Get the max ratio of a group of pointers.
@param {string} pointers - The target pointers.
@returns {number} The result ratio.
function getMaxZoomRatio(pointers) {
  var pointers2 = assign({}, pointers);
  var ratios = [];
  forEach(pointers, function (pointer, pointerId) {
    delete pointers2[pointerId];
    forEach(pointers2, function (pointer2) {
      var x1 = Math.abs(pointer.startX - pointer2.startX);
      var y1 = Math.abs(pointer.startY - pointer2.startY);
      var x2 = Math.abs(pointer.endX - pointer2.endX);
      var y2 = Math.abs(pointer.endY - pointer2.endY);
      var z1 = Math.sqrt(x1 * x1 + y1 * y1);
      var z2 = Math.sqrt(x2 * x2 + y2 * y2);
      var ratio = (z2 - z1) / z1;
      ratios.push(ratio);
    });
  });
  ratios.sort(function (a, b) {
    return Math.abs(a) < Math.abs(b);
  });
  return ratios[0];
}