c++
class TextEncoderStream {
private:
    std::u16string pendingHighSurrogate;
    std::wstring_convert<std::codecvt_utf8_utf16<char16_t>, char16_t> converter;
    std::stringstream stream;

public:
    TextEncoderStream() {
    }

    std::string encode(const std::u16string& chunk) {
        std::u16string finalChunk;

        for (const auto& item : chunk) {
            uint16_t codeUnit = item;

            if (!pendingHighSurrogate.empty()) {
                const auto highSurrogate = pendingHighSurrogate;
                pendingHighSurrogate.clear();

                if (0xDC00 <= codeUnit && codeUnit <= 0xDFFF) {
                    finalChunk += highSurrogate + item;
                    continue;
                }

                finalChunk += u'\uFFFD';
            }

            if (0xD800 <= codeUnit && codeUnit <= 0xDBFF) {
                pendingHighSurrogate = item;
                continue;
            }

            if (0xDC00 <= codeUnit && codeUnit <= 0xDFFF) {
                finalChunk += u'\uFFFD';
                continue;
            }

            finalChunk += item;
        }

        if (!finalChunk.empty()) {
            stream << converter.to_bytes(finalChunk);
        }

        return stream.str();
    }

    std::string flush() {
        if (!pendingHighSurrogate.empty()) {
            return "\xEF\xBF\xBD";
        }

        return "";
    }

    std::string getEncoding() const {
        return "utf-8";
    }

    std::string getReadable() const {
        return "ReadableStream";
    }

    std::string getWritable() const {
        return "WritableStream";
    }
};