codecvt

// codecvt standard header

// Copyright (c) Microsoft Corporation.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception

#pragma once
#ifndef _CODECVT_
#define _CODECVT_
#include <yvals_core.h>
#if _STL_COMPILER_PREPROCESSOR
#include <cwchar>
#include <locale>

#pragma pack(push, _CRT_PACKING)
#pragma warning(push, _STL_WARNING_LEVEL)
#pragma warning(disable : _STL_DISABLED_WARNINGS)
_STL_DISABLE_CLANG_WARNINGS
#pragma push_macro("new")
#undef new

_STD_BEGIN
_INLINE_VAR constexpr int _Little_first = 1;
_INLINE_VAR constexpr int _Big_first    = 2;

enum _CXX17_DEPRECATE_CODECVT_HEADER codecvt_mode { consume_header = 4, generate_header = 2, little_endian = 1 };

using _Statype = _CSTD mbstate_t;

_STL_DISABLE_DEPRECATED_WARNING
template <class _Elem, unsigned long _Mymax = 0x10ffff, codecvt_mode _Mymode = codecvt_mode{}>
class _CXX17_DEPRECATE_CODECVT_HEADER codecvt_utf8 : public codecvt<_Elem, char, _Statype> {
    // facet for converting between _Elem and UTF-8 byte sequences
public:
    using _Mybase     = codecvt<_Elem, char, _Statype>;
    using result      = typename _Mybase::result;
    using _Byte       = char;
    using intern_type = _Elem;
    using extern_type = _Byte;
    using state_type  = _Statype;

    explicit codecvt_utf8(size_t _Refs = 0) : _Mybase(_Refs) {}

    __CLR_OR_THIS_CALL ~codecvt_utf8() noexcept override {}

protected:
    result __CLR_OR_THIS_CALL do_in(_Statype& _State, const _Byte* _First1, const _Byte* _Last1, const _Byte*& _Mid1,
        _Elem* _First2, _Elem* _Last2, _Elem*& _Mid2) const override {
        // convert bytes [_First1, _Last1) to [_First2, _Last2)
        char* _Pstate = reinterpret_cast<char*>(&_State);
        _Mid1         = _First1;
        _Mid2         = _First2;

        while (_Mid1 != _Last1 && _Mid2 != _Last2) { // convert a multibyte sequence
            unsigned long _By = static_cast<unsigned char>(*_Mid1);
            unsigned long _Ch;
            int _Nextra;

            if (_By < 0x80u) {
                _Ch     = _By;
                _Nextra = 0;
            } else if (_By < 0xc0u) { // 0x80-0xbf not first byte
                ++_Mid1;
                return _Mybase::error;
            } else if (_By < 0xe0u) {
                _Ch     = _By & 0x1f;
                _Nextra = 1;
            } else if (_By < 0xf0u) {
                _Ch     = _By & 0x0f;
                _Nextra = 2;
            } else if (_By < 0xf8u) {
                _Ch     = _By & 0x07;
                _Nextra = 3;
            } else {
                _Ch     = _By & 0x03;
                _Nextra = _By < 0xfc ? 4 : 5;
            }

            if (_Nextra == 0) {
                ++_Mid1;
            } else if (_Last1 - _Mid1 < _Nextra + 1) {
                break; // not enough input
            } else {
                for (++_Mid1; 0 < _Nextra; --_Nextra, ++_Mid1) {
                    if ((_By = static_cast<unsigned char>(*_Mid1)) < 0x80u || 0xc0u <= _By) {
                        return _Mybase::error; // not continuation byte
                    } else {
                        _Ch = _Ch << 6 | (_By & 0x3f);
                    }
                }
            }

            if (*_Pstate == 0) { // first time, maybe look for and consume header
                *_Pstate = 1;

                constexpr bool _Consuming = (_Mymode & consume_header) != 0;
                if constexpr (_Consuming) {
                    if (_Ch == 0xfeff) { // drop header and retry
                        const result _Ans = do_in(_State, _Mid1, _Last1, _Mid1, _First2, _Last2, _Mid2);

