Method encode

common/base64.hpp:80–144

    */
    template<typename Input_iterator, typename Output_iterator>
    static Output_iterator encode(Input_iterator in_begin, Input_iterator in_end, Output_iterator out,
                                  alphabet alphabet = alphabet::standard)
    {
        constexpr auto pad = '=';
        const char* alpha  = alphabet == alphabet::url_filename_safe
                                ? "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789-_"
                                : "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/";

        while (in_begin != in_end) {
            std::uint8_t i0 = 0, i1 = 0, i2 = 0;

            // first character
            i0 = static_cast<std::uint8_t>(*in_begin);
            ++in_begin;

            *out = alpha[i0 >> 2 & 0x3f];
            ++out;

            // part of first character and second
            if (in_begin != in_end) {
                i1 = static_cast<std::uint8_t>(*in_begin);
                ++in_begin;

                *out = alpha[((i0 & 0x3) << 4) | (i1 >> 4 & 0x0f)];
                ++out;
            } else {
                *out = alpha[(i0 & 0x3) << 4];
                ++out;

                // last padding
                *out = pad;
                ++out;

                // last padding
                *out = pad;
                ++out;

                break;
            }

            // part of second character and third
            if (in_begin != in_end) {
                i2 = static_cast<std::uint8_t>(*in_begin);
                ++in_begin;

                *out = alpha[((i1 & 0xf) << 2) | (i2 >> 6 & 0x03)];
                ++out;
            } else {
                *out = alpha[(i1 & 0xf) << 2];
                ++out;

                // last padding
                *out = pad;
                ++out;

                break;
            }