Re: Good hash for pointers

From	Michael S <already5chosen@yahoo.com>
Newsgroups	comp.lang.c
Subject	Re: Good hash for pointers
Date	2024-06-03 20:16 +0300
Organization	A noiseless patient Spider
Message-ID	<20240603201646.0000319d@yahoo.com> (permalink)
References	(13 earlier) <20240602104506.000072e4@yahoo.com> <v3kk9p$3u8qu$1@raubtier-asyl.eternal-september.org> <20240603174604.000014d4@yahoo.com> <v3kovm$3v2ie$1@raubtier-asyl.eternal-september.org> <v3kqo3$3v9m2$1@dont-email.me>

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On Mon, 3 Jun 2024 17:24:36 +0100
bart <bc@freeuk.com> wrote:

> On 03/06/2024 16:54, Bonita Montero wrote:
> > Am 03.06.2024 um 16:46 schrieb Michael S:  
> >> On Mon, 3 Jun 2024 16:34:37 +0200
> >> Bonita Montero <Bonita.Montero@gmail.com> wrote:
> >>  
> >>> Am 02.06.2024 um 09:45 schrieb Michael S:
> >>>  
> >>>> So, what were your conclusions?
> >>>> Ignoring the speed of computation, would something like
> >>>> cryptographic hash scaled to bucket size be a best hash for this
> >>>> type of application? Or some sort of less thorough grinding of
> >>>> the bits is better?  
> >>>
> >>> There's no need for a crypto-hash here.
> >>>  
> >>
> >> Do you think I don't know?
> >> Crypto hash is just an example of near-ideal pseudo-random
> >> uniformity. 
> > 
> > As I've shown for pointers you get a perfect equal distribution with
> > multiplying by an appropriate prime.
> >   
> 
> A pointer with 8-byte or 16-byte alignment will have the bottom 3-4
> bits zero.
> 
> No matter what number you multiply them by, prime or otherwise, those 
> 3-4 bits will always be zero.
> 
> If you mask the result to fit a table of size power-of-two, then the 
> resulting index will can only ever refer to every 8th or every 16th 
> slot; there will 8-16x as many clashes as there ought to be.
> 
> So some extra work is needed to get around that, for example 
> right-shifting before masking as some here have done, something you
> have never mentioned.
> 
> 

According to my understanding, Bonita and Tim are discussing hash
generator which output is not used as is. They assume that index of
the slot will be calculated as (Hash(key)*bucket_size)/(Hash_max+1).
For big enough Hash_max (Bonita suggests 2**63-1), poor quality of
few LS bits of Hash(key) does not matter.

