Re: hash function over IP address

From	blp@cs.stanford.edu (Ben Pfaff)
Newsgroups	comp.programming, comp.unix.programmer
Subject	Re: hash function over IP address
Date	2012-04-05 07:42 -0700
Message-ID	<877gxukzo5.fsf@blp.benpfaff.org> (permalink)
References	<jliber$ocn$1@speranza.aioe.org> <87hawzyy3z.fsf@blp.benpfaff.org> <87398jw4ep.fsf@sapphire.mobileactivedefense.com> <87d37nyvjw.fsf@blp.benpfaff.org> <87k41uo2ph.fsf@sapphire.mobileactivedefense.com>

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Rainer Weikusat <rweikusat@mssgmbh.com> writes:

> blp@cs.stanford.edu (Ben Pfaff) writes:
>> Rainer Weikusat <rweikusat@mssgmbh.com> writes:
>>
>>> Ben Pfaff <blp@cs.stanford.edu> writes:
>>>> "Mark" <mark_cruzNOTFORSPAM@hotmail.com> writes:
>>>>
>>>>> I'm trying to find a way create a hash value over multiple IP addresses (I 
>>>>> keep a list of IP addresses and its number is variable. The simplest method 
>>>>> I've found so far is (obviously not the most effective and collision free):
>>>>
>>>> Why not use one of many available high-quality hash functions?
>>>> Three that come to mind without even going to a search engine:
>>>> Jenkins lookup3, murmurhash, FNV.
>>>
>>> The simple answer is these hash functions are designed to be used for
>>> large, variable length string keys NOT for small, fixed-size
>>> quantities which can be compared with a single machine instruction.
>>>
>>> http://burtleburtle.net/bob/hash/integer.html
>>> http://www.cris.com/~Ttwang/tech/inthash.htm
>>
>> He has a list of multiple IP addresses.  A hash function for
>> variable length keys may well be appropriate.
>
> Calculating the hash value must be cheaper than the full comparisons
> which will be needed for a linear search. And performing extensive
> byte-shuffling on machine words is not a good strategy to achieve
> that.

You suggested http://burtleburtle.net/bob/hash/integer.html.
Take a look at the first suggested hash function for an integer
there.  GCC 4.4 with -O3 compiles this into 12 instructions.  And
then you have to figure out a way to combine multiple hashes for
subsequent integers.

Now look at, say, http://burtleburtle.net/bob/c/lookup3.c.  It
takes 12*3 instructions (according to the comments) to hash three
32-bit integers, which, per-integer, is the same rate, and you
don't have to invent a way to combine them.
-- 
"While the Melissa license is a bit unclear, Melissa aggressively
 encourages free distribution of its source code."
--Kevin Dalley <kevin@seti.org>

