/// <summary>
/// Returns a list of valid squares a king can move to. The list can be empty
/// </summary>
/// <param name="piece">ChessPiece to examine</param>
/// <param name="board">ChessBoard the piece exists within</param>
/// <returns>List of valid squares, or empty list</returns>
public static List <BoardSquare> GetLegalMoves_King(ChessPiece piece, ChessBoard board)
{
List <BoardSquare> moves = new List <BoardSquare>();
/* The King is special. He can move one square in any direction
* (provided it is on the board) so long as the square is empty or
* it has an opponent piece on it. However, the King can never
* move into check, even if the square or capture would be legal
* otherwise, so it requires some extra checking.
*
* Further complicating things, if the king is castling, he cannot
* move THROUGH check either (basically check those squares as if
* they were final destinations)
*
* +---+---+---+---+---+---+---+---+
* | | | | | | | | |
* +---+---+---+---+---+---+---+---+ r = enemy rook (block)
* | | | | | | p | | | T = Move or capture
* +---+---+---+---+---+---+---+---+ p = enemy pawn (block)
* | | | T | T | X | | b | | b = enemy bishop (block)
* +---+---+---+---+---+---+---+---+ X = illegal move
* | | | T | K | T | | | |
* +---+---+---+---+---+---+---+---+ +---+---+---+---+---+---+
* | | | T | T | X | | | | | K | T | | | | |
* +---+---+---+---+---+---+---+---+ +---+---+---+---+---+---+
* | | | | | | | | | | X | X | | | | r |
* +---+---+---+---+---+---+---+---+ +---+---+---+---+---+---+
* | | | | | | | | | | | | | | | |
* +---+---+---+---+---+---+---+---+ +---+---+---+---+---+---+
*/
// Cannot castle if in check
if (!IsSquareInCheck(board, piece.File, piece.Rank, piece.Color))
{
/* Castling may only be done if the king has never moved, the rook involved has never moved,
* the squares between the king and the rook involved are unoccupied, the king is not in check,
* and the king does not cross over or end on a square in which it would be in check.
*
* The ChessBoard will keep track of the castling rights when various pieces move, but it
* won't constantly update the legality of the move
*/
// Build a list of squares to check
BoardSide castlingRights = (piece.Color == PieceColor.White) ? board.WhiteCastlingRights : board.BlackCastlingRights;
BoardSide[] sidesToCheck = new BoardSide[2] {
BoardSide.King, BoardSide.Queen
};
foreach (BoardSide sideToCheck in sidesToCheck)
{
// Backrank depends on color
int kingRank = (piece.Color == PieceColor.White) ? 1 : 8;
BoardSquare[] squares = null;
// First check if we still have the right, if not, no need to persue it
if (castlingRights.HasFlag(sideToCheck))
{
squares = new BoardSquare[2];
// The target Files depend on the side of the board we're checking
// put the final target in [0]
if (sideToCheck == BoardSide.King)
{
squares[0] = new BoardSquare(new PieceFile(7), kingRank);
squares[1] = new BoardSquare(new PieceFile(6), kingRank);
}
else // Queenside
{
squares[0] = new BoardSquare(new PieceFile(3), kingRank);
squares[1] = new BoardSquare(new PieceFile(4), kingRank);
}
}
// There should be 2 and only 2 from above if we found potential targets
if (squares != null)
{
// must be empty and not in check - empty is faster so verify it first
if ((board.FindPieceAt(squares[0].File, squares[0].Rank) == null) &&
(board.FindPieceAt(squares[1].File, squares[1].Rank) == null))
{
// Now make sure neither square is in check
if (!IsSquareInCheck(board, squares[0].File, squares[0].Rank, piece.Color) &&
!IsSquareInCheck(board, squares[1].File, squares[1].Rank, piece.Color))
{
// King can still castle to this side, add the move option
moves.Add(squares[0]);
}
}
}
}
}
// Check each of the 8 squares around the king. If it's free or has
// an enemy piece, then check if it's targetable by the opponent
// (moving into check) If not, then add it to the list
CheckPieceTargets checkKingTargets = (p, fileDelta, rankDelta, b, m) =>
{
// Verify targets are reachable (not off edge)
int startCol = p.File.ToInt();
int startRow = p.Rank;
int endCol = startCol + (fileDelta);
int endRow = startRow + (rankDelta);
bool occupied; // ignored here
if (SquareIsFreeOrContainsOpponent(endCol, endRow, b, p.Color, out occupied))
{
m.Add(new BoardSquare(new PieceFile(endCol), endRow));
}
};
// Check all 8 squares around the king
checkKingTargets(piece, 0, 1, board, moves);
checkKingTargets(piece, 0, -1, board, moves);
checkKingTargets(piece, 1, 0, board, moves);
checkKingTargets(piece, -1, 0, board, moves);
checkKingTargets(piece, 1, 1, board, moves);
checkKingTargets(piece, -1, -1, board, moves);
checkKingTargets(piece, 1, -1, board, moves);
checkKingTargets(piece, -1, 1, board, moves);
// Check violations are handled by the common caller for regulatr moves
return(moves);
}
/// <summary>
/// Returns a list of valid squares a pawn can move to. The list can be empty
/// </summary>
/// <param name="piece">ChessPiece to examine</param>
/// <param name="board">ChessBoard the piece exists within</param>
/// <returns>List of valid squares, or empty list</returns>
public static List <BoardSquare> GetLegalMoves_Pawn(ChessPiece piece, ChessBoard board)
{
List <BoardSquare> moves = new List <BoardSquare>();
/* Pawns can move one space forward on any move provided the square is empty
* and 2 squares if it's the first move the pawn has ever made.
