checkpoint
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390
Shogi.Domain/ValueObjects/Rules/ShogiBoard.cs
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390
Shogi.Domain/ValueObjects/Rules/ShogiBoard.cs
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using Shogi.Domain.ValueObjects.Movement;
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using Shogi.Domain.YetToBeAssimilatedIntoDDD;
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namespace Shogi.Domain.ValueObjects;
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/// <summary>
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/// Facilitates Shogi board state transitions, cognisant of Shogi rules.
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/// The board is always from Player1's perspective.
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/// [0,0] is the lower-left position, [8,8] is the higher-right position
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/// </summary>
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public sealed class ShogiBoard(BoardState initialState)
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{
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private static readonly int[] zeroToEight = [0, 1, 2, 3, 4, 5, 6, 7, 8];
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private static readonly Vector2 BoardSize = new(9, 9);
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public BoardState BoardState { get; } = initialState;
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/// <summary>
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/// Move a piece from a board position to another board position, potentially capturing an opponents piece. Respects all rules of the game.
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/// </summary>
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/// <remarks>
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/// The strategy involves simulating a move on a throw-away board state that can be used to
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/// validate legal vs illegal moves without having to worry about reverting board state.
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/// </remarks>
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/// <exception cref="InvalidOperationException"></exception>
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public MoveResult Move(string from, string to, bool isPromotion)
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{
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// Validate the move
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var moveResult = IsMoveValid(Notation.FromBoardNotation(from), Notation.FromBoardNotation(to));
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if (!moveResult.IsSuccess)
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{
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return moveResult;
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}
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// Move is valid, but is it legal?
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// Check for correct player's turn.
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if (BoardState.WhoseTurn != BoardState[from]!.Owner)
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{
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return new MoveResult(false, "Not allowed to move the opponent's pieces.");
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}
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// Simulate the move on a throw-away state and look for "check" and "check-mate".
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var simState = new BoardState(BoardState);
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moveResult = simState.Move(from, to, isPromotion);
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if (!moveResult.IsSuccess)
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{
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return moveResult;
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}
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// Look for threats against the kings.
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InCheckResult inCheckResult = IsEitherPlayerInCheck(simState);
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var playerPutThemselfInCheck = BoardState.WhoseTurn == WhichPlayer.Player1
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? inCheckResult.HasFlag(InCheckResult.Player1InCheck)
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: inCheckResult.HasFlag(InCheckResult.Player2InCheck);
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if (playerPutThemselfInCheck)
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{
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return new MoveResult(false, "This move puts the moving player in check, which is illegal.");
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}
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var playerPutOpponentInCheck = BoardState.WhoseTurn == WhichPlayer.Player1
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? inCheckResult.HasFlag(InCheckResult.Player2InCheck)
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: inCheckResult.HasFlag(InCheckResult.Player1InCheck);
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// Move is legal; mutate the real state.
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if (playerPutOpponentInCheck)
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{
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BoardState.InCheck = BoardState.WhoseTurn == WhichPlayer.Player1
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? WhichPlayer.Player2
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: WhichPlayer.Player1;
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}
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else if (inCheckResult == InCheckResult.NobodyInCheck)
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{
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BoardState.InCheck = null;
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}
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BoardState.Move(from, to, isPromotion);
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if (BoardState.InCheck.HasValue)
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{
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var gameOverResult = EvaluateGameOver();
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BoardState.IsCheckmate = gameOverResult switch
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{
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GameOverResult.GameIsNotOver => false,
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GameOverResult.Player1Wins => true,
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GameOverResult.Player2Wins => true,
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_ => throw new InvalidOperationException("Unexpected GameOverResult value.")
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};
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}
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return new MoveResult(true);
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}
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public MoveResult Move(WhichPiece pieceInHand, string to)
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{
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if (BoardState.IsCheckmate)
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{
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return new MoveResult(false, "The game is over. A winner has been decided.");
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}
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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, $"Tried to play a piece from the hand to an occupied position.");
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}
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var toVector = Notation.FromBoardNotation(to);
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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 && toVector.Y > 6
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|| BoardState.WhoseTurn == WhichPlayer.Player2 && toVector.Y < 2)
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{
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return new MoveResult(false, "Illegal move. Knight has no valid moves after placement.");
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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 && toVector.Y == 8
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|| BoardState.WhoseTurn == WhichPlayer.Player2 && toVector.Y == 0)
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{
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return new MoveResult(false, $"Illegal move. {pieceInHand} has no valid moves after placement.");
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}
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break;
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}
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}
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if (pieceInHand == WhichPiece.Pawn)
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{
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// Pawns cannot be placed into a column with another unpromoted pawn controlled by the moving player.
