std::string urlencode(const std::string &str)
{
    static std::string nonresv =
        "-."
        "0123456789"
        "ABCDEFGHIJKLMNOPQRSTUVWXYZ"
        "_"
        "abcdefghijklmnopqrstuvwxyz"
        "~";

    std::string res;

    for(size_t n = 0; n < str.length(); ++n) {
        const char& c = str[n];

        if(nonresv.find(c) != std::string::npos) {
            res += c;
            continue;
        }

        char buf[4];
        snprintf(buf, sizeof(buf), "%%%02X", c);
        res += buf;
    }

    return res;
}