                        if (_Ans == _Mybase::partial) { // roll back header determination
                            *_Pstate = 0;
                            _Mid1    = _First1;
                        }

                        return _Ans;
                    }
                }
            }

            if (_Mymax < _Ch) {
                return _Mybase::error; // code too large
            }

            *_Mid2++ = static_cast<_Elem>(_Ch);
        }

        return _First1 == _Mid1 ? _Mybase::partial : _Mybase::ok;
    }

    result __CLR_OR_THIS_CALL do_out(_Statype& _State, const _Elem* _First1, const _Elem* _Last1, const _Elem*& _Mid1,
        _Byte* _First2, _Byte* _Last2, _Byte*& _Mid2) const override {
        // convert [_First1, _Last1) to bytes [_First2, _Last2)
        char* _Pstate = reinterpret_cast<char*>(&_State);
        _Mid1         = _First1;
        _Mid2         = _First2;

        while (_Mid1 != _Last1 && _Mid2 != _Last2) { // convert and put a wide char
            _Byte _By;
            int _Nextra;
            unsigned long _Ch = static_cast<unsigned long>(*_Mid1);

            if (_Mymax < _Ch) {
                return _Mybase::error;
            }

            if (_Ch < 0x0080u) {
                _By     = static_cast<_Byte>(_Ch);
                _Nextra = 0;
            } else if (_Ch < 0x0800u) {
                _By     = static_cast<_Byte>(0xc0 | _Ch >> 6);
                _Nextra = 1;
            } else if (_Ch < 0x00010000u) {
                _By     = static_cast<_Byte>(0xe0 | _Ch >> 12);
                _Nextra = 2;
            } else if (_Ch < 0x00200000u) {
                _By     = static_cast<_Byte>(0xf0 | _Ch >> 18);
                _Nextra = 3;
            } else if (_Ch < 0x04000000u) {
                _By     = static_cast<_Byte>(0xf8 | _Ch >> 24);
                _Nextra = 4;
            } else {
                _By     = static_cast<_Byte>(0xfc | (_Ch >> 30 & 0x03));
                _Nextra = 5;
            }

            if (*_Pstate == 0) { // first time, maybe generate header
                *_Pstate                   = 1;
                constexpr bool _Generating = (_Mymode & generate_header) != 0;
                if constexpr (_Generating) {
                    if (_Last2 - _Mid2 < 3 + 1 + _Nextra) {
                        return _Mybase::partial; // not enough room for both
                    }

                    // prepend header
                    *_Mid2++ = '\xef';
                    *_Mid2++ = '\xbb';
                    *_Mid2++ = '\xbf';
                }
            }

            if (_Last2 - _Mid2 < 1 + _Nextra) {
                break; // not enough room for output
            }

            ++_Mid1;
            for (*_Mid2++ = _By; 0 < _Nextra;) {
                *_Mid2++ = static_cast<_Byte>((_Ch >> 6 * --_Nextra & 0x3f) | 0x80);
            }
        }

        return _First1 == _Mid1 ? _Mybase::partial : _Mybase::ok;
    }

    result __CLR_OR_THIS_CALL do_unshift(_Statype&, _Byte* _First2, _Byte*, _Byte*& _Mid2) const override {
        // generate bytes to return to default shift state
        _Mid2 = _First2;
        return _Mybase::noconv;
    }

    friend int _Codecvt_do_length<>(const codecvt_utf8&, _Statype&, const _Byte*, const _Byte*, size_t);

    int __CLR_OR_THIS_CALL do_length(
        _Statype& _State, const _Byte* _First1, const _Byte* _Last1, size_t _Count) const noexcept override {
        return _Codecvt_do_length(*this, _State, _First1, _Last1, _Count);
    }

    bool __CLR_OR_THIS_CALL do_always_noconv() const noexcept override {
        // return true if conversions never change input
        return false;
    }

    int __CLR_OR_THIS_CALL do_max_length() const noexcept override {
        // return maximum length required for a conversion
        if constexpr ((_Mymode & (consume_header | generate_header)) != 0) {
            return 9;
        } else {
            return 6;
        }
    }

