clean-tictactoe/TicTacToe/Grid.cs

using System;
using System.Text;

namespace TicTacToe {
    public class Grid {
        public const int GRIDSIZE = 3;
        private const Player GameBeginner = Player.Cross;
        private Player NextGameBeginner = GameBeginner;
        private readonly Player[,] _field = new Player[GRIDSIZE, GRIDSIZE];
        private Player? winner;
        private ((int x, int y) p1, (int x, int y) p2)? winningLine;
        public event EventHandler<Player> ExpectNextTurn;
        public event EventHandler<Player> GameOver;
        public Player this[int x, int y] { get => _field[x, y]; }
        public bool IsGameOver { get; private set; } = false;
        public Player ActivePlayer { get; private set; } = GameBeginner;
        public Player Winner { get => winner.Value; private set => winner = value; }
        public ((int x, int y) p1, (int x, int y) p2) WinningLine { get => winningLine.Value; private set => winningLine = value; }
        public Grid() {
            Reset();
        }
        /// <summary>
        /// Checks if a given field is empty.
        /// </summary>
        /// <param name="x">Column index</param>
        /// <param name="y">Row index</param>
        /// <returns>True if the field is empty</returns>
        public bool IsEmpty(int x, int y) => _field[x, y] == Player.Empty;

        public void Reset() {
            ActivePlayer = NextGameBeginner;
            NextGameBeginner = EnemyOf(NextGameBeginner);
            IsGameOver = false;
            winner = null;
            winningLine = null;
            for (var y = 0; y < GRIDSIZE; y++)
                for (var x = 0; x < GRIDSIZE; x++)
                    _field[x, y] = Player.Empty;
            ExpectNextTurn?.Invoke(this, ActivePlayer);
        }

        /// <summary>
        /// Checks if a given field is not empty.
        /// </summary>
        /// <param name="x">Column index</param>
        /// <param name="y">Row index</param>
        /// <returns>True if the field is set</returns>
        public bool IsSet(int x, int y) => _field[x, y] != Player.Empty;

        public void PickSpot(int x, int y) {
            if (IsGameOver)
                throw new InvalidOperationException("Cannot pick spot on finished game!");
            if (!IsEmpty(x, y))
                throw new InvalidOperationException("Cannot pick already picked spot!");
            _field[x, y] = ActivePlayer;
            TurnOver();
            if (!IsGameOver)
                ExpectNextTurn?.Invoke(this, ActivePlayer);
        }

        /// <summary>
        /// Tests if the game is over and updates the properties.
        /// </summary>
        /// <see cref="IsGameOver"/>
        /// <see cref="Winner"/>
        private void TurnOver() {
            if (CheckRows() ||
                CheckColumns() ||
                CheckDiagonalPrimary() ||
                CheckDiagonalSecondary() ||
                CheckDraw()) { // Important to check after all other cases as there might be a full field with a winner
                IsGameOver = true;
                GameOver?.Invoke(this, Winner);
            } else {
                IsGameOver = false;
                ActivePlayer = EnemyOf(ActivePlayer);
            }
        }

        private static Player EnemyOf(Player player) {
            switch (player) {
            case Player.Cross:
                return Player.Circle;
            case Player.Circle:
                return Player.Cross;
            default:
                throw new Exception("Unexpected enumerable state!");
            }
        }

        #region GameOverTests
        /// <summary>
        /// Checks if a field is full and there is no winner
        /// </summary>
        /// <returns></returns>
        private bool CheckDraw() {
            for (var y = 0; y < GRIDSIZE; y++) {
                for (var x = 0; x < GRIDSIZE; x++) {
                    if (IsEmpty(x, y))
                        return false;
                }
            }
            Winner = Player.Empty;
            return true;
        }
        /// <summary>
        /// Checks primary diagonal on a win.
        /// </summary>
        /// <returns>True if a win was found</returns>
        private bool CheckDiagonalPrimary() {
            var circlecount = 0;
            var crosscount = 0;
            for (var x = 0; x < GRIDSIZE; x++) {
                switch (_field[x, x]) {
                case Player.Circle:
                    circlecount++;
                    break;
                case Player.Cross:
                    crosscount++;
                    break;
                case Player.Empty:
                    return false;
                default:
                    throw new InvalidProgramException("Unexpected enumerable state!");
                }
            }
            if (crosscount == GRIDSIZE) {
                Winner = Player.Cross;
                WinningLine = ((0, 0), (GRIDSIZE, GRIDSIZE));
                return true;
            }
            if (circlecount == GRIDSIZE) {
                Winner = Player.Circle;
                WinningLine = ((0, 0), (GRIDSIZE, GRIDSIZE));
                return true;
            }
            return false;
        }
        /// <summary>
        /// Checks secondary diagonal on a win.
        /// </summary>
        /// <returns>True if a win was found</returns>
        private bool CheckDiagonalSecondary() {
            var circlecount = 0;
            var crosscount = 0;
            for (var x = 0; x < GRIDSIZE; x++) {
                switch (_field[x, GRIDSIZE - 1 - x]) {
                case Player.Circle:
                    circlecount++;
                    break;
                case Player.Cross:
                    crosscount++;
                    break;
                case Player.Empty:
                    return false;
                default:
                    throw new InvalidProgramException("Unexpected enumerable state!");
                }
            }
            if (crosscount == GRIDSIZE) {
                Winner = Player.Cross;
                WinningLine = ((0, GRIDSIZE), (GRIDSIZE, 0));
                return true;
            }
            if (circlecount == GRIDSIZE) {
                Winner = Player.Circle;
                WinningLine = ((0, GRIDSIZE), (GRIDSIZE, 0));
                return true;
            }
            return false;
        }

