private static int capacityWithReserve(int length) {
    int capacity = length + 3 * (length / 5);
    if (capacity < 0) {
      throw new IllegalArgumentException("length(=" + length + ") is too big -- i.e. id range is too large to keep in bitset");
    }
    return capacity;
  }
private static int capacityWithReserve(int length)