code: private boolean isClockwise(Point center, Point a, Point b) {
          double cross = (a.x - center.x) * (b.y - center.y) - (b.x - center.x) * (a.y - center.y);
          return cross > 0;
      }
docs: judge a->b is ordered clockwise
  
  @param center
  @param a
  @param b
  @return