Split Shogi into ShogiBoardState and StandardRules

Shogi.Domain/Shogi.cs

@@ -0,0 +1,90 @@
+using System.Numerics;
+
+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 board;
+		private readonly StandardRules rules;
+		public string Error { get; private set; }
+
+		public Shogi(ShogiBoardState board)
+		{
+			this.board = board;
+			rules = new StandardRules(this.board);
+			Error = string.Empty;
+		}
+
+		public Shogi(IList<Move> moves) : this()
+		{
+			for (var i = 0; i < moves.Count; i++)
+			{
+				if (!Move(moves[i]))
+				{
+					// Todo: Add some smarts to know why a move was invalid. In check? Piece not found? etc.
+					throw new InvalidOperationException($"Unable to construct ShogiBoard with the given move at index {i}. {Error}");
+				}
+			}
+		}
+
+		public bool Move(Move move)
+		{
+			var moveSuccess = TryMove(move);
+
+			if (!moveSuccess)
+			{
+				return false;
+			}
+
+			var otherPlayer = WhoseTurn == WhichPlayer.Player1 ? WhichPlayer.Player2 : WhichPlayer.Player1;
+			if (EvaluateCheckAfterMove(move, otherPlayer))
+			{
+				InCheck = otherPlayer;
+				IsCheckmate = EvaluateCheckmate();
+			}
+			else
+			{
+				InCheck = null;
+			}
+			return true;
+		}
+
+		/// <summary>
+		/// Attempts a given move. Returns false if the move is illegal.
+		/// </summary>
+		private bool TryMove(Move move)
+		{
+			// Try making the move in a "throw away" board.
+			var simulator = new StandardRules(new ShogiBoardState(this.board));
+
+			var simulatedMoveResults = move.PieceFromHand.HasValue
+				? simulator.PlaceFromHand(move)
+				: simulator.PlaceFromBoard(move);
+			if (!simulatedMoveResults)
+			{
+				// Surface the error description.
+				Error = simulationBoard.Error;
+				return false;
+			}
+			// If already in check, assert the move that resulted in check no longer results in check.
+			if (InCheck == WhoseTurn)
+			{
+				if (simulationBoard.EvaluateCheckAfterMove(MoveHistory[^1], WhoseTurn))
+				{
+					// Sneakily using this.WhoseTurn instead of validationBoard.WhoseTurn;
+					return false;
+				}
+			}
+
+			// The move is valid and legal; update board state.
+			if (move.PieceFromHand.HasValue) PlaceFromHand(move);
+			else PlaceFromBoard(move);
+			return true;
+		}
+	}
+}