Shogi/Shogi.Domain/ValueObjects/StandardRules.cs

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

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

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

        /// <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 DidPlayerPutThemselfInCheck()
        {
            if (boardState.PreviousMove.From == null)
            {
                // You can't place yourself in check by placing a piece from your hand.
                return false;
            }

            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 isDiscoverCheck = false;
            // Get line equation from king through the now-unoccupied location.
            var direction = Vector2.Subtract(kingPosition, boardState.PreviousMove.From.Value);
            var slope = Math.Abs(direction.Y / direction.X);
            var path = BoardState.GetPathAlongDirectionFromStartToEdgeOfBoard(boardState.PreviousMove.From.Value, 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))
            {
                isDiscoverCheck = threat.WhichPiece switch
                {
                    WhichPiece.Lance => !threat.IsPromoted,
                    WhichPiece.Rook => true,
                    _ => false
                };
            }
            else if (slope == 1)
            {
                isDiscoverCheck = threat.WhichPiece switch
                {
                    WhichPiece.Bishop => true,
                    _ => false
                };
            }
            else if (slope == 0)
            {
                isDiscoverCheck = threat.WhichPiece switch
                {
                    WhichPiece.Rook => true,
                    _ => false
                };
            }
            return isDiscoverCheck;
        }

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

            }

            return true;
        }

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

						var paths = boardState[kingPosition]!.MoveSet;
            return paths
                .Select(path => path.Step + kingPosition)
								// Because the king could be on the edge of the board, where some of its paths do not make sense.
                .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);
        }
    }
}