Shogi/Shogi.Domain/StandardRules.cs

using System.Numerics;

namespace Shogi.Domain
{
	internal class StandardRules
	{
		private readonly ShogiBoardState board;
		private Vector2 player1KingPosition;
		private Vector2 player2KingPosition;

		public StandardRules(ShogiBoardState board)
		{
			this.board = board;
			CacheKingPositions();
		}

		private void CacheKingPositions()
		{
			this.board.ForEachNotNull((tile, position) =>
			{
				if (tile.WhichPiece == WhichPiece.King)
				{
					if (tile.Owner == WhichPlayer.Player1)
					{
						player1KingPosition = position;
					}
					else if (tile.Owner == WhichPlayer.Player2)
					{
						player2KingPosition = position;
					}
				}
			});
		}

		/// <summary>
		/// Move a piece from a board tile to another board tile.
		/// </summary>
		/// <param name="from">The position of the piece being moved expressed in board notation.</param>
		/// <param name="to">The target position expressed in board notation.</param>
		/// <returns>A <see cref="MoveResult" /> describing the success or failure of the simulation.</returns>
		public MoveResult Move(string from, string to, bool isPromotion = false)
		{
			var fromPiece = board[from];
			if (fromPiece == null)
			{
				return new MoveResult(false, $"Tile [{from}] is empty. There is no piece to move.");
			}
			if (fromPiece.Owner != board.WhoseTurn)
			{
				return new MoveResult(false, "Not allowed to move the opponents piece");
			}
			if (IsPathable(move.From.Value, move.To) == false)
			{
				return new MoveResult(false, $"Proposed move is not part of the move-set for piece {fromPiece.WhichPiece}.");
			}

			var captured = board[to];
			if (captured != null)
			{
				if (captured.Owner == board.WhoseTurn)
				{
					return new MoveResult(false, "Capturing your own piece is not allowed.");
				}
				captured.Capture();
				board.Hand.Add(captured);
			}

			//Mutate the board.
			if (isPromotion)
			{
				var fromVector = ShogiBoardState.FromBoardNotation(from);
				var toVector = ShogiBoardState.FromBoardNotation(to);
				if (board.WhoseTurn == WhichPlayer.Player1 && (toVector.Y > 5 || fromVector.Y > 5))
				{
					fromPiece.Promote();
				}
				else if (board.WhoseTurn == WhichPlayer.Player2 && (toVector.Y < 3 || fromVector.Y < 3))
				{
					fromPiece.Promote();
				}
			}
			board[to] = fromPiece;
			board[from] = null;
			if (fromPiece.WhichPiece == WhichPiece.King)
			{
				if (fromPiece.Owner == WhichPlayer.Player1)
				{
					player1King.X = move.To.X;
					player1King.Y = move.To.Y;
				}
				else if (fromPiece.Owner == WhichPlayer.Player2)
				{
					player2King.X = move.To.X;
					player2King.Y = move.To.Y;
				}
			}
			MoveHistory.Add(move);
			return true;
		}

		/// <summary>
		/// Move a piece from the hand to the board.
		/// </summary>
		/// <param name="pieceInHand"></param>
		/// <param name="to">The target position expressed in board notation.</param>
		/// <returns>A <see cref="MoveResult" /> describing the success or failure of the simulation.</returns>
		public void Move(WhichPiece pieceInHand, string to)
		{
			var index = Hand.FindIndex(p => p.WhichPiece == move.PieceFromHand);
			if (index == -1)
			{
				Error = $"{move.PieceFromHand} does not exist in the hand.";
				return false;
			}
			if (Board[move.To] != null)
			{
				Error = $"Illegal move - attempting to capture while playing a piece from the hand.";
				return false;
			}

			switch (move.PieceFromHand!.Value)
			{
				case WhichPiece.Knight:
					{
						// Knight cannot be placed onto the farthest two ranks from the hand.
						if ((WhoseTurn == WhichPlayer.Player1 && move.To.Y > 6)
							|| (WhoseTurn == WhichPlayer.Player2 && move.To.Y < 2))
						{
							Error = $"Knight has no valid moves after placed.";
							return false;
						}
						break;
					}
				case WhichPiece.Lance:
				case WhichPiece.Pawn:
					{
						// Lance and Pawn cannot be placed onto the farthest rank from the hand.
						if ((WhoseTurn == WhichPlayer.Player1 && move.To.Y == 8)
							|| (WhoseTurn == WhichPlayer.Player2 && move.To.Y == 0))
						{
							Error = $"{move.PieceFromHand} has no valid moves after placed.";
							return false;
						}
						break;
					}
			}

