Re: Random numbers in tasks

["Robert I. Eachus" <rieachus@attbi.com>]

Papastefanos Serafeim wrote:
> There is a small problem in that solution, I cannot just
> declare the generator in the task type: There is a function in
> a different package that is used to return a random
> number in a given range and that function is used at
> other places too...

There is magic in Ada for especially this purpose.  It is in Annex C, so 
it is possible that some compilers don't support it.  Look at C.7.2 The 
Package Task_Attributes.  Here is an example and output using GNAT. 
Notice that this version uses a fixed seed for each different task.  If 
you want different seeds for every run modify the body of 
My_Float_Random.Gen_Source.  But notice that just using a call to get 
seeds based on the clock may not be sufficient, unless you put a delay in.

with Ada.Task_Attributes;
with Ada.Numerics.Float_Random;
package My_Float_Random is
   function Random return Ada.Numerics.Float_Random.Uniformly_Distributed;
end My_Float_Random;

package body My_Float_Random is
    type Generator_Pointer is access Ada.Numerics.Float_Random.Generator;

    protected Gen_Source is
      procedure Initialize_Generator(Pointer: in out Generator_Pointer);
    private
      Init: Integer := 10;
    end Gen_Source;

    protected body Gen_Source is
      procedure Initialize_Generator
                  (Pointer: in out Generator_Pointer) is
       begin
         Pointer := new Ada.Numerics.Float_Random.Generator;
         Ada.Numerics.Float_Random.Reset(Pointer.all, Init);
         Init := Init + 10;
       end  Initialize_Generator;

    end Gen_Source;

    function Initialize return Generator_Pointer is
      GP : Generator_Pointer;
    begin
      Gen_Source.Initialize_Generator(GP);
      return GP;
    end Initialize;

    package Per_Task is new
      Ada.Task_Attributes(Generator_Pointer, Initialize);

    function Random return
       Ada.Numerics.Float_Random.Uniformly_Distributed is
    begin
      return Ada.Numerics.Float_Random.Random(Per_Task.Value.all);
    end Random;

end My_Float_Random;

with Ada.Text_IO;
with My_Float_Random;
procedure Test_Task_Random is

    package Rand_IO is new Ada.Text_IO.Float_IO(Float);

    task type Rand_Task (ID: Integer) is end Rand_Task;

    type Task_Pointer is access Rand_Task;
    TP : Task_Pointer;

    protected IO_Serialize is
      procedure Write_Line(S: in String);
    end IO_Serialize;

    protected body IO_Serialize is
      procedure Write_Line (S: in String)
      is begin Ada.Text_IO.Put_Line(S); end Write_Line;
    end IO_Serialize;

    task body Rand_Task is
      Rand_String: String(1..10);
    begin
      for I in 1..10 loop
        Rand_IO.Put(Rand_String, My_Float_Random.Random, 7, 0);
        IO_Serialize.Write_Line( "Task" & Integer'Image(ID) &
           " Random number: " & Rand_String);
      end loop;
    end Rand_Task;

begin

    for I in 1..10 loop
      TP := new Rand_Task(I);
    end loop;

end Test_Task_Random;

E:\Ada\Random>test_task_random
test_task_random
Task 1 Random number:  0.7443592
Task 1 Random number:  0.1400007
Task 1 Random number:  0.9659407
Task 1 Random number:  0.6070659
...
Task 6 Random number:  0.3006703
Task 7 Random number:  0.5094006
Task 8 Random number:  0.9858680
Task 10 Random number:  0.2958182
Task 9 Random number:  0.7662218
Task 10 Random number:  0.4117813
Task 9 Random number:  0.9709954
Task 10 Random number:  0.2093689
Task 9 Random number:  0.3808535
Task 10 Random number:  0.6235309