!standard A.5.2(29)                                            11-11-12  AI05-0280-1/01
!class binding interpretation 11-11-12
!status Amendment 2012 11-11-11
!status ARG Approved 9-0-0  11-11-11
!status work item 11-11-09
!status received 11-09-28
!priority Low
!difficulty Easy
!qualifier Omission
!subject Behavior of Random.Reset with default initialized State

!summary

The implicit initial value of the type State is such that it will set a generator to the
implicit initial value used for all generators.

!question

The packages Ada.Numerics.Float_Random and Ada.Numerics.Discrete_Random have the
following procedures;

procedure Save (Gen : in Generator;
                To_State : out State);

and

procedure Reset (Gen : in Generator;
                 From_State : in State);

The intent is that one must call Save first to save the State of the random number
generator, and then one can restore the state using the Reset call.

What happens if Reset is called, without having previously called Save, to save the state?
That is, if From_State is default initialized?

!recommendation

(See summary.)

!wording

Add to the end of A.5.2(29):

The implicit initial value of type State corresponds to the implicit initial value of
all generators.

!discussion

The default value for State can be set using default expressions or (if it is a scalar
or array type), with one of the new Ada 2012 Default_Value/Default_Component_Value aspects.

There is a slight chance that this new requirement would cause an inconsistency in a program,
but only if the compiler initialized these to something else useful, and the program depended
upon that initialization. Both parts of this seem unlikely (its much more likely that the
compiler simply left these uninitialized, which would not be useful), and any such usage
surely would not have been portable to other compilers.

!corrigendum A.5.2(29)

@drepl
An object of the private type State can be used to hold the internal state of a generator.
Such objects are only needed if the application is designed to save and restore generator
states or to examine or manufacture them.
@dby
An object of the private type State can be used to hold the internal state of a generator.
Such objects are only needed if the application is designed to save and restore generator
states or to examine or manufacture them. The implicit initial value of type State
corresponds to the implicit initial value of all generators.

!ACATS Test

An ACATS C-Test should be created to check that State is initialized. This is fairly low priority,
however. (The main reason it is important is that it reflects a change, and a test would tell
if implementations did indeed make the change.)

!ASIS

No ASIS impact.

!appendix

!topic Behavior unclear if Ada.Numerics.???_Random.Reset is called with default initialized State.
!reference Ada 2005 RM/A.5.2(12, 35)/
!from /Brad Moore 11-09-27/
!keywords Random Number Generation
!discussion

The packages Ada.Numerics.Float_Random and Ada.Numerics.Discrete_Random have the following procedures;

*procedure* Save (Gen : *in* Generator;
                               To_State : *out* State);

and

*procedure* Reset (Gen : *in* Generator;
                                From_State : *in* State);

The intent is that one must call Save first to save the State of the random number generate, and the one
can restore the state using the Reset call.

What happens if Reset is called, without having previously called Save, to save the state. ie. From_State
is default initialized? The RM does not specify the behaviour. Would this be equivalent to the time-dependent
Reset procedure? Does an exception get raised? Should this be specified?

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