Shogi/Shogi.Domain/Shogi.cs

using Shogi.Domain.Pieces;
using System;
using System.Text;

namespace Shogi.Domain
{
	/// <summary>
	/// Facilitates Shogi board state transitions, cognisant of Shogi rules.
	/// The board is always from Player1's perspective.
	/// [0,0] is the lower-left position, [8,8] is the higher-right position
	/// </summary>
	public sealed class Shogi
	{
		private readonly ShogiBoardState boardState;
		private readonly StandardRules rules;

		public Shogi() : this(new ShogiBoardState())
		{
		}

		public Shogi(ShogiBoardState board)
		{
			this.boardState = board;
			rules = new StandardRules(this.boardState);
		}

		/// <summary>
		/// Move a piece from a board position to another board position, potentially capturing an opponents piece. Respects all rules of the game.
		/// </summary>
		/// <remarks>
		/// The strategy involves simulating a move on a throw-away board state that can be used to 
		/// validate legal vs illegal moves without having to worry about reverting board state.
		/// </remarks>
		/// <exception cref="InvalidOperationException"></exception>
		public void Move(string from, string to, bool isPromotion)
		{
			var simulationState = new ShogiBoardState(boardState);
			var simulation = new StandardRules(simulationState);
			var moveResult = simulation.Move(from, to, isPromotion);
			if (!moveResult.Success)
			{
				throw new InvalidOperationException(moveResult.Reason);
			}

			// If already in check, assert the move that resulted in check no longer results in check.
			if (boardState.InCheck == boardState.WhoseTurn
				&& simulation.IsOpposingKingThreatenedByPosition(boardState.PreviousMoveTo))
			{
				throw new InvalidOperationException("Unable to move because you are still in check.");
			}

			var otherPlayer = boardState.WhoseTurn == WhichPlayer.Player1 ? WhichPlayer.Player2 : WhichPlayer.Player1;
			if (simulation.IsPlayerInCheckAfterMove())
			{
				throw new InvalidOperationException("Illegal move. This move places you in check.");
			}

			_ = rules.Move(from, to, isPromotion);
			if (rules.IsOpponentInCheckAfterMove())
			{
				boardState.InCheck = otherPlayer;
				// TODO: evaluate checkmate.
				//if (rules.IsOpponentInCheckMate())
				//{
				//	boardState.IsCheckmate = true;
				//}
			}
			else
			{
				boardState.InCheck = null;
			}
			boardState.WhoseTurn = otherPlayer;
		}

		public void Move(WhichPiece pieceInHand, string to)
		{
			var index = boardState.ActivePlayerHand.FindIndex(p => p.WhichPiece == pieceInHand);
			if (index == -1)
			{
				throw new InvalidOperationException($"{pieceInHand} does not exist in the hand.");
			}

			if (boardState[to] != null)
			{
				throw new InvalidOperationException("Illegal placement of piece from the hand. Destination is not empty.");
			}

			var toVector = ShogiBoardState.FromBoardNotation(to);
			switch (pieceInHand)
			{
				case WhichPiece.Knight:
					{
						// Knight cannot be placed onto the farthest two ranks from the hand.
						if ((boardState.WhoseTurn == WhichPlayer.Player1 && toVector.Y > 6)
							|| (boardState.WhoseTurn == WhichPlayer.Player2 && toVector.Y < 2))
						{
							throw new InvalidOperationException("Illegal move. Knight has no valid moves after placement.");
						}
						break;
					}
				case WhichPiece.Lance:
				case WhichPiece.Pawn:
					{
						// Lance and Pawn cannot be placed onto the farthest rank from the hand.
						if ((boardState.WhoseTurn == WhichPlayer.Player1 && toVector.Y == 8)
							|| (boardState.WhoseTurn == WhichPlayer.Player2 && toVector.Y == 0))
						{
							throw new InvalidOperationException($"Illegal move. {pieceInHand} has no valid moves after placement.");
						}
						break;
					}
			}

