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Subject: Re: Non power of two hash table sizes

Author: Heiner Marxen

Date: 14:37:27 02/18/02

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On February 18, 2002 at 17:07:11, Dann Corbit wrote:

>On February 18, 2002 at 16:56:37, Benny Antonsson wrote:
>
>>Why must it be a prime number ?
>
>When a composite number is chosen, factors in the number can cause a bad
>[non-uniform] distribution.
>
>See Knuth's "The Art of Computer Programming", Volume 3 'Sorting and Searching'
>page 516 for a detailed explanation.

IMO this is only an issue if you start with a non-uniform distribution.
If your hash key is already very good (Zobrist should be good enough),
you may take any hash table size and do

   hash_index = hash_key % hash_table_size;

(After all, taking the low N bits is just an efficient way to do key % (1<<N))

In doubt (probably anyhow) just measure the quality of the distribution.
It is of high quality, it behaves similar to a true random distribution.
Many years ago I've done such experiments, but I do not remember the details
any more, sorry.

Cheers,
Heiner

• Re: Non power of two hash table sizes Dann Corbit 15:58:54 02/18/02
  • Re: Non power of two hash table sizes Heiner Marxen 17:19:17 02/18/02
    • Re: Non power of two hash table sizes Dan Andersson 23:47:41 02/18/02
      • Re: Non power of two hash table sizes Heiner Marxen 06:23:18 02/19/02
        • Re: Non power of two hash table sizes Dan Andersson 11:21:15 02/19/02
    • Re: Non power of two hash table sizes Dann Corbit 17:34:09 02/18/02
      • Re: Non power of two hash table sizes Keith Evans 15:11:20 02/19/02

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