code: func (h *histogram) getRandomBoundIdx() int {
  	cnt := h.Buckets[len(h.Buckets)-1].Count
  	randCnt := randInt64(0, cnt)
  	for i, bkt := range h.Buckets {
  		if bkt.Count >= randCnt {
  			if bkt.Count-bkt.Repeat > randCnt {
  				return 2 * i
  			}
  			return 2*i + 1
  		}
  	}
  	return 0
  }
docs: // When the randCnt falls in the middle of bucket, we return the idx of lower bound which is an even number.
  // When the randCnt falls in the end of bucket, we return the upper bound which is odd.