对于给定的棋盘位置，大多数国际象棋引擎仅从生成伪合法移动开始。通过伪法律，我的意思是即使它会产生一个动作：
这样做的原因是性能。由于 beta 修剪实际上不会搜索许多移动，因此您可以通过避免全面检查移动有效性来节省时间。
对于 搜索的每一步，您都需要检查它是否真的有效。这通常通过将国王的颜色和方格（以及国王旁边的方格用于易位移动）传递给 IsAttacked 方法来完成。如果该方法返回 true，则您知道该移动无效，因此不应将其包含在搜索中。
这是我自己的 C# 国际象棋引擎中的 IsAttacked 方法。请记住，我的引擎基于magic bitboard，因此代码不会直接适用于您链接到的国际象棋初学者工具包。除非您熟悉魔术位板，否则翻译不会是微不足道的。
// IsAttacked is primarily used as a move legality test to see if a set of
// one or more squares is under attack from the side to move.
// It returns true as soon as an attack is detected, otherwise returns false.
// It can be used for check detection, castling legality, or simply to
// detect whether a specific square is attacked.
internal bool IsAttacked(Board board, UInt64 targetSquares, bool whiteAttacking)
{
UInt64 slidingAttackers; Int32 targetSquare;
UInt64 remainingTargetSquares = targetSquares;
// Test for attacks by WHITE on any of the target squares.
if (whiteAttacking)
{
// For the remaining target squares...
while (remainingTargetSquares != 0)
{
// Find the next square in the list.
targetSquare = BitOperations.BitScanForward(remainingTargetSquares);
// Is this square attacked by a pawn, knight, or king?
if ((board.WhitePawns & Constants.BLACK_PAWN_ATTACKS[targetSquare]) != 0) return true;
if ((board.WhiteKnights & Constants.KNIGHT_ATTACKS[targetSquare]) != 0) return true;
if ((board.WhiteKing & Constants.KING_ATTACKS[targetSquare]) != 0) return true;
// Is this square attacked by a queen or rook along a file or rank?
slidingAttackers = board.WhiteQueens | board.WhiteRooks;
if (slidingAttackers != 0)
{
if (this.RankMoves(board.OccupiedSquares, slidingAttackers, targetSquare) != 0) return true;
if (this.FileMoves(board.OccupiedSquares, slidingAttackers, targetSquare) != 0) return true;
}
// Is this square attacked by a queen or bishop along a diagonal?
slidingAttackers = board.WhiteQueens | board.WhiteBishops;
if (slidingAttackers != 0)
{
if (this.DiagonalA8H1Moves(board.OccupiedSquares, slidingAttackers, targetSquare) != 0) return true;
if (this.DiagonalA1H8Moves(board.OccupiedSquares, slidingAttackers, targetSquare) != 0) return true;
}
// This square isn't attacked - remove and move on to next target square.
remainingTargetSquares ^= Constants.BITSET[targetSquare];
}
}
// Test for attacks by BLACK on any of the target squares.
else
{
// For the remaining target squares...
while (remainingTargetSquares != 0)
{
// Find the next square in the list.
targetSquare = BitOperations.BitScanForward(remainingTargetSquares);
// Is this square attacked by a pawn, knight, or king?
if ((board.BlackPawns & Constants.WHITE_PAWN_ATTACKS[targetSquare]) != 0) return true;
if ((board.BlackKnights & Constants.KNIGHT_ATTACKS[targetSquare]) != 0) return true;
if ((board.BlackKing & Constants.KING_ATTACKS[targetSquare]) != 0) return true;
// Is this square attacked by a queen or rook along a file or rank?
slidingAttackers = board.BlackQueens | board.BlackRooks;
if (slidingAttackers != 0)
{
if (this.RankMoves(board.OccupiedSquares, slidingAttackers, targetSquare) != 0) return true;
if (this.FileMoves(board.OccupiedSquares, slidingAttackers, targetSquare) != 0) return true;
}
// Is this square attacked by a queen or bishop along a diagonal?
slidingAttackers = board.BlackQueens | board.BlackBishops;
if (slidingAttackers != 0)
{
if (this.DiagonalA8H1Moves(board.OccupiedSquares, slidingAttackers, targetSquare) != 0) return true;
if (this.DiagonalA1H8Moves(board.OccupiedSquares, slidingAttackers, targetSquare) != 0) return true;
}
// This square isn't attacked - remove and move on to next target square.
remainingTargetSquares ^= Constants.BITSET[targetSquare];
}
}
// None of the target squares are attacked.
return false;
}
这是为怀特生成伪合法易位动作的代码片段：
// If White can still castle kingside...
if ((board.WhiteCastlingStatus & Board.EnumCastlingStatus.CanCastleOO) != 0)
{
// And the White kingside castling squares (F1/G1) aren't occupied...
if ((Constants.MASK_FG[Constants.WHITE_MOVE] & board.OccupiedSquares) == 0)
{
board.MoveBuffer[moveIndex++] = Constants.WHITE_CASTLING_OO;
}
}
// If White can still castle queenside...
if ((board.WhiteCastlingStatus & Board.EnumCastlingStatus.CanCastleOOO) != 0)
{
// And the White queenside castling squares (D1/C1/B1) aren't occupied...
if ((Constants.MASK_BD[Constants.WHITE_MOVE] & board.OccupiedSquares) == 0)
{
board.MoveBuffer[moveIndex++] = Constants.WHITE_CASTLING_OOO;
}
}
下面是检查伪合法易位移动是否真正合法的代码：
// Checks whether the King is moving from or into check.
// Checks whether the King is moving across attacked squares.
internal bool IsCastlingMoveLegal(Board board, Move move)
{
if (move.IsCastlingOO)
{
if (move.IsWhiteMove)
{
// Are any of the White kingside castling squares (E1/F1/G1) attacked?
return !this.IsAttacked(board, Constants.MASK_EG[Constants.WHITE_MOVE], false);
}
else
{
// Are any of the Black kingside castling squares (E8/F8/G8) attacked?
return !this.IsAttacked(board, Constants.MASK_EG[Constants.BLACK_MOVE], true);
}
}
else if (move.IsCastlingOOO)
{
if (move.IsWhiteMove)
{
// Are any of the White queenside castling squares (E1/D1/C1) attacked?
return !this.IsAttacked(board, Constants.MASK_CE[Constants.WHITE_MOVE], false);
}
else
{
// Are any of the Black queenside castling squares (E8/D8/C8) attacked?
return !this.IsAttacked(board, Constants.MASK_CE[Constants.BLACK_MOVE], true);
}
}
// Not a castling move!
else
{
Debug.Assert(false, "Not a castling move!");
return true;
}
}