using System.Numerics;
using System.Runtime.CompilerServices;

[assembly: InternalsVisibleTo("Shogi.Domain.UnitTests")]
namespace Shogi.Domain
{
	internal class StandardRules
	{
		/// <param name="element">Guaranteed to be non-null.</param>
		/// <param name="position"></param>
		public delegate void Callback(Piece collider, Vector2 position);

		private readonly ShogiBoardState board;
		private Vector2 player1KingPosition;
		private Vector2 player2KingPosition;

		public StandardRules(ShogiBoardState board)
		{
			this.board = board;
			CacheKingPositions();
		}

		private void CacheKingPositions()
		{
			this.board.ForEachNotNull((tile, position) =>
			{
				if (tile.WhichPiece == WhichPiece.King)
				{
					if (tile.Owner == WhichPlayer.Player1)
					{
						player1KingPosition = position;
					}
					else if (tile.Owner == WhichPlayer.Player2)
					{
						player2KingPosition = position;
					}
				}
			});
		}

		/// <summary>
		/// Move a piece from a board tile to another board tile.
		/// </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>
		/// <returns>A <see cref="MoveResult" /> describing the success or failure of the simulation.</returns>
		public MoveResult Move(string fromNotation, string toNotation, bool isPromotion = false)
		{
			var from = ShogiBoardState.FromBoardNotation(fromNotation);
			var to = ShogiBoardState.FromBoardNotation(toNotation);
			var fromPiece = board[from];
			if (fromPiece == null)
			{
				return new MoveResult(false, $"Tile [{fromNotation}] is empty. There is no piece to move.");
			}

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

			if (ShogiIsPathable(from, to) == false)
			{
				return new MoveResult(false, $"Proposed move is not part of the move-set for piece {fromPiece.WhichPiece}.");
			}

			var captured = board[to];
			if (captured != null)
			{
				if (captured.Owner == board.WhoseTurn)
				{
					return new MoveResult(false, "Capturing your own piece is not allowed.");
				}
				captured.Capture();
				board.Hand.Add(captured);
			}

			//Mutate the board.
			if (isPromotion)
			{
				if (board.WhoseTurn == WhichPlayer.Player1 && (to.Y > 5 || from.Y > 5))
				{
					fromPiece.Promote();
				}
				else if (board.WhoseTurn == WhichPlayer.Player2 && (to.Y < 3 || from.Y < 3))
				{
					fromPiece.Promote();
				}
			}
			board[to] = fromPiece;
			board[from] = null;
			if (fromPiece.WhichPiece == WhichPiece.King)
			{
				if (fromPiece.Owner == WhichPlayer.Player1)
				{
					player1KingPosition = from;
				}
				else if (fromPiece.Owner == WhichPlayer.Player2)
				{
					player2KingPosition = from;
				}
			}
			//MoveHistory.Add(move);
			return new MoveResult(true);
		}

		/// <summary>
		/// Move a piece from the hand to the board.
		/// </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>
		public MoveResult Move(WhichPiece pieceInHand, string toNotation)
		{
			var to = ShogiBoardState.FromBoardNotation(toNotation);
			var index = board.Hand.FindIndex(p => p.WhichPiece == pieceInHand);
			if (index == -1)
			{
				return new MoveResult(false, $"{pieceInHand} does not exist in the hand.");
			}
			if (board[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 ((board.WhoseTurn == WhichPlayer.Player1 && to.Y > 6)
							|| (board.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 ((board.WhoseTurn == WhichPlayer.Player1 && to.Y == 8)
							|| (board.WhoseTurn == WhichPlayer.Player2 && to.Y == 0))
						{
							return new MoveResult(false, $"{pieceInHand} has no valid moves after placed.");
						}
						break;
					}
			}

