Author: Brian Richardson
Date: 05:01:46 12/07/02
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On December 07, 2002 at 06:55:39, David Rasmussen wrote: >In my incremental move generator, I first check if there is a hash move, and if >there is, then I check it for pseudo-legality. If it's not pseudo-legal, I >conclude that there is a hash collision. The position that this move was stored >from cannot be the same as the current, and still they have the same hash >signature. In my program when this happens, I exit. I do this because this >basically never happens. Until now. It was playing a game on ICC, and it >suddenly exited. I could see from the log that this is what happened. So I was >wondering: How often does this kind of collision happen for your engines? >I think Bob Hyatt has mentioned that this happens in 1 of 100 games. It doesn't >for me. > >/David There are two situations that are sometimes called hash collisions. One is when just the first n bits of the hash index or key are the same. This happens quite a lot, and I don't personally consider it a collision. The other is when all bits of the hash signature are the same. I have never seen this with 64 bit values. I would not exit in either case, but just continue searching.
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