Re: Implementing Memorize

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

* Lutz Donnerhacke wrote:
> The following version does not the job:

Simple logic error:

> with Ada.Text_IO, Ada.Calendar;
> 
> 
> procedure t is
>    package Fibs is
>       type Direct is tagged null record;
>       
>       function Fib (c1 : Direct; c2 : Direct'Class; n : Natural) return Natural;
>    end Fibs;
>    
>    package body Fibs is
>       function Fib (c1 : Direct; c2 : Direct'Class; n : Natural) return Natural is
>       begin
>          case n is
>             when 0 | 1  => return 1;
>             when others => return Fib (c2, c2, n-1) + Fib (c2, c2, n-2);
>          end case;
>       end Fib;
>    end Fibs;
>    
>    use Fibs;
>    
>    package Memorizes is
>       type Memorize is new Fibs.Direct with null record;
>       function Fib (c1 : Memorize; c2 : Fibs.Direct'Class; 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 (c1 : Memorize; c2 : Fibs.Direct'Class; n : Natural) return Natural is
>       begin
>          if n not in hash'Range then
>             return Fib(Direct(c1), c2, n);
>          else
>             declare
>                x : Natural renames hash(n).value;
>             begin
>                if not hash(n).valid then
>                   x := Fib(Direct(c1), c2, n);
                    hash(n).valid := 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, 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;

Does the job.