Author: Richard A. Fowell (fowell@netcom.com)
Date: 15:51:46 12/26/98
Do hash tables confuse your matefinding routines? I tried this mate in 7: 8/krK5/p7/P7/6P1/8/7Q/8 w - - on two different chess programs, by two different authors, and both reported erroneously short mates when their hash tables were on. With hash tables off, the correct solution was given by both programs. In both cases, the programs were using special-purpose matefinding logic that had no known problems before. Can anyone shed any light on this? Since this problem showed up in the only two programs I tested for this, it seems like it may be a widespread issue. When I use Sigma Chess 4.01 Lite with hash off, using the "mate in 7" setting, I get the right answer: Hash off (Mate in 7) - Kc7-c8, Mate in 7, 948.7 seconds Kc7-c8 Rb7-b3 Qh2-h7 Ka7-a8 Qh7-e4 Ka8-a7 Qe4-d4 Ka7-a8 Qd4-d5 Ka8-a7 Qd5xb3 Ka7-a8 Qb3-b7 When I enable hash, Sigma Chess (erroneously) reports mate in 6 - with pretty much a different PV for every hash table setting I tried. The key move is right, and the time is much shorter (2.4 sec, 6.9 sec, 8.0 sec, 11.1 sec, 12.5 sec, 32.6 sec in the cases I tried), but the evaluation is wrong. When I use MacChess 5.0 EN, at level "mate in 7", (which uses hash tables) it reports a mate in 4. When I use MacChess 4.0 at level "mate in 7", (which does not use hash tables) it reports the correct mate in 7, e.g.: MacChess 4.0 EN 1. Kc8 Rb3 2. Qc7+ Ka8 3. Qc6+ Ka7 4. Qd7+ Ka8 5. Qd5+ Ka7 6. Qxb3 Ka8 7. Qb7# So ... what's going on, here? Richard A. Fowell (fowell@netcom.com)
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