    int __CLR_OR_THIS_CALL do_encoding() const noexcept override {
        // return length of code sequence (from codecvt)
        if constexpr ((_Mymode & (consume_header | generate_header)) != 0) {
            return -1; // -1 => state dependent
        } else {
            return 0; // 0 => varying length
        }
    }
};

template <class _Elem, unsigned long _Mymax = 0x10ffff, codecvt_mode _Mymode = codecvt_mode{}>
class _CXX17_DEPRECATE_CODECVT_HEADER codecvt_utf16 : public codecvt<_Elem, char, _Statype> {
    // facet for converting between _Elem and UTF-16 multibyte sequences
private:
    enum { _Bytes_per_word = 2 };

public:
    using _Mybase     = codecvt<_Elem, char, _Statype>;
    using result      = typename _Mybase::result;
    using _Byte       = char;
    using intern_type = _Elem;
    using extern_type = _Byte;
    using state_type  = _Statype;

    explicit codecvt_utf16(size_t _Refs = 0) : _Mybase(_Refs) {}

    __CLR_OR_THIS_CALL ~codecvt_utf16() noexcept override {}

protected:
    result __CLR_OR_THIS_CALL do_in(_Statype& _State, const _Byte* _First1, const _Byte* _Last1, const _Byte*& _Mid1,
        _Elem* _First2, _Elem* _Last2, _Elem*& _Mid2) const override {
        // convert bytes [_First1, _Last1) to [_First2, _Last2)
        char* _Pstate = reinterpret_cast<char*>(&_State);
        _Mid1         = _First1;
        _Mid2         = _First2;

        while (_Bytes_per_word <= _Last1 - _Mid1 && _Mid2 != _Last2) { // convert a multibyte sequence
            const auto _Ptr = reinterpret_cast<const unsigned char*>(_Mid1);
            unsigned long _Ch;
            unsigned short _Ch0;
            unsigned short _Ch1;

            if (*_Pstate == _Little_first) {
                _Ch0 = static_cast<unsigned short>(_Ptr[1] << 8 | _Ptr[0]);
            } else if (*_Pstate == _Big_first) {
                _Ch0 = static_cast<unsigned short>(_Ptr[0] << 8 | _Ptr[1]);
            } else { // no header seen yet, try preferred mode
                constexpr bool _Prefer_LE      = (_Mymode & little_endian) != 0;
                constexpr char _Default_endian = _Prefer_LE ? _Little_first : _Big_first;

                if constexpr (_Prefer_LE) {
                    _Ch0 = static_cast<unsigned short>(_Ptr[1] << 8 | _Ptr[0]);
                } else {
                    _Ch0 = static_cast<unsigned short>(_Ptr[0] << 8 | _Ptr[1]);
                }

                *_Pstate                  = _Default_endian;
                constexpr bool _Consuming = (_Mymode & consume_header) != 0;
                if constexpr (_Consuming) {
                    if (_Ch0 == 0xfffeu) {
                        *_Pstate = 3 - _Default_endian;
                    }

                    if (_Ch0 == 0xfffeu || _Ch0 == 0xfeffu) { // consume header, fixate on endianness, and retry
                        _Mid1 += _Bytes_per_word;
                        result _Ans = do_in(_State, _Mid1, _Last1, _Mid1, _First2, _Last2, _Mid2);

                        if (_Ans == _Mybase::partial) { // not enough bytes, roll back header
                            *_Pstate = 0;
                            _Mid1    = _First1;
                        }

                        return _Ans;
                    }
                }
            }

            if (_Ch0 < 0xd800u || 0xdc00u <= _Ch0) { // one word, consume bytes
                _Mid1 += _Bytes_per_word;
                _Ch = _Ch0;
            } else if (_Last1 - _Mid1 < 2 * _Bytes_per_word) {
                break;
            } else { // get second word
                if (*_Pstate == _Little_first) {
                    _Ch1 = static_cast<unsigned short>(_Ptr[3] << 8 | _Ptr[2]);
                } else {
                    _Ch1 = static_cast<unsigned short>(_Ptr[2] << 8 | _Ptr[3]);
                }

                if (_Ch1 < 0xdc00u || 0xe000u <= _Ch1) {
                    return _Mybase::error;
                }

