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Subject: Re: Hashing in distributed perft

Author: Steffen Jakob

Date: 05:06:28 12/19/03

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On December 19, 2003 at 05:45:00, Uri Blass wrote:

>On December 19, 2003 at 02:27:17, Steffen Jakob wrote:
>
>>On December 19, 2003 at 01:24:27, Russell Reagan wrote:
>>
>>>On December 19, 2003 at 01:00:31, Steffen Jakob wrote:
>>>
>>>>I repeat my posting from below because the ruffian thread pushed it very fast to
>>>>the bottom of the message list. :)
>>>>
>>>>What are you using for the hash key in your distributed perft implementation?
>>>>How do you make sure that there are no hash key collisions which are possible in
>>>>the usual zobrist key approach? Those collisions are too rare to influence the
>>>>playing strength of a chess engine but would make the result of your perft
>>>>project invalid.
>>>
>>>I remember Albert saying that he uses 128-bit hash keys, which is not
>>>theoretically sound, but should work in practice. Deiter also uses hash tables
>>>for this I think. Maybe he can tell us what he does.
>>
>>I like this distributed perft project very much (and contributed 4 solutions to
>>subproblems ;-) but the only reason why we are doing this is to get the *exact*
>>number of lines. Even if it is wrong by one line then the result is wrong and
>>the whole effort was rather useless. Even if the result is correct then we
>>cannot be sure about it. Therefore I would propose to run a validation without
>>hash tables. Can it be estimated how long this would take?
>
>I do not see a reason not to use hash tables when it is possible to use hash
>tables and be safe with 192 bytes.

Can you tell me the likelihood that an error will occur because of an undetected
hash key collision? If you can then you can say "perft(n) == x with a likelihood
of p%". If you canĀ“t then how can I trust a result from which I know that it
might be incorrect? Why not make p=100 for the case of the hash table errors
(e.g. by storing the complete board information in the hash entry [but not in
the key])?

Greetings,
Steffen.



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