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

		private bool ShogiIsPathable(Vector2 from, Vector2 to)
		{
			var piece = board[from];
			if (piece == null) return false;

			var isObstructed = false;
			var isPathable = PathTo(from, to, (other, position) =>
			{
				if (other.Owner == piece.Owner) isObstructed = true;
			});
			return !isObstructed && isPathable;
		}

		public bool EvaluateCheckAfterMove(WhichPiece pieceInHand, Vector2 to, WhichPlayer whichPlayer)
		{
			if (whichPlayer == board.InCheck) return true; // If we already know the player is in check, don't bother.

			var isCheck = false;
			var kingPosition = whichPlayer == WhichPlayer.Player1 ? player1KingPosition : player2KingPosition;

			// Check if the move put the king in check.
			if (PathTo(to, kingPosition)) return true;

			// TODO: Check for illegal move from hand. It is illegal to place from the hand such that you check-mate your opponent.
			// Go read the shogi rules to be sure this is true.

			return isCheck;
		}

		public bool EvaluateCheckAfterMove(Vector2 from, Vector2 to, WhichPlayer whichPlayer)
		{
			if (whichPlayer == board.InCheck) return true; // If we already know the player is in check, don't bother.

			var isCheck = false;
			var kingPosition = whichPlayer == WhichPlayer.Player1 ? player1KingPosition : player2KingPosition;

			// Check if the move put the king in check.
			if (PathTo(to, kingPosition)) return true;

			// Get line equation from king through the now-unoccupied location.
			var direction = Vector2.Subtract(kingPosition, from);
			var slope = Math.Abs(direction.Y / direction.X);
			// 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))
			{
				// if slope of the move is also infinity...can skip this?
				LinePathTo(kingPosition, direction, (piece, position) =>
				{
					if (piece.Owner != whichPlayer)
					{
						switch (piece.WhichPiece)
						{
							case WhichPiece.Rook:
								isCheck = true;
								break;
							case WhichPiece.Lance:
								if (!piece.IsPromoted) isCheck = true;
								break;
						}
					}
				});
			}
			else if (slope == 1)
			{
				LinePathTo(kingPosition, direction, (piece, position) =>
				{
					if (piece.Owner != whichPlayer && piece.WhichPiece == WhichPiece.Bishop)
					{
						isCheck = true;
					}
				});
			}
			else if (slope == 0)
			{
				LinePathTo(kingPosition, direction, (piece, position) =>
				{
					if (piece.Owner != whichPlayer && piece.WhichPiece == WhichPiece.Rook)
					{
						isCheck = true;
					}
				});
			}

			return isCheck;
		}

		public bool EvaluateCheckmate()
		{
			if (!board.InCheck.HasValue) return false;

			// Assume true and try to disprove.
			var isCheckmate = true;
			board.ForEachNotNull((piece, from) => // For each piece...
			{
				// Short circuit
				if (!isCheckmate) return;

				if (piece.Owner == board.InCheck) // ...owned by the player in check...
				{
					// ...evaluate if any move gets the player out of check.
					PathEvery(from, (other, position) =>
					{
						var simulationBoard = new StandardRules(new ShogiBoardState(board));
						var fromNotation = ShogiBoardState.ToBoardNotation(from);
						var toNotation = ShogiBoardState.ToBoardNotation(position);
						var simulationResult = simulationBoard.Move(fromNotation, toNotation, false);
						if (simulationResult.Success)
						{
							if (!EvaluateCheckAfterMove(from, position, board.InCheck.Value))
							{
								isCheckmate = false;
							}
						}
					});
				}
				// TODO: Assert that a player could not place a piece from their hand to avoid check.
			});
			return isCheckmate;
		}

