Re: Ada.Containers Hash function for a set of small integers

[Simon Wright <simon@pushface.org>]

Michael R <michael@zanyblue.com> writes:

> Hi Folks,
>
> I'd like to use the generic Hashed_Maps container to store mappings
> from a set of small integers (three) to Wide_Strings
> (Indefinite_Hashed_Maps):
>
> (1, 10, 4) => "ABC",
> (10, 3, 5) => "XYZ",
>
> Does anyone have recommendations on how best to implement the Hash
> function for keys like this?

I don't know about 'best', but in ColdFrame
(http://coldframe.sourceforge.net/coldframe/) I would generate this hash
for use with Booch Maps, where the key components of Instance are
{A:Integer, B:Integer, C:Integer}:

   function Instance_Hash (I : Instance) return Natural is
      type M is mod 2**31;
      Result : M := 0;
   begin
      Result := Result xor M (I.A mod 2**31);
      Result := Result * 10019;
      Result := Result xor M (I.B mod 2**31);
      Result := Result * 10019;
      Result := Result xor M (I.C mod 2**31);
      Result := Result * 10019;
      return Natural (Result);
   end Instance_Hash;

I believe the 10019 came from Knuth, but can't see a reference.