// Textbook fragment 03.10
/** Simulation of a Tic-Tac-Toe game (does not do strategy). */
public class TicTacToe {
  protected static final int X = 1, O = -1;     // players
  protected static final int EMPTY = 0;         // empty cell
  protected int board[][] = new int[3][3];      // game board
  protected int player;                         // current player
  /** Constructor */
  public TicTacToe() { clearBoard(); }
  /** Clears the board */
  public void clearBoard() {
    for (int i = 0; i < 3; i++)
      for (int j = 0; j < 3; j++)
        board[i][j] = EMPTY;            // every cell should be empty
    player = X;                         // the first player is 'X'
  }
  /** Puts an X or O mark at position i,j */
  public void putMark(int i, int j) throws IllegalArgumentException {
    if ((i < 0) || (i > 2) || (j < 0) || (j > 2))
      throw new IllegalArgumentException("Invalid board position");
    if (board[i][j] != EMPTY)
      throw new IllegalArgumentException("Board position occupied");
    board[i][j] = player;       // place the mark for the current player
    player = - player;          // switch players (uses fact that O = - X)
  }
  /** Checks whether the board configuration is a win for the given player */
  public boolean isWin(int mark) {
    return ((board[0][0] + board[0][1] + board[0][2] == mark*3)   // row 0
            || (board[1][0] + board[1][1] + board[1][2] == mark*3)   // row 1
            || (board[2][0] + board[2][1] + board[2][2] == mark*3)   // row 2
            || (board[0][0] + board[1][0] + board[2][0] == mark*3)   // column 0
            || (board[0][1] + board[1][1] + board[2][1] == mark*3)   // column 1
            || (board[0][2] + board[1][2] + board[2][2] == mark*3)   // column 2
            || (board[0][0] + board[1][1] + board[2][2] == mark*3)   // diagonal
            || (board[2][0] + board[1][1] + board[0][2] == mark*3)); // diagonal
  }
  /** Returns the winning player or 0 to indicate a tie */
  public int winner() {
    if (isWin(X))
      return(X);
    else if (isWin(O))
      return(O);
    else
      return(0);
  }