Fix critical SEE bug

[stockfish] / src / position.cpp

diff --git a/src/position.cpp b/src/position.cpp
index 1273a62fb3d0a0e975e2d06cdbf590359d9c473d..3526a3d023a5ef38a9e214193bb15820cd30f780 100644 (file)
--- a/src/position.cpp
+++ b/src/position.cpp
@@ -59,17 +59,18 @@ Key Position::exclusion_key() const { return st->key ^ Zobrist::exclusion;}
 
 namespace {
 
-// next_attacker() is an helper function used by see() to locate the least
+// min_attacker() is an helper function used by see() to locate the least
 // valuable attacker for the side to move, remove the attacker we just found
-// from the 'occupied' bitboard and scan for new X-ray attacks behind it.
+// from the bitboards and scan for new X-ray attacks behind it.
 
 template<int Pt> FORCE_INLINE
-PieceType next_attacker(const Bitboard* bb, const Square& to, const Bitboard& stmAttackers,
-                        Bitboard& occupied, Bitboard& attackers) {
+PieceType min_attacker(const Bitboard* bb, const Square& to, const Bitboard& stmAttackers,
+                       Bitboard& occupied, Bitboard& attackers) {
 
-  if (stmAttackers & bb[Pt])
+  Bitboard b = stmAttackers & bb[Pt];
+
+  if (b)
   {
-      Bitboard b = stmAttackers & bb[Pt];
       occupied ^= b & ~(b - 1);
 
       if (Pt == PAWN || Pt == BISHOP || Pt == QUEEN)
@@ -78,13 +79,15 @@ PieceType next_attacker(const Bitboard* bb, const Square& to, const Bitboard& st
       if (Pt == ROOK || Pt == QUEEN)
           attackers |= attacks_bb<ROOK>(to, occupied) & (bb[ROOK] | bb[QUEEN]);
 
+      attackers &= occupied; // Remove the just found attacker
+
       return (PieceType)Pt;
   }
-  return next_attacker<Pt+1>(bb, to, stmAttackers, occupied, attackers);
+  return min_attacker<Pt+1>(bb, to, stmAttackers, occupied, attackers);
 }
 
 template<> FORCE_INLINE
-PieceType next_attacker<KING>(const Bitboard*, const Square&, const Bitboard&, Bitboard&, Bitboard&) {
+PieceType min_attacker<KING>(const Bitboard*, const Square&, const Bitboard&, Bitboard&, Bitboard&) {
   return KING; // No need to update bitboards, it is the last cycle
 }
 
@@ -690,9 +693,9 @@ bool Position::move_gives_check(Move m, const CheckInfo& ci) const {
       Square rfrom = to; // 'King captures the rook' notation
       Square kto = relative_square(us, rfrom > kfrom ? SQ_G1 : SQ_C1);
       Square rto = relative_square(us, rfrom > kfrom ? SQ_F1 : SQ_D1);
-      Bitboard b = (pieces() ^ kfrom ^ rfrom) | rto | kto;
 
-      return attacks_bb<ROOK>(rto, b) & ksq;
+      return   (PseudoAttacks[ROOK][rto] & ksq)
+            && (attacks_bb<ROOK>(rto, (pieces() ^ kfrom ^ rfrom) | rto | kto) & ksq);
   }
   default:
       assert(false);
@@ -1133,7 +1136,7 @@ int Position::see_sign(Move m) const {
   // Early return if SEE cannot be negative because captured piece value
   // is not less then capturing one. Note that king moves always return
   // here because king midgame value is set to 0.
-  if (PieceValue[MG][piece_on(to_sq(m))] >= PieceValue[MG][piece_moved(m)])
+  if (PieceValue[MG][piece_moved(m)] <= PieceValue[MG][piece_on(to_sq(m))])
       return 1;
 
   return see(m);
@@ -1194,26 +1197,21 @@ int Position::see(Move m, int asymmThreshold) const {
   do {
       assert(slIndex < 32);
 
+      if (captured == KING) // Stop before processing a king capture
+      {
+          swapList[slIndex++] = QueenValueMg * 16;
+          break;
+      }
+
       // Add the new entry to the swap list
       swapList[slIndex] = -swapList[slIndex - 1] + PieceValue[MG][captured];
       slIndex++;
 
-      // Locate and remove from 'occupied' the next least valuable attacker
-      captured = next_attacker<PAWN>(byTypeBB, to, stmAttackers, occupied, attackers);
-
-      attackers &= occupied; // Remove the just found attacker
+      // Locate and remove the next least valuable attacker
+      captured = min_attacker<PAWN>(byTypeBB, to, stmAttackers, occupied, attackers);
       stm = ~stm;
       stmAttackers = attackers & pieces(stm);
 
-      if (captured == KING)
-      {
-          // Stop before processing a king capture
-          if (stmAttackers)
-              swapList[slIndex++] = QueenValueMg * 16;
-
-          break;
-      }
-
   } while (stmAttackers);
 
   // If we are doing asymmetric SEE evaluation and the same side does the first