
is certainly not true in Ada because of the overhead of calling the generic
formal subprogram for each element. In Janus/Ada, the overhead of calling a
formal subprogram is at least double of a normal subprogram (we have to save
and restore display information, because you could be calling into a more
nested scope than the generic body -- something that normally isn't possible
in Ada).

Other compilers may not have that overhead, but they'll certainly have call
overhead. Whereas, the explicit loop iterator for Vectors only needs to call
Element. So the call overhead is at best a wash, and at worst much worse for
the passive iterator. Moreover, the compiler is a lot more likely to be able
to in-line the call to Element (which likely has a pretty simple
implementation and thus will meet the in-lining qualifications), than the
bunch of arbitrary code in the Process formal routine.

So, a passive iterator will only be faster in complex containers (where you
have to separate the  Element and Successor functions). For a Vector (where
the language already has the needed iteration mechanism built-in), it's
going to be slower (or, if you're really lucky, the same speed) and it
certainly is a lot harder to write.

So I think having it on Vector would simply be for consistency; you'd never
actually use it if you know you're dealing with a Vector.

****************************************************************

From: Robert A. Duff
Sent: Saturday, February 7, 2004  7:22 PM

> Other compilers may not have that overhead, but they'll certainly have call
> overhead. Whereas, the explicit loop iterator for Vectors only needs to call
> Element. So the call overhead is at best a wash, and at worst much worse for
> the passive iterator. Moreover, the compiler is a lot more likely to be able
> to in-line the call to Element (which likely has a pretty simple
> implementation and thus will meet the in-lining qualifications), than the
> bunch of arbitrary code in the Process formal routine.

I don't see why the compiler shouldn't inline the Process routine,
assuming the compiler isn't doing shared generics.  They're usually
small, but anyway, the Process routine is typically called exactly
once, so it shouldn't matter how big it is.

****************************************************************

From: Randy Brukardt
Sent: Saturday, February 7, 2004  7:33 PM

Most compilers have limitations on what can be inlined; Process (which
contains arbitrary code) is far more likely to violate one of those
limitations than Element (which never changes and is likely to be very
simple). In addition, many compilers only inline when you give pragma
Inline, and you can't do that on a generic formal.

****************************************************************

From: Robert A. Duff
Sent: Saturday, February 7, 2004  7:43 PM

If Process violates whatever these arbitrary restrictions are, then
sure, you can't get it inlined.  But typically Process is very simple --
often just one line of code that calls some other procedure to do the
real work, passing some additional parameters.  Process isn't a "real"
procedure, conceptually -- it's just the body of a loop.

In my current project, we make heavy use of the generic iterator
pattern, and I think that in many many cases, Process is just
a line or two of code.  (And if it's more, inlining is relatively
less important.)

>... In addition, many compilers only inline when you give pragma
> Inline, and you can't do that on a generic formal.

You give the inline on the actual.  In non-sharing implementations,
that should apply inside the instance.  And the iterator procedure
itself can be inlined, too.

****************************************************************

From: Randy Brukardt
Sent: Saturday, February 7, 2004  8:04 PM

Certainly it's not real (which is one thing I dislike about passive
iterators in Ada - but we've discussed that before), but if it is very short
(or the bodies of your loops are typically very short), then you're
programming style must be very different from mine. The only loops that I
write that are very short are those that I probably shouldn't have written
in the first place (like the one finding the last '.' in a string) --
there's a routine somewhere in Ada.Strings that will do the job, but looking
it up is more work than writing the loop. (And a lot of them would be
replaced by a Vector/List/Sequence container if I had one.)

But just looking at the spam filter I'm working on at this moment: The
average loop length is about 25 lines, the mean is around 8 lines. (There
are more short loops than I would have guessed. But most of them wouldn't
exist if I had a container to use instead - most of them are insert-at-end
or delete-specific-item from a list.)

...
> You give the inline on the actual.  In non-sharing implementations,
> that should apply inside the instance.  And the iterator procedure
> itself can be inlined, too.

At which point, you *equal* the performance of the active iterator. And only
if *everything* goes right. The OP claimed that the passive iterator would
always have better performance, and that's certainly not true for the vector
container. I doubt that it would be true for the Map container, either. It
could be true for a complex container, but those aren't commonly used.

****************************************************************

From: Alexandre E. Kopilovitch
Sent: Saturday, February 7, 2004  7:55 PM

Martin Krischik wrote:

> > > The only sequence container in the proposal is a vector,
> >
> > Ah, yes, it's Sequence - quite right name for that container (and not Vector).
>
> No, in my book elements in a Sequence have only a relative positions, or at
> least the relative position is the primary position and absolut position is
> only the secondary.

I don't know in which domain your book was grown up, but I can assure you that
in mathematics (and by extension in physics and other natural sciences as they
use mathematical apparatus) elements of a sequence are commonly indexed, and
those indices are always treated as absolute position (which may be zero or
even negative). By the way, your book is also certainly not from Biology/Genetics,
where term "sequence" is used heavily, and they often speak about both absolute
and relative positions in sequences.

