C で文字列を 16 進数に、またはその逆に効率的に変換するにはどうすればよいですか?

std::string string_to_hex(const std::string& input) {
    static const char hex_digits[] = "0123456789ABCDEF";

    std::string output;
    output.reserve(input.length() * 2);

    for (unsigned char c : input) {
        output.push_back(hex_digits[c >> 4]);
        output.push_back(hex_digits[c & 15]);
    }

    return output;
}

std::string hex_to_string(const std::string& input) {
    const auto len = input.length();
    if (len & 1) throw std::invalid_argument("odd length");

    std::string output;
    output.reserve(len / 2);

    for (auto it = input.begin(); it != input.end(); ) {
        int hi = hex_value(*it++);
        int lo = hex_value(*it++);
        output.push_back(hi << 4 | lo);
    }

    return output;
}

int hex_value(unsigned char hex_digit) {
    static const signed char hex_values[256] = {...};
    int value = hex_values[hex_digit];
    if (value == -1) throw std::invalid_argument("invalid hex digit");
    return value;
}