/in/foo.cc: In function 'std::string encode(std::string)':
/in/foo.cc:6:23: warning: comparison of integer expressions of different signedness: 'int' and 'std::__cxx11::basic_string<char>::size_type' {aka 'long unsigned int'} [-Wsign-compare]
    6 |     for (int i = 0; i < s.length(); i++) {
      |                     ~~^~~~~~~~~~~~