Author: Robert Hyatt
Date: 15:26:16 02/03/04
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On February 03, 2004 at 18:11:23, Matthew White wrote: >This message is probably directed to Bob. I was reading through the eval code in >Crafty 19.9, and I noticed the function EvaluateStalemate which only tests for >stalemate if the side to move has no pawns and no pieces. There are many cases >when the side to move could have both pawns and pieces on the board and still be >stalemated. For example, in the following position, white cannot take black's >queen with Qxf3 because an immediate stalemate results (after Kh4 Qg3 or Qxg4, a >stalemate is forced anyway). By the way, adding a black bishop on h8, it is >still a stalemate (see the second diagram). I think that the correct way to >detect stalemate is the side to move has no legal moves and is not in check. I'm >guessing that there was some optimization that led to this code, but it causes >Crafty to miss out on draws by stalemate in some hopeless positions. >Incidentally, the position that brought about the stalemate defense is included >as the third diagram (from Gelfand-Kramnik, Candidates Match Game 6 1994). The >key move is 67... Qc1! because Qxd8 leads to rapid stalemate. Gelfand played 68. >d5 in the actual game, but still only drew. Note that the code is only used for one purpose... To handle the problems that occur in certain types of drawn positions that get evaluated wrongly. All I care about is a lone king that has no moves, primarily, as it wrecks a few special-case evaluations I do such as bishop + rp. It is not intended to catch all stalemates, that is the purpose of the search. In general, if one side has been stalemated, then the search would have already found it and returned the right score. This just prevents thinking one side can win an ending if the other king can not move, in special cases... > >[D]3Q4/4R1pk/p4p1p/P4P1P/3P2P1/5qK1/8/8 w - - 0 4 > >Crafty happily takes the queen though, and a stalemate results. This has nothing to do with the stalemate evaluation code. This is a result of the searching deciding that a draw is the best score it can produce. IE comment the code out and try it. Nothing will change. The main intent here is to catch the case of (say) king + bishop + right rook pawn that is not a win if the opponent king has been stalemated in the position being evaluated... >White(3): go > clearing hash tables > time surplus 19.76 time limit 30.34 (3:32) [easy move] > depth time score variation (1) > 15-> 0.16 0.01 3. Kxf3 > 16 0.16 0.01 3. Kxf3 > 16-> 0.24 0.01 3. Kxf3 > 17 0.24 0.01 3. Kxf3 > 17-> 0.35 0.01 3. Kxf3 > 18 0.35 0.01 3. Kxf3 > 18-> 0.58 0.01 3. Kxf3 > 19 0.58 0.01 3. Kxf3 > 19-> 0.96 0.01 3. Kxf3 > 20 0.96 0.01 3. Kxf3 > 20-> 1.66 0.01 3. Kxf3 > 21 1.66 0.01 3. Kxf3 > 21-> 3.57 0.01 3. Kxf3 > 22 3.57 0.01 3. Kxf3 > 22-> 7.46 0.01 3. Kxf3 > 23 7.46 0.01 3. Kxf3 > 23-> 26.47 0.01 3. Kxf3 > 24 26.47 0.01 3. Kxf3 > time=49.63 cpu=388% mat=6 n=108272028 fh=87% nps=2.18M > ext-> chk=14554635 cap=169258 pp=210432 1rep=876371 mate=112511 > predicted=1 nodes=108272028 evals=29355913 > endgame tablebase-> probes=0 hits=0 > hashing-> 34%(raw) 33%(depth) 99%(sat) 99%(pawn) > hashing-> 0%(exact) 30%(lower) 3%(upper) > SMP-> split=22850 stop=3198 data=16/128 cpu=3:12 elap=49.63 > >White(3): Kxf3 > time used: 49.63 > puzzling over a move to ponder. > clearing hash tables > depth time score variation > (no moves) >Stalemate with a bishop on board: >[D]3Q3b/4R1pk/p4p1p/P4P1P/3P2P1/5K2/8/8 b - - 0 2 >The original position (key move is Qc1!): >[D]2qr4/4R1pk/pQ3p1p/P4P1P/3P2P1/5P1K/8/8 b - - 0 1
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