                _Mid1 += 2 * _Bytes_per_word;
                _Ch = static_cast<unsigned long>(_Ch0 - 0xd800 + 0x0040) << 10 | (_Ch1 - 0xdc00);
            }

            if (_Mymax < _Ch) {
                return _Mybase::error; // code too large
            }

            *_Mid2++ = static_cast<_Elem>(_Ch);
        }

        return _First1 == _Mid1 ? _Mybase::partial : _Mybase::ok;
    }

    result __CLR_OR_THIS_CALL do_out(_Statype& _State, const _Elem* _First1, const _Elem* _Last1, const _Elem*& _Mid1,
        _Byte* _First2, _Byte* _Last2, _Byte*& _Mid2) const override {
        // convert [_First1, _Last1) to bytes [_First2, _Last2)
        char* _Pstate = reinterpret_cast<char*>(&_State);
        _Mid1         = _First1;
        _Mid2         = _First2;

        if (*_Pstate == 0) { // determine endianness once, maybe generate header
            if constexpr ((_Mymode & little_endian) != 0) {
                *_Pstate = _Little_first;
            } else {
                *_Pstate = _Big_first;
            }

            constexpr bool _Generating = (_Mymode & generate_header) != 0;
            if constexpr (_Generating) {
                if (_Last2 - _Mid2 < 3 * _Bytes_per_word) {
                    return _Mybase::partial; // not enough room for all
                }

                if (*_Pstate == _Little_first) { // put header LS byte first
                    *_Mid2++ = '\xff';
                    *_Mid2++ = '\xfe';
                } else { // put header MS byte first
                    *_Mid2++ = '\xfe';
                    *_Mid2++ = '\xff';
                }
            }
        }

        while (_Mid1 != _Last1 && _Bytes_per_word <= _Last2 - _Mid2) { // convert and put a wide char
            bool _Extra       = false;
            unsigned long _Ch = static_cast<unsigned long>(*_Mid1++);

            if ((_Mymax < 0x10ffffu ? _Mymax : 0x10ffffu) < _Ch) {
                return _Mybase::error; // value too large
            }

            if (_Ch <= 0xffffu) { // one word, can't be code for first of two
                if (0xd800u <= _Ch && _Ch < 0xdc00u) {
                    return _Mybase::error;
                }
            } else if (_Last2 - _Mid2 < 2 * _Bytes_per_word) { // not enough room for two-word output, back up
                --_Mid1;
                return _Mybase::partial;
            } else {
                _Extra = true;
            }

            if (*_Pstate == _Little_first) {
                if (_Extra) { // put a pair of words LS byte first
                    unsigned short _Ch0 =
                        static_cast<unsigned short>(0xd800 | static_cast<unsigned short>(_Ch >> 10) - 0x0040);
                    *_Mid2++ = static_cast<_Byte>(_Ch0);
                    *_Mid2++ = static_cast<_Byte>(_Ch0 >> 8);

                    _Ch0     = static_cast<unsigned short>(0xdc00 | (static_cast<unsigned short>(_Ch) & 0x03ff));
                    *_Mid2++ = static_cast<_Byte>(_Ch0);
                    *_Mid2++ = static_cast<_Byte>(_Ch0 >> 8);
                } else { // put a single word LS byte first
                    *_Mid2++ = static_cast<_Byte>(_Ch);
                    *_Mid2++ = static_cast<_Byte>(_Ch >> 8);
                }
            } else {
                if (_Extra) { // put a pair of words MS byte first
                    unsigned short _Ch0 =
                        static_cast<unsigned short>(0xd800 | static_cast<unsigned short>(_Ch >> 10) - 0x0040);
                    *_Mid2++ = static_cast<_Byte>(_Ch0 >> 8);
                    *_Mid2++ = static_cast<_Byte>(_Ch0);

