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From: Duncan Sands <baldrick@free.fr>
Newsgroups: comp.lang.ada
Subject: Re: Hashing on System.Address
Date: Tue, 14 Jun 2005 12:14:13 +0200
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Date: 2005-06-14T12:14:13+02:00

On Tuesday 14 June 2005 12:03, Marius Amado Alves wrote:
> > ... The other thing I haven't really figured out is what to do
> > if you have a 64-bit address, and Hash_Type is only 32...
> 
> I gave a general solution to this class of cases before:
> 
> <<
> Treat both the input and output values as bit strings X (1 .. M) and Y 
> (1 .. N) respectively. (Use representation clauses, packed arrays, and 
> unchecked conversion for this.) Make N small, say 16. Set Y bits to the 
> value of X bits as follows:
> 
> if M > N, Y (J) = X (1 + (J - 1) * M / N), for J in 1 .. N
> 
> if M = N, Y = X
> 
> if M < N, initialize Y to all zeros,
>   then let Y (1 + (I - 1) * N / M) = X (I), for I in 1 .. M
>  >>

Hi Marius, thanks for the suggestion.  A few comments though:
if M > N (say M = 2*N) then you're skipping every second bit
in X.  Why not use them - mix them in somehow?  If M < N then
you are trying to spread the bits of X evenly throughout Y.  Are
there any theoretical reasons to think this is a good strategy?
Finally, I'm betting the code produced is kind of yucky.  It would
be nice to have a solution that resulted in a small number of
simple bit operations.

All the best,

Duncan.