		/// <summary>
		/// Navigate the collection such that each "step" is always towards the destination, respecting the Paths available to the element at origin.
		/// </summary>
		/// <param name="element">The pathing element.</param>
		/// <param name="origin">The starting location.</param>
		/// <param name="destination">The destination.</param>
		/// <param name="callback">Do cool stuff here.</param>
		/// <returns>True if the element reached the destination.</returns>
		public bool PathTo(Vector2 origin, Vector2 destination, Callback? callback = null)
		{
			if (destination.X > 8 || destination.Y > 8 || destination.X < 0 || destination.Y < 0)
			{
				return false;
			}
			var piece = board[origin];
			if (piece == null) return false;

			var path = FindDirectionTowardsDestination(GetMoveSet(piece.WhichPiece).GetMoves(piece.IsUpsideDown), origin, destination);
			if (!IsPathable(origin, destination, path.Direction))
			{
				// Assumption: if a single best-choice step towards the destination cannot happen, no pathing can happen.
				return false;
			}

			var shouldPath = true;
			var next = origin;
			while (shouldPath && next != destination)
			{
				next = Vector2.Add(next, path.Direction);
				var collider = board[next];
				if (collider != null)
				{
					callback?.Invoke(collider, next);
					shouldPath = false;
				}
				else if (path.Distance == Distance.OneStep)
				{
					shouldPath = false;
				}
			}
			return next == destination;
		}

		public void PathEvery(Vector2 from, Callback callback)
		{
			var piece = board[from];
			if (piece == null)
			{
				return;
			}
			foreach (var path in GetMoveSet(piece.WhichPiece).GetMoves(piece.IsUpsideDown))
			{
				var shouldPath = true;
				var next = Vector2.Add(from, path.Direction); ;
				while (shouldPath && next.X < 8 && next.Y < 8 && next.X >= 0 && next.Y >= 0)
				{
					var collider = board[(int)next.Y, (int)next.X];
					if (collider != null)
					{
						callback(collider, next);
						shouldPath = false;
					}
					if (path.Distance == Distance.OneStep)
					{
						shouldPath = false;
					}
					next = Vector2.Add(next, path.Direction);
				}
			}
		}

		public static bool IsPathable(Vector2 origin, Vector2 destination, Vector2 direction)
		{
			var next = Vector2.Add(origin, direction);
			if (Vector2.Distance(next, destination) >= Vector2.Distance(origin, destination)) return false;

			var slope = (destination.Y - origin.Y) / (destination.X - origin.X);
			if (float.IsInfinity(slope))
			{
				return next.X == destination.X;
			}
			else
			{
				// b = -mx + y
				var yIntercept = -slope * origin.X + origin.Y;
				// y = mx + b
				return next.Y == slope * next.X + yIntercept;
			}
		}

		/// <summary>
		/// Path the line from origin to destination, ignoring any Paths defined by the element at origin.
		/// </summary>
		public void LinePathTo(Vector2 origin, Vector2 direction, Callback callback)
		{
			direction = Vector2.Normalize(direction);

			var next = Vector2.Add(origin, direction);
			while (next.X >= 0 && next.X < 8 && next.Y >= 0 && next.Y < 8)
			{
				var element = board[next];
				if (element != null) callback(element, next);
				next = Vector2.Add(next, direction);
			}
		}

		public static Move FindDirectionTowardsDestination(ICollection<Move> paths, Vector2 origin, Vector2 destination) =>
			paths.Aggregate((a, b) =>
			{
				var distanceA = Vector2.Distance(destination, Vector2.Add(origin, a.Direction));
				var distanceB = Vector2.Distance(destination, Vector2.Add(origin, b.Direction));
				return distanceA < distanceB ? a : b;
			});

		public static MoveSet GetMoveSet(WhichPiece whichPiece)
		{
			return whichPiece switch
			{
				WhichPiece.King => MoveSet.King,
				WhichPiece.GoldGeneral => MoveSet.GoldGeneral,
				WhichPiece.SilverGeneral => MoveSet.SilverGeneral,
				WhichPiece.Bishop => MoveSet.Bishop,
				WhichPiece.Rook => MoveSet.Rook,
				WhichPiece.Knight => MoveSet.Knight,
				WhichPiece.Lance => MoveSet.Lance,
				WhichPiece.Pawn => MoveSet.Pawn,
				_ => throw new ArgumentException($"{nameof(WhichPiece)} not recognized."),
			};
		}
	}
}