code: public long finalize64() {
      permuteAndUpdate();
      permuteAndUpdate();
      permuteAndUpdate();
      permuteAndUpdate();
      done = true;
      return v0[0] + v1[0] + mul0[0] + mul1[0];
    }
docs: Computes the hash value after all bytes were processed. Invalidates the
  state.
  
  NOTE: The 64-bit HighwayHash algorithm is declared stable and no longer subject to change.
  
  @return 64-bit hash