Author: Robert Hyatt
Date: 08:31:12 02/07/01
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On February 07, 2001 at 10:59:31, Pat King wrote: >I have seen it written here that with 64 bit Zobrist hashing, the perfect key >should change 32 bits. When I had what I thought to be hashing problems, I >captured some stats on my hash keys. I found that most of them changed 28-36 >bits (within 4) with a few outliers as far as 13 bits from "perfection". I also >checked that I was not generating duplicate keys. How good or bad is this? >Should I work on the average, or the outliers? Any comments appreciated :) > >Pat The main issue is hamming distance between any two positions you search. If each move changes 10 bits, then after 6 moves, you have potentially changed 60. After 12 you _could_ be back to where you started. The place to start working is on your random numbers. When I first did mine, I simply checked the hamming distance between any two of the numbers and if it was unacceptably low (say < 16 bits different) I culled one of them. I doubt you can do really bad random numbers unless you make the classic mistake of using two 32-bit floating point numbers and sticking them together to make one 64 bit random number. The problem with this is that the 'exponent' part of each number will be close to the same since FP random number generators usually produce a number N such that 0 <= N < 1.0 and that will mean your 64 bit numbers are really maybe on 44 bits of significant bits.
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