code: int addKey(BufferedString str) {
      // _map is shared and be cast to null (if categorical is killed) -> grab local copy
      IcedHashMap<BufferedString, Integer> m = _map;
      if( m == null ) return Integer.MAX_VALUE;     // Nuked already
      Integer res = m.get(str);
      if( res != null ) return res; // Recorded already
      assert str.length() < 65535; // Length limit so 65535 can be used as a sentinel
      int newVal = _id.incrementAndGet();
      res = m.putIfAbsent(new BufferedString(str), newVal);
      if( res != null ) return res;
      if( m.size() > MAX_CATEGORICAL_COUNT) maxDomainExceeded = true;
      return newVal;
    }
docs: Add key to this map (treated as hash set in this case).