Author: Larry Griffiths
Date: 05:18:34 02/24/99
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On February 23, 1999 at 15:23:14, Dan Newman wrote: >On February 23, 1999 at 14:43:29, Larry Griffiths wrote: > >>On February 23, 1999 at 14:33:03, Larry Griffiths wrote: >> >>>Dan, >>> >>>I read your post on hash collisions. I have tried some hamming distance >>>code and I see little difference when compared to generating random >>>numbers. I have been thinking of writing code that does what you said >>>where 64 bits must have 32 bits (or 50%) turned on in each number. I also see >>>where there are many references to the 12 pieces times 64 squares = 768 >>>hash codes. Pawns only use 48 squares for each side, so 32 squares are unused by pawns. >> >>Forget my reference to the Bishops. >> >>>768-48 leaves 720 hashcodes for the piece square table. >>>Food for thought? Errors in my thinking? >>> >>>Larry. > >That's interesting. You can generate a smaller set of random numbers and >fill out only those portions of the table that are actually used. And a >smaller set can be better optimized in the same period of time. (Currently, >IIRC, I use 1024 numbers for the piece-square part since I use Bob Hyatt's >numbering scheme for pieces: P=1,N=2,K=3,B=5,R=6,Q=7 -- which is quite >nice for distiguishing sliders from non-sliders.) > >-Dan. > >-Dan. Dan, I actually only use a power of 2 for my hash keys. If my hash table is 262144 entries, then I only use 18 bits for a hash key. I currently store a 64 bit bitboard in the hash entry for collision checking. I did create some code to choose numbers that have exactly 9 bits on with equal distances between the numbers. I ran 6 ply test using these keys but it did not seem to make much difference. I did try small number of bits and large number of bits also. I have a graph in my program to display the hash table usage. I saw bands (16 of them) occur when using very few bits per number or very many bits per number. I think this is caused by the 16 pawn moves that can be made on the very first ply but I am not sure of this. I have "holes" in my hash table that only get used after several moves by each side. Larry.
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