Introduce ProbCut for check evasions

Vizvezdenec · MichaelB7 · commit 991d7d66282e · 2021-04-12T22:49:53.000-04:00
The idea of this patch can be described as follows: if we are in check and the transposition table move is a capture that returns a value far above beta, we can assume that the opponent just blundered a piece by giving check, and we return the transposition table value. This is similar to the usual probCut logic for quiet moves, but with a different threshold. Passed STC LLR: 2.94 (-2.94,2.94) {-0.25,1.25} Total: 33440 W: 3056 L: 2891 D: 27493 Ptnml(0-2): 110, 2338, 11672, 2477, 123 https://tests.stockfishchess.org/tests/view/602cd1087f517a561bc49bda Passed LTC LLR: 2.98 (-2.94,2.94) {0.25,1.25} Total: 10072 W: 401 L: 309 D: 9362 Ptnml(0-2): 2, 288, 4365, 378, 3 https://tests.stockfishchess.org/tests/view/602ceea57f517a561bc49bf0 The committed version has an additional fix to never return unproven wins in the tablebase range or the mate range. This fix passed tests for non- regression at STC and LTC: STC: LLR: 2.93 (-2.94,2.94) {-1.25,0.25} Total: 26240 W: 2354 L: 2280 D: 21606 Ptnml(0-2): 85, 1763, 9372, 1793, 107 https://tests.stockfishchess.org/tests/view/602d86a87f517a561bc49c7a LTC: LLR: 2.95 (-2.94,2.94) {-0.75,0.25} Total: 35304 W: 1299 L: 1256 D: 32749 Ptnml(0-2): 14, 1095, 15395, 1130, 18 https://tests.stockfishchess.org/tests/view/602d98d17f517a561bc49c83 Closes official-stockfish#3362 Bench: 3830215
diff --git a/src/search.cpp b/src/search.cpp
@@ -1364,6 +1364,23 @@ namespace {
 
 moves_loop: // When in check, search starts from here
 
+    ttCapture = ttMove && pos.capture_or_promotion(ttMove);
+
+    // Step 11. A small Probcut idea, when we are in check
+    probCutBeta = beta + 400;
+    if (   ss->inCheck
+        && !PvNode
+        && depth >= 4
+        && ttCapture
+        && (tte->bound() & BOUND_LOWER)
+        && tte->depth() >= depth - 3
+        && ttValue >= probCutBeta
+        && abs(ttValue) <= VALUE_KNOWN_WIN
+        && abs(beta) <= VALUE_KNOWN_WIN
+       )
+        return probCutBeta;
+
+
     const PieceToHistory* contHist[] = { (ss-1)->continuationHistory, (ss-2)->continuationHistory,
                                           nullptr                   , (ss-4)->continuationHistory,
                                           nullptr                   , (ss-6)->continuationHistory };
@@ -1380,12 +1397,11 @@ namespace {
 
     value = bestValue;
     singularQuietLMR = moveCountPruning = false;
-    ttCapture = ttMove && pos.capture_or_promotion(ttMove);
 
     // Mark this node as being searched
     ThreadHolding th(thisThread, posKey, ss->ply);
 
-    // Step 11. Loop through all pseudo-legal moves until no moves remain
+    // Step 12. Loop through all pseudo-legal moves until no moves remain
     // or a beta cutoff occurs.
     while ((move = mp.next_move(moveCountPruning)) != MOVE_NONE)
     {
@@ -1483,7 +1499,7 @@ namespace {
           }
       }
 
-      // Step 13. Extensions (~75 Elo)
+      // Step 14. Extensions (~75 Elo)
 
       // Singular extension search (~70 Elo). If all moves but one fail low on a
       // search of (alpha-s, beta-s), and just one fails high on (alpha, beta),
