Author: Frank Schneider
Date: 21:04:19 07/09/02
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On July 09, 2002 at 17:59:17, Robert Hyatt wrote: >I have been running some tests after prompting by Bruce, and the results >have been interesting. > >The question posed by him was "how many hash collisions (signatures match >but positions do not) can we tolerate in a search without having serious >problems?" > >I did the following: I made my HashProbe() routine use the node counter, >and every N nodes, I would "match" a hash entry even if the signatures were >not the same, simulating a collision. I got all the way down to a collision >every 1000 nodes without seeing even a score change at the root, which was >surprising. > >I would like to ask others to try this as well. It only adds a couple of >lines to your HashProbe() code. I started at one "collision" every 100K >nodes, but that did nothing. Then one every 10K. And finally one every >1K. This seems surprising to me. I ran several positions, with and without >the "collision problem" and compared my logfile output. Scores did not change >at all. I used tactical positions like Kopec22, opening positions, and even >the classic fine #70. > >Seems that the search is far more resistant to such errors than anybody has >thought previously. It would be interesting to see if this is just a result >of Crafty, or if it holds for others. Particularly for someone that hashes >in the q-search, which I do not do... Note that I did not false match every >N calls to HashProbe() rather I false matched on the next call to HashProbe() >after having searched N nodes since the previous call. Sometimes this would >be less frequently than once per 1000 nodes obviously, since I could burn >several thousand nodes in the q-search and not do another false match until I >get back out of that mess and back into the normal non-qsearch part. As far as i know older versions of Fritz (< Fritz 5) used a 32-bit hashkey, so it's likely they had collisions now and then. Frank
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