We have clearly different usage of terms "vector" and "sequence": substantial
part of today's software engineering (tools and books) use them one way, while
mathematics (and all natural sciences that use it heavily) always use them another
way.

So all the argument here about Vector/Sequence here is about Ada's choice of
preference: will Ada choose software engineering (effectively, Java and C++
libraries) side or mathematical/scientific side on this issue.

I suppose (or hope) that the thesis "Ada is for problem space, not for solution
space" implies the latter.

****************************************************************

From: Martin Krischik
Sent: Sunday, February 8, 2004  11:40 AM

> I don't know in which domain your book was grown up, but I can assure you

It's the english dictornary: "Aufeinanderfolge, Reihenfolge, Szene,
Zeitfolge". Ah, you don't speak german. Well let's look for "Reihenfolge" in
a rushian dictornary (and have a fight with my wives rushian keyboard):
"???????????".

Asking my wives what it means she said "one after the other, queue".

> that in mathematics (and by extension in physics and other natural sciences
> as they use mathematical apparatus) elements of a sequence are commonly
> indexed, and those indices are always treated as absolute position (which
> may be zero or even negative). By the way, your book is also certainly not
> from Biology/Genetics, where term "sequence" is used heavily, and they
> often speak about both absolute and relative positions in sequences.

I have spend 4 years in Great Britain I am shure if I ask anyone on the street
there "what is a sequence" he or she will answer somthing like "one after the
other" - and that is relativ positioning.

> We have clearly different usage of terms "vector" and "sequence":
> substantial part of today's software engineering (tools and books) use them
> one way, while mathematics (and all natural sciences that use it heavily)
> always use them another way.

Even when it comes done to software engineering: IBM's Open Class Library has
a Sequence - for relativ positioning  getFirst, getNext, insertAfter. Usualy
used to fill listboxes.

> So all the argument here about Vector/Sequence here is about Ada's choice
> of preference: will Ada choose software engineering (effectively, Java and
> C++ libraries) side or mathematical/scientific side on this issue.

I don't like the STL that much. So I am not realy defending "vector".

> I suppose (or hope) that the thesis "Ada is for problem space, not for
> solution space" implies the latter.

I agree with you on that too.

But I think we are off topic here.

****************************************************************

From: Marius Amado Alves
Sent: Saturday, February 7, 2004  8:41 PM

Randy Brukardt wrote:

>The 'committee' primarily adopted the existing proposal submitted by Matt
>Heaney. We decided not to change any of the major design decisions of that
>proposal - because no package will suit everyone or every need, and we felt
>it was more important to standardize something coherently designed for most
>needs than to fiddle endlessly with it and risk introducing serious bugs.
>
>Which is to say, I don't know. :-)

I do: there is none (except perhaps the implicit one: ease of
implementation). On the other hand, there is a rationale for indefinite
elements. This requirement has been largely felt and voiced since ever,
and I included it in my Bases document (I think stored in alternative
1), and even formulated it as an Annex (stored in alternative 2 but
applicable to any alternative). But I've always seemed to feel some
resistance from Matt and the ARG. Which resistance I find inexplicable.
I really don't see how making the element type indefinite may
"compromise coherence" or "introduce bugs". Sure it complicates the
implementation. But the increase in power for the user is a quantum
leap, as it frees him from doing tricky memory management in many
situations. In my proposed Annex I included this passage from someone
who should be dear to at least one person in that group--perhaps in the
hope of making those strange walls of resistance just shiver a bit:

<<If I ask a student whether her design is as good as Chartres, she often smiles tolerantly
at me as if to say, "Of course not, that isnt't what I am trying to do.... I could never do
that." Then, I express my disagreement, and tell her: "That standard *must* be our
standard. If you are going to be a builder, no other standard is worthwhile.">>
  -- Cristopher Alexander, Foreword to [Gabriel 1996]

****************************************************************

From: Randy Brukardt
Sent: Saturday, February 7, 2004  9:20 PM

> I do: there is none (except perhaps the implicit one: ease of
> implementation). On the other hand, there is a rationale for indefinite
> elements.

Perhaps. But that wasn't the question. The question was why aren't there
indefinite *keys*.

...
> But I've always seemed to feel some
> resistance from Matt and the ARG.

Given that the "ARG" (other than the subcommittee) has not yet looked at
these proposals, that's a pretty bizarre statement.

...
> I really don't see how making the element type indefinite may
> "compromise coherence" or "introduce bugs". Sure it complicates the
> implementation.

And, on most implementations, I would expect it to make it *many* times
slower. (It wouldn't have any effect on Janus/Ada, I don't think, because we
already have to allocate an element at a time anyway.) I would guess that it
is that efficiency concern that Matt is responding to. But I'll let him
respond himself...

