264 lines
12 KiB
C#
264 lines
12 KiB
C#
using Shogi.Domain.ValueObjects;
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using BoardTile = System.Collections.Generic.KeyValuePair<System.Numerics.Vector2, Shogi.Domain.ValueObjects.Piece>;
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namespace Shogi.Domain
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{
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internal class StandardRules
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{
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private readonly BoardState boardState;
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internal StandardRules(BoardState board)
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{
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boardState = board;
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}
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/// <summary>
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/// Move a piece from a board tile to another board tile ignorant of check or check-mate.
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/// </summary>
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/// <param name="fromNotation">The position of the piece being moved expressed in board notation.</param>
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/// <param name="toNotation">The target position expressed in board notation.</param>
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/// <param name="isPromotionRequested">True if a promotion is expected as a result of this move.</param>
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/// <returns>A <see cref="MoveResult" /> describing the success or failure of the move.</returns>
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internal MoveResult Move(string fromNotation, string toNotation, bool isPromotionRequested = false)
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{
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var from = Notation.FromBoardNotation(fromNotation);
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var to = Notation.FromBoardNotation(toNotation);
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var fromPiece = boardState[from];
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if (fromPiece == null)
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{
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return new MoveResult(false, $"Tile [{fromNotation}] is empty. There is no piece to move.");
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}
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if (fromPiece.Owner != boardState.WhoseTurn)
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{
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return new MoveResult(false, "Not allowed to move the opponents piece");
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}
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var path = fromPiece.GetPathFromStartToEnd(from, to);
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if (boardState.IsPathBlocked(path))
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{
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return new MoveResult(false, "Another piece obstructs the desired move.");
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}
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if (boardState[to] != null)
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{
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boardState.Capture(to);
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}
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if (isPromotionRequested && (boardState.IsWithinPromotionZone(to) || boardState.IsWithinPromotionZone(from)))
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{
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fromPiece.Promote();
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}
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boardState[to] = fromPiece;
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boardState[from] = null;
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boardState.PreviousMove = new Move(from, to);
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var otherPlayer = boardState.WhoseTurn == WhichPlayer.Player1 ? WhichPlayer.Player2 : WhichPlayer.Player1;
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boardState.WhoseTurn = otherPlayer;
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return new MoveResult(true);
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}
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/// Move a piece from the hand to the board ignorant if check or check-mate.
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/// </summary>
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/// <param name="pieceInHand"></param>
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/// <param name="to">The target position expressed in board notation.</param>
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/// <returns>A <see cref="MoveResult" /> describing the success or failure of the simulation.</returns>
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internal MoveResult Move(WhichPiece pieceInHand, string toNotation)
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{
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var to = Notation.FromBoardNotation(toNotation);
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var index = boardState.ActivePlayerHand.FindIndex(p => p.WhichPiece == pieceInHand);
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if (index == -1)
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{
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return new MoveResult(false, $"{pieceInHand} does not exist in the hand.");
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}
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if (boardState[to] != null)
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{
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return new MoveResult(false, "Illegal move - attempting to capture while playing a piece from the hand.");
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}
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switch (pieceInHand)
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{
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case WhichPiece.Knight:
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{
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// Knight cannot be placed onto the farthest two ranks from the hand.
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if ((boardState.WhoseTurn == WhichPlayer.Player1 && to.Y > 6)
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|| (boardState.WhoseTurn == WhichPlayer.Player2 && to.Y < 2))
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{
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return new MoveResult(false, "Knight has no valid moves after placed.");
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}
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break;
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}
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case WhichPiece.Lance:
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case WhichPiece.Pawn:
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{
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// Lance and Pawn cannot be placed onto the farthest rank from the hand.
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if ((boardState.WhoseTurn == WhichPlayer.Player1 && to.Y == 8)
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|| (boardState.WhoseTurn == WhichPlayer.Player2 && to.Y == 0))
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{
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return new MoveResult(false, $"{pieceInHand} has no valid moves after placed.");
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}
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break;
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}
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}
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// Mutate the board.
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boardState[to] = boardState.ActivePlayerHand[index];
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boardState.ActivePlayerHand.RemoveAt(index);
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boardState.PreviousMove = new Move(pieceInHand, to);
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return new MoveResult(true);
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}
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/// <summary>
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/// Determines if the last move put the player who moved in check.
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/// </summary>
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/// <remarks>
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/// This strategy recognizes that a "discover check" could only occur from a subset of pieces: Rook, Bishop, Lance.
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/// In this way, only those pieces need to be considered when evaluating if a move placed the moving player in check.
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/// </remarks>
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internal bool DidPlayerPutThemselfInCheck()
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{
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if (boardState.PreviousMove.From == null)
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{
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// You can't place yourself in check by placing a piece from your hand.
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return false;
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}
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var previousMovedPiece = boardState[boardState.PreviousMove.To];
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if (previousMovedPiece == null) throw new ArgumentNullException(nameof(previousMovedPiece), $"No piece exists at position {boardState.PreviousMove.To}.");
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var kingPosition = previousMovedPiece.Owner == WhichPlayer.Player1 ? boardState.Player1KingPosition : boardState.Player2KingPosition;
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var isDiscoverCheck = false;
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// Get line equation from king through the now-unoccupied location.
