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;
		}
	}
}