****************************************************************

From: Marius Amado Alves
Sent: Sunday, February 8, 2004  6:26 AM

>... that wasn't the question. The question was why aren't there
>indefinite *keys*.
>
Oops... sorry.

Curiously enough if you have indefinite elements the requirement for
indefinite keys looses strength: you can then use elementary containers
or indefinite element positions as keys.

>...
>
>>But I've always seemed to feel some
>>resistance from Matt and the ARG.
>
>Given that the "ARG" (other than the subcommittee) has not yet looked at
>these proposals, that's a pretty bizarre statement.

Just a feeling. The proposals are there in the AI, and there was some
discussion.

>>I really don't see how making the element type indefinite may
>>"compromise coherence" or "introduce bugs". Sure it complicates the
>>implementation.
>
>And, on most implementations, I would expect it to make it *many* times
>slower....

No. The system should chose at compile time a specific body according to
the 'Definite attribute of the actual element type.

Aside. Of course there is still no standard means to do this, but it
would be a nice extension. Conditional compilation of generic bodies
based on instantiation properties. Variant units :-)
  generic
    type T is private;
    ...
  package G is
    when T'Definite =>
      ...;
    when others =>
      ...;
  end;
(On the subject of conditional compilation, see also the recent Ada
Preprocessor thread on CLA.)
In the meanwhile, there is no requirement that Ada.Containers be
implemented strictly in Ada, is there? I doubt any Ada 95 container
(arrays, files) is.
End of aside.

So no coherence problem, nor bugs, nor efficiency problem :-)

****************************************************************

[Editor's note: For continuing mail on this AI, see AI-00302-04.]

****************************************************************

Possible improvements from the original author (Matt Heaney), February 18, 2004

o The vector container declares a subtype of its generic formal index type:

   subtype Index_Subtype is Index_Type;

This turns out to be very useful when you need to keep track of what is
the range of the vector container index type.  I had a real headache
when writing an app when I switched from Positive to Natural, and it's
because I didn't use an index subtype.

We could generalize this for all the containers.  For example the map
container would look like this:

   subtype Key_Subtype is Key_Type;
   subtype Element_Subtype is Element_Type;

This turns out to be useful when you need to instantiate the
Generic_Element function, which you can do using just the subtypes:

   type Element_Access is
      access all XXX_Maps.Element_Subtype;

   function To_Access is
      new XXX_Maps.Generic_Element (Element_Access);


o There is an open issue about what value Sorted_Sets.Find should return
if Find fails to find the search item.  It could either be Null_Cursor
or it could be the value Back (Set).  (The API now says that it's the
value Null_Cursor.)

The benefit of Null_Cursor is that detecting accidental deferences of
the return value of a failed search is easy, since the internal access
object is null.  However, that breaks symmetry with searches over a
sequence, which fall off the end onto Back when the search fails.

If we were interested in trying to prevent any dereference of the Back
sentinel node of a set, one possibility would be to give the sentinel
node a special color (assuming that the set is implemented as a
red-black tree, of course), and then test that in the dereference
operations Element and Generic_Element.

But this might be too paranoid, and there are probably many wrong ways
to dereference a cursor that I can't even imagine.  And an implementor
might use some other data structure that makes the detection difficult
or impossible.

So we could just return Back and not worry about accidental
dereferences.  (I don't have any data to suggest that accidental
deferences would even be a problem.) I'm beginning to think that I was
being a too conservative when I said Find should return Null_Cursor
instead of Back, and I'm concerned now about the inconsistency.


o This also illustrates the fact that support for aggregate-style
operations in the current spec is weak, mostly supporting manipulation
of elements one-at-a-time.

If we are interesting in a vector being a component for manipulation of
unbounded arrays then there are operations in Ada.Strings.Unbounded that
might be useful, such as Replace_Slice and Overwrite.

One of the errors in Ada.Strings.Unbounded was that to insert an
aggregate of elements (characters) into the string you have to insert an
array type.  But suppose you only have another Unbounded_String?  We can
generalize insert to allow insertion of a vector or a vector slice into
another vector, like this:

   procedure Insert (Vector   : in out Vector_Type;
                     Before   : in     Index_Type'Base;
                     New_Item : in     Vector_Type);

   procedure Insert (Vector   : in out Vector_Type;
                     Before   : in     Index_Type'Base;
                     New_Item : in     Vector_Type;
                     First    : in     Index_Type'Base;
                     Last     : in     Index_Type'Base);

That's just insertion.  Slice assignment could be done using:

   procedure Replace_Elements (Vector : in Vector_Type;
                               Low    : in Index_Type'Base;
                               High   : in Index_Type'Base;
                               By     : in Element_Type);

which provides the vector analog of

   V (I .. J) := (others => E);

We have also discussed assignment operations.  A vector is non-limited
so of course you can say:

   V1 := V2;

This replaces the target with the entire range of source.  Another
possibility is to assign to the target just a slice of the source:

   V1 := V2 (I .. J);

Both of these operations would look like this:

   procedure Assign (Target : in out Vector_Type;
                     Source : in     Vector_Type);

   procedure Assign (Target : in out Vector_Type;
                     Source : in     Vector_Type;
                     Low    : in     Index_Type'Base;
                     High   : in     Index_Type'Base);

We could generalize further still.  The operation

   procedure Assign (Target : in out Vector_Type;
                     Source : in     Element_Type;
                     Count  : in     Element_Count);

is the vector analog of

   subtype Array_Subtype is Array_Type (1 .. Count);

   V1 := Array_Subtype'(others => Source);


Assign operations might also be more efficient than constructor
functions, since the target can be built in place, without any
controlled finalization and adjustment.

