Micro-optmize castling moves

Pre compute castle path so to quickly test
for impeded rule.

This speeds up perft on starting position
of more than 2%.

No functional change

Signed-off-by: Marco Costalba <mcostalba@gmail.com>

src/movegen.cpp

@@ -40,7 +40,7 @@ namespace {
     const CastleRight CR[] = { Side ? WHITE_OOO : WHITE_OO,
                                Side ? BLACK_OOO : BLACK_OO };
 
-    if (!pos.can_castle(CR[us]))
+    if (pos.castle_impeded(CR[us]) || !pos.can_castle(CR[us]))
         return mlist;
 
     // After castling, the rook and king final positions are the same in Chess960
@@ -48,24 +48,15 @@ namespace {
     Square kfrom = pos.king_square(us);
     Square rfrom = pos.castle_rook_square(CR[us]);
     Square kto = relative_square(us, Side == KING_SIDE ? SQ_G1 : SQ_C1);
-    Square rto = relative_square(us, Side == KING_SIDE ? SQ_F1 : SQ_D1);
     Bitboard enemies = pos.pieces(~us);
 
     assert(!pos.in_check());
     assert(pos.piece_on(kfrom) == make_piece(us, KING));
     assert(pos.piece_on(rfrom) == make_piece(us, ROOK));
 
-    // Unimpeded rule: All the squares between the king's initial and final squares
-    // (including the final square), and all the squares between the rook's initial
-    // and final squares (including the final square), must be vacant except for
-    // the king and castling rook.
-    for (Square s = std::min(rfrom, rto), e = std::max(rfrom, rto); s <= e; s++)
-        if (s != kfrom && s != rfrom && !pos.square_is_empty(s))
-            return mlist;
-
     for (Square s = std::min(kfrom, kto), e = std::max(kfrom, kto); s <= e; s++)
-        if (  (s != kfrom && s != rfrom && !pos.square_is_empty(s))
+        if (    s != kfrom // We are not in check
-            ||(pos.attackers_to(s) & enemies))
+            && (pos.attackers_to(s) & enemies))
             return mlist;
 
     // Because we generate only legal castling moves we need to verify that

src/position.cpp

@@ -242,14 +242,27 @@ void Position::from_fen(const string& fenStr, bool isChess960) {
 /// Position::set_castle_right() is an helper function used to set castling
 /// rights given the corresponding color and the rook starting square.
 
-void Position::set_castle_right(Color c, Square rsq) {
+void Position::set_castle_right(Color c, Square rfrom) {
 
-  int f = (rsq < king_square(c) ? WHITE_OOO : WHITE_OO) << c;
+  Square kfrom = king_square(c);
+  bool kingSide = kfrom < rfrom;
+  int cr = (kingSide ? WHITE_OO : WHITE_OOO) << c;
 
-  st->castleRights |= f;
+  st->castleRights |= cr;
-  castleRightsMask[king_square(c)] |= f;
+  castleRightsMask[kfrom] |= cr;
-  castleRightsMask[rsq] |= f;
+  castleRightsMask[rfrom] |= cr;
-  castleRookSquare[f] = rsq;
+  castleRookSquare[cr] = rfrom;
+
+  Square kto = relative_square(c, kingSide ? SQ_G1 : SQ_C1);
+  Square rto = relative_square(c, kingSide ? SQ_F1 : SQ_D1);
+
+  for (Square s = std::min(rfrom, rto); s <= std::max(rfrom, rto); s++)
+      if (s != kfrom && s != rfrom)
+          castlePath[cr] |= s;
+
+  for (Square s = std::min(kfrom, kto); s <= std::max(kfrom, kto); s++)
+      if (s != kfrom && s != rfrom)
+          castlePath[cr] |= s;
 }
 
 

src/position.h

@@ -127,6 +127,7 @@ public:
   // Castling rights
   bool can_castle(CastleRight f) const;
   bool can_castle(Color c) const;
+  bool castle_impeded(CastleRight f) const;
   Square castle_rook_square(CastleRight f) const;
 
   // Bitboards for pinned pieces and discovered check candidates
@@ -207,7 +208,7 @@ private:
   // Initialization helper functions (used while setting up a position)
   void clear();
   void put_piece(Piece p, Square s);
-  void set_castle_right(Color c, Square rsq);
+  void set_castle_right(Color c, Square rfrom);
   bool move_is_legal(const Move m) const;
 
   // Helper template functions
@@ -242,6 +243,7 @@ private:
   // Other info
   int castleRightsMask[64];    // [square]
   Square castleRookSquare[16]; // [castleRight]
+  Bitboard castlePath[16];     // [castleRight]
   StateInfo startState;
   int64_t nodes;
   int startPosPly;
@@ -331,6 +333,10 @@ inline bool Position::can_castle(Color c) const {
   return st->castleRights & ((WHITE_OO | WHITE_OOO) << c);
 }
 
+inline bool Position::castle_impeded(CastleRight f) const {
+  return occupied & castlePath[f];
+}
+
 inline Square Position::castle_rook_square(CastleRight f) const {
   return castleRookSquare[f];
 }