using Shogi.Domain.ValueObjects;
using BoardTile = System.Collections.Generic.KeyValuePair<System.Numerics.Vector2, Shogi.Domain.ValueObjects.Piece>;

namespace Shogi.Domain
{
	internal class StandardRules
	{
		private readonly BoardState boardState;

		internal StandardRules(BoardState board)
		{
			boardState = board;
		}

		/// <summary>
		/// Move a piece from a board tile to another board tile ignorant of check or check-mate.
		/// </summary>
		/// <param name="fromNotation">The position of the piece being moved expressed in board notation.</param>
		/// <param name="toNotation">The target position expressed in board notation.</param>
		/// <param name="isPromotionRequested">True if a promotion is expected as a result of this move.</param>
		/// <returns>A <see cref="MoveResult" /> describing the success or failure of the move.</returns>
		internal MoveResult Move(string fromNotation, string toNotation, bool isPromotionRequested = false)
		{
			var from = Notation.FromBoardNotation(fromNotation);
			var to = Notation.FromBoardNotation(toNotation);
			var fromPiece = boardState[from];
			if (fromPiece == null)
			{
				return new MoveResult(false, $"Tile [{fromNotation}] is empty. There is no piece to move.");
			}

			if (fromPiece.Owner != boardState.WhoseTurn)
			{
				return new MoveResult(false, "Not allowed to move the opponents piece");
			}

			var path = fromPiece.GetPathFromStartToEnd(from, to);

			if (boardState.IsPathBlocked(path))
			{
				return new MoveResult(false, "Another piece obstructs the desired move.");
			}

			if (boardState[to] != null)
			{
				boardState.Capture(to);
			}

			if (isPromotionRequested && (boardState.IsWithinPromotionZone(to) || boardState.IsWithinPromotionZone(from)))
			{
				fromPiece.Promote();
			}

			boardState[to] = fromPiece;
			boardState[from] = null;

			boardState.PreviousMove = new Move(from, to);
			var otherPlayer = boardState.WhoseTurn == WhichPlayer.Player1 ? WhichPlayer.Player2 : WhichPlayer.Player1;
			boardState.WhoseTurn = otherPlayer;

			return new MoveResult(true);
		}

		/// Move a piece from the hand to the board ignorant if check or check-mate.
		/// </summary>
		/// <param name="pieceInHand"></param>
		/// <param name="to">The target position expressed in board notation.</param>
		/// <returns>A <see cref="MoveResult" /> describing the success or failure of the simulation.</returns>
		internal MoveResult Move(WhichPiece pieceInHand, string toNotation)
		{
			var to = Notation.FromBoardNotation(toNotation);
			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, $"Illegal move - attempting to capture while playing a piece from the hand.");
			}

			switch (pieceInHand)
			{
				case WhichPiece.Knight:
					{
						// Knight cannot be placed onto the farthest two ranks from the hand.
						if ((boardState.WhoseTurn == WhichPlayer.Player1 && to.Y > 6)
							|| (boardState.WhoseTurn == WhichPlayer.Player2 && to.Y < 2))
						{
							return new MoveResult(false, "Knight has no valid moves after placed.");
						}
						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 && to.Y == 8)
							|| (boardState.WhoseTurn == WhichPlayer.Player2 && to.Y == 0))
						{
							return new MoveResult(false, $"{pieceInHand} has no valid moves after placed.");
						}
						break;
					}
			}

			// Mutate the board.
			boardState[to] = boardState.ActivePlayerHand[index];
			boardState.ActivePlayerHand.RemoveAt(index);
			//MoveHistory.Add(move);
			return new MoveResult(true);
		}

		/// <summary>
		/// Determines if the last move put the player who moved in check.
		/// </summary>
		/// <remarks>
		/// This strategy recognizes that a "discover check" could only occur from a subset of pieces: Rook, Bishop, Lance.
		/// In this way, only those pieces need to be considered when evaluating if a move placed the moving player in check.
		/// </remarks>
		internal bool IsPlayerInCheckAfterMove()
		{
			var previousMovedPiece = boardState[boardState.PreviousMove.To];
			if (previousMovedPiece == null) throw new ArgumentNullException(nameof(previousMovedPiece), $"No piece exists at position {boardState.PreviousMove.To}.");
			var kingPosition = previousMovedPiece.Owner == WhichPlayer.Player1 ? boardState.Player1KingPosition : boardState.Player2KingPosition;

			var isCheck = false;