Assignment operations are probably more efficient but it might make
sense to have constructor functions too:

   function To_Vector (Length : Element_Count)
     return Vector_Type;

   function To_Vector (Source : Vector_Type;
                       Low    : Index_Type'Base;
                       High   : Index_Type'Base)
     return Vector_Type;

   function To_Vector (Source : Element_Type;
                       Count : Element_Count)
     return Vector_Type;

Another possibility is to provide concatenation operators, too.


o The aggregate Delete operation looks like this:

  procedure Delete (Vector  : in out Vector_Type;
                    First   : in     Index_Type'Base;
                    Count   : in     Natural);

But this also feels wrong, since it specifies the first index and a
count.  Count should probably be reserved for aggregate-style
insertions.  The analog in Ada.Strings.Unbounded is declared like this:

   procedure Delete (Vector  : in out Vector_Type;
                     From    : in     Index_Type'Base;
                     Through : in     Index_Type'Base);

This seems better since it specifies the range using a more traditional
Ada closed-range style.


o The Resize operation is the analog of the reserve() operation in the
STL std::vector class.  However, there is no real analog of the resize()
operation in std::vector, a member function which expands the internal
array as necessary, and sets the number of elements to the specified
value.

You can do this using other operations but its awkward.  It might be
better to provide an operation like:

   procedure Set_Length
     (Vector : in out Vector_Type;
      Length : in     Element_Count);

which sets the length directly.

Randy asked whether this would leave "holes" in the vector.  But there
are no real wholes, since it's just a contiguous array under the hood
and so the normal initialization rules for array components apply.


o There has been interest in passive iterators for the vector.  We
should probably include them.  Here's what they look like:

   generic
      with procedure Process
         (Element : in Element_Type) is <>;
   procedure Generic_Constant_Iteration
      (Vector : in Vector_Type);

   generic
      with procedure Process
         (Element : in out Element_Type) is <>;
   procedure Generic_Iteration
     (Vector : in Vector_Type);

   generic
      with procedure Process
         (Element : in Element_Type) is <>;
   procedure Generic_Constant_Reverse_Iteration
     (Vector : in Vector_Type);

   generic
      with procedure Process
          (Element : in out Element_Type) is <>;
   procedure Generic_Reverse_Iteration
     (Vector : in Vector_Type);


o I discussed aggregate-style operations for vectors above.  Another
possibility is to declare an array type, and provide operations that
operation that have an array parameter.  Something like:

   type Vector_Elements is
      array (Index_Type range <>) of aliased Element_Type;

[Editor's note: This only works for definite elements, of course.]

and then provide operations like:

   function To_Vector (Source : Vector_Elements)
      return Vector_Type;

   function To_Array (Source : Vector_Type)
      return Vector_Elements;

   function Slice (Vector : Vector_Type;
                   Low    : Index_Type'Base;
                   High   : Index_Type'Base)
     return Array_Type;

   procedure Assign (Target : in out Vector_Type;
                     Source : in     Array_Type);

   procedure Copy (Source : in     Vector_Type;
                   Target :    out Array_Type;
                   Last   :    out Index_Type'Base);

etc.

Yet another possibility is to declare a nested generic that declares a
generic formal array type, so the user can specify his own array type:

   generic

      type Array_Type is
         array (Index_Type range <>) of Element_Type;

   package Generic_Arrays is

      function To_Vector (Source : Array_Type)
         return Vector_Type;

      function To_Array (Source : Vector_Type)
         return Array_Type;
      ...
   end Generic_Arrays;


o There is still some debate about the exact nature and purpose of the
vector container.  My model (and I think Bob Duff's model) is that a
vector is implemented internally using an unconstrained array.  A vector
allows insertion at any position, but it is specifically optimized for
insertion at the back end.

One of the benefits of the STL std::vector is that you can do this:

   vector<HANDLE> v;

   v.push_back(h1);
   v.push_back(h2);

   const HANDLE* const ph = &v[0];
   const DWORD n = v.size();

   WaitForMultipleObjects(n, ph, INFINITE);

In other words under the hood a vector is just a normal C-style array.
You're allowed to take the address of a vector element and the language
guarantees that the elements are in contiguous memory, as if they had
been declared as elements of a plain array.

