X-Git-Url: https://git.sesse.net/?p=stockfish;a=blobdiff_plain;f=src%2Fposition.cpp;h=c09a953b62e3a7612d53f03069266a2a00493d6d;hp=9fe5f893c9da5f199577abeb283fa7942d4bee6e;hb=073eed590edf992ed3aeb6c754cb0b3b394fe79d;hpb=1e586288ca4a0ae567c8d9b9b77e804f25fe6719;ds=sidebyside

diff --git a/src/position.cpp b/src/position.cpp
index 9fe5f893..c09a953b 100644
--- a/src/position.cpp
+++ b/src/position.cpp
@@ -955,102 +955,83 @@ Key Position::key_after(Move m) const {
 }
 
 
-/// Position::see() is a static exchange evaluator: It tries to estimate the
-/// material gain or loss resulting from a move.
+/// Position::see_ge (Static Exchange Evaluation Greater or Equal) tests if the
+/// SEE value of move is greater or equal to the given value. We'll use an
+/// algorithm similar to alpha-beta pruning with a null window.
 
-Value Position::see_sign(Move m) const {
+bool Position::see_ge(Move m, Value v) const {
 
   assert(is_ok(m));
 
-  // 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][moved_piece(m)] <= PieceValue[MG][piece_on(to_sq(m))])
-      return VALUE_KNOWN_WIN;
-
-  return see(m);
-}
-
-Value Position::see(Move m) const {
-
-  Square from, to;
-  Bitboard occupied, attackers, stmAttackers;
-  Value swapList[32];
-  int slIndex = 1;
-  PieceType nextVictim;
-  Color stm;
-
-  assert(is_ok(m));
-
-  from = from_sq(m);
-  to = to_sq(m);
-  swapList[0] = PieceValue[MG][piece_on(to)];
-  stm = color_of(piece_on(from));
-  occupied = pieces() ^ from;
-
-  // Castling moves are implemented as king capturing the rook so cannot
-  // be handled correctly. Simply return VALUE_ZERO that is always correct
-  // unless in the rare case the rook ends up under attack.
+  // Castling moves are implemented as king capturing the rook so cannot be
+  // handled correctly. Simply assume the SEE value is VALUE_ZERO that is always
+  // correct unless in the rare case the rook ends up under attack.
   if (type_of(m) == CASTLING)
-      return VALUE_ZERO;
+      return VALUE_ZERO >= v;
+
+  Square from = from_sq(m), to = to_sq(m);
+  PieceType nextVictim = type_of(piece_on(from));
+  Color stm = ~color_of(piece_on(from)); // First consider opponent's move
+  Value balance; // Values of the pieces taken by us minus opponent's ones
+  Bitboard occupied, stmAttackers;
 
   if (type_of(m) == ENPASSANT)
   {
-      occupied ^= to - pawn_push(stm); // Remove the captured pawn
-      swapList[0] = PieceValue[MG][PAWN];
+      occupied = SquareBB[to - pawn_push(~stm)]; // Remove the captured pawn
+      balance = PieceValue[MG][PAWN];
+  }
+  else
+  {
+      balance = PieceValue[MG][piece_on(to)];
+      occupied = 0;
   }
 
-  // Find all attackers to the destination square, with the moving piece
-  // removed, but possibly an X-ray attacker added behind it.
-  attackers = attackers_to(to, occupied) & occupied;
+  if (balance < v)
+      return false;
 
-  // If the opponent has no attackers we are finished
-  stm = ~stm;
-  stmAttackers = attackers & pieces(stm);
-  occupied ^= to; // For the case when captured piece is a pinner
+  if (nextVictim == KING)
+      return true;
+
+  balance -= PieceValue[MG][nextVictim];
+
+  if (balance >= v)
+      return true;
 
-  // Don't allow pinned pieces to attack pieces except the king as long all
-  // pinners are on their original square.
-  if (!(st->pinnersForKing[stm] & ~occupied))
-      stmAttackers &= ~st->blockersForKing[stm];
+  bool relativeStm = true; // True if the opponent is to move
+  occupied ^= pieces() ^ from ^ to;
 
-  if (!stmAttackers)
-        return swapList[0];
+  // Find all attackers to the destination square, with the moving piece removed,
+  // but possibly an X-ray attacker added behind it.
+  Bitboard attackers = attackers_to(to, occupied) & occupied;
 
-  // The destination square is defended, which makes things rather more
-  // difficult to compute. We proceed by building up a "swap list" containing
-  // the material gain or loss at each stop in a sequence of captures to the
-  // destination square, where the sides alternately capture, and always
-  // capture with the least valuable piece. After each capture, we look for
-  // new X-ray attacks from behind the capturing piece.
-  nextVictim = type_of(piece_on(from));
+  while (true)
+  {
+      stmAttackers = attackers & pieces(stm);
 
-  do {
-      assert(slIndex < 32);
+      // Don't allow pinned pieces to attack pieces except the king as long all
+      // pinners are on their original square.
+      if (!(st->pinnersForKing[stm] & ~occupied))
+          stmAttackers &= ~st->blockersForKing[stm];
 
-      // Add the new entry to the swap list
-      swapList[slIndex] = -swapList[slIndex - 1] + PieceValue[MG][nextVictim];
+      if (!stmAttackers)
+          return relativeStm;
 
       // Locate and remove the next least valuable attacker
       nextVictim = min_attacker<PAWN>(byTypeBB, to, stmAttackers, occupied, attackers);
-      stm = ~stm;
-      stmAttackers = attackers & pieces(stm);
 
-      // Don't allow pinned pieces to attack pieces except the king
-      if (   nextVictim != KING
-          && !(st->pinnersForKing[stm] & ~occupied))
-          stmAttackers &= ~st->blockersForKing[stm];
+      if (nextVictim == KING)
+          return relativeStm == bool(attackers & pieces(~stm));
 
-      ++slIndex;
+      balance += relativeStm ?  PieceValue[MG][nextVictim]
+                             : -PieceValue[MG][nextVictim];
 
-  } while (stmAttackers && (nextVictim != KING || (--slIndex, false))); // Stop before a king capture
+      relativeStm = !relativeStm;
 
-  // Having built the swap list, we negamax through it to find the best
-  // achievable score from the point of view of the side to move.
-  while (--slIndex)
-      swapList[slIndex - 1] = std::min(-swapList[slIndex], swapList[slIndex - 1]);
+      if (relativeStm == (balance >= v))
+          return relativeStm;
 
-  return swapList[0];
+      stm = ~stm;
+  }
 }