Author: Robert Hyatt
Date: 07:34:40 12/27/98
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On December 26, 1998 at 18:51:46, Richard A. Fowell (fowell@netcom.com) wrote: >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) This has to be a bug. Hash tables can't find a "shorter mate" than actually exists on the board, unless there is a bug. Hash tables can let a shallow search see "much deeper" than you would expect (position fine#70 is the classic example that programs solve at a much shallower depth than is actually possible).
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