Do more futility pruning in qsearch

[stockfish] / src / search.cpp

diff --git a/src/search.cpp b/src/search.cpp
index 76d0545ac50bd7f619f88db3785ee6c0f3e4c316..beb1cb54822856df2c7522a0a3f0f0f07e0d88b2 100644 (file)
--- a/src/search.cpp
+++ b/src/search.cpp
@@ -80,8 +80,9 @@ namespace {
   int Reductions[MAX_MOVES]; // [depth or moveNumber]
 
   Depth reduction(bool i, Depth d, int mn, Value delta, Value rootDelta) {
-    int r = Reductions[d] * Reductions[mn];
-    return (r + 1372 - int(delta) * 1073 / int(rootDelta)) / 1024 + (!i && r > 936);
+    int reductionScale = Reductions[d] * Reductions[mn];
+    return  (reductionScale + 1372 - int(delta) * 1073 / int(rootDelta)) / 1024
+          + (!i && reductionScale > 936);
   }
 
   constexpr int futility_move_count(bool improving, Depth depth) {
@@ -1130,7 +1131,7 @@ moves_loop: // When in check, search starts here
       // Decrease further on cutNodes. (~1 Elo)
       if (   ss->ttPv
           && !likelyFailLow)
-          r -= cutNode && tte->depth() >= depth + 3 ? 3 : 2;
+          r -= cutNode && tte->depth() >= depth ? 3 : 2;
 
       // Decrease reduction if opponent's move count is high (~1 Elo)
       if ((ss-1)->moveCount > 8)
@@ -1548,17 +1549,29 @@ moves_loop: // When in check, search starts here
 
                 futilityValue = futilityBase + PieceValue[pos.piece_on(to_sq(move))];
 
+                // If static eval + value of piece we are going to capture is much lower
+                // than alpha we can prune this move
                 if (futilityValue <= alpha)
                 {
                     bestValue = std::max(bestValue, futilityValue);
                     continue;
                 }
 
+                // If static eval is much lower than alpha and move is not winning material
+                // we can prune this move
                 if (futilityBase <= alpha && !pos.see_ge(move, VALUE_ZERO + 1))
                 {
                     bestValue = std::max(bestValue, futilityBase);
                     continue;
                 }
+
+                // If static exchange evaluation is much worse than what is needed to not
+                // fall below alpha we can prune this move
+                if (futilityBase > alpha && !pos.see_ge(move, (alpha - futilityBase) * 4))
+                {
+                    bestValue = alpha;
+                    continue;
+                }
             }
 
             // We prune after the second quiet check evasion move, where being 'in check' is