Pinned aware SEE

Don't allow pinned pieces to attack the exchange-square as long all
pinners (this includes also potential ones) are on their original
square.
As soon a pinner moves to the exchange-square or get captured on it, we
fall back to standard SEE behaviour.

This correctly handles the majority of cases with absolute pins.

bench: 6883133

src/evaluate.cpp

@@ -360,7 +360,8 @@ namespace {
         if (Pt == QUEEN)
         {
             // Penalty if any relative pin or discovered attack against the queen
-            if (pos.slider_blockers(pos.pieces(Them, ROOK, BISHOP), s))
+            Bitboard pinners;
+            if (pos.slider_blockers(pos.pieces(Them, ROOK, BISHOP), s, pinners))
                 score -= WeakQueen;
         }
     }

src/position.cpp

@@ -291,8 +291,8 @@ void Position::set_castling_right(Color c, Square rfrom) {
 
 void Position::set_check_info(StateInfo* si) const {
 
-  si->blockersForKing[WHITE] = slider_blockers(pieces(BLACK), square<KING>(WHITE));
-  si->blockersForKing[BLACK] = slider_blockers(pieces(WHITE), square<KING>(BLACK));
+  si->blockersForKing[WHITE] = slider_blockers(pieces(BLACK), square<KING>(WHITE), si->pinnersForKing[WHITE]);
+  si->blockersForKing[BLACK] = slider_blockers(pieces(WHITE), square<KING>(BLACK), si->pinnersForKing[BLACK]);
 
   Square ksq = square<KING>(~sideToMove);
 
@@ -415,24 +415,25 @@ Phase Position::game_phase() const {
 }
 
 
-/// Position::slider_blockers() returns a bitboard of all the pieces (both colors) that
-/// are blocking attacks on the square 's' from 'sliders'. A piece blocks a slider
-/// if removing that piece from the board would result in a position where square 's'
-/// is attacked. For example, a king-attack blocking piece can be either a pinned or
-/// a discovered check piece, according if its color is the opposite or the same of
-/// the color of the slider.
+/// Position::slider_blockers() returns a bitboard of all the pieces (both colors)
+/// that are blocking attacks on the square 's' from 'sliders'. A piece blocks a
+/// slider if removing that piece from the board would result in a position where
+/// square 's' is attacked. For example, a king-attack blocking piece can be either
+/// a pinned or a discovered check piece, according if its color is the opposite
+/// or the same of the color of the slider. The pinners bitboard get filled with
+/// real and potential pinners.
 
-Bitboard Position::slider_blockers(Bitboard sliders, Square s) const {
+Bitboard Position::slider_blockers(Bitboard sliders, Square s, Bitboard& pinners) const {
 
-  Bitboard b, pinners, result = 0;
+  Bitboard b, p, result = 0;
 
   // Pinners are sliders that attack 's' when a pinned piece is removed
-  pinners = (  (PseudoAttacks[ROOK  ][s] & pieces(QUEEN, ROOK))
-             | (PseudoAttacks[BISHOP][s] & pieces(QUEEN, BISHOP))) & sliders;
+  pinners = p = (  (PseudoAttacks[ROOK  ][s] & pieces(QUEEN, ROOK))
+                 | (PseudoAttacks[BISHOP][s] & pieces(QUEEN, BISHOP))) & sliders;
 
-  while (pinners)
+  while (p)
   {
-      b = between_bb(s, pop_lsb(&pinners)) & pieces();
+      b = between_bb(s, pop_lsb(&p)) & pieces();
 
       if (!more_than_one(b))
           result |= b;
@@ -999,6 +1000,18 @@ Value Position::see(Move m) const {
   if (!stmAttackers)
       return swapList[0];
 
+  // Don't allow pinned pieces to attack as long all pinners (this includes also
+  // potential ones) are on their original square. When a pinner moves to the
+  // exchange-square or get captured on it, we fall back to standard SEE behaviour.
+  else if (   (stmAttackers & st->blockersForKing[stm])
+           && ((st->pinnersForKing[stm] & (occupied ^ (occupied & to))) == st->pinnersForKing[stm]))
+  {
+      // Pinned pieces can't attack so remove them from attackers
+      stmAttackers ^= (stmAttackers & st->blockersForKing[stm]);
+      if (!stmAttackers)
+          return swapList[0];
+  }
+
   // 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
@@ -1017,6 +1030,11 @@ Value Position::see(Move m) const {
       captured = min_attacker<PAWN>(byTypeBB, to, stmAttackers, occupied, attackers);
       stm = ~stm;
       stmAttackers = attackers & pieces(stm);
+      if (    stmAttackers
+          && (stmAttackers & st->blockersForKing[stm])
+          && ((st->pinnersForKing[stm] & (occupied ^ (occupied & to))) == st->pinnersForKing[stm]))
+          stmAttackers ^= (stmAttackers & st->blockersForKing[stm]);
+
       ++slIndex;
 
   } while (stmAttackers && (captured != KING || (--slIndex, false))); // Stop before a king capture

src/position.h

@@ -64,6 +64,7 @@ struct StateInfo {
   Piece      capturedPiece;
   StateInfo* previous;
   Bitboard   blockersForKing[COLOR_NB];
+  Bitboard   pinnersForKing[COLOR_NB];
   Bitboard   checkSquares[PIECE_TYPE_NB];
 };
 
@@ -121,7 +122,7 @@ public:
   Bitboard attacks_from(Piece pc, Square s) const;
   template<PieceType> Bitboard attacks_from(Square s) const;
   template<PieceType> Bitboard attacks_from(Square s, Color c) const;
-  Bitboard slider_blockers(Bitboard sliders, Square s) const;
+  Bitboard slider_blockers(Bitboard sliders, Square s, Bitboard& pinners) const;
 
   // Properties of moves
   bool legal(Move m) const;