* A pawn may move diagonally if the square is occupied by an opponent piece (capture)
* or if the space behind the diagonal is occuped by an opponent pawn that
* just moved 2 spaces (en-passant)
* +---+---+---+---+---+---+---+---+
* | | | | | | | | |
* +---+---+---+---+---+---+---+---+
* | | | | | | | | |
* +---+---+---+---+---+---+---+---+
* | | | | | | | | | C = Capture only
* +---+---+---+---+---+---+---+---+ T = Move or capture
* | | | | M | | | | | M = Move Only
* +---+---+---+---+---+---+---+---+
* | | | C | M | C | | | |
* +---+---+---+---+---+---+---+---+
* | | | | P | | | | |
* +---+---+---+---+---+---+---+---+
* | | | | | | | | |
* +---+---+---+---+---+---+---+---+
*/
// One rank "forward" which depends on your color
int rank = (piece.Color == PieceColor.White) ? piece.Rank + 1 : piece.Rank - 1;
if (null == board.FindPieceAt(piece.File, rank))
{
moves.Add(new BoardSquare(piece.File, rank));
// The 2nd move is only valid of the 1st one was (i.e. you can't move through
// another piece on your first pawn move)
if (piece.Deployed == false)
{
rank += (piece.Color == PieceColor.White) ? 1 : -1;
if (null == board.FindPieceAt(piece.File, rank))
{
moves.Add(new BoardSquare(piece.File, rank));
}
}
}
// Get the en-passant target if it exists, most of the time it will not
// it only exists the move after an enemy pawn has jumped 2 spaces on
// its initial move.
BoardSquare enPassantTarget;
bool enPassantValid = board.GetEnPassantTarget(out enPassantTarget);
// Targets will ALWAYS be 1 rank away (enPassant target is behind piece captured)
rank = (piece.Color == PieceColor.White) ? piece.Rank + 1 : piece.Rank - 1;
// Lambda helper
CheckPieceTargets checkPawnTargets = (p, fileOffset, rankOffset, b, m) =>
{
int newFileIndex = p.File.ToInt() + fileOffset;
// Can't have diagonal targets on the back rank, or if we're up
// against the edge we want to check towards
if (!ChessPiece.IsOnBackRank(piece) && (newFileIndex > 0) && (newFileIndex <= 8))
{
PieceFile tFile = new PieceFile(p.File.ToInt() + fileOffset);
BoardSquare targetSquare = new BoardSquare(tFile, rank);
ChessPiece tPiece = b.FindPieceAt(tFile, rank);
// Either there is a piece of the opposing color on this square
// or the sqaure is a valid enpassant target
if (((tPiece != null) && (tPiece.Color != p.Color)) ||
((targetSquare == enPassantTarget) && enPassantValid))
{
m.Add(new BoardSquare(tFile, rank));
}
}
};
// There are 2 possible Files (L,R or Kingside, Queenside, etc)
// Diagonal left (lower file) PieceFile.ToInt() is 0-based since the
// drawing code used it first...so adjust by 1 here
checkPawnTargets(piece, -1, 0, board, moves);
checkPawnTargets(piece, 1, 0, board, moves);
return(moves);
}
/// <summary>
/// Checks if a given piece can target a specified square. It is not required
/// that the square be occupied by an enemy piece, just potentially reachable
/// </summary>
/// <param name="board">ChessBoard to check against</param>
/// <param name="piece">ChessPiece to check</param>
/// <param name="targetFile">file to target</param>
/// <param name="targetRank">rank to target</param>
/// <returns>true if the piece can target the given square</returns>
public static bool CanPieceTargetSquare(ChessBoard board, ChessPiece piece, PieceFile targetFile, int targetRank)
{
bool result = false;
if (piece.Captured == true)
{
return(false);
}
BoardSquare targetSquare = new BoardSquare(targetFile, targetRank);
List <BoardSquare> moves = new List <BoardSquare>();
switch (piece.Job)
{
