Author: Dan Newman

Date: 15:19:23 02/24/99

Go up one level in this thread

On February 24, 1999 at 08:18:34, Larry Griffiths wrote: >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. I've been playing around with trying to get a better set of numbers (from the point of view of reducing hashing errors rather than filling in the holes). But so far I've only managed to increase the error rate :). (I went from about 1 error per second to 20/s using 32-bit hashcodes when I tried to generate a good set of numbers with exactly 16 one bits. I'm beginning to think that that wasn't such a great idea.) I also tried Bob's idea of adding in ~N for every N (I put them right next to each other in my table) and got no change in error rate for a very short test. This is also contrary to my expectations... but I need to do some more extensive tests. -Dan.

- Re: Hash Collisions (Dan)
**Larry Griffiths***15:33:19 02/24/99*

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