Method get_codepoint

lesson6-Segmentation/json.hpp:6040–6071

! @brief get codepoint from 4 hex characters following `\u` For input "\u c1 c2 c3 c4" the codepoint is: (c1 * 0x1000) + (c2 * 0x0100) + (c3 * 0x0010) + c4 = (c1 << 12) + (c2 << 8) + (c3 << 4) + (c4 << 0) Furthermore, the possible characters '0'..'9', 'A'..'F', and 'a'..'f' must be converted to the integers 0x0..0x9, 0xA..0xF, 0xA..0xF, resp. The conversion is done

            non-hex character)
    */
    int get_codepoint()
    {
        // this function only makes sense after reading `\u`
        JSON_ASSERT(current == 'u');
        int codepoint = 0;

        const auto factors = { 12u, 8u, 4u, 0u };
        for (const auto factor : factors)
        {
            get();

            if (current >= '0' && current <= '9')
            {
                codepoint += static_cast<int>((static_cast<unsigned int>(current) - 0x30u) << factor);
            }
            else if (current >= 'A' && current <= 'F')
            {
                codepoint += static_cast<int>((static_cast<unsigned int>(current) - 0x37u) << factor);
            }
            else if (current >= 'a' && current <= 'f')
            {
                codepoint += static_cast<int>((static_cast<unsigned int>(current) - 0x57u) << factor);
            }
            else
            {
                return -1;
            }
        }

        JSON_ASSERT(0x0000 <= codepoint && codepoint <= 0xFFFF);
        return codepoint;
    }

    /*!
    @brief check if the next byte(s) are inside a given range