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From: Kilgallen@SpamCop.net (Larry Kilgallen)
Newsgroups: comp.lang.ada
Subject: Re: Hashing on System.Address
Date: 14 Jun 2005 03:42:47 -0500
Organization: LJK Software
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 <wcc3brlrfj5.fsf@shell01.TheWorld.com>
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Date: 2005-06-14T03:42:47-05:00
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In article <wcc3brlrfj5.fsf@shell01.TheWorld.com>, Robert A Duff <bobduff@shell01.TheWorld.com> writes:
> "Matthew Heaney" <mheaney@on2.com> writes:
> 
>> Duncan Sands wrote:
>> > Does anyone have an efficient portable algorithm for hashing
>> > on a System.Address?
>> >
>> > The Ada 2005 package Ada.Containers defines Hash_Type as
>> >
>> > 	type Hash_Type is mod implementation-defined;
>> >
>> > This is the target type I would like for the hash.
>> 
>> Use System.Storage_Elements.To_Integer to convert the address to an
>> Integer_Address, and then convert that to type Hash_Type.
> 
> It might be a good idea to divide that by 8, since most addresses have
> zeros in the low three bits.

Or it might not be, since he said portable.  Looking at addresses of
_instructions_ (which are the closest to that pattern) on the machine
architectures on which VMS runs:

	VAX - No restriction, 1 bit (odd and even) just as likely to be
		one as the other
	Alpha - 4 byte boundary, 4 bit (100) just as likely to be one
		as the other
	Itanium - 8 byte boundary for VLIW instructions, but in software
		the last octet of the address is 0, 1 or 2 depending on
		which instruction in the bundle is addressed.