Method decode

common/base64.hpp:200–244

    */
    template<typename Input_iterator, typename Output_iterator>
    static Output_iterator decode(Input_iterator in_begin, Input_iterator in_end, Output_iterator out,
                                  alphabet alphabet          = alphabet::auto_,
                                  decoding_behavior behavior = decoding_behavior::moderate)
    {
        //constexpr auto pad = '=';
        std::uint8_t last  = 0;
        auto bits          = 0;

        while (in_begin != in_end) {
            auto c = *in_begin;
            ++in_begin;

            if (c == '=') {
                break;
            }

            auto part = _base64_value(alphabet, c);

            // enough bits for one byte
            if (bits + 6 >= 8) {
                *out = (last << (8 - bits)) | (part >> (bits - 2));
                ++out;

                bits -= 2;
            } else {
                bits += 6;
            }

            last = part;
        }

        // check padding
        if (behavior != decoding_behavior::loose) {
            while (in_begin != in_end) {
                auto c = *in_begin;
                ++in_begin;

                if (c != '=') {
                    throw base64_error("invalid base64 character.");
                }
            }
        }

        return out;
    }
    /**
     Decodes a string.