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Thread

Re: Good hash for pointers Bonita Montero <Bonita.Montero@gmail.com> - 2024-05-24 20:28 +0200
  Re: Good hash for pointers Malcolm McLean <malcolm.arthur.mclean@gmail.com> - 2024-05-24 19:57 +0100
    Re: Good hash for pointers bart <bc@freeuk.com> - 2024-05-25 00:54 +0100
      Re: Good hash for pointers Tim Rentsch <tr.17687@z991.linuxsc.com> - 2024-05-25 02:12 -0700
        Re: Good hash for pointers bart <bc@freeuk.com> - 2024-05-25 12:28 +0100
          Re: Good hash for pointers Tim Rentsch <tr.17687@z991.linuxsc.com> - 2024-05-25 11:12 -0700
            Re: Good hash for pointers bart <bc@freeuk.com> - 2024-05-25 20:31 +0100
              Re: Good hash for pointers Tim Rentsch <tr.17687@z991.linuxsc.com> - 2024-05-25 22:54 -0700
        Re: Good hash for pointers Bonita Montero <Bonita.Montero@gmail.com> - 2024-05-25 17:00 +0200
          Re: Good hash for pointers Tim Rentsch <tr.17687@z991.linuxsc.com> - 2024-05-25 10:40 -0700
            Re: Good hash for pointers Malcolm McLean <malcolm.arthur.mclean@gmail.com> - 2024-05-25 18:56 +0100
              Re: Good hash for pointers Tim Rentsch <tr.17687@z991.linuxsc.com> - 2024-05-25 11:23 -0700
              Re: Good hash for pointers Janis Papanagnou <janis_papanagnou+ng@hotmail.com> - 2024-05-25 23:13 +0200
                Re: Good hash for pointers Malcolm McLean <malcolm.arthur.mclean@gmail.com> - 2024-05-25 23:07 +0100
                Re: Good hash for pointers bart <bc@freeuk.com> - 2024-05-25 23:42 +0100
                Re: Good hash for pointers Richard Harnden <richard.nospam@gmail.invalid> - 2024-05-26 19:58 +0100
                Re: Good hash for pointers Kaz Kylheku <643-408-1753@kylheku.com> - 2024-05-26 22:42 +0000
              Re: Good hash for pointers Bonita Montero <Bonita.Montero@gmail.com> - 2024-05-26 18:05 +0200
                Re: Good hash for pointers Bonita Montero <Bonita.Montero@gmail.com> - 2024-05-26 18:07 +0200
            Re: Good hash for pointers Bonita Montero <Bonita.Montero@gmail.com> - 2024-05-26 18:04 +0200
              Re: Good hash for pointers Tim Rentsch <tr.17687@z991.linuxsc.com> - 2024-05-26 09:24 -0700
                Re: Good hash for pointers Bonita Montero <Bonita.Montero@gmail.com> - 2024-05-26 18:36 +0200
                Re: Good hash for pointers Tim Rentsch <tr.17687@z991.linuxsc.com> - 2024-05-26 10:20 -0700
                Re: Good hash for pointers Bonita Montero <Bonita.Montero@gmail.com> - 2024-05-26 19:39 +0200
                Re: Good hash for pointers Bonita Montero <Bonita.Montero@gmail.com> - 2024-05-26 19:54 +0200
                Re: Good hash for pointers Bonita Montero <Bonita.Montero@gmail.com> - 2024-05-27 08:07 +0200
                Re: Good hash for pointers Ben Bacarisse <ben@bsb.me.uk> - 2024-05-28 11:07 +0100
                Re: Good hash for pointers Bonita Montero <Bonita.Montero@gmail.com> - 2024-05-30 10:10 +0200
                Re: Good hash for pointers Bonita Montero <Bonita.Montero@gmail.com> - 2024-05-30 11:27 +0200
                Re: Good hash for pointers Tim Rentsch <tr.17687@z991.linuxsc.com> - 2024-05-30 19:26 -0700
                Re: Good hash for pointers Tim Rentsch <tr.17687@z991.linuxsc.com> - 2024-05-30 19:27 -0700
                Re: Good hash for pointers Michael S <already5chosen@yahoo.com> - 2024-06-02 10:45 +0300
                Re: Good hash for pointers "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2024-06-02 12:42 -0700
                Re: Good hash for pointers "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2024-06-03 12:35 -0700
                Re: Good hash for pointers Tim Rentsch <tr.17687@z991.linuxsc.com> - 2024-06-02 16:02 -0700
                Re: Good hash for pointers Michael S <already5chosen@yahoo.com> - 2024-06-03 10:50 +0300
                Re: Good hash for pointers Tim Rentsch <tr.17687@z991.linuxsc.com> - 2024-06-03 18:02 -0700