			var tempBoard = new ShogiBoardState(boardState);
			var simulation = new StandardRules(tempBoard);
			var moveResult = simulation.Move(pieceInHand, to);
			if (!moveResult.Success)
			{
				throw new InvalidOperationException(moveResult.Reason);
			}

			var otherPlayer = tempBoard.WhoseTurn == WhichPlayer.Player1 ? WhichPlayer.Player2 : WhichPlayer.Player1;
			if (boardState.InCheck == boardState.WhoseTurn)
			{
				//if (simulation.IsPlayerInCheckAfterMove(boardState.PreviousMoveTo, toVector, boardState.WhoseTurn))
				//{
				//	throw new InvalidOperationException("Illegal move. You're still in check!");
				//}
			}

			var kingPosition = otherPlayer == WhichPlayer.Player1 ? tempBoard.Player1KingPosition : tempBoard.Player2KingPosition;
			//if (simulation.IsPlayerInCheckAfterMove(toVector, kingPosition, otherPlayer))
			//{

			//}

			//rules.Move(from, to, isPromotion);
			//if (rules.IsPlayerInCheckAfterMove(fromVector, toVector, otherPlayer))
			//{
			//	board.InCheck = otherPlayer;
			//	board.IsCheckmate = rules.EvaluateCheckmate();
			//}
			//else
			//{
			//	board.InCheck = null;
			//}
			boardState.WhoseTurn = otherPlayer;
		}

		/// <summary>
		/// Prints a ASCII representation of the board for debugging board state.
		/// </summary>
		/// <returns></returns>
		public string ToStringStateAsAscii()
		{
			var builder = new StringBuilder();
			builder.Append("                    ");
			builder.Append("Player 2(.)");
			builder.AppendLine();
			for (var rank = 8; rank >= 0; rank--)
			{
				// Horizontal line
				builder.Append("     - ");
				for (var file = 0; file < 8; file++) builder.Append("- - ");
				builder.Append("- -");

				// Print Rank ruler.
				builder.AppendLine();
				builder.Append($"{rank + 1}  ");

				// Print pieces.
				builder.Append("  |");
				for (var x = 0; x < 9; x++)
				{
					var piece = boardState[x, rank];
					if (piece == null)
					{
						builder.Append("   ");
					}
					else
					{
						builder.AppendFormat("{0}", ToAscii(piece));
					}
					builder.Append('|');
				}
				builder.AppendLine();
			}

			// Horizontal line
			builder.Append("     - ");
			for (var x = 0; x < 8; x++) builder.Append("- - ");
			builder.Append("- -");
			builder.AppendLine();
			builder.Append("                    ");
			builder.Append("Player 1");

			builder.AppendLine();
			builder.AppendLine();
			// Print File ruler.
			builder.Append("     ");
			builder.Append("  A   B   C   D   E   F   G   H   I  ");

			return builder.ToString();
		}

		/// <summary>
		/// 
		/// </summary>
		/// <param name="piece"></param>
		/// <returns>
		/// A string with three characters. 
		/// The first character indicates promotion status. 
		/// The second character indicates piece. 
		/// The third character indicates ownership.
		/// </returns>
		public static string ToAscii(Piece piece)
		{
			var builder = new StringBuilder();
			if (piece.IsPromoted) builder.Append('^');
			else builder.Append(' ');

			var name = piece.WhichPiece switch
			{
				WhichPiece.King => "K",
				WhichPiece.GoldGeneral => "G",
				WhichPiece.SilverGeneral => "S",
				WhichPiece.Bishop => "B",
				WhichPiece.Rook => "R",
				WhichPiece.Knight => "k",
				WhichPiece.Lance => "L",
				WhichPiece.Pawn => "P",
				_ => throw new ArgumentException($"Unknown value for {nameof(WhichPiece)}."),
			};
			builder.Append(name);

			if (piece.Owner == WhichPlayer.Player2) builder.Append('.');
			else builder.Append(' ');

			return builder.ToString();
		}
	}
}