The Ada vector container should follow the same model.  It doesn't make
any sense to try to improve (say) insertions in the middle of a vector,
since there are other perfectly-good containers for that (like a list).


o There was some discussion about providing a stable sort, too.  We
could even provide a partial sort.


o There seems to be interest in being able to sort arrays, too.  A
generic operation for sorting an array would look like:

generic

   type Index_Type is (<>);

   type Element_Type is private;

   type Array_Type is array (Index_Type range <>) of Element_Type;

   with function "<" (Left, Right : Element_Type)
      return Boolean is <>;

procedure Ada.Containers.Generic_Sort_Unconstrained_Array
  (Source : in out Array_Type);

pragma Pure (Ada.Containers.Generic_Sort_Unconstrained_Array);


There would be an analogous operation for sorting a constrained array.
You could generalize further still, and provide an operation for sorting
any container having a cursor with the requisite properties (such as a
difference operator, etc).


o We could generalize the vector sort by allowing the user to pass in a
Swap operation for vector elements. The generic operation could have a
named default (that does swapping using normal assignment):

   procedure Swap
     (Vector : in Vector_Type;
      I, J   : in Index_Type'Base);

   generic

      with function "<" (Left, Right : Element_Type)
         return Boolean is <>;

      with procedure Swap
        (Vector : in Vector_Type;
         I, J   : in Index_Type'Base) is Vectors.Swap;

   procedure Generic_Sort (Vector : in Vector_Type);

This would be useful for elements that are expensive to copy, say that
are implemented as a controlled type that contains a value by
reference.  The swap for that element could then just swap the pointers,
instead of finalizing and then adjusting controlled objects.

****************************************************************

Comments on parameter names, Randy Brukardt, April 16, 2004.

Most of the parameter names in the containers libraries are "Container".

We considered using more functional names. We started with Insert:

   procedure Insert (Container : in out Vector;
                     Before    : in     Cursor;
                     New_Item  : in     Element_Type;
                     Count     : in     Size_Type := 1);

One possibility would be to use what Ada.Strings.Unbounded uses. The parameter
there is named "Source". That appears to be for compatibility with the
function version of the routine, but it is clearly inappropriate for a
procedure. So no help there.

Another possibility would be to use "Into":

   procedure Insert (Into     : in out Vector;
                     Before   : in     Cursor;
                     New_Item : in     Element_Type;
                     Count    : in     Size_Type := 1);

which makes nice named calls:

   Insert (Into => My_Vector, Before => Blob, New_Item => An_Item);

Similarly, Append and Prepend could use "To":

   procedure Append (To       : in out Vector;
                     Before   : in     Cursor;
                     New_Item : in     Element_Type;
                     Count    : in     Size_Type := 1);

And Delete could use "From":

   procedure Delete (From     : in out Vector;
                     Position : in     Cursor;
                     Count    : in     Size_Type := 1);

but then "Position" should really be "At" -- except that it's reserved in Ada.

And Replace_Element and Find should use "In", but that is clearly not going
to work. We could use "Inside", but that's a stretch.

Then, we'd have to find something to use for the various functions. The
existing:
    function Last (Container : Vector) return Cursor;
makes as much sense as anything I can come up with.

I don't think that the above buy enough to change Matt's choices (except for
a couple obvious mistakes, already fixed).

****************************************************************

From: Randy Brukardt
Sent: Friday, September 3, 2004  7:43 PM

This document lists significant changes between AI-302-3/05 (which for the
most part reflects the Palma meetings conclusions) and AI-302-3/06 (which
reflects more recent e-mail discussions and experience). Simple typos aren't
listed. I've numbered the changes for easy reference. Most of these require
no discussion. A few questions about issues that require some discussion are
at the end.

 1) !summary: Changed (vectors) => (vectors and lists) to reflect both kinds
of sequence containers.

 2) !proposal: The summary of the semantics of hashed containers is
simplified to be less precise (it was too specific to a particular
implementation).

 3) The name of the Count parameter of To_Vector was changed to Length,
given that that is what it signifies.

 4) The name of the Count parameter of Set_Capacity for Vectors was changed
to Capacity, given that that is what it signifies. (Note that the Map one
already called it Capacity in the spec., so there also was a consistency
issue.)

 5) The container to Generic_Sort has been made class-wide. Generic units
are not primitive operations, and it is usually a mistake to use specific
tagged types in non-primitive operations. By making it class-wide,
instantiations are directly usable by any extensions. Note that
implementations should take care not to dispatch to primitive operations of
Vectors and Lists in the implementations of these routines; the routines are
defined in terms of semantics, not primitive routines, and thus any
overridings could have the wrong semantics.

 6) For vectors, the two Find operations could be ambiguous if the default
parameters are used. For instance:
    Op (Find (V, E));  --  index-based or cursor-based Find?
Therefore, the index-returning Find was renamed to Find_Index. This is
similar to the way the ambiguity was handled for First and Last. A similar
change was made for Reverse_Find. (Note: We change the index version, so
that the cursor-based version has the same name and profile for all of the
containers.)

