code: function(box1, box2) {
              var controlArea         = box1.width * box1.height,
                  intersectionX       = -1,
                  intersectionY       = -1,
                  intersectionArea    = -1,
                  ratio               = -1;
  
              intersectionX =
                  Math.max(0, Math.min(box1.left + box1.width, box2.left + box2.width) - Math.max(box1.left, box2.left));
  
              intersectionY =
                  Math.max(0, Math.min(box1.top + box1.height, box2.top + box2.height) - Math.max(box1.top, box2.top));
  
              intersectionArea = intersectionY * intersectionX;
  
              ratio = intersectionArea / controlArea;
  
              return ratio;
          }
docs: Calcuates the area of intersection between two rectangles and expresses it as
  a ratio in comparison to the area of the first rectangle.
  
  @private
  @param   {Rect}  box1
  @param   {Rect}  box2
  @return  {number}