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var columnAlreadyHasPawn = zeroToEight
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.Select(y => BoardState[new Vector2(toVector.X, y)])
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.Where(piece => piece?.WhichPiece == WhichPiece.Pawn)
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.Where(piece => piece?.Owner == BoardState.WhoseTurn)
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.Where(piece => piece?.IsPromoted == false)
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.Any();
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if (columnAlreadyHasPawn)
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{
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return new MoveResult(false, "A player may not have two unpromoted pawns in the same file.");
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}
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}
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var simState = new BoardState(BoardState);
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var moveResult = simState.Move(pieceInHand, to);
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if (!moveResult.IsSuccess)
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{
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return moveResult;
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}
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var inCheckResult = IsEitherPlayerInCheck(simState);
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var playerPutThemselfInCheck = BoardState.WhoseTurn == WhichPlayer.Player1
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? inCheckResult.HasFlag(InCheckResult.Player1InCheck)
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: inCheckResult.HasFlag(InCheckResult.Player2InCheck);
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if (playerPutThemselfInCheck)
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{
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return new MoveResult(false, "This move puts the moving player in check, which is illegal.");
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}
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var playerPutOpponentInCheck = BoardState.WhoseTurn == WhichPlayer.Player1
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? inCheckResult.HasFlag(InCheckResult.Player2InCheck)
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: inCheckResult.HasFlag(InCheckResult.Player1InCheck);
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// Move is legal; mutate the real state.
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if (playerPutOpponentInCheck)
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{
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BoardState.InCheck = BoardState.WhoseTurn == WhichPlayer.Player1
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? WhichPlayer.Player2
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: WhichPlayer.Player1;
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}
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else if (inCheckResult == InCheckResult.NobodyInCheck)
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{
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BoardState.InCheck = null;
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}
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BoardState.Move(pieceInHand, to);
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// A pawn, placed from the hand, cannot be the cause of checkmate.
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if (BoardState.InCheck.HasValue && pieceInHand != WhichPiece.Pawn)
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{
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var gameOverResult = EvaluateGameOver();
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BoardState.IsCheckmate = gameOverResult switch
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{
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GameOverResult.GameIsNotOver => false,
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GameOverResult.Player1Wins => true,
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GameOverResult.Player2Wins => true,
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_ => throw new InvalidOperationException("Unexpected GameOverResult value.")
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};
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}
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return new MoveResult(true);
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}
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private GameOverResult EvaluateGameOver()
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{
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if (!BoardState.InCheck.HasValue)
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{
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return GameOverResult.GameIsNotOver;
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}
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var kingInCheck = BoardState.State
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.Single(kvp => kvp.Value?.WhichPiece == WhichPiece.King && kvp.Value?.Owner == BoardState.InCheck);
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var kingInCheckVectorPosition = Notation.FromBoardNotation(kingInCheck.Key);
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var piecesOfPlayerInCheck = BoardState.State
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.Where(kvp => kvp.Value?.Owner == BoardState.InCheck)
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.Cast<KeyValuePair<string, Piece>>();
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foreach (var (notation, piece) in piecesOfPlayerInCheck)
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{
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// Get possible locations this piece could move to.
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var allPossibleMoves = GetPossiblePositionsForPiece(Notation.FromBoardNotation(notation), piece);
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// Try to make a legal move, disproving checkmate.
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foreach (var move in allPossibleMoves)
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{
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var simState = new BoardState(BoardState);
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simState.Move(notation, Notation.ToBoardNotation(move), false);
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var inCheckResult = IsEitherPlayerInCheck(simState);
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var isStillInCheck = BoardState.InCheck == WhichPlayer.Player1
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? inCheckResult.HasFlag(InCheckResult.Player1InCheck)
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: inCheckResult.HasFlag(InCheckResult.Player2InCheck);
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if (!isStillInCheck)
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{
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return GameOverResult.GameIsNotOver;
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}
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}
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}
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return BoardState.InCheck == WhichPlayer.Player1
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? GameOverResult.Player2Wins
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: GameOverResult.Player1Wins;
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Vector2[] GetPossiblePositionsForPiece(Vector2 piecePosition, Piece piece)
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{
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var paths = piece.MoveSet;
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return
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paths
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.SelectMany(path =>
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{
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var list = new List<Vector2>(10);
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var position = path.Step + piecePosition;
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if (path.Distance == Distance.MultiStep)
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{
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while (position.IsInsideBoardBoundary())
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{
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list.Add(position);
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position += path.Step;
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}
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}
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else if (position.IsInsideBoardBoundary())
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{
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list.Add(position);
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}
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return list;
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})
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// Where tile at position is empty, meaning the piece could move there.
