code: private static int getOffsetY(Collision collision, Mirror mirror)
      {
          if (mirror == Mirror.VERTICAL)
          {
              return -collision.getOffsetY();
          }
          return collision.getOffsetY();
      }
docs: Get the vertical offset.
  
  @param collision The collision reference.
  @param mirror The mirror used.
  @return The offset value depending of mirror.