Reintroduce SEE verification against discovered attacks

[stockfish] / src / search.cpp

diff --git a/src/search.cpp b/src/search.cpp
index 593fdc722e0b13a2a1c3d3b8c6087e9b5e613612..b2c2344ac0aaff10a249bfb7f64a22fa0e648d6f 100644 (file)
--- a/src/search.cpp
+++ b/src/search.cpp
@@ -990,9 +990,28 @@ moves_loop: // When in check, search starts here
                    + captureHistory[movedPiece][to_sq(move)][type_of(pos.piece_on(to_sq(move)))] / 7 < alpha)
                   continue;
 
+              Bitboard occupied;
               // SEE based pruning (~11 Elo)
-              if (!pos.see_ge(move, Value(-205) * depth))
-                      continue;
+              if (!pos.see_ge(move, occupied, Value(-205) * depth))
+              {
+                 if (depth < 2 - capture)
+                    continue;
+                 // Don't prune the move if opponent Queen/Rook is under discovered attack after the exchanges
+                 // Don't prune the move if opponent King is under discovered attack after or during the exchanges
+                 Bitboard leftEnemies = (pos.pieces(~us, KING, QUEEN, ROOK)) & occupied;
+                 Bitboard attacks = 0;
+                 occupied |= to_sq(move);
+                 while (leftEnemies && !attacks)
+                 {
+                      Square sq = pop_lsb(leftEnemies);
+                      attacks |= pos.attackers_to(sq, occupied) & pos.pieces(us) & occupied;
+                      // don't consider pieces which were already threatened/hanging before SEE exchanges
+                      if (attacks && (sq != pos.square<KING>(~us) && (pos.attackers_to(sq, pos.pieces()) & pos.pieces(us))))
+                         attacks = 0;
+                 }
+                 if (!attacks)
+                    continue;
+              }
           }
           else
           {