        /// <summary>
        /// Checks all columns on a win.
        /// </summary>
        /// <returns>True if a win was found</returns>
        private bool CheckColumns() {
            for (var x = 0; x < GRIDSIZE; x++) {
                if (CheckColumn(in x)) {
                    WinningLine = ((x, 0), (x, GRIDSIZE));
                    return true;
                }
            }
            return false;
        }

        /// <summary>
        /// Checks a specific column on a win.
        /// </summary>
        /// <param name="y">Column index</param>
        /// <returns>True if a win was found</returns>
        private bool CheckColumn(in int y) {
            var circlecount = 0;
            var crosscount = 0;
            for (var x = 0; x < GRIDSIZE; x++) {
                switch (_field[y, x]) {
                case Player.Circle:
                    circlecount++;
                    break;
                case Player.Cross:
                    crosscount++;
                    break;
                case Player.Empty:
                    return false;
                default:
                    throw new InvalidProgramException("Unexpected enumerable state!");
                }
            }
            if (crosscount == GRIDSIZE) {
                Winner = Player.Cross;
                return true;
            }
            if (circlecount == GRIDSIZE) {
                Winner = Player.Circle;
                return true;
            }
            return false;
        }

        /// <summary>
        /// Checks all rows on a win.
        /// </summary>
        /// <returns>True if a win was found</returns>
        private bool CheckRows() {
            for (var y = 0; y < GRIDSIZE; y++) {
                if (CheckRow(in y)) {
                    WinningLine = ((0, y), (GRIDSIZE, y));
                    return true;
                }
            }
            return false;
        }

        /// <summary>
        /// Checks a specific row on a win.
        /// </summary>
        /// <param name="y">Row index</param>
        /// <returns>True if a win was found</returns>
        private bool CheckRow(in int y) {
            var circlecount = 0;
            var crosscount = 0;
            for (var x = 0; x < GRIDSIZE; x++) {
                switch (_field[x, y]) {
                case Player.Circle:
                    circlecount++;
                    break;
                case Player.Cross:
                    crosscount++;
                    break;
                case Player.Empty:
                    return false;
                default:
                    throw new InvalidProgramException("Unexpected enumerable state!");
                }
            }
            if (crosscount == GRIDSIZE) {
                Winner = Player.Cross;
                return true;
            }
            if (circlecount == GRIDSIZE) {
                Winner = Player.Circle;
                return true;
            }
            return false;
        }
        #endregion

        /// <summary>
        /// Writes the grid data into a string. All rows are printed pipe ('|') separated and all row items are printed comma (',') separated.
        /// </summary>
        /// <returns>A string uniquely identifying the grid</returns>
        public override string ToString() {
            var sb = new StringBuilder();
            for (var y = 0; y < GRIDSIZE; y++) {
                if (y > 0)
                    sb.Append("|");
                for (var x = 0; x < GRIDSIZE; x++) {
                    if (x > 0)
                        sb.Append(",");
                    sb.Append(GetPlayerChar(_field[x, y]));
                }
            }
            return sb.ToString();
        }

        public static char GetPlayerChar(Player player) {
            switch (player) {
            case Player.Empty:
                return ' ';
            case Player.Cross:
                return 'X';
            case Player.Circle:
                return 'O';
            default:
                throw new Exception("Unexpected enumerable state!");
            };
        }
    }
}