private void removed(int index) {
    usage -= keys[index].length;
    keys[index] = null;
    if (values[index] != null) {
      usage -= values[index].length;
      values[index] = null;
    }
    hashes[index] = -1;
    distKeyLengths[index] = -1;
  }
returns index for key/value/hashcode if it's acceptable.
-1, -2, -3, -4 can be returned for other actions.
<p/>
-1 for FORWARD   : should be forwarded to output collector (for GBY)
-2 for EXCLUDED  : not in top-k. ignore it
ql/src/java/org/apache/hadoop/hive/ql/exec/TopNHash.java