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hash function over IP address "Mark" <mark_cruzNOTFORSPAM@hotmail.com> - 2012-04-04 16:37 -0400
  Re: hash function over IP address Barry Margolin <barmar@alum.mit.edu> - 2012-04-04 16:49 -0400
    Re: hash function over IP address China Blue Water Navy <chine.bleu@yahoo.com> - 2012-04-04 14:02 -0700
    Re: hash function over IP address "Mark" <mark_cruzNOTFORSPAM@hotmail.com> - 2012-04-04 17:21 -0400
      Re: hash function over IP address Daniel Pitts <newsgroup.nospam@virtualinfinity.net> - 2012-04-04 14:47 -0700
  Re: hash function over IP address Ben Pfaff <blp@cs.stanford.edu> - 2012-04-04 14:40 -0700
    Re: hash function over IP address Rainer Weikusat <rweikusat@mssgmbh.com> - 2012-04-04 22:52 +0100
      Re: hash function over IP address blp@cs.stanford.edu (Ben Pfaff) - 2012-04-04 15:35 -0700
        Re: hash function over IP address Rainer Weikusat <rweikusat@mssgmbh.com> - 2012-04-05 12:08 +0100
          Re: hash function over IP address blp@cs.stanford.edu (Ben Pfaff) - 2012-04-05 07:42 -0700
            Re: hash function over IP address Rainer Weikusat <rweikusat@mssgmbh.com> - 2012-04-05 16:29 +0100
          Re: hash function over IP address Daniel Pitts <newsgroup.nospam@virtualinfinity.net> - 2012-04-05 10:05 -0700
            Re: hash function over IP address Rainer Weikusat <rweikusat@mssgmbh.com> - 2012-04-05 18:12 +0100
              Re: hash function over IP address Daniel Pitts <newsgroup.nospam@virtualinfinity.net> - 2012-04-05 10:23 -0700
                Re: hash function over IP address Rainer Weikusat <rweikusat@mssgmbh.com> - 2012-04-05 18:34 +0100
                Re: hash function over IP address Daniel Pitts <newsgroup.nospam@virtualinfinity.net> - 2012-04-05 11:11 -0700
                Re: hash function over IP address Rainer Weikusat <rweikusat@mssgmbh.com> - 2012-04-05 19:34 +0100
                Re: hash function over IP address Rainer Weikusat <rweikusat@mssgmbh.com> - 2012-04-05 19:38 +0100
                Re: hash function over IP address Daniel Pitts <newsgroup.nospam@virtualinfinity.net> - 2012-04-05 12:18 -0700
            Re: hash function over IP address scott@slp53.sl.home (Scott Lurndal) - 2012-04-05 18:45 +0000
              Re: hash function over IP address "Mark" <mark_cruzNOTFORSPAM@hotmail.com> - 2012-04-05 15:02 -0400
  Re: hash function over IP address Rick Jones <rick.jones2@hp.com> - 2012-04-04 23:18 +0000
  Re: hash function over IP address William Ahern <william@wilbur.25thandClement.com> - 2012-04-04 16:57 -0700
  Re: hash function over IP address Eric Sosman <esosman@ieee-dot-org.invalid> - 2012-04-04 21:05 -0400
    Re: hash function over IP address Barry Margolin <barmar@alum.mit.edu> - 2012-04-04 22:52 -0400
      Re: hash function over IP address Eric Sosman <esosman@ieee-dot-org.invalid> - 2012-04-05 08:46 -0400
    Re: hash function over IP address "Mark" <mark_cruzNOTFORSPAM@hotmail.com> - 2012-04-05 08:42 -0400
      Re: hash function over IP address Rainer Weikusat <rweikusat@mssgmbh.com> - 2012-04-05 15:04 +0100
        Re: hash function over IP address "Mark" <mark_cruzNOTFORSPAM@hotmail.com> - 2012-04-05 10:43 -0400
          Re: hash function over IP address Eric Sosman <esosman@ieee-dot-org.invalid> - 2012-04-05 22:24 -0400
          Re: hash function over IP address Rainer Weikusat <rweikusat@mssgmbh.com> - 2012-04-06 17:13 +0100
            Re: hash function over IP address Rainer Weikusat <rweikusat@mssgmbh.com> - 2012-04-06 17:21 +0100
            Re: hash function over IP address "Mark" <mark_cruzNOTFORSPAM@hotmail.com> - 2012-04-09 10:04 -0400
              Re: hash function over IP address Rainer Weikusat <rweikusat@mssgmbh.com> - 2012-04-09 15:54 +0100
  Re: hash function over IP address BGB <cr88192@hotmail.com> - 2012-04-04 20:21 -0700
  Re: hash function over IP address Udit Gangwani <uditg22@gmail.com> - 2012-04-05 00:26 -0700
  Re: hash function over IP address Rainer Weikusat <rweikusat@mssgmbh.com> - 2012-04-05 12:15 +0100
    Re: hash function over IP address Barry Margolin <barmar@alum.mit.edu> - 2012-04-05 07:52 -0400

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