JavaScript function to determine whether a game of tic-tac-toe has been finished.

Tic-Tac-Toe-Winner.js

  // Game_Board will have two slots:
  //   .boardString represents the contents of the 9 cells of the board
  //   .winner indicates whether or not the game has been won, and by whom
  //
  // Each cell in the game board has a cell #, as shown in this chart:
  //     |   |
  //   0 | 1 | 2
  // ----+---+----
  //   3 | 4 | 5
  // ----+---+----
  //   6 | 7 | 8
  //     |   |
  // GameBoard.boardString.charAt(cellNumber) is '-' if cellNumber is
  // unplayed, 'X' if played in by player X, and 'O' if played in by
  // player O.
  //
  // Game_Board.winner is:
  //   '?' if game is not finished
  //   'X' if game has been won by X
  //   'O' if game has been won by O
  //   'C' if game has been won by 'cat' (no moves left, but no winner)
  
  // game_overQ()
  //   checks the game in Game_Board.boardString to see if the game is 
  //   finished. If the game is done, Game_Board.winner is set to 'X',
  //   'O', or 'C' according to whether the game was won by player X,
  //   player O, or 'cat'. If the game is not yet finished, Game_Board.winner
  //   is set to '?'.  winnerQ() and stalemateQ() do most of the work -- both
  //   functions set Game_Board.winner if and only if they return true.
  function game_overQ()
  {
    Game_Board.winner = '?'; // Assume the outcome is in doubt, then check.
    return winnerQ(0,1,2)  // check for 3-in-a-row horizontally
       ||  winnerQ(3,4,5) 
       ||  winnerQ(6,7,8) 
       ||  winnerQ(0,3,6)  // check for 3-in-a-row vertically
       ||  winnerQ(1,4,7) 
       ||  winnerQ(2,5,8) 
       ||  winnerQ(0,4,8)  // check for 3-in-a-row diagonally
       ||  winnerQ(6,4,2)
       ||  stalemateQ();   // check for win by 'cat'
  }

  // winnerQ(p1, p2, p3)
  //   p1, p2, p3: three cell numbers (integers in the range 0..8).
  // Returns a boolean indicating whether the indicated cells in 
  // Game_Board.boardString contain a three-in-a-row win for either player. 
  // If they do, Game_Board.winner will be set to 'X' or 'O' as appropriate 
  // before winnerQ() returns.   
  //
  // The result will be nonsense unless the cell numbers are distinct cells 
  // which form three-in-a-row on the game board, since winnerQ() merely 
  // checks whether all three cells are occupied by 'X' or all three by 'O'.
  function winnerQ(p1, p2, p3)
  {
    var s = Game_Board.boardString;
    var c1 = s.charAt(p1);
    if (c1 == '-') return false;
    var c2 = s.charAt(p2);
    if (c1 != c2) return false;
    var c3 = s.charAt(p3);
    if (c1 != c3) return false;
    Game_Board.winner = c1;
    return true;
  }

  // stalemateQ()
  // Returns true if all the cells in Game_Board.boardString are occupied, and
  // false otherwise. (Technically, this condition does not guarantee stalemate,
  // but this routine is called only after all possible wins have been checked.)
  // As a side effect, Game_Board.winner is set to 'C' (for 'cat') if the game
  // is over.
  function stalemateQ()
  {
    var s = Game_Board.boardString;
    for (var i=0; i<9; i++) {
      if (s.charAt(i) == '-') return false;
    }
    Game_Board.winner = "C";
    return true;
  }