Re: Implementing Memorize

[Lutz Donnerhacke <lutz@iks-jena.de>]

* Dmitry A Kazakov wrote:
> On Wed, 22 Oct 2003 11:07:43 +0000 (UTC), Lutz Donnerhacke
>>Calculating Fibbonacci numbers recursivly is easy but braindead. Languages
>>with access to the symbol table at execution time are able to change the the
>>reccuring call with a memorize wrapper which returns already computed values
>>immediatly instead of recalculating them. In the referenced article I wrote
>>an C-Implementation of such a wrapper. Now I'm looking for a more elegant
>>version in Ada.
>
> Solution depends on how exposed should be the context used to store
> results:
>
> 1. Completely hidden (allocated upon elaboration of the body)
> 2. Partially exposed as a generic parameter of the compilation unit
> 3. Fully exposed as a parameter of the subprogram

It's more or less irrelevant. A full generic module would fine.
The following might be a useful start. (Unnecessary copies removed.)

with Ada.Text_IO, Ada.Calendar;

procedure t is
   package Fibs is
      type Direct is tagged null record;
      
      function Fib (c : Direct; n : Natural) return Natural;
   end Fibs;
   
   package body Fibs is
      function Fib (c : Direct; n : Natural) return Natural is
      begin
         case n is
            when 0 | 1  => return 1;
            when others => return Fib (Direct'Class(c), n-1) + Fib (Direct'Class(c), n-2);
         end case;
      end Fib;
   end Fibs;
   
   use Fibs;
   
   package Memorizes is
      type Memorize is new Fibs.Direct with null record;
      function Fib (c : Memorize; n : Natural) return Natural;
   end Memorizes;
   
   package body Memorizes is
      type Item is record
         value : Natural;
         valid : Boolean := False;
      end record;
      
      hash : array(0 .. 100) of Item;
      
      function Fib (c : Memorize; n : Natural) return Natural is
      begin
         if n not in hash'Range then
            return Fib(Direct(c), n);
         else
            declare
               x : Natural renames hash(n).value;
               v : Boolean renames hash(n).valid;
            begin
               if not v then
                  x := Fib(Direct(c), n);
                  v := True;
               end if;
               return x;
            end;
         end if;
      end Fib;
   end Memorizes;

   use Memorizes;
   
   procedure Time_It (ctx : Direct'Class; n : Natural) is
      use Ada.Text_IO, Ada.Calendar;
      start, ende : Time;
      result : Natural;
   begin
      start := Clock;
      result := Fib (ctx, n);
      ende := Clock;
      Put_Line ("Result =" & Natural'Image(result) &
        " in" & Duration'Image(ende-start) & " seconds.");
   end Time_It;

   d : Direct;
   m : Memorize;
begin
   Time_It (d, 30);
   Time_It (m, 30);
end t;