encoding_range_adaptor.c++

        struct adaptor : public ranges::adaptor_base {
        public:
            adaptor() = default;
            explicit adaptor(ranges::range_sentinel_t<Rng> end)
            : state(), end(end) {}

            typename Encoding::code_unit get(ranges::range_iterator_t<Rng> it) const {
                if(position < 0) {
                    if(depleted) {
                        encoded = Encoding::flush(state);
                    } else {
                        encoded = Encoding::encode_one(*it, state);
                    }
                    position = 0;
                }
                return encoded[position];
            }
            void next(ranges::range_iterator_t<Rng>& it) {
                ++position;
                if(position == static_cast<ptrdiff_t>(encoded.size())) {
                if(it == end) {
                    depleted = true;
                }
                    if(!depleted) ++it;
                    position = -1;
                }
            }
            bool equal(ranges::range_iterator_t<Rng> it0, ranges::range_iterator_t<Rng> it1, adaptor const & other) const {
                return it0 == it1 && position == other.position && depleted == other.depleted;
            }

        private:
            mutable std::vector<typename Encoding::code_unit> encoded;
            mutable std::ptrdiff_t position = -1;
            mutable typename Encoding::state state;
            bool depleted = false;
            ranges::range_sentinel_t<Rng> end;
        };