Dev Builds » 20191112-0009

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Commit ID 5ae195ee7e3ccac01b8145f9b7022e352a288f07
Author Joost VandeVondele
Date 2019-11-12 00:09:57 UTC
Fix incorrect mate score. Current master 648c7ec25db2040c0af34dd846dfa3f57af5ad0a will generate an incorrect mate score for: ``` setoption name Hash value 8 setoption name Threads value 1 position fen 8/1p2KP2/1p4q1/1Pp5/2P5/N1Pp1k2/3P4/1N6 b - - 76 40 go depth 49 ``` even though the position is a draw. Generally, SF tries to display only proven mate scores, so this is a bug. This was posted by Uri Blass, with the correct analysis that this must be related to the 50 moves draw rule being ignored somewhere. Indeed, this is possible as positions and there eval are stored in the TT, without reference to the 50mr counter. Depending on the search path followed a position can thus be mate or draw in the TT (GHI or Graph history interaction). Therefore, to prove mate lines, the TT content has to be used with care. Rather than ignoring TT content in general or for mate scores (which impact search or mate finding), it is possible to be more selective. In particular, @WOnder93 suggested to only ignore the TT if the 50mr draw ply is closer than the mate ply. This patch implements this idea, by clamping the eval in the TT to +-VALUE_MATED_IN_MAX_PLY. This retains the TTmove, but causes a research of these lines (with the current 50mr counter) as needed. This patch hardly ever affects search (as indicated by the unchanged bench), but fixes the testcase. As the conditions are very specific, also mate finding will almost never be less efficient (testing welcome). It was also shown to pass STC and LTC non-regression testing, in a form using if/then/else instead of ternary operators: STC: LLR: 2.96 (-2.94,2.94) [-3.00,1.00] Total: 93605 W: 15346 L: 15340 D: 62919 LTC: LLR: 2.96 (-2.94,2.94) [-3.00,1.00] Total: 33873 W: 7359 L: 7261 D: 19253 closes Bench: 4362323
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