LibChess: Early check of legal move

One of the conditions for legal move is that the target square is not
occupied by a piece of the same color as the moving piece.

Instead of checking this for each piece separately at the end, we can
check this at the beginning and avoid more expensive checks.

Userland/Libraries/LibChess/Chess.cpp

@@ -421,6 +421,27 @@ bool Board::is_legal_no_check(const Move& move, Color color) const
         // attempted move outside of board
         return false;
 
+    // Check castling first to allow dragging king onto the rook.
+    if (piece.type == Type::King) {
+        if (color == Color::White) {
+            if ((move.to == Square("a1") || move.to == Square("c1")) && m_white_can_castle_queenside && get_piece(Square("b1")).type == Type::None && get_piece(Square("c1")).type == Type::None && get_piece(Square("d1")).type == Type::None) {
+                return true;
+            } else if ((move.to == Square("h1") || move.to == Square("g1")) && m_white_can_castle_kingside && get_piece(Square("f1")).type == Type::None && get_piece(Square("g1")).type == Type::None) {
+                return true;
+            }
+        } else {
+            if ((move.to == Square("a8") || move.to == Square("c8")) && m_black_can_castle_queenside && get_piece(Square("b8")).type == Type::None && get_piece(Square("c8")).type == Type::None && get_piece(Square("d8")).type == Type::None) {
+                return true;
+            } else if ((move.to == Square("h8") || move.to == Square("g8")) && m_black_can_castle_kingside && get_piece(Square("f8")).type == Type::None && get_piece(Square("g8")).type == Type::None) {
+                return true;
+            }
+        }
+    }
+
+    if (piece.color == get_piece(move.to).color)
+        // Attempted move to a square occupied by a piece of the same color.
+        return false;
+
     if (piece.type == Type::Pawn) {
         int dir = (color == Color::White) ? +1 : -1;
         int start_rank = (color == Color::White) ? 1 : 6;
@@ -459,7 +480,7 @@ bool Board::is_legal_no_check(const Move& move, Color color) const
     } else if (piece.type == Type::Knight) {
         int rank_delta = abs(move.to.rank - move.from.rank);
         int file_delta = abs(move.to.file - move.from.file);
-        if (get_piece(move.to).color != color && max(rank_delta, file_delta) == 2 && min(rank_delta, file_delta) == 1) {
+        if (max(rank_delta, file_delta) == 2 && min(rank_delta, file_delta) == 1) {
             return true;
         }
     } else if (piece.type == Type::Bishop) {
@@ -473,10 +494,7 @@ bool Board::is_legal_no_check(const Move& move, Color color) const
                     return false;
                 }
             }
-
-            if (get_piece(move.to).color != color) {
-                return true;
-            }
+            return true;
         }
     } else if (piece.type == Type::Rook) {
         int rank_delta = move.to.rank - move.from.rank;
@@ -489,10 +507,7 @@ bool Board::is_legal_no_check(const Move& move, Color color) const
                     return false;
                 }
             }
-
-            if (get_piece(move.to).color != color) {
-                return true;
-            }
+            return true;
         }
     } else if (piece.type == Type::Queen) {
         int rank_delta = move.to.rank - move.from.rank;
@@ -505,36 +520,13 @@ bool Board::is_legal_no_check(const Move& move, Color color) const
                     return false;
                 }
             }
-
-            if (get_piece(move.to).color != color) {
-                return true;
-            }
+            return true;
         }
     } else if (piece.type == Type::King) {
         int rank_delta = move.to.rank - move.from.rank;
         int file_delta = move.to.file - move.from.file;
         if (abs(rank_delta) <= 1 && abs(file_delta) <= 1) {
-            if (get_piece(move.to).color != color) {
-                return true;
-            }
-        }
-
-        if (color == Color::White) {
-            if ((move.to == Square("a1") || move.to == Square("c1")) && m_white_can_castle_queenside && get_piece(Square("b1")).type == Type::None && get_piece(Square("c1")).type == Type::None && get_piece(Square("d1")).type == Type::None) {
-
-                return true;
-            } else if ((move.to == Square("h1") || move.to == Square("g1")) && m_white_can_castle_kingside && get_piece(Square("f1")).type == Type::None && get_piece(Square("g1")).type == Type::None) {
-
-                return true;
-            }
-        } else {
-            if ((move.to == Square("a8") || move.to == Square("c8")) && m_black_can_castle_queenside && get_piece(Square("b8")).type == Type::None && get_piece(Square("c8")).type == Type::None && get_piece(Square("d8")).type == Type::None) {
-
-                return true;
-            } else if ((move.to == Square("h8") || move.to == Square("g8")) && m_black_can_castle_kingside && get_piece(Square("f8")).type == Type::None && get_piece(Square("g8")).type == Type::None) {
-
-                return true;
-            }
+            return true;
         }
     }