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var direction = Vector2.Subtract(kingPosition, boardState.PreviousMove.From.Value);
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var slope = Math.Abs(direction.Y / direction.X);
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var path = BoardState.GetPathAlongDirectionFromStartToEdgeOfBoard(boardState.PreviousMove.From.Value, Vector2.Normalize(direction));
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var threat = boardState.QueryFirstPieceInPath(path);
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if (threat == null || threat.Owner == previousMovedPiece.Owner) return false;
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// If absolute slope is 45°, look for a bishop along the line.
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// If absolute slope is 0° or 90°, look for a rook along the line.
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// if absolute slope is 0°, look for lance along the line.
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if (float.IsInfinity(slope))
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{
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isDiscoverCheck = threat.WhichPiece switch
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{
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WhichPiece.Lance => !threat.IsPromoted,
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WhichPiece.Rook => true,
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_ => false
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};
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}
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else if (slope == 1)
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{
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isDiscoverCheck = threat.WhichPiece switch
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{
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WhichPiece.Bishop => true,
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_ => false
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};
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}
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else if (slope == 0)
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{
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isDiscoverCheck = threat.WhichPiece switch
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{
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WhichPiece.Rook => true,
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_ => false
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};
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}
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return isDiscoverCheck;
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}
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internal bool IsOpponentInCheckAfterMove() => IsOpposingKingThreatenedByPosition(boardState.PreviousMove.To);
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internal bool IsOpposingKingThreatenedByPosition(Vector2 position)
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{
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var previousMovedPiece = boardState[position];
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if (previousMovedPiece == null) return false;
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var kingPosition = previousMovedPiece.Owner == WhichPlayer.Player1 ? boardState.Player2KingPosition : boardState.Player1KingPosition;
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var path = previousMovedPiece.GetPathFromStartToEnd(position, kingPosition);
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var threatenedPiece = boardState.QueryFirstPieceInPath(path);
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if (!path.Any() || threatenedPiece == null) return false;
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return threatenedPiece.WhichPiece == WhichPiece.King;
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}
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internal bool IsOpponentInCheckMate()
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{
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// Assume checkmate, then try to disprove.
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if (!boardState.InCheck.HasValue) return false;
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// Get all pieces from opponent who threaten the king in question.
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var opponent = boardState.WhoseTurn == WhichPlayer.Player1 ? WhichPlayer.Player2 : WhichPlayer.Player1;
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var tilesOccupiedByOpponent = boardState.GetTilesOccupiedBy(opponent);
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var kingPosition = boardState.WhoseTurn == WhichPlayer.Player1
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? boardState.Player1KingPosition
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: boardState.Player2KingPosition;
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var threats = tilesOccupiedByOpponent.Where(tile => PieceHasLineOfSight(tile, kingPosition)).ToList();
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if (threats.Count == 1)
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{
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/* If there is exactly one threat it is possible to block the check.
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* Foreach piece owned by whichPlayer
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* if piece can intercept check, return false;
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*/
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var threat = threats.Single();
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var pathFromThreatToKing = threat.Value.GetPathFromStartToEnd(threat.Key, kingPosition);
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var tilesThatCouldBlockTheThreat = boardState.GetTilesOccupiedBy(boardState.WhoseTurn);
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foreach (var threatBlockingPosition in pathFromThreatToKing)
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{
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var tilesThatDoBlockThreat = tilesThatCouldBlockTheThreat
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.Where(tile => PieceHasLineOfSight(tile, threatBlockingPosition))
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.ToList();
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if (tilesThatDoBlockThreat.Any())
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{
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return false; // Cannot be check-mate if a piece can intercept the threat.
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}
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}
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}
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else
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{
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/*
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* If no ability to block the check, maybe the king can evade check by moving.
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*/
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foreach (var maybeSafePosition in GetPossiblePositionsForKing(this.boardState.WhoseTurn))
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{
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threats = tilesOccupiedByOpponent
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.Where(tile => PieceHasLineOfSight(tile, maybeSafePosition))
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.ToList();
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if (!threats.Any())
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{
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return false;
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}
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}
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}
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return true;
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}
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private IList<Vector2> GetPossiblePositionsForKing(WhichPlayer whichPlayer)
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{
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var kingPosition = whichPlayer == WhichPlayer.Player1
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? boardState.Player1KingPosition
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: boardState.Player2KingPosition;
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return King.KingPaths
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.Select(path => path.Direction + kingPosition)
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.Where(newPosition => newPosition.IsInsideBoardBoundary())
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// Where tile at position is empty, meaning the king could move there.
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.Where(newPosition => boardState[newPosition] == null)
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.ToList();
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}
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private bool PieceHasLineOfSight(BoardTile tile, Vector2 lineOfSightTarget)
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{
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var path = tile.Value.GetPathFromStartToEnd(tile.Key, lineOfSightTarget);
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return path
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.SkipLast(1)
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.All(position => boardState[Notation.ToBoardNotation(position)] == null);
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}
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}
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}
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