Author: Robert Hyatt
Date: 13:08:23 09/16/99
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On September 16, 1999 at 12:10:29, Bas Hamstra wrote: >I don't agree. I store 32 bits, but use the Index as to store 48 or more bits >*effectively*. I store a hashmove for extra checking. If the hashmove is not >legal it prints a message. I never see such messages. > >Regards, >Bas Hamstra. I am not sure what you "don't agree" with. Storing 32 bits is fine (main hash) but it will _not_ work if that is all you compute for the hash signature. If you store the left-most 32 bits, and use the rightmost log2(size_of_table) bits for a probe address, you are effectively using far more than 32 bits, which was the point.. you _must_ use more than 32 or you will get collisions like nobody's business... > >On September 16, 1999 at 09:44:56, Robert Hyatt wrote: > >>On September 16, 1999 at 04:00:43, Tom Kerrigan wrote: >> >>>If you have a program with 64 bit hash keys, is it a good idea to devote n bits >>>to pawns and the rest to pieces and side-to-move? >>> >>>I was thinking a good balance may be 24 bits for pawns, but I have no data to >>>back this up. Does anybody else? >>> >>>-Tom >> >>If you are going to use them independently to probe tables (ie using the 24 >>bits to probe the pawn hash table) then this won't work well. 24 bits is only >>16 million possibilities... that will get more than a few collisions. And >>the same for the upper 40 bits and pieces. I know that 32 bits is not enough >>for normal hashing. Stanback/I ran a bunch of tests years ago in a discussion >>on r.g.c.c and we got collisions by the bucketload with 32 bit keys. 64 was >>totally safe back then, although speeds are way up now compared to then. I >>am using a 32 bit pawn hash signature, but a full 64 bit total hash signature.
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