Generate UUID

inline int setbits(const int a, const int b, const int mask)
{
    return a | (b & mask);
}

static unsigned char random_char()
{
    std::random_device rd;
    std::mt19937 gen(rd());
    std::uniform_int_distribution<> dis(0, 255);
    return static_cast<unsigned char>(dis(gen));
}

static std::string generate_hex(const unsigned int len)
{
    std::stringstream ss;
    for (auto i = 0; static_cast<const unsigned>(i) < len; i++) {
        auto rc = random_char();
        std::stringstream hexstream;
        hexstream << std::hex << int(rc);
        auto hex = hexstream.str();
        ss << (hex.length() < 2 ? '0' + hex : hex);
    }
    return ss.str();
}

std::string uuid()
{
    // The UUID is of the form: xxxxxxxx-xxxx-Mxxx-Nxxx-xxxxxxxxxxxx
    //
    // Where the 4 bits of digit M indicate the UUID version, and the 1–3
    // most significant bits of digit N indicate the UUID variant.
    //
    // UUIDs generated here are version 4 variant 1.

    std::ostringstream ss;
    const int version = std::stoi(generate_hex(2), nullptr, 16);
    const int variant = std::stoi(generate_hex(2), nullptr, 16);
    ss << generate_hex(4) << "-" << generate_hex(2) << "-"
       << std::hex << setbits(0x4000, version, 0x0fff) << "-"
       << std::hex << setbits(0x8000, variant, 0x3fff) << "-"
       << generate_hex(6);
    return ss.str();
}