Author: Robert Hyatt
Date: 06:35:02 09/10/99
Go up one level in this thread
On September 10, 1999 at 07:48:44, Ed Schröder 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... :) > >A simple solution: do not store a position in the hash table if there is >no best-move. It solves the mate-cases and also repetition cases. Also >there is no speed loss of the search. > >Ed I played with this yesterday. This means you will _never_ store a fail-low position, where you tried all the moves and each one was refuted by the move below it in the tree. Not storing those will definitely slow you down. The trick I used above solves this cleanly without losing the fail-low (UPPER bound) cases that still continue to work. Assuming I read what you meant correctly?
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