case PieceClass.Pawn:
// Can't reuse the normal helper as it requires the space to be occupied
// also w/o en-passant and double moves, this can be simpler
int pawnTargetRank = (piece.Color == PieceColor.White) ? piece.Rank + 1 : piece.Rank - 1;
if (targetRank == pawnTargetRank)
{
if (((piece.File.ToInt() - 1) == targetFile.ToInt()) ||
((piece.File.ToInt() + 1) == targetFile.ToInt()))
{
result = true;
}
}
break;
case PieceClass.Knight:
moves = GetLegalMoves_Knight(piece, board);
break;
case PieceClass.Bishop:
moves = GetLegalMoves_Bishop(piece, board);
break;
case PieceClass.Rook:
moves = GetLegalMoves_Rook(piece, board);
break;
case PieceClass.Queen:
moves = GetLegalMoves_Queen(piece, board);
break;
case PieceClass.King:
// don't recurse into the normal call, also alternate method to examine
// These are pairs of offsets (-1, 0), (-1, 1),...etc so there are twice
// as many of these as squares to check
int[] offsets = new int[] { -1, 0, -1, 1, -1, -1, 1, 0, 1, 1, 1, -1, 0, 1, 0, -1 };
for (int index = 0; index < offsets.Length / 2; index++)
{
int fileOffset = offsets[index * 2];
int rankOffset = offsets[(index * 2) + 1];
// Test the validity of the square offset
if (BoardSquare.IsValid(piece.File.ToInt() + fileOffset, piece.Rank + rankOffset))
{
BoardSquare testSquare =
new BoardSquare(new PieceFile(piece.File.ToInt() + fileOffset), piece.Rank + rankOffset);
if (testSquare == targetSquare)
{
result = true;
break;
}
}
}
break;
default:
throw new ArgumentOutOfRangeException();
}
// King is special above
if (piece.Job != PieceClass.King)
{
// Check moves for the target square
foreach (BoardSquare square in moves)
{
if (square == targetSquare)
{
result = true;
break;
}
}
}
return(result);
}
/// <summary>
/// Helper to return all legal moves for the given piece.
/// </summary>
/// <param name="piece">piece to check, assumed to be valid</param>
/// <param name="board">board to check agains, also assumed valid</param>
/// <returns>List of squares the piece can legally move to</returns>
public static List <BoardSquare> GetLegalMoves(ChessPiece piece, ChessBoard board)
{
List <BoardSquare> preCheckMoves = new List <BoardSquare>();
// Get a list of legal moves ignoring check violations
switch (piece.Job)
{
case PieceClass.Pawn:
preCheckMoves = GetLegalMoves_Pawn(piece, board);
break;
case PieceClass.Knight:
preCheckMoves = GetLegalMoves_Knight(piece, board);
break;
case PieceClass.Bishop:
preCheckMoves = GetLegalMoves_Bishop(piece, board);
break;
case PieceClass.Rook:
preCheckMoves = GetLegalMoves_Rook(piece, board);
break;
case PieceClass.Queen:
preCheckMoves = GetLegalMoves_Queen(piece, board);
break;
case PieceClass.King:
preCheckMoves = GetLegalMoves_King(piece, board);
break;
default:
throw new ArgumentOutOfRangeException();
}
// Eliminate any move found that would place the King in check
List <BoardSquare> legalMoves = new List <BoardSquare>();
ChessPiece playerKing = board.GetKing(piece.Color);
foreach (BoardSquare square in preCheckMoves)
{
// Move the piece for the check - also remove any capture for the test
ChessPiece tempCapture = board.FindPieceAt(square.File, square.Rank);
piece.TempMove(square.File, square.Rank);
if (tempCapture != null)
{
tempCapture.Captured = true;
}
if (!IsSquareInCheck(board, playerKing.File, playerKing.Rank, playerKing.Color))
{
legalMoves.Add(square);
}
if (tempCapture != null)
{
tempCapture.Captured = false;
}
// reset the piece (this bypasses the ChessBoard class)
piece.ResetTempMove();
}
return(legalMoves);
}
