Use capture history to better judge which sacrifices to explore

This idea has been bouncing around a while. @Vizvezdenec tried it a
couple years ago in Stockfish without results, but its recent arrival in
Ethereal inspired him and thence me to try it afresh in Stockfish.

(Also factor out the now-common code with futpruning for captures.)

STC:
https://tests.stockfishchess.org/tests/view/662355bc3fe04ce4cefc18ac
LLR: 2.92 (-2.94,2.94) <0.00,2.00>
Total: 45760 W: 11970 L: 11640 D: 22150
Ptnml(0-2): 124, 5371, 11625, 5571, 189

LTC:
https://tests.stockfishchess.org/tests/view/662dda396115ff6764c817c9
LLR: 2.94 (-2.94,2.94) <0.50,2.50>
Total: 243828 W: 62042 L: 61287 D: 120499
Ptnml(0-2): 211, 27202, 66329, 27965, 207

closes https://github.com/official-stockfish/Stockfish/pull/5200

Bench: 1480008

diff --git a/src/search.cpp b/src/search.cpp
index 3718c37813bdc912e89492b5e05d4540f1cecab3..e4f170be61d187a796a1d1429d04ced546aa2e22 100644 (file)
--- a/src/search.cpp
+++ b/src/search.cpp
@@ -967,20 +967,22 @@ moves_loop:  // When in check, search starts here
 
             if (capture || givesCheck)
             {
+                Piece capturedPiece = pos.piece_on(move.to_sq());
+                int   captHist =
+                  thisThread->captureHistory[movedPiece][move.to_sq()][type_of(capturedPiece)];
+
                 // Futility pruning for captures (~2 Elo)
                 if (!givesCheck && lmrDepth < 7 && !ss->inCheck)
                 {
-                    Piece capturedPiece = pos.piece_on(move.to_sq());
-                    Value futilityValue =
-                      ss->staticEval + 285 + 277 * lmrDepth + PieceValue[capturedPiece]
-                      + thisThread->captureHistory[movedPiece][move.to_sq()][type_of(capturedPiece)]
-                          / 7;
+                    Value futilityValue = ss->staticEval + 285 + 277 * lmrDepth
+                                        + PieceValue[capturedPiece] + captHist / 7;
                     if (futilityValue <= alpha)
                         continue;
                 }
 
                 // SEE based pruning for captures and checks (~11 Elo)
-                if (!pos.see_ge(move, -203 * depth))
+                int seeHist = std::clamp(captHist / 32, -199 * depth, 199 * depth);
+                if (!pos.see_ge(move, -203 * depth - seeHist))
                     continue;
             }
             else