			// Get line equation from king through the now-unoccupied location.
			var direction = Vector2.Subtract(kingPosition, boardState.PreviousMove.From);
			var slope = Math.Abs(direction.Y / direction.X);
			var path = BoardState.GetPathAlongDirectionFromStartToEdgeOfBoard(boardState.PreviousMove.From, Vector2.Normalize(direction));
			var threat = boardState.QueryFirstPieceInPath(path);
			if (threat == null || threat.Owner == previousMovedPiece.Owner) return false;
			// If absolute slope is 45°, look for a bishop along the line.
			// If absolute slope is 0° or 90°, look for a rook along the line.
			// if absolute slope is 0°, look for lance along the line.
			if (float.IsInfinity(slope))
			{
				isCheck = threat.WhichPiece switch
				{
					WhichPiece.Lance => !threat.IsPromoted,
					WhichPiece.Rook => true,
					_ => false
				};
			}
			else if (slope == 1)
			{
				isCheck = threat.WhichPiece switch
				{
					WhichPiece.Bishop => true,
					_ => false
				};
			}
			else if (slope == 0)
			{
				isCheck = threat.WhichPiece switch
				{
					WhichPiece.Rook => true,
					_ => false
				};
			}

			return isCheck;
		}

		internal bool IsOpponentInCheckAfterMove() => IsOpposingKingThreatenedByPosition(boardState.PreviousMove.To);

		internal bool IsOpposingKingThreatenedByPosition(Vector2 position)
		{
			var previousMovedPiece = boardState[position];
			if (previousMovedPiece == null) return false;

			var kingPosition = previousMovedPiece.Owner == WhichPlayer.Player1 ? boardState.Player2KingPosition : boardState.Player1KingPosition;
			var path = previousMovedPiece.GetPathFromStartToEnd(position, kingPosition);
			var threatenedPiece = boardState.QueryFirstPieceInPath(path);
			if (!path.Any() || threatenedPiece == null) return false;

			return threatenedPiece.WhichPiece == WhichPiece.King;
		}

		internal bool IsOpponentInCheckMate()
		{
			// Assume checkmate, then try to disprove.
			if (!boardState.InCheck.HasValue) return false;
			// Get all pieces from opponent who threaten the king in question.
			var opponent = boardState.WhoseTurn == WhichPlayer.Player1 ? WhichPlayer.Player2 : WhichPlayer.Player1;
			var tilesOccupiedByOpponent = boardState.GetTilesOccupiedBy(opponent);
			var kingPosition = boardState.WhoseTurn == WhichPlayer.Player1
					? boardState.Player1KingPosition
					: boardState.Player2KingPosition;
			var threats = tilesOccupiedByOpponent.Where(tile => PieceHasLineOfSight(tile, kingPosition)).ToList();
			if (threats.Count == 1)
			{
				/* If there is exactly one threat it is possible to block the check.
				 * Foreach piece owned by whichPlayer
				 *	if piece can intercept check, return false;
				 */
				var threat = threats.Single();
				var pathFromThreatToKing = threat.Value.GetPathFromStartToEnd(threat.Key, kingPosition);
				var tilesThatCouldBlockTheThreat = boardState.GetTilesOccupiedBy(boardState.WhoseTurn);
				foreach (var threatBlockingPosition in pathFromThreatToKing)
				{
					var tilesThatDoBlockThreat = tilesThatCouldBlockTheThreat
						.Where(tile => PieceHasLineOfSight(tile, threatBlockingPosition))
						.ToList();

					if (tilesThatDoBlockThreat.Any())
					{
						return false; // Cannot be check-mate if a piece can intercept the threat.
					}
				}
			}
			else
			{
				/*
				 * If no ability to block the check, maybe the king can evade check by moving.
				*/

				foreach (var maybeSafePosition in GetPossiblePositionsForKing(this.boardState.WhoseTurn))
				{
					threats = tilesOccupiedByOpponent
						.Where(tile => PieceHasLineOfSight(tile, maybeSafePosition))
						.ToList();
					if (!threats.Any())
					{
						return false;
					}
				}

			}

			return true;
		}

		private IList<Vector2> GetPossiblePositionsForKing(WhichPlayer whichPlayer)
		{
			var kingPosition = whichPlayer == WhichPlayer.Player1
				? boardState.Player1KingPosition
				: boardState.Player2KingPosition;

			return King.KingPaths
				.Select(path => path.Direction + kingPosition)
				.Where(newPosition => newPosition.IsInsideBoardBoundary())
				// Where tile at position is empty, meaning the king could move there.
				.Where(newPosition => boardState[newPosition] == null)
				.ToList();
		}

		private bool PieceHasLineOfSight(BoardTile tile, Vector2 lineOfSightTarget)
		{
			var path = tile.Value.GetPathFromStartToEnd(tile.Key, lineOfSightTarget);
			return path
				.SkipLast(1)
				.All(position => boardState[Notation.ToBoardNotation(position)] == null);
		}
	}
}