Re: Simple hash or pseudo-random function

[gautier_niouzes@hotmail.com]

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.