From mboxrd@z Thu Jan  1 00:00:00 1970
X-Spam-Checker-Version: SpamAssassin 3.4.4 (2020-01-24) on polar.synack.me
X-Spam-Level: 
X-Spam-Status: No, score=-1.9 required=5.0 tests=BAYES_00 autolearn=ham
	autolearn_force=no version=3.4.4
X-Google-Language: ENGLISH,ASCII-7-bit
X-Google-Thread: 103376,d74d61525ce986e9
X-Google-Attributes: gid103376,public
X-Google-ArrivalTime: 2003-10-22 04:33:04 PST
Path: 
 archiver1.google.com!news2.google.com!news.maxwell.syr.edu!news.tele.dk!news.tele.dk!small.news.tele.dk!news-FFM2.ecrc.net!news.iks-jena.de!not-for-mail
From: Lutz Donnerhacke <lutz@iks-jena.de>
Newsgroups: comp.lang.ada
Subject: Re: Implementing Memorize
Date: Wed, 22 Oct 2003 11:33:03 +0000 (UTC)
Organization: IKS GmbH Jena
Message-ID: <slrnbpcqnf.oa.lutz@taranis.iks-jena.de>
References: <mailman.165.1066815754.25614.comp.lang.ada@ada-france.org>
 <slrnbpcn5h.oa.lutz@taranis.iks-jena.de>
 <mailman.167.1066819733.25614.comp.lang.ada@ada-france.org>
 <slrnbpcp7v.oa.lutz@taranis.iks-jena.de>
NNTP-Posting-Host: taranis.iks-jena.de
X-Trace: branwen.iks-jena.de 1066822383 4900 217.17.192.37 (22 Oct 2003
 11:33:03 GMT)
X-Complaints-To: usenet@iks-jena.de
NNTP-Posting-Date: Wed, 22 Oct 2003 11:33:03 +0000 (UTC)
User-Agent: slrn/0.9.7.4 (Linux)
Xref: archiver1.google.com comp.lang.ada:1402
Date: 2003-10-22T11:33:03+00:00
List-Id: <comp.lang.ada>

* Lutz Donnerhacke wrote:
> 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.

The following version does not the job:
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);
               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;