using Shogi.Domain.YetToBeAssimilatedIntoDDD;
namespace Shogi.Domain.ValueObjects;

/// <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 ShogiBoard
{
	private readonly StandardRules rules;
	private static readonly Vector2 BoardSize = new Vector2(9, 9);

	public ShogiBoard(BoardState initialState)
	{
		BoardState = initialState;
		rules = new StandardRules(BoardState);
	}

	public BoardState BoardState { get; }

	/// <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 MoveResult Move(string from, string to, bool isPromotion = false)
	{
		// Validate the move
		var moveResult = IsMoveValid(Notation.FromBoardNotation(from), Notation.FromBoardNotation(to));
		if (!moveResult.IsSuccess)
		{
			return moveResult;
		}

		// Move is valid, but is it legal?
		// Check for correct player's turn.
		if (BoardState.WhoseTurn != BoardState[from]!.Owner)
		{
			return new MoveResult(false, "Not allowed to move the opponent's pieces.");
		}

		// Simulate the move on a throw-away state and look for "check" and "check-mate".
		var simState = new BoardState(BoardState);
		moveResult = simState.Move(from, to, isPromotion);
		if (!moveResult.IsSuccess)
		{
			return moveResult;
		}

		var kings = simState.State
			.Where(kvp => kvp.Value?.WhichPiece == WhichPiece.King)
			.Cast<KeyValuePair<string, Piece>>()
			.ToArray();

		if (kings.Length != 2) throw new InvalidOperationException("Unexpected scenario: board does not have two kings in play.");

		// Look for threats against the kings.
		var inCheckResult = simState.State
			.Where(kvp => kvp.Value != null)
			.Cast<KeyValuePair<string, Piece>>()
			.Aggregate(InCheckResult.NobodyInCheck, (inCheckResult, kvp) =>
			{
				var newInCheckResult = inCheckResult;
				var threatPiece = kvp.Value;
				var opposingKingPosition = Notation.FromBoardNotation(kings.Single(king => king.Value.Owner != threatPiece.Owner).Key);
				var candidatePositions = threatPiece.GetPathFromStartToEnd(Notation.FromBoardNotation(kvp.Key), opposingKingPosition);

				foreach (var position in candidatePositions)
				{
					// No piece at this position, so pathing is unobstructed. Continue pathing.
					if (simState[position] == null) continue;

					var pieceAtPosition = simState[position]!;
					if (pieceAtPosition.WhichPiece == WhichPiece.King && pieceAtPosition.Owner != threatPiece.Owner)
					{
						newInCheckResult &= pieceAtPosition.Owner == WhichPlayer.Player1 ? InCheckResult.Player2InCheck : InCheckResult.Player1InCheck;
					}
					else
					{
						break;
					}
				}

				return newInCheckResult;
			});

		var playerPutThemselfInCheck = BoardState.WhoseTurn == WhichPlayer.Player1
			? inCheckResult.HasFlag(InCheckResult.Player1InCheck)
			: inCheckResult.HasFlag(InCheckResult.Player2InCheck);

		if (playerPutThemselfInCheck)
		{
			return new MoveResult(false, "This move puts the moving player in check, which is illega.");
		}

		// Move is legal; mutate the real state.
		BoardState.Move(from, to, isPromotion);
		var playerPutOpponentInCheck = BoardState.WhoseTurn == WhichPlayer.Player1
				? inCheckResult.HasFlag(InCheckResult.Player2InCheck)
				: inCheckResult.HasFlag(InCheckResult.Player1InCheck);
		if (playerPutOpponentInCheck)
		{
			BoardState.InCheck = BoardState.WhoseTurn == WhichPlayer.Player1
				? WhichPlayer.Player2
				: WhichPlayer.Player1;
		}

		// TODO: Look for check-mate.
		return new MoveResult(true);










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

		//if (simulation.DidPlayerPutThemselfInCheck())
		//{
		//	throw new InvalidOperationException("Illegal move. This move places you in check.");
		//}

