Re: Ada.Numerics.Float_Random.Generator question

[rieachus@comcast.net]

On Saturday, September 17, 2016 at 3:40:13 PM UTC-4, Andrew Shvets wrote:
 
> This works.  However, as I'm now looking into the Ada.Numerics.Discrete_Random package, I've noticed that it can generate a random value based on the type that is passed in.  This appeals to me and I'd like to do the same for a custom float type (say I want the delta to be 0.01.)
> 
> As Ada.Numerics.Float_Random.Generator is not a generic package, I can't use the same approach.  The best way that I can think of is to create the same result using From and To and then cast the result to the custom float type.
> 
> I hope that there is a better solution to this.  Is there?

Since working has to be better, yes.  First, what you seem to be asking for is a PRNG that selects from a uniform distribution of fixed point values.  To do this take the generic discrete random package, and redeclare it with a fixed point type:

generic
   type Result_Subtype is delta <>;
package My_Fixed_Random is

   -- Basic facilities

   type Generator is limited private;

   function Random (Gen : Generator) return Result_Subtype;

   procedure Reset (Gen       : in Generator;
                    Initiator : in Integer);
   procedure Reset (Gen       : in Generator);

   -- Advanced facilities

   type State is private;

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

   Max_Image_Width : constant := implementation-defined integer value;

   function Image (Of_State    : State)  return String;
   function Value (Coded_State : String) return State;

private
   
end My_Fixed_Random;

Now in the body, put an instance of generic discrete random fit to your fixed point type:

package body My_Fixed_Random is
  type Hidden is range Int64(Result_Subtype'First/Result_Subtype'Small)     ..Int64(Result_Subtype'Last/Result_Subtype'Small);

  package Hidden_Pkg is new Ada.Numerics.Discrete_Random(Hidden);

  Complete with the operations from My_Fixed_Random, multiplying by 'Small and converting to Result_Subtype where needed. All done. ;-)  I wish I had gotten this package added to the Numerics section, as you can see it is about a page of very easy code.  But there were lots of other issues that needed resolving, and not enough time to resolve them all.

Now for the evils that I hope you never meet.  With this package, you should generate only numbers that are multiples of 'Small, and each legal value for Result_Subtype should occur with equal probability.  You might have an issue with a value next below 'First, but usually if you have a fixed point type that officially has 2^n - 1 values, you will know what to do to avoid Constraint_Error.  More troubling is that the language rules allow for fixed point types where the specified 'Small or 'Delta does not evenly divide one.  I do think that permission is right--but if you use it, you had better know a lot about numerics.