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

[Michael R <michael@zanyblue.com>]

On Apr 23, 2:39 pm, Simon Wright <si...@pushface.org> wrote:
> Michael R <mich...@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.

Hi,

Thank you for this suggestion.  Just fyi, my GNAT 4.4.3 compiler
generated

xtest.adb:20:40: value not in range of type "Standard.Integer"
xtest.adb:20:40: static expression fails Constraint_Check

for "M (I.A mod 2**31)" expressions.  Rephrasing as

Result := Result xor M'Mod (I.A);

compiled OK.

Take care,
Michael.