Author: Robert Hyatt
Date: 06:57:58 12/09/02
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On December 09, 2002 at 00:42:49, Russell Reagan wrote: >On December 07, 2002 at 23:21:11, Robert Hyatt wrote: > >>I don't remember the results any longer, but several of us ran some tests years >>ago in a discussion on r.g.c, and found that 32 bits is useless, and that 64 >>bits has so few errors they can be ignored. With recent testing I did that >>suggests that a bad hash collision every 10K nodes has no effect on the final >>score, we seem to be "ok". :) > >Do you think any additional verification would be an improvement? For example, >checking to see if the pseudo-legal moves are the same for the position (maybe >a pseudo-legal moves hash key, or pseudo-pawn moves hash key). It would seem >impossible to have a 64-bit hash key be the same for two different positions, >and their pseudo-legal moves hash key also be the same. I don't think it is necessary. I check the hash move for legality because I will end up making that move later, prior to generating all moves. If the move is illegal, it will corrupt the board (ie a O-O move will result in an extra king if the king is not currently on E1 or E8...) > >You could generate it at move generation time, and have it ready to go, and you >could use whichever piece sets you wanted. You could even have several keys. One >for (say), rook and bishop moves, another for pawn and queen moves, etc. If the >regular hash key is the same, check to see if the next key is the same, and if >you have several keys, it seems like you could be sure that a position was the >same. I imagine with such a scheme, you might get a single false result in the >entire life of your program. > >Would that be too expensive and slow things down too much? It would for me since I try the hash move before I do anything, and if it produces a cutoff I am done without generating any moves.
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