Author: Uri Blass
Date: 05:21:32 12/19/03
Go up one level in this thread
On December 19, 2003 at 08:18:43, Steffen Jakob wrote: >On December 19, 2003 at 08:11:51, Uri Blass wrote: > >>On December 19, 2003 at 08:06:28, Steffen Jakob wrote: >> >>>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 can´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. >> >>192 bits are enough to get different hash key for different positions so there >>is no problem with hash tables. > >If you use pseudo-random numbers for zobrist hashing it is always possible to >get a collision. Yes, this is paranoid. :-) No need to use pseudo random numbers. It is possible to store all the board with 192 bits easily as was explained in another post. Uri
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