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From: "Randy Brukardt" <randy@rrsoftware.com>
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 <mailman.22.1118733848.17633.comp.lang.ada@ada-france.org>
Subject: Re: Hashing on System.Address
Date: Tue, 14 Jun 2005 15:18:25 -0500
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Date: 2005-06-14T15:18:25-05:00
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"Duncan Sands" <baldrick@free.fr> wrote in message
news:mailman.22.1118733848.17633.comp.lang.ada@ada-france.org...
> Hi Mark,
>
> > You can always convert a value of type System.Address to a value of a
> > signed or modular (implementation-defined) type. That should help when
> > you want to hash it into a value of type Hash_Type.
>
> yes that helps - thanks for the suggestion.  In fact that is what I'm
already
> doing.  The problem is the next part...  That can be simplified to: I have
two
> modular types (binary modulus), but I don't know their modulus.  Values of
> the first type should be hashed into values of the second type.  This can
be
> done (I've done it) but somehow the code is much more horrible than I feel
> it should be.  One annoying point is that a program with a static
expression
> that would raise Constraint_Error is illegal, even if you would never get
to the
> Contraint_Error raising part in run-time.  If you don't understand the
relevance
> of this remark, try to code this kind of hash thing yourself and you will
soon
> see :)

That's one of the reasons that Ada 2006 adds the "Mod" attribute. You could
use that to discard any extra bits. (If you want to keep them, well, that
gets messier.).

So I'd use something like:

    if Integer_Address'Last < Hash_Type'Modulus then
            return Hash_Type'Mod(Addr);
    elsif Integer_Address'Last / Hash_Type'Modulus < Hash_Type'Modulus then
            return Hash_Type'Mod(Addr) xor Hash_Type'Mod(Addr /
Hash_Type'Modulus);
    else
            raise Program_Error;
    end if;

You could of course continue to handle larger multiples in the same way.

I don't remember if those static expressions are illegal because of typing.
That can be fixed by making sure the entire expression is of type
Universal_Integer:

    if Integer_Address'Pos(Integer_Address'Last) < Hash_Type'Modulus then
            return Hash_Type'Mod(Addr);
    elsif Integer_Address'Pos(Integer_Address'Last) / Hash_Type'Modulus <
Hash_Type'Modulus then
            return Hash_Type'Mod(Addr) xor Hash_Type'Mod(Addr /
Hash_Type'Modulus);
    else
            raise Program_Error;
    end if;

                                   Randy.