                Re: Good hash for pointers Michael S <already5chosen@yahoo.com> - 2024-06-04 11:38 +0300
                Re: Good hash for pointers Tim Rentsch <tr.17687@z991.linuxsc.com> - 2024-06-04 22:10 -0700
                Re: Good hash for pointers Bonita Montero <Bonita.Montero@gmail.com> - 2024-06-03 16:34 +0200
                Re: Good hash for pointers Michael S <already5chosen@yahoo.com> - 2024-06-03 17:46 +0300
                Re: Good hash for pointers Bonita Montero <Bonita.Montero@gmail.com> - 2024-06-03 17:54 +0200
                Re: Good hash for pointers bart <bc@freeuk.com> - 2024-06-03 17:24 +0100
                Re: Good hash for pointers Michael S <already5chosen@yahoo.com> - 2024-06-03 20:16 +0300
                Re: Good hash for pointers bart <bc@freeuk.com> - 2024-06-03 19:48 +0100
                Re: Good hash for pointers Michael S <already5chosen@yahoo.com> - 2024-06-03 22:41 +0300
                Re: Good hash for pointers Michael S <already5chosen@yahoo.com> - 2024-06-03 22:51 +0300
                Re: Good hash for pointers Tim Rentsch <tr.17687@z991.linuxsc.com> - 2024-06-03 16:51 -0700
                Re: Good hash for pointers Tim Rentsch <tr.17687@z991.linuxsc.com> - 2024-06-03 17:01 -0700
                Re: Good hash for pointers Bonita Montero <Bonita.Montero@gmail.com> - 2024-06-03 20:25 +0200
                Re: Good hash for pointers Michael S <already5chosen@yahoo.com> - 2024-06-03 19:50 +0300
                Re: Good hash for pointers Bonita Montero <Bonita.Montero@gmail.com> - 2024-06-03 20:31 +0200
                Re: Good hash for pointers Michael S <already5chosen@yahoo.com> - 2024-06-05 00:59 +0300
                Re: Good hash for pointers Bonita Montero <Bonita.Montero@gmail.com> - 2024-06-05 11:10 +0200
                Re: Good hash for pointers Michael S <already5chosen@yahoo.com> - 2024-06-05 12:34 +0300
                Re: Good hash for pointers Bonita Montero <Bonita.Montero@gmail.com> - 2024-06-05 12:05 +0200
                Re: Good hash for pointers Michael S <already5chosen@yahoo.com> - 2024-06-05 13:11 +0300
                Re: Good hash for pointers Tim Rentsch <tr.17687@z991.linuxsc.com> - 2024-06-05 08:58 -0700
                AES problem (was: Good hash for pointers) Michael S <already5chosen@yahoo.com> - 2024-06-05 19:51 +0300
                Re: AES problem (was: Good hash for pointers) Tim Rentsch <tr.17687@z991.linuxsc.com> - 2024-06-05 21:24 -0700
                Re: Good hash for pointers Michael S <already5chosen@yahoo.com> - 2024-06-05 19:59 +0300
                Re: Good hash for pointers Tim Rentsch <tr.17687@z991.linuxsc.com> - 2024-06-05 21:40 -0700
                Re: Good hash for pointers Michael S <already5chosen@yahoo.com> - 2024-06-06 11:00 +0300
                Re: Good hash for pointers Michael S <already5chosen@yahoo.com> - 2024-06-06 13:35 +0300
                Re: Good hash for pointers Bonita Montero <Bonita.Montero@gmail.com> - 2024-06-07 20:53 +0200
                Re: Good hash for pointers Bonita Montero <Bonita.Montero@gmail.com> - 2024-06-09 13:35 +0200
                Re: Good hash for pointers Richard Harnden <richard.nospam@gmail.invalid> - 2024-06-09 12:40 +0100
                Re: Good hash for pointers Bonita Montero <Bonita.Montero@gmail.com> - 2024-06-09 15:09 +0200
                Re: Good hash for pointers Malcolm McLean <malcolm.arthur.mclean@gmail.com> - 2024-06-10 01:34 +0100
                Re: Good hash for pointers Tim Rentsch <tr.17687@z991.linuxsc.com> - 2024-06-09 18:31 -0700
                Re: Good hash for pointers Michael S <already5chosen@yahoo.com> - 2024-06-10 15:14 +0300
                Re: Good hash for pointers Bonita Montero <Bonita.Montero@gmail.com> - 2024-06-10 14:40 +0200
                Re: Good hash for pointers Bonita Montero <Bonita.Montero@gmail.com> - 2024-06-10 09:35 +0200

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