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Commit ID bae019b53e5c2bfcf0d69b4ebfc52b4f4de762eb
Author joergoster
Date 2020-01-09 18:48:47 UTC
50-moves rule improvement for transposition table User "adentong" reported recently of a game where Stockfish blundered a game in a tournament because during a search there was an hash-table issue for positions inside the tree very close to the 50-moves draw rule. This is part of a problem which is commonly referred to as the Graph History Interaction (GHI), and is difficult to solve in computer chess because storing the 50-moves counter in the hash-table loses Elo in general. Links: Issue 2451 : About the GHI : This patch tries to address the issue in this particular game and similar reported games: it prevents that values from the transposition table are getting used when the 50-move counter is close to reaching 100 (). The idea is that in such cases values from previous searches, with a much lower 50-move count, become less and less reliable. More precisely, the heuristic we use in this patch is that we don't take the transposition table cutoff when we have reached a 45-moves limit, but let the search continue doing its job. There is a possible slowdown involved, but it will also help to find either a draw when it thought to be losing, or a way to avoid the draw by 50-move rule. This heuristics probably will not fix all possible cases, but seems to be working reasonably well in practice while not losing too much Elo. Passed non-regression tests: STC: LLR: 2.95 (-2.94,2.94) [-3.00,1.00] Total: 274452 W: 59700 L: 60075 D: 154677 LTC: LLR: 2.95 (-2.94,2.94) [-3.00,1.00] Total: 95235 W: 15297 L: 15292 D: 64646 Closes Bench: 4586187
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