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(9, 9);

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

	public BoardState BoardState { get; }

	private static readonly int[] zeroToEight = [0, 1, 2, 3, 4, 5, 6, 7, 8];

	/// <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)
	{
		// 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;
		}

		// Look for threats against the kings.
		InCheckResult inCheckResult = IsEitherPlayerInCheck(simState);

		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 illegal.");
		}
		var playerPutOpponentInCheck = BoardState.WhoseTurn == WhichPlayer.Player1
				? inCheckResult.HasFlag(InCheckResult.Player2InCheck)
				: inCheckResult.HasFlag(InCheckResult.Player1InCheck);

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

		return new MoveResult(true);
	}

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

		if (BoardState[to] != null)
		{
			return new MoveResult(false, $"Tried to play a piece from the hand to an occupied position.");
		}

		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)
					{
						return new MoveResult(false, "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)
					{
						return new MoveResult(false, $"Illegal move. {pieceInHand} has no valid moves after placement.");
					}
					break;
				}
		}

		if (pieceInHand == WhichPiece.Pawn)
		{
			// Pawns cannot be placed into a column with another unpromoted pawn controlled by the moving player.
			var columnAlreadyHasPawn = zeroToEight
				.Select(y => BoardState[new Vector2(toVector.X, y)])
				.Where(piece => piece?.WhichPiece == WhichPiece.Pawn)
				.Where(piece => piece?.Owner == BoardState.WhoseTurn)
				.Where(piece => piece?.IsPromoted == false)
				.Any();

			if (columnAlreadyHasPawn)
			{
				return new MoveResult(false, "A player may not have two unpromoted pawns in the same file.");
			}
		}

		var simState = new BoardState(BoardState);
		var moveResult = simState.Move(pieceInHand, to);
		if (!moveResult.IsSuccess)
		{
			return moveResult;
		}

		var inCheckResult = IsEitherPlayerInCheck(simState);
		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 illegal.");
		}
		var playerPutOpponentInCheck = BoardState.WhoseTurn == WhichPlayer.Player1
				? inCheckResult.HasFlag(InCheckResult.Player2InCheck)
				: inCheckResult.HasFlag(InCheckResult.Player1InCheck);

		// Move is legal; mutate the real state.
		BoardState.Move(pieceInHand, to);
		if (playerPutOpponentInCheck)
		{
			BoardState.InCheck = BoardState.WhoseTurn == WhichPlayer.Player1
				? WhichPlayer.Player2
				: WhichPlayer.Player1;
		}

		//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;
		//	}
		//}

		return new MoveResult(true);
	}


	public GameOverResult EvaluateGameOver()
	{
		return GameOverResult.GameIsNotOver;
	}

	private static InCheckResult IsEitherPlayerInCheck(BoardState simState)
	{
		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.");

		return 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 positionsThreatened = threatPiece.GetPathFromStartToEnd(Notation.FromBoardNotation(kvp.Key), opposingKingPosition);

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

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

						return newInCheckResult;
					});
	}

	/// <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;
	}
}