Author: Robert Hyatt
Date: 06:38:37 09/10/99
Go up one level in this thread
On September 10, 1999 at 05:47:32, allan johnson wrote: >On September 10, 1999 at 00:19:37, Robert Hyatt wrote: > >>Here is an interesting position given to me by Steffen Jakob: >> >> /p/P5p/7p/7P/4kpK/// w >> >> +---+---+---+---+---+---+---+---+ >> 8 | | | | | | | | | >> +---+---+---+---+---+---+---+---+ >> 7 | *P| | | | | | | | >> +---+---+---+---+---+---+---+---+ >> 6 | P | | | | | | *P| | >> +---+---+---+---+---+---+---+---+ >> 5 | | | | | | | | *P| >> +---+---+---+---+---+---+---+---+ >> 4 | | | | | | | | P | >> +---+---+---+---+---+---+---+---+ >> 3 | | | | | *K| *P| K | | >> +---+---+---+---+---+---+---+---+ >> 2 | | | | | | | | | >> +---+---+---+---+---+---+---+---+ >> 1 | | | | | | | | | >> +---+---+---+---+---+---+---+---+ >> a b c d e f g h >> >> >>Obviously black is getting crushed. He has one move, Kh3, which leads to a >>mate in 6. Steffen asked me to try this and Crafty found a mate in 4, which >>doesn't exist. I spent the entire day debugging this thing and here is what >>I found: >> >>If you recall the discussion here a couple of weeks ago, I reported that I store >>absolute mate scores (EXACT scores) in the hash table, and that I adjust them >>so that they are always stored as "mate in N from the current position". This >>has always worked flawlessly for me, and still does. >> >>For bounds, I once tried adjusting the bounds as well, but found quirks, and >>left them alone. Wrong answer. To fix this mate in 4 problem, I decided to >>adjust the bounds as well, but I now set any bound value that is larger than >>MATE-300, by reducing it to exactly MATE-300, but still using the "LOWER" >>flag to say that this is the lowest value this position could have. For bound >>values < -MATE+300, I set them to exactly -MATE+300 and leave the flag as is. >> >>This position is cute. Because not only is it a mate in 6, but there are >>transpositions that lead to mate in 7, mate in 8, and there are shorter (but >>non-forced) mates in 4 and 5. And there are stalemates, and positions with >>1 legal move, and so forth. >> >>You ought to find the following variation as one mate in 6: >> >>Kh3, f2, Kg2, Ke2, Kg3, f1=Q, Kh2, g5, hg, Kf3, g6, Qg2# >> >>If you find a shorter mate, it is wrong. If you find a longer mate, you >>are probably just extending like mad on checks (crafty finds a mate in 8 at >>shallow depths (9 plies, 2 secs on my PII/300 notebook), and doesn't find the >>mate in 6 until depth 10, 3 seconds. >> >>It is a good test as the transpositions are real cute with white's king caught >>in a tiny box, but with several different moves that triangulate and transpose >>into other variations... >> >>If you get it right, you have either handled the bounds right, or else you are >>very lucky. IE Crafty 16.17 gets this dead right. But if I disable the eval, >>it goes bananas, yet the eval is not important when mate is possible. >> >>Have fun... >> >>I did... :) >Robert The Mephisto Risc 1mb has no trouble in finding mate in 6 for black. >It plays Kh3 after 45sec and announces mate in 6 for black.It's move orderis >in the order you have listed. >Regards Allan The problem is, that doesn't mean that code is correct. Every version of Crafty I tested gets this one right, yet _all_ had the same mate bound bug. Only the current code would fail, and it was move-ordering related. In older versions if I just made the eval return(0), they would start failing as well... because order changed, but the mate score doesn't come from eval...
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