                    _Ch0     = static_cast<unsigned short>(0xdc00 | (static_cast<unsigned short>(_Ch) & 0x03ff));
                    *_Mid2++ = static_cast<_Byte>(_Ch0 >> 8);
                    *_Mid2++ = static_cast<_Byte>(_Ch0);
                } else { // put a single word MS byte first
                    *_Mid2++ = static_cast<_Byte>(_Ch >> 8);
                    *_Mid2++ = static_cast<_Byte>(_Ch);
                }
            }
        }

        return _First1 == _Mid1 ? _Mybase::partial : _Mybase::ok;
    }

    result __CLR_OR_THIS_CALL do_unshift(_Statype&, _Byte* _First2, _Byte*, _Byte*& _Mid2) const override {
        // generate bytes to return to default shift state
        _Mid2 = _First2;
        return _Mybase::noconv;
    }

    friend int _Codecvt_do_length<>(const codecvt_utf16&, _Statype&, const _Byte*, const _Byte*, size_t);

    int __CLR_OR_THIS_CALL do_length(
        _Statype& _State, const _Byte* _First1, const _Byte* _Last1, size_t _Count) const noexcept override {
        return _Codecvt_do_length(*this, _State, _First1, _Last1, _Count);
    }

    bool __CLR_OR_THIS_CALL do_always_noconv() const noexcept override {
        // return true if conversions never change input
        return false;
    }

    int __CLR_OR_THIS_CALL do_max_length() const noexcept override {
        // return maximum length required for a conversion
        if constexpr ((_Mymode & (consume_header | generate_header)) != 0) {
            return 3 * _Bytes_per_word;
        } else {
            return 6 * _Bytes_per_word;
        }
    }

    int __CLR_OR_THIS_CALL do_encoding() const noexcept override {
        // return length of code sequence (from codecvt)
        if constexpr ((_Mymode & (consume_header | generate_header)) != 0) {
            return -1; // -1 => state dependent
        } else {
            return 0; // 0 => varying length
        }
    }
};

template <class _Elem, unsigned long _Mymax = 0x10ffff, codecvt_mode _Mymode = codecvt_mode{}>
class _CXX17_DEPRECATE_CODECVT_HEADER codecvt_utf8_utf16
    : public codecvt<_Elem, char, _Statype> { // facet for converting between UTF-16 _Elem and UTF-8 byte sequences
public:
    using _Mybase     = codecvt<_Elem, char, _Statype>;
    using result      = typename _Mybase::result;
    using _Byte       = char;
    using intern_type = _Elem;
    using extern_type = _Byte;
    using state_type  = _Statype;

    static_assert(sizeof(unsigned short) <= sizeof(state_type), "state_type too small");

    explicit codecvt_utf8_utf16(size_t _Refs = 0) : _Mybase(_Refs) {}

    __CLR_OR_THIS_CALL ~codecvt_utf8_utf16() noexcept override {}

protected:
    result __CLR_OR_THIS_CALL do_in(_Statype& _State, const _Byte* _First1, const _Byte* _Last1, const _Byte*& _Mid1,
        _Elem* _First2, _Elem* _Last2, _Elem*& _Mid2) const override {
        // convert bytes [_First1, _Last1) to [_First2, _Last2)
        unsigned short* _Pstate = reinterpret_cast<unsigned short*>(&_State);
        _Mid1                   = _First1;
        _Mid2                   = _First2;

        while (_Mid1 != _Last1 && _Mid2 != _Last2) { // convert a multibyte sequence
            unsigned long _By = static_cast<unsigned char>(*_Mid1);
            unsigned long _Ch;
            int _Nextra;
            int _Nskip;

            if (*_Pstate > 1u) {
                if (_By < 0x80u || 0xc0u <= _By) {
                    return _Mybase::error; // not continuation byte
                }

