code: public void updateStats(int fIdx, long count) {
          synchronized (pq) {
              frmtCount[fIdx] += count;
              Stats tmp = new Stats(fIdx, frmtCount[fIdx]);
              // remove the stats object with the same fIdx
              pq.remove(tmp);
              pq.offer(tmp);
          }
          if (LOG.isTraceEnabled()) {
              LOG.trace("update forest " + fIdx);
          }
      }
docs: add count to forest with index fIdx, which may not be the forest with
  minimum frmtCount. Used by fragment count rollback.
  
  @param fIdx
  @param count