 7) function Is_In (Item      : Element_Type;
                    Container : Vector) return Boolean;
was replaced by
    function Contains (Container : Vector; Item : Element_Type) return
Boolean;
This new profile allows users to use prefix notation on calls:
   if Obj.Contains (Element) then ...
This was discussed on Ada-Comment; the only advantage of the first profile
is that it is similar to that used in Ada.Strings. But most people thought
"Contains" sounds more natural, and they want to be able to use prefix
notation.

 8) The names Iteration and Reverse_Iteration were changed to Iterate and
Reverse_Iterate; they should be a verb. The process routines should have a
parameter of Cursor. The latter was purely a cut-and-paste error (repeated
everywhere). Similarly, the text about Elements following Iteration for maps
should be after Update_Element for maps.

 9) Added text to insure that inserting a null vector into a vector never
raises an exception. This is needed to insure that all of the inserts are
consistent on this point. (The base Insert already has this wording.)
Otherwise, the shorter Inserts
would raise an exception for an index out of range, and the basic one would
not. This behavior is needed so that inserting a empty vector into another
empty vector does not fail (we don't want to have to require testing for
non-empty before doing an Insert).

10) The description of Delete_Last for Vector was simplified to use Clear
instead of Delete when everything will be deleted.

11) The AARM note after Delete for Vector was corrected to use Last_Index
rather than length; the note as written presumed that First_Index = 1 (the
classic incorrect Ada assumption about arrays).

12) Added wording to say that Last_Index returns Index_Type'First - 1 if the
container is empty. That makes it clear what the result is, as the position
of the last element isn't defined in that case (since there are no elements
in the container).

13) Insert for lists had a wording correction to plural so that it is
obvious that multiple elements can be inserted (the wording originally was
copied from the single element version, which was dropped in favor of a
default parameter of 1 for the Count).

14) Corrected AARM notes for Insert, Delete, Find for Hash to say "should
only compare keys" instead of "should only compare elements", as it is the
keys that are compared, not the elements.

15) function Key (Position : Cursor) return Key_Type was added to the
specification
of Generic_Keys inside of Ordered_Sets. This operation matches one declared
in Hashed_Maps, and we want these containers to be as similar as possible.
The operation was originally omitted because there was no key read function
in the generic spec; now that one has been added, there is no reason to omit
this function.

16) Added wording so that if something outside of the package Hashed_Maps
changes a
key value, the behavior of the package is unspecified. If a key includes an
access-to-variable component, and the designated object of the access
component takes part in hashing or equality, then it is impossible to
guarantee the behavior of the package. The key could be changed even without
any call into the package at all (if a copy of the access is saved
somewhere).

This really is a user error, similar to passing in a hash function that
doesn't provide the same results when called with the same key. Moreover,
implementations can't protect against it even if they wanted to, since it
can happen without any access to the Hashed_Maps package.

Similar wording was added to Ordered_Sets, as elements could have similar
problems.

17) Changed the name of the parameters of Is_Disjoint to Left and Right.
This operation is communitive, and might was well have parameter names
similar to Union and Intersection. OTOH, Is_Subset is neither communitive
nor "common knowledge". Thus, its parameters were named "Subset" and
"Of_Set", so it is clear which is the subset that is being tested. This is
clearly preferable in named prefix calls:
    Obj.Is_Subset (Of_Set => Obj2);
which is crystal-clear which is being tested to be a subset of which set.
(This operation is not common enough for readers of code to "know" which
operand is which.)

18) Pragma Pure was added to the array sort subprograms.

19) Matt made a number of improvements to the !examples section, adding and
reordering examples.

20) The following was added to all of the containers:
   procedure Query_Element
     (Position : in Cursor;
      Process  : not null access procedure (Element : in Element_Type));
This is valuable for reading large elements without copying them. For Sets,
this also avoids the need to check if the key was modified (meaning it can
be used even without defining a key). Since the key check could be
expensive, this is a significant savings. The routine was added to all of
the containers for consistency.

21) Nick Roberts points out that now that Index_Type is an integer type, it
is a lot simpler and clearer to say Index_Type'First-1 rather than
Index_Type'Pred(Index_Type'First). I've made this change generally.

22) Nick also pointed out that "any exceptions raised ... are propagated"
should be singular, as only one exception can be raised by a routine.
(Unless, of course, there are multiple operations in the "...").

23) Nick asked for an AARM note to clarify the purpose of the pragma Assert
in the
Vectors package. This was added.

24) Nick asked for the introductory description of length and capacity for
vectors to
be clarified (as it used "length" in the description of capacity, among
other sins).

25) Nick asked that the list of containers in Language Design Principles be
put in the order that they're defined in the text. He also asked that the
statement about
separate definite and indefinite versions be stated more clearly.