			// Mutate the board.
			Board[move.To] = Hand[index];
			Hand.RemoveAt(index);
			MoveHistory.Add(move);

			return true;
		}

		private bool IsPathable(Vector2 from, Vector2 to)
		{
			var piece = Board[from];
			if (piece == null) return false;

			var isObstructed = false;
			var isPathable = pathFinder.PathTo(from, to, (other, position) =>
			{
				if (other.Owner == piece.Owner) isObstructed = true;
			});
			return !isObstructed && isPathable;
		}

		private bool EvaluateCheckAfterMove(Move move, WhichPlayer WhichPerspective)
		{
			if (WhichPerspective == InCheck) return true; // If we already know the player is in check, don't bother.

			var isCheck = false;
			var kingPosition = WhichPerspective == WhichPlayer.Player1 ? player1King : player2King;

			// Check if the move put the king in check.
			if (pathFinder.PathTo(move.To, kingPosition)) return true;

			if (move.From.HasValue)
			{
				// Get line equation from king through the now-unoccupied location.
				var direction = Vector2.Subtract(kingPosition, move.From!.Value);
				var slope = Math.Abs(direction.Y / direction.X);
				// If absolute slope is 45°, look for a bishop along the line.
				// If absolute slope is 0° or 90°, look for a rook along the line.
				// if absolute slope is 0°, look for lance along the line.
				if (float.IsInfinity(slope))
				{
					// if slope of the move is also infinity...can skip this?
					pathFinder.LinePathTo(kingPosition, direction, (piece, position) =>
					{
						if (piece.Owner != WhichPerspective)
						{
							switch (piece.WhichPiece)
							{
								case WhichPiece.Rook:
									isCheck = true;
									break;
								case WhichPiece.Lance:
									if (!piece.IsPromoted) isCheck = true;
									break;
							}
						}
					});
				}
				else if (slope == 1)
				{
					pathFinder.LinePathTo(kingPosition, direction, (piece, position) =>
					{
						if (piece.Owner != WhichPerspective && piece.WhichPiece == WhichPiece.Bishop)
						{
							isCheck = true;
						}
					});
				}
				else if (slope == 0)
				{
					pathFinder.LinePathTo(kingPosition, direction, (piece, position) =>
					{
						if (piece.Owner != WhichPerspective && piece.WhichPiece == WhichPiece.Rook)
						{
							isCheck = true;
						}
					});
				}
			}
			else
			{
				// TODO: Check for illegal move from hand. It is illegal to place from the hand such that you check-mate your opponent.
				// Go read the shogi rules to be sure this is true.
			}

			return isCheck;
		}

		private bool EvaluateCheckmate()
		{
			if (!InCheck.HasValue) return false;

			// Assume true and try to disprove.
			var isCheckmate = true;
			Board.ForEachNotNull((piece, from) => // For each piece...
			{
				// Short circuit
				if (!isCheckmate) return;

				if (piece.Owner == InCheck) // ...owned by the player in check...
				{
					// ...evaluate if any move gets the player out of check.
					pathFinder.PathEvery(from, (other, position) =>
					{
						var simulationBoard = new Shogi(this);
						var moveToTry = new Move(from, position);
						var moveSuccess = simulationBoard.TryMove(moveToTry);
						if (moveSuccess)
						{
							if (!EvaluateCheckAfterMove(moveToTry, InCheck.Value))
							{
								isCheckmate = false;
							}
						}
					});
				}
			});
			return isCheckmate;
		}
	}
}