if( 0 == ids.size() ) { return digest_type(); }

while( ids.size() > 1 ) {
  if( ids.size() % 2 )
     ids.push_back(ids.back());

  for (size_t i = 0; i < ids.size() / 2; i++) {
     ids[i] = digest_type::hash(make_canonical_pair(ids[2 * i], ids[(2 * i) + 1]));
  }

  ids.resize(ids.size() / 2);
}

if( ids.size() % 2 ) // If the remainder of size() / 2 is not 0
     ids.push_back(ids.back());  //Push back a duplicate of the last element

return ids.front(); // Returns the only element left in the vector