docs: public double getEstimate(final byte[] key) {
      if (key == null) { return Double.NaN; }
      checkMethodKeySize(key);
      final double est = maps_[0].getEstimate(key);
      if (est >= 0.0) { return est; }
      //key has been promoted
      final int level = -(int)est;
      final Map map = maps_[level];
      return map.getEstimate(key);
    }