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Date: Wed, 18 Jul 2018 06:38:07 -0700 (PDT)
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Subject: Re: Simple hash or pseudo-random function
From: gautier_niouzes@hotmail.com
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Xref: reader02.eternal-september.org comp.lang.ada:53891
Date: 2018-07-18T06:38:07-07:00
List-Id: <comp.lang.ada>

Am Dienstag, 17. Juli 2018 08:09:10 UTC+2 schrieb Jeffrey R. Carter:

> Another possibility would be to try (calling your 64-bit type U64)
> 
> subtype S8 is String (1 .. 8);
> 
> function To_S8 is new Ada.Unchecked_Conversion (Source => U64, Target => S8);
> 
> Ada.Strings.Hash (To_S8 (N) );
> 
> If Hash is a decent hash function ("It should be unlikely for similar strings to 
> return the same value.") then it might be sufficiently random.

Brillant, works as expected (after a few adjustments on the final range to be really uniform).

Looking at GNAT's s-strhas.adb :

      H := 0;
      for J in Key'Range loop
         H := Char_Type'Pos (Key (J))
                + Shift_Left (H, 6) + Shift_Left (H, 16) - H;
      end loop;

I guess it is "random" enough on small changes of Key (Key'First .. Key'Last - 1) or on small changes (+1 or -1) of N if U64's are stored as little-endian.