26) Update_Element for Sets has been renamed Checked_Update_Element, as it
is different from the other Update_Elements both in profile (the Set is
included in the
call) and in semantics. This is necessary because the routine may be forced
to delete the element and raise Constraint_Error if it is modified to match
an existing key. In that case, the set itself will need to be modified;
potentially including pointers in the implementation such as First or Last.
Thus the profile must be different.

27) At Palma, we decided that Swap should always swap elements, not nodes.
(Thus, the cursors would point at the changed elements afterwards). However,
this suffers from two major problems: First, it isn't necessarily possible
for indefinite types. It is not possible to copy the elements between nodes
(because they are potentially different shapes or even types), and it is
possible (although one would have to leave Ada to do it) to put the elements
directly in the nodes. Second, the purpose of the swap operations is to
provide better performance than the user can get from manually copying the
elements. But, that clearly requires swapping the nodes for a list. Only a
novice programmer would write a list swap that *copied* the designated
objects.

Similarly, if there is inherent indirectness in a container implementation,
we want to allow Swap to take advantage of it.

Therefore, we didn't change the semantics of Swap for lists. Rather, because
the vector operation is different, we renamed it to Swap_Elements. This will
prevent problems if a container is switched from a vector to a list or vice
versa.

I added some AARM notes to explain the difference and the reasons.

Matt pointed out that the Container parameter to Swap_Elements for cursors
is unnecessary; it violates our meta-rule that Container parameters are only
included on  cursor operations if the cardinality of the Container may
change. It was removed, since there is no longer a need to be the same as
the list Swap.

28) The Success parameter of Insert for Maps and Sets was changed to
Inserted. Success is a boolean parameter, and Success = False implies some
sort of failure or error. But it is not an error (necessarily) for Insert to
return this value; it just means that the item was previously in the Map or
Set. The caller of Insert must decide if Inserted = False is an error
(failure) or if it is intended behavior. Thus we select a name that doesn't
have a strong connotation.

29) The Replace operation of Maps was renamed Insert_Or_Replace. This
operation has insertion semantics (if the key is not in the map) or
replacement semantics otherwise. Thus the name change; it's confusing
without it. (The suggestion of renaming it to Insert has the same problem,
in the other direction). It's a useful shorthand, especially as it does not
have the Out parameters of regular Insert. If all you need to do is stick
something into the map, and only care that it ends up there, this is the
operation to use.

A similar operation was defined for Sets. This operation is named Insert,
because it has no replacement function (the element is unchanged if it is
already in the map). Again, it is useful if you want to add an element to a
Set, and have no need to worry about whether it is already there or where it
ended up. (This operation was requested by users on Ada-Comment, noting that
defining 'dead' objects to receive return values that they don't care about
is ugly and possibly dangerous.) For more on this subject, see Question Q5.


Questions:

Q1) Find_Index returns Last_Index (Container) + 1 if the element is not
found. This seems consistent to me (it's past the end of the container in a
forward search), but Matt worries that First_Index (Container) - 1 might be
thought of as better. The trouble with First_Index (Container - 1 is that
you can't put it into an object:

    declare
        I : Index_Type := Index_Type'First;
    begin
        I := Find_Index (Vect, Item, I);
        while I <= Last_Index (Vect) loop
            -- Do something to the element I.
            I := Find_Index (Vect, Item, I+1);
        end loop;
    end;

If Find_Index returned Index_Type'First - 1, saving the result of Find_Index
would raise Constraint_Error if the item is not found. That's not what we
want, I think.

Q2) The parameters to Generic_Merge have not been made class-wide (even
though the comments about non-primitive operations with specific tagged
parameters mentioned for Generic_Sort hold here, too). That's because both
parameters need to be the same type. An alternative would be to make them
class-wide, and then have a runtime check (of the actual tags) that they
actually are the same type. But that is not very O-O. A third possibility
would be to repeat the type in the generic spec:
   generic
       type List_Type is new List with private;
       with function "<" (Left, Right : Element_Type)
          return Boolean is <>;
    procedure Generic_Merge (Target : in out List_Type;
                             Source : in out List_Type);

But that is not very consistent with the rest of the specification. Some
guidance would be helpful here.


Q3) The generic formal part for maps has:
   with function "=" (Left, Right : Key_Type)
      return Boolean is <>;
   with function Is_Equal_Key (Left, Right : Key_Type)
      return Boolean is "=";
Matt wonders why both operations are needed; his reference implementation
never uses "=". This came from the Palma meeting; it's mentioned explicitly
in the meeting minutes. The intent is that Map equality use "=" rather than
Is_Equal_Key. The idea is that Maps that compare equal have no visible
differences. Consider a map with keys that are strings representing numbers.
Is_Equal_Key probably would compare the 'Values of the keys (so "0046" and
"46" would be the same), but "=" would just compare the strings.

The other reason is that Is_Equal_Key often will not be native equality (as
in the example above). But it will be often enough that we want to be able
to provide "=" as a result.