                // deliver second half of two-word value
                ++_Mid1;
                *_Mid2++ = static_cast<_Elem>(*_Pstate | (_By & 0x3f));
                *_Pstate = 1;
                continue;
            }

            if (_By < 0x80u) {
                _Ch     = _By;
                _Nextra = 0;
            } else if (_By < 0xc0u) { // 0x80-0xbf not first byte
                ++_Mid1;
                return _Mybase::error;
            } else if (_By < 0xe0u) {
                _Ch     = _By & 0x1f;
                _Nextra = 1;
            } else if (_By < 0xf0u) {
                _Ch     = _By & 0x0f;
                _Nextra = 2;
            } else if (_By < 0xf8u) {
                _Ch     = _By & 0x07;
                _Nextra = 3;
            } else {
                _Ch     = _By & 0x03;
                _Nextra = _By < 0xfc ? 4 : 5;
            }

            _Nskip  = _Nextra < 3 ? 0 : 1; // leave a byte for 2nd word
            _First1 = _Mid1; // roll back point

            if (_Nextra == 0) {
                ++_Mid1;
            } else if (_Last1 - _Mid1 < _Nextra + 1 - _Nskip) {
                break; // not enough input
            } else {
                for (++_Mid1; _Nskip < _Nextra; --_Nextra, ++_Mid1) {
                    if ((_By = static_cast<unsigned char>(*_Mid1)) < 0x80u || 0xc0u <= _By) {
                        return _Mybase::error; // not continuation byte
                    }

                    _Ch = _Ch << 6 | (_By & 0x3f);
                }
            }

            if (0 < _Nskip) {
                _Ch <<= 6; // get last byte on next call
            }

            if ((_Mymax < 0x10ffffu ? _Mymax : 0x10ffffu) < _Ch) {
                return _Mybase::error; // value too large
            }

            if (0xffffu < _Ch) { // deliver first half of two-word value, save second word
                unsigned short _Ch0 = static_cast<unsigned short>(0xd800 | (_Ch >> 10) - 0x0040);

                *_Mid2++ = static_cast<_Elem>(_Ch0);
                *_Pstate = static_cast<unsigned short>(0xdc00 | (_Ch & 0x03ff));
                continue;
            }

            if (_Nskip != 0) {
                if (_Mid1 == _Last1) { // not enough bytes, noncanonical value
                    _Mid1 = _First1;
                    break;
                }

                if ((_By = static_cast<unsigned char>(*_Mid1++)) < 0x80u || 0xc0u <= _By) {
                    return _Mybase::error; // not continuation byte
                }

                _Ch |= _By & 0x3f; // complete noncanonical value
            }

            if (*_Pstate == 0u) { // first time, maybe look for and consume header
                *_Pstate = 1;

                constexpr bool _Consuming = (_Mymode & consume_header) != 0;
                if constexpr (_Consuming) {
                    if (_Ch == 0xfeffu) { // drop header and retry
                        result _Ans = do_in(_State, _Mid1, _Last1, _Mid1, _First2, _Last2, _Mid2);

                        if (_Ans == _Mybase::partial) { // roll back header determination
                            *_Pstate = 0;
                            _Mid1    = _First1;
                        }

                        return _Ans;
                    }
                }
            }

            *_Mid2++ = static_cast<_Elem>(_Ch);
        }

        return _First1 == _Mid1 ? _Mybase::partial : _Mybase::ok;
    }

    result __CLR_OR_THIS_CALL do_out(_Statype& _State, const _Elem* _First1, const _Elem* _Last1, const _Elem*& _Mid1,
        _Byte* _First2, _Byte* _Last2, _Byte*& _Mid2) const override {
        // convert [_First1, _Last1) to bytes [_First2, _Last2)
        unsigned short* _Pstate = reinterpret_cast<unsigned short*>(&_State);
        _Mid1                   = _First1;
        _Mid2                   = _First2;

        while (_Mid1 != _Last1 && _Mid2 != _Last2) { // convert and put a wide char
            unsigned long _Ch;
            unsigned short _Ch1 = static_cast<unsigned short>(*_Mid1);
            bool _Save          = false;

            if (1u < *_Pstate) { // get saved MS 11 bits from *_Pstate
                if (_Ch1 < 0xdc00u || 0xe000u <= _Ch1) {
                    return _Mybase::error; // bad second word
                }

