tic_tac_toe_game.dart

// One possible solution
enum TicTacToeMark { none, x, o }

enum TicTacToeState { xTurn, oTurn, xWon, oWon, tie }

class TicTacToeGame {
  var board = List<TicTacToeMark>.filled(9, TicTacToeMark.none);
  var state = TicTacToeState.xTurn;

  void pressedSquare(int index) {
    if (index < 0 || index >= board.length) {
      return;
    }
    if (board[index] != TicTacToeMark.none) {
      return;
    }
    if (state == TicTacToeState.xTurn) {
      board[index] = TicTacToeMark.x;
      state = TicTacToeState.oTurn;
      checkForGameOver();
    } else if (state == TicTacToeState.oTurn) {
      board[index] = TicTacToeMark.o;
      state = TicTacToeState.xTurn;
      checkForGameOver();
    }
  }

  List<String> get linesOf3 {
    final lines = <String>[];
    final b = boardString;
    lines.add(b[0] + b[1] + b[2]);
    lines.add(b[3] + b[4] + b[5]);
    lines.add(b[6] + b[7] + b[8]);

    lines.add(b[0] + b[3] + b[6]);
    lines.add(b[1] + b[4] + b[7]);
    lines.add(b[2] + b[5] + b[8]);

    lines.add(b[0] + b[4] + b[8]);
    lines.add(b[2] + b[4] + b[6]);
    return lines;
  }

  void checkForGameOver() {
    if (!board.contains(TicTacToeMark.none)) {
      state = TicTacToeState.tie;
    }
    for (final line in linesOf3) {
      if (line == "XXX") {
        state = TicTacToeState.xWon;
      } else if (line == "OOO") {
        state = TicTacToeState.oWon;
      }
    }
  }

  String get stateString => state.toString().split('.').last;

  String get boardString {
    var s = "";
    for (final mark in board) {
      s +=
          mark == TicTacToeMark.x ? "X" : (mark == TicTacToeMark.o ? "O" : "-");
    }
    return s;
  }

  @override
  String toString() {
    return "$stateString $boardString";
  }
}