However, Map equality is not an important operation; it exists simply
because the type is non-limited and thus we have to define it. Arguably, the
extra operation doesn't buy much. It doesn't cost much, either, as it will
never need to be specified in an instantiation as long as named notation is
used.

Matt points out that the reason this operation is called "Is_Equal_Key" is
that otherwise there would be two "=" operators in the formal part, and that
would prevent using named notation for either. Moreover, it is not
unreasonable (see above) for the user to override the key equality. So, if
one is dropped, it would have to be the "=" operation.

I did add some text specifying the required relationship between "=" and
Is_Equal_Key: if "=" returns True for some operands, then Is_Equal_Key also
returns True for those operands. I also added some text about the design
need to give key equality a different name than "=" in order to allow named
notation in instantiations.


Q4) Set_Capacity is defined to raise Constraint_Error if Length (Container)
> Count. Matt would prefer that this case is not separately handled. He
would like
    Set_Capacity (M, 0)
to be a shorthand for setting the Map or Vector to the smallest reasonable
size. (I find this a bit odd, as Matt never wanted this routine to even
allow smaller values. But whatever.) Note that just dropping the check would
not be enough; we'd have to redo the description of the operation to say
that the capacity is set to at least Count_Type'Max (Count, Length
(Container)) -- because we don't want this operation to drop elements. I'm
unsure that the benefit is worth the change, and it seems like a bug to me
to try to set the capacity of a container to be smaller than the number of
elements it holds.


Q5) There is a lot of confusion about the meaning of the Insert operations
for maps and sets. The changes outlined in (29) above should help.

However, the semantics of the Insert that does not return an Inserted value
remain controversial. Five possible behaviors were identified for Maps:
   1) Insert raises an exception when the key is already in the map;
   2) Insert does nothing when the key is already in the map;
   3) Insert replaces the existing element when the key is already in the
map;
   4) Replace raises an exception when the key is not already in the map;
   5) Replace does nothing when the key is not already in the map.
(One could imagine a Replace that inserts when not found, but that is just
choice 3.)

Convincing examples were shown for virtually all of these semantics. Note
that replacement semantics (choices 3 through 5) make less sense for Sets,
as you would be replacing an equivalent element. However, since it certainly
possible for an element to have components that do not participate in the
equivalence test, it is easy to imagine examples where such replacements
make sense.

It was suggested to provide all of these operations with different names,
but that requires coming up with five sets of names, with the resultant
clutter and confusion. (And this is on top of the Insert that returns a
Position and Inserted result.) Moreover, all of these routines can be
written using other primitives. We therefore decided to provide (3) for Maps
and (2) for Sets.

Another suggestion was to use an enumeration similar to the ones in
Ada.Strings. Then we'd have:

type Exists_Action is (Error, Ignore, Replace);

procedure Insert (Container : in out Map;
                  Key       : in     Key_Type;
                  New_Item  : in     Element_Type;
                  If_Exists : in     Exists_Action := Error);

type Nonexistent_Action is (Error, Ignore, Insert);

procedure Replace (Container : in out Map;
                   Key       : in     Key_Type;
                   New_Item  : in     Element_Type;
                   If_Nonexistent : in     Nonexistent_Action := Error);

The objections to this idea boiled down to two: this is control-coupling,
and that some people hate the design of Ada.Strings; they find it hard to
find or remember the names of the parameters.

Several people suggested that control-coupling was preferable to a forest of
routines, or one routine with non-intutive semantics. Moreover,
control-coupling is mainly a problem for the implementer of a routine; it is
not bad for the user of a routine, and make in fact increase the options
available to them.

I personally am in favor of this sort of solution, because (1) it allows
using simple names for the routines, without surprise semantics; (2) it
allows dodgy cases to be errors by default, while allowing the specification
of other behaviors when needed (which is makes using the library safer, by
cutting surprise behavior); (3) it doesn't force users into using the
Position-returning Insert when they don't need the Position for a later
operation.

However, it does slightly complicate the specifications of the packages, and
it only got lukewarm support, so the decision has been left to the ARG.


Q6) Tucker has mentioned that he often has components in the key of a map
beyond the actual key participating ones. (This is similar to the behavior
of a set; if we had a Hashed_Set this probably would be less of an issue.)
For that to be effective, it would be necessary to change a key that is
already in a map. Currently, neither Replace_Element nor Insert_or_Replace
change the value of a key that is in the map; only the element is changed.

In order to get the sort of semantics that Tucker seems to be suggesting,
we'd need a way to change the value of a key. But such an operation would
potentially change the location of the element, so it could be fairly
expensive. Moreover, it would likely require allocation even if the hash
didn't change for the indefinite form of the container.

Finally, whether or not the key is replaced would seem to be another
(orthogonal) option for the Insert routine "6) Insert replaces the key and
the element when the key is already in the map; 7) Insert replaces the key,
leaving the element unchanged when the key is already in the map".

This complication doesn't seem worth it to me, but as it came up very late,
the entire ARG needs to discuss the issue.

****************************************************************


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