                _Ch = static_cast<unsigned long>((*_Pstate << 10) | (_Ch1 - 0xdc00));
            } else if (0xd800u <= _Ch1 && _Ch1 < 0xdc00u) { // get new first word
                _Ch   = static_cast<unsigned long>((_Ch1 - 0xd800 + 0x0040) << 10);
                _Save = true; // put only first byte, rest with second word
            } else {
                _Ch = _Ch1; // not first word, just put it
            }

            _Byte _By;
            int _Nextra;

            if (_Ch < 0x0080u) {
                _By     = static_cast<_Byte>(_Ch);
                _Nextra = 0;
            } else if (_Ch < 0x0800u) {
                _By     = static_cast<_Byte>(0xc0 | _Ch >> 6);
                _Nextra = 1;
            } else if (_Ch < 0x10000u) {
                _By     = static_cast<_Byte>(0xe0 | _Ch >> 12);
                _Nextra = 2;
            } else {
                _By     = static_cast<_Byte>(0xf0 | _Ch >> 18);
                _Nextra = 3;
            }

            int _Nput = _Nextra < 3 ? _Nextra + 1 : _Save ? 1 : 3;

            if (_Last2 - _Mid2 < _Nput) {
                break; // not enough room, even without header
            }

            if constexpr ((_Mymode & generate_header) != 0) { // header to put
                if (*_Pstate == 0u) {
                    if (_Last2 - _Mid2 < 3 + _Nput) {
                        break; // not enough room for header + output
                    }

                    // prepend header
                    *_Mid2++ = '\xef';
                    *_Mid2++ = '\xbb';
                    *_Mid2++ = '\xbf';
                }
            }

            ++_Mid1;
            if (_Save || _Nextra < 3) { // put first byte of sequence, if not already put
                *_Mid2++ = _By;
                --_Nput;
            }

            for (; 0 < _Nput; --_Nput) {
                *_Mid2++ = static_cast<_Byte>((_Ch >> 6 * --_Nextra & 0x3f) | 0x80);
            }

            *_Pstate = static_cast<unsigned short>(_Save ? _Ch >> 10 : 1);
        }

        return _First1 == _Mid1 ? _Mybase::partial : _Mybase::ok;
    }

    result __CLR_OR_THIS_CALL do_unshift(_Statype&, _Byte* _First2, _Byte*, _Byte*& _Mid2) const override {
        // generate bytes to return to default shift state
        _Mid2 = _First2;
        return _Mybase::noconv;
    }

    friend int _Codecvt_do_length<>(const codecvt_utf8_utf16&, _Statype&, const _Byte*, const _Byte*, size_t);

    int __CLR_OR_THIS_CALL do_length(
        _Statype& _State, const _Byte* _First1, const _Byte* _Last1, size_t _Count) const noexcept override {
        return _Codecvt_do_length(*this, _State, _First1, _Last1, _Count);
    }

    bool __CLR_OR_THIS_CALL do_always_noconv() const noexcept override {
        // return true if conversions never change input
        return false;
    }

    int __CLR_OR_THIS_CALL do_max_length() const noexcept override {
        // return maximum length required for a conversion
        if constexpr ((_Mymode & consume_header) != 0) {
            return 9; // header + max input
        } else if constexpr ((_Mymode & generate_header) != 0) {
            return 7; // header + max output
        } else {
            return 6; // 6-byte max input sequence, no 3-byte header
        }
    }

    int __CLR_OR_THIS_CALL do_encoding() const noexcept override {
        // return length of code sequence (from codecvt)
        return 0; // 0 => varying length
    }
};
_STL_RESTORE_DEPRECATED_WARNING
_STD_END
#pragma pop_macro("new")
_STL_RESTORE_CLANG_WARNINGS
#pragma warning(pop)
#pragma pack(pop)
#endif // _STL_COMPILER_PREPROCESSOR
#endif // _CODECVT_