code: private static int getMapperId(long key, int numReducers, int maxKeySpace) {
      key = key - getFirstSumKey(numReducers, maxKeySpace);
      return (int) (key / numReducers);
    }
docs: Which mapper sent this key sum?
  @param key Key to check
  @param numReducers Number of reducers
  @param maxKeySpace Max key space
  @return Mapper that send this key sum