unsigned char hs2i(char c)
{
  if ('0' <= c && c <= '9') {
    return ((unsigned char)c - '0');
  } else if ('a' <= c && c <= 'f') {
    return ((unsigned char)c - 'a' + 10);
  } else if ('A' <= c && c <= 'F') {
    return ((unsigned char)c - 'A' + 10);
  } else {
    return 0;
  }
}

String decodeUri(String s) {
  String a = "";
  char c;
  std::pair<char, char> h;
  for (int i = 0; i < s.length(); i++) {
    c = s.charAt(i);
    if (c == '+') {
      a += ' ';
    } else if (c == '%') {
      h.first = s.charAt(++i);
      h.second = s.charAt(++i);
      c = (hs2i(h.first) << 4) | hs2i(h.second);
      a += c;
    } else {
      a += c;
    }
    yield();
  }
  return a;
}