Tic Tacs Toe

script.js

function evaluateBoard(data) {
  /*
   *  Setup
   */
  // Don't want to keep calling .charAt, so save all the values with easier-to-use names up front
  var topLeft = data.charAt(0);
  var topMiddle = data.charAt(1);
  var topRight = data.charAt(2);
  var centerLeft = data.charAt(3);
  var centerMiddle = data.charAt(4);
  var centerRight = data.charAt(5);
  var bottomLeft = data.charAt(6);
  var bottomMiddle = data.charAt(7);
  var bottomRight = data.charAt(8);
  
  // Ways to win: Rows
  var topRow = [topLeft, topMiddle, topRight];
  var centerRow = [centerLeft, centerMiddle, centerRight];
  var bottomRow = [bottomLeft, bottomMiddle, bottomRight];
  
  // Ways to win: Columns
  var leftColumn = [topLeft, centerLeft, bottomLeft];
  var middleColumn = [topMiddle, centerMiddle, bottomMiddle];
  var rightColumn = [topRight, centerRight, bottomRight];
  
  // Ways to win: Diagonals
  var firstDiagonal = [topLeft, centerMiddle, bottomRight];
  var secondDiagonal = [bottomLeft, centerMiddle, topRight];
  
  // Put all the ways to win together in an array of "ways to win"
  var waysToWin = [topRow, centerRow, bottomRow, leftColumn, middleColumn, rightColumn, firstDiagonal, secondDiagonal];
  
  /*
   *  Start actual algorithm to evaluate the board
   */
  // Is there a winner?  Go through each way to win and see if either side won that way
  var isWinner = false;
  var winningSide = null;
  for (var wayToWinIndex = 0; wayToWinIndex < waysToWin.length; wayToWinIndex++) {
    // Which group are we testing in this iteration?
    var targetGroup = waysToWin[wayToWinIndex];
    
    // Test the group to see if it's a winner
    isWinner = groupIsWinner(targetGroup);
    
    // did someone win?
    if (isWinner) {
      // Which side won?
      winningSide = targetGroup[0]; // x or o
      break; // don't have to keep going
    }
  }
  
  // If we have a winner, declare it!
  if (isWinner) {
    return winningSide + ' wins!';
  }
  else { // we don't have a winner
    // is the game incomplete or tie?
    
    // If it's a tie, there won't be any empty squares
    if (data.indexOf('-') === -1) {
      // No empty squares, it's a tie
      return 'The game is a tie';
    }
    else { // there are empty sqaures.  Game is incomplete
      return 'The game is still in progress';
    }
  }
}

function groupIsWinner (group) {
  // Only a winner if the three spaces match and they're not empty
  return group[0] !== '-' &&
         group[0] === group[1] &&
         group[1] === group[2];
}

Tic Tacs Toe.markdown

Tic Tacs Toe

A Pen by Andy Terranova on CodePen.

License.

@paulevans, awesome! Just getting a chance to read through this now - I'd say keep it here for now, if you want to generalize into something more easily digestible for the thread though, that would be awesome.