Question

I’m trying to program my own chess engine (no AI).
I know there is Chess Game Starter Kit and I watched it for start inspiration.

But what I didn’t catch is where are validated moves (here is moves validation) for my non-king pieces preventing to get myself to check?

Imagine situation:
A5 – opponents rook
A4 – my bishop
A3 – my king

I can’t move my bishop now since I would get to check.

Or how would you suggest to check this situation?

Thank you

Answer 1

For a given board position, most chess engines start by generating pseudo-legal moves only. By pseudo-legal, I mean a move will be generated even if it:

• Leaves the King in check
• Moves the King into check
• Castles across squares that are being attacked

The reason for this is performance. As many moves won’t actually be searched due to beta pruning, you save time by avoiding the full check of move validity.

For every move that is searched, you need to check that it’s really valid. This is normally done by passing the King’s colour and square (and squares next to the King for a castling move) into an IsAttacked method. If that method returns true, you know the move isn’t valid and therefore you shouldn’t include it in your search.

This is the IsAttacked method from my own C# chess engine. Bear in mind that my engine is magic bitboard-based, so the code won’t be directly applicable to the chess starter kit to which you linked. Unless you’re familiar with magic bitboards, the translation won’t be trivial.

// 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;
}

Here’s a fragment of code that generates pseudo-legal castling moves for White:

// 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;
    }
}

And here’s the code that checks whether a pseudo-legal castling move is actually legal:

// 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;
    }
}