docs: private void reduce() {
          Long[] keys = counter.asMap().keySet().toArray(ArrayUtils.EMPTY_LONG_OBJECT_ARRAY);
          Arrays.sort(keys);
          for (int i = 0, n = keys.length - maxNumBlocks + BUFFER_NUM_BLOCKS; i < n; i++) {
              counter.remove(keys[i]);
          }
      }