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A.18.21 The Generic Package Containers.Bounded_Hashed_Maps

{AI05-0001-1} The language-defined generic package Containers.Bounded_Hashed_Maps provides a private type Map and a set of operations. It provides the same operations as the package Containers.Hashed_Maps (see A.18.5), with the difference that the maximum storage is bounded. 

Static Semantics

{AI05-0001-1} The declaration of the generic library package Containers.Bounded_Hashed_Maps has the same contents and semantics as Containers.Hashed_Maps except:
{AI12-0112-1} The aspect pragma Preelaborate is replaced with aspect pragma Pure. Aspect Global is deleted.
The type Map is declared with discriminants that specify both the capacity (number of elements) and modulus (number of distinct hash values) of the hash table as follows:
{AI12-0112-1}   type Map (Capacity : Count_Type;
            Modulus  : Hash_Type) is tagged private...;
{AI12-0409-1} The aspect_definition for Preelaborable_Initialization for type Map is changed to: 
  Preelaborable_Initialization =>
The type Map needs finalization if and only if type Key_Type or type Element_Type needs finalization.
Implementation Note: {AI05-0212-1} The type Map cannot depend on package Ada.Finalization unless the element or key type depends on that package. The objects returned from the Iterator and Reference functions probably do depend on package Ada.Finalization. Restricted environments may need to avoid use of those functions and their associated types. 
{AI12-0339-1} In function Empty, the postcondition is altered to:
   Post =>
      Empty'Result.Capacity = Capacity and then
      Empty'Result.Modulus = Default_Modulus (Capacity) and then
      not Tampering_With_Elements_Prohibited (Empty'Result) and then
      not Tampering_With_Cursors_Prohibited (Empty'Result) and then
      Length (Empty'Result) = 0;
The description of Reserve_Capacity is replaced with:
{AI12-0112-1}    procedure Reserve_Capacity (Container : in out Map;
                               Capacity  : in     Count_Type)
      with Pre => Capacity <= Container.Capacity 
                      or else raise Capacity_Error;
{AI12-0112-1} This If the specified Capacity is larger than the capacity of Container, then Reserve_Capacity propagates Capacity_Error. Otherwise, the operation has no effect, [other than checking the precondition]
An additional operation is added immediately following Reserve_Capacity: 
  function Default_Modulus (Capacity : Count_Type) return Hash_Type;
Default_Modulus returns an implementation-defined value for the number of distinct hash values to be used for the given capacity (maximum number of elements). 
{AI12-0112-1} For procedures Insert and Include, the part of the precondition reading:
     (<some length> <= Count_Type'Last - <some other length>
        or else raise Constraint_Error)
is replaced by:
     (<some length> <= Count_Type'Last - <some other length>
        or else raise Constraint_Error) and then
     (<some length> > Container.Capacity - <some other length>
        or else raise Capacity_Error)
{AI12-0112-1} In procedure Assign, the precondition is altered to: 
   Pre => (not Tampering_With_Cursors_Prohibited (Target)
             or else raise Program_Error) and then
          (Length (Source) <= Target.Capacity
             or else raise Capacity_Error),
The function Copy is replaced with: 
{AI12-0112-1}   function Copy (Source   : Map;
                 Capacity : Count_Type := 0;
                 Modulus  : Hash_Type := 0) return Map
     with Pre  => Capacity = 0 or else Capacity >= Length (Source)
                     or else raise Capacity_Error,
          Post => 
             Length (Copy'Result) = Length (Source) and then
             not Tampering_With_Elements_Prohibited (Copy'Result) and then
             not Tampering_With_Cursors_Prohibited (Copy'Result) and then
             Copy'Result.Capacity = (if Capacity = 0 then
                Length (Source) else Capacity) and then
             Copy'Result.Modulus = (if Modulus = 0 then
                Default_Modulus (Capacity) else Modulus)
{AI05-0264-1} Returns a map with key/element pairs initialized from the values in Source. If Capacity is 0, then the map capacity is the length of Source; if Capacity is equal to or greater than the length of Source, the map capacity is the value of the Capacity parameter; otherwise, the operation propagates Capacity_Error. If the Modulus argument is 0, then the map modulus is the value returned by a call to Default_Modulus with the map capacity as its argument; otherwise, the map modulus is the value of the Modulus parameter. 

Bounded (Run-Time) Errors

{AI05-0160-1} {AI05-0265-1} It is a bounded error to assign from a bounded map object while tampering with elements [or cursors] of that object is prohibited. Either Program_Error is raised by the assignment, execution proceeds with the target object prohibiting tampering with elements [or cursors], or execution proceeds normally. 
Proof: Tampering with elements includes tampering with cursors, so we only really need to talk about tampering with elements here; we mention cursors for clarity. 

Erroneous Execution

{AI05-0265-1} When a bounded map object M is finalized, if tampering with cursors is prohibited for M other than due to an assignment from another map, then execution is erroneous.
Reason: This is a tampering event, but since the implementation is not allowed to use Ada.Finalization, it is not possible in a pure Ada implementation to detect this error. (There is no Finalize routine that will be called that could make the check.) Since the check probably cannot be made, the bad effects that could occur (such as an iterator going into an infinite loop or accessing a nonexistent element) cannot be prevented and we have to allow anything. We do allow re-assigning an object that only prohibits tampering because it was copied from another object as that cannot cause any negative effects. 

Implementation Requirements

{AI05-0184-1} {AI05-0264-1} For each instance of Containers.Hashed_Maps and each instance of Containers.Bounded_Hashed_Maps, if the two instances meet the following conditions, then the output generated by the Map'Output or Map'Write subprograms of either instance shall be readable by the Map'Input or Map'Read of the other instance, respectively:
{AI05-0184-1} {AI05-0248-1} the Element_Type parameters of the two instances are statically matching subtypes of the same type; and
{AI05-0184-1} the output generated by Element_Type'Output or Element_Type'Write is readable by Element_Type'Input or Element_Type'Read, respectively (where Element_Type denotes the type of the two actual Element_Type parameters); and
{AI05-0184-1} the preceding two conditions also hold for the Key_Type parameters of the instances. 

Implementation Advice

{AI05-0001-1} {AI05-0269-1} Bounded hashed map objects should be implemented without implicit pointers or dynamic allocation. 
Implementation Advice: Bounded hashed map objects should be implemented without implicit pointers or dynamic allocation.
{AI05-0001-1} The implementation advice for procedure Move to minimize copying does not apply. 
Implementation Advice: The implementation advice for procedure Move to minimize copying does not apply to bounded hashed maps.

Extensions to Ada 2005

{AI05-0001-1} {AI05-0160-1} {AI05-0184-1} The generic package Containers.Bounded_Hashed_Maps is new. 

Inconsistencies With Ada 2012

{AI12-0111-1} Correction: Tampering with elements is now defined to be equivalent to tampering with cursors for bounded containers. If a program requires tampering detection to work, it might fail in Ada 2022. Needless to say, this shouldn't happen outside of test programs. See Inconsistencies With Ada 2012 in A.18.2 for more details. 

Incompatibilities With Ada 2012

{AI12-0409-1} Correction:A bounded map now only has Preelaborable_Initialization (abbreviated PI in this note) when the actuals for the Element_Type and the Key_Type have PI. If an program used a map whose actual Element_Type or Key_Type does not have PI in a context when PI is required (such as a library-level object in a preelaborated unit or as a component of a type with PI), the program would be illegal in Ada 2022 but legal in original Ada 2012. This situation is unlikely, especially as some existing Ada 2012 implementations reject the instance in this case. 

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