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.Where(newPosition => BoardState[newPosition] == null)
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.ToArray();
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}
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}
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private static InCheckResult IsEitherPlayerInCheck(BoardState simState)
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{
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var kings = simState.State
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.Where(kvp => kvp.Value?.WhichPiece == WhichPiece.King)
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.Cast<KeyValuePair<string, Piece>>()
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.ToArray();
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if (kings.Length != 2) throw new InvalidOperationException("Unexpected scenario: board does not have two kings in play.");
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return simState.State
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.Where(kvp => kvp.Value != null)
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.Cast<KeyValuePair<string, Piece>>()
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.Aggregate(InCheckResult.NobodyInCheck, (inCheckResult, kvp) =>
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{
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var newInCheckResult = inCheckResult;
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var threatPiece = kvp.Value;
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var opposingKingPosition = Notation.FromBoardNotation(kings.Single(king => king.Value.Owner != threatPiece.Owner).Key);
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var positionsThreatened = threatPiece.GetPathFromStartToEnd(Notation.FromBoardNotation(kvp.Key), opposingKingPosition);
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foreach (var position in positionsThreatened)
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{
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// No piece at this position, so pathing is unobstructed. Continue pathing.
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if (simState[position] == null) continue;
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var threatenedPiece = simState[position]!;
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if (threatenedPiece.WhichPiece == WhichPiece.King && threatenedPiece.Owner != threatPiece.Owner)
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{
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newInCheckResult |= threatenedPiece.Owner == WhichPlayer.Player1 ? InCheckResult.Player1InCheck : InCheckResult.Player2InCheck;
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}
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else
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{
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break;
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}
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}
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return newInCheckResult;
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});
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}
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/// <summary>
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/// The purpose is to ensure a proposed board move is valid with regard to the moved piece's rules.
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/// This event does not worry about check or check-mate, or if a move is legal according to all Shogi rules.
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/// It asserts that a proposed move is possible and worthy of further validation (check, check-mate, etc).
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/// </summary>
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private MoveResult IsMoveValid(Vector2 from, Vector2 to)
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{
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if (IsWithinBounds(from) && IsWithinBounds(to))
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{
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if (BoardState[to]?.WhichPiece == WhichPiece.King)
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{
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return new MoveResult(false, "Kings may not be captured.");
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}
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var piece = BoardState[from];
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if (piece == null)
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{
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return new MoveResult(false, $"There is no piece at position {from}.");
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}
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var clampedFromTo = Vector2.Clamp(to - from, -Vector2.One, Vector2.One);
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var matchingPaths = piece.MoveSet.Where(p => p.Step == clampedFromTo);
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if (Vector2.Distance(to, from) < 2)
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{
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if (!matchingPaths.Any())
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{
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return new MoveResult(false, "Piece cannot move like that.");
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}
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}
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else
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{
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var multiStepPaths = matchingPaths
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.Where(path => path.Distance == Distance.MultiStep)
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.ToArray();
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if (multiStepPaths.Length == 0)
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{
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return new MoveResult(false, "Piece cannot move like that");
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}
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foreach (var path in multiStepPaths)
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{
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// Assert that no pieces exist along the from -> to path.
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var isPathObstructed = GetPositionsAlongPath(from, to, path)
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.SkipLast(1)
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.Any(pos => BoardState[pos] != null);
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if (isPathObstructed)
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{
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return new MoveResult(false, "Piece cannot move through other pieces.");
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}
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}
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}
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var pieceAtTo = BoardState[to];
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if (pieceAtTo?.Owner == piece.Owner)
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{
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return new MoveResult(false, "Cannot capture your own pieces.");
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}
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}
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return new MoveResult(true);
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}
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private static IEnumerable<Vector2> GetPositionsAlongPath(Vector2 from, Vector2 to, Path path)
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{
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var next = from;
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while (next != to && next.X >= 0 && next.X < 9 && next.Y >= 0 && next.Y < 9)
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{
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next += path.Step;
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yield return next;
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}
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}
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private static bool IsWithinBounds(Vector2 position)
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{
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var isPositive = position - position == Vector2.Zero;
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return isPositive && position.X <= BoardSize.X && position.Y <= BoardSize.Y;
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}
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}
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