		//var otherPlayer = BoardState.WhoseTurn == WhichPlayer.Player1
		//		? WhichPlayer.Player2
		//		: WhichPlayer.Player1;
		//_ = BoardState.Move(from, to, isPromotion); // "Rules" should not be doing any data changes. "State" should do that.
		//if (rules.IsOpponentInCheckAfterMove())
		//{
		//	BoardState.InCheck = otherPlayer;
		//	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 = Notation.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 BoardState(BoardState);
		//var simulation = new StandardRules(tempBoard);
		//var moveResult = simulation.Move(pieceInHand, to);
		//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.PreviousMove.To))
		//{
		//	throw new InvalidOperationException("Unable to drop piece becauase you are still in check.");
		//}

		//if (simulation.DidPlayerPutThemselfInCheck())
		//{
		//	throw new InvalidOperationException("Illegal move. This move places you in check.");
		//}

		//// Update the non-simulation board.
		//var otherPlayer = tempBoard.WhoseTurn == WhichPlayer.Player1
		//		? WhichPlayer.Player2
		//		: WhichPlayer.Player1;
		//_ = rules.Move(pieceInHand, to);
		//if (rules.IsOpponentInCheckAfterMove())
		//{
		//	BoardState.InCheck = otherPlayer;
		//	// A pawn, placed from the hand, cannot be the cause of checkmate.
		//	if (rules.IsOpponentInCheckMate() && pieceInHand != WhichPiece.Pawn)
		//	{
		//		BoardState.IsCheckmate = true;
		//	}
		//}

		//var kingPosition = otherPlayer == WhichPlayer.Player1
		//		? tempBoard.Player1KingPosition
		//		: tempBoard.Player2KingPosition;
		//BoardState.WhoseTurn = otherPlayer;
	}

	/// <summary>
	/// The purpose is to ensure a proposed board move is valid with regard to the moved piece's rules.
	/// This event does not worry about check or check-mate, or if a move is legal according to all Shogi rules.
	/// It asserts that a proposed move is possible and worthy of further validation (check, check-mate, etc).
	/// </summary>
	private MoveResult IsMoveValid(Vector2 from, Vector2 to)
	{
		if (IsWithinBounds(from) && IsWithinBounds(to))
		{
			if (BoardState[to]?.WhichPiece == WhichPiece.King)
			{
				return new MoveResult(false, "Kings may not be captured.");
			}

			var piece = BoardState[from];
			if (piece == null)
			{
				return new MoveResult(false, $"There is no piece at position {from}.");
			}
			var matchingPaths = piece.MoveSet.Where(p => p.NormalizedStep == Vector2.Normalize(to - from));
			if (!matchingPaths.Any())
			{
				return new MoveResult(false, "Piece cannot move like that.");
			}

			var multiStepPaths = matchingPaths.Where(path => path.Distance == YetToBeAssimilatedIntoDDD.Pathing.Distance.MultiStep).ToArray();
			foreach (var path in multiStepPaths)
			{
				// Assert that no pieces exist along the from -> to path.
				var isPathObstructed = GetPositionsAlongPath(from, to, path)
																.Any(pos => BoardState[pos] != null);
				if (isPathObstructed)
				{
					return new MoveResult(false, "Piece cannot move through other pieces.");
				}

			}

			var pieceAtTo = BoardState[to];
			if (pieceAtTo?.Owner == piece.Owner)
			{
				return new MoveResult(false, "Cannot capture your own pieces.");
			}

		}
		return new MoveResult(true);
	}

	private static IEnumerable<Vector2> GetPositionsAlongPath(Vector2 from, Vector2 to, YetToBeAssimilatedIntoDDD.Pathing.Path path)
	{
		var next = from;
		while (next != to && next.X >= 0 && next.X < 9 && next.Y >= 0 && next.Y < 9)
		{
			next += path.Step;
			yield return next;
		}
	}

	private static bool IsWithinBounds(Vector2 position)
	{
		var isPositive = position - position == Vector2.Zero;
		return isPositive && position.X <= BoardSize.X && position.Y <= BoardSize.Y;
	}
}