50-moves rule improvement for transposition table

joergoster · snicolet · commit bae019b53e5c · 2020-01-09T19:48:47.000+01:00
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 : #2451 About the GHI : https://www.chessprogramming.org/Graph_History_Interaction 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 http://tests.stockfishchess.org/tests/view/5df546116932658fe9b451bf LTC: LLR: 2.95 (-2.94,2.94) [-3.00,1.00] Total: 95235 W: 15297 L: 15292 D: 64646 http://tests.stockfishchess.org/tests/view/5df69c926932658fe9b4520e Closes #2453 Bench: 4586187
diff --git a/src/search.cpp b/src/search.cpp
@@ -716,7 +716,9 @@ namespace {
                 update_continuation_histories(ss, pos.moved_piece(ttMove), to_sq(ttMove), penalty);
             }
         }
-        return ttValue;
+
+        if (pos.rule50_count() < 90)
+            return ttValue;
     }
 
     // Step 5. Tablebases probe

Original file line number	Diff line number	Diff line change
`@@ -716,7 +716,9 @@ namespace {`
`716`	`716`	`update_continuation_histories(ss, pos.moved_piece(ttMove), to_sq(ttMove), penalty);`
`717`	`717`	`}`
`718`	`718`	`}`
`719`		`- return ttValue;`
	`719`	`+`
	`720`	`+ if (pos.rule50_count() < 90)`
	`721`	`+ return ttValue;`
`720`	`722`	`}`
`721`	`723`
`722`	`724`	`// Step 5. Tablebases probe`