generic_codecvt.hpp

//
// Copyright (c) 2015 Artyom Beilis (Tonkikh)
// Copyright (c) 2021-2023 Alexander Grund
//
// Distributed under the Boost Software License, Version 1.0.
// https://www.boost.org/LICENSE_1_0.txt
 
#ifndef BOOST_LOCALE_GENERIC_CODECVT_HPP
#define BOOST_LOCALE_GENERIC_CODECVT_HPP
 
#include <boost/locale/utf.hpp>
#include <cstdint>
#include <locale>
 
namespace boost { namespace locale {
 
    static_assert(sizeof(std::mbstate_t) >= 2, "std::mbstate_t is to small to store an UTF-16 codepoint");
    namespace detail {
        // Avoid including cstring for std::memcpy
        inline void copy_uint16_t(void* dst, const void* src)
        {
            unsigned char* cdst = static_cast<unsigned char*>(dst);
            const unsigned char* csrc = static_cast<const unsigned char*>(src);
            cdst[0] = csrc[0];
            cdst[1] = csrc[1];
        }
        inline uint16_t read_state(const std::mbstate_t& src)
        {
            uint16_t dst;
            copy_uint16_t(&dst, &src);
            return dst;
        }
        inline void write_state(std::mbstate_t& dst, const uint16_t src)
        {
            copy_uint16_t(&dst, &src);
        }
    } // namespace detail

    class generic_codecvt_base {
    public:
        enum initial_convertion_state {
            to_unicode_state,  
            from_unicode_state 
        };
    };

    template<typename CharType, typename CodecvtImpl, int CharSize = sizeof(CharType)>
    class generic_codecvt;

    template<typename CharType, typename CodecvtImpl>
    class generic_codecvt<CharType, CodecvtImpl, 2> : public std::codecvt<CharType, char, std::mbstate_t>,
                                                      public generic_codecvt_base {
    public:
        typedef CharType uchar;
 
        generic_codecvt(size_t refs = 0) : std::codecvt<CharType, char, std::mbstate_t>(refs) {}
        const CodecvtImpl& implementation() const { return *static_cast<const CodecvtImpl*>(this); }
 
    protected:
        std::codecvt_base::result do_unshift(std::mbstate_t& s, char* from, char* /*to*/, char*& next) const override
        {
            if(*reinterpret_cast<char*>(&s) != 0)
                return std::codecvt_base::error;
            next = from;
            return std::codecvt_base::ok;
        }
        int do_encoding() const noexcept override { return 0; }
        int do_max_length() const noexcept override { return implementation().max_encoding_length(); }
        bool do_always_noconv() const noexcept override { return false; }
 
        int do_length(std::mbstate_t& std_state, const char* from, const char* from_end, size_t max) const override
        {
            bool state = *reinterpret_cast<char*>(&std_state) != 0;
            const char* save_from = from;
 
            auto cvt_state = implementation().initial_state(to_unicode_state);
            while(max > 0 && from < from_end) {
                const char* prev_from = from;
                const utf::code_point ch = implementation().to_unicode(cvt_state, from, from_end);
                if(ch == boost::locale::utf::incomplete || ch == boost::locale::utf::illegal) {
                    from = prev_from;
                    break;
                }
                max--;
                if(ch > 0xFFFF) {
                    if(!state)
                        from = prev_from;
                    state = !state;
                }
            }
            *reinterpret_cast<char*>(&std_state) = state;
            return static_cast<int>(from - save_from);
        }
 
        std::codecvt_base::result do_in(std::mbstate_t& std_state,
                                        const char* from,
                                        const char* from_end,
                                        const char*& from_next,
                                        uchar* to,
                                        uchar* to_end,
                                        uchar*& to_next) const override
        {
            std::codecvt_base::result r = std::codecvt_base::ok;
 
            // mbstate_t is POD type and should be initialized to 0 (i.a. state = stateT())
            // according to standard. We use it to keep a flag 0/1 for surrogate pair writing
            //
            // If true then only the high surrogate of a codepoint > 0xFFFF was written, but no input consumed.
            bool low_surrogate_pending = *reinterpret_cast<char*>(&std_state) != 0;
            auto cvt_state = implementation().initial_state(to_unicode_state);
            while(to < to_end && from < from_end) {
                const char* from_saved = from;
 
                utf::code_point ch = implementation().to_unicode(cvt_state, from, from_end);
 
                if(ch == boost::locale::utf::illegal) {
                    from = from_saved;
                    r = std::codecvt_base::error;
                    break;
                }
                if(ch == boost::locale::utf::incomplete) {
                    from = from_saved;
                    r = std::codecvt_base::partial;
                    break;
                }
                // Normal codepoints go directly to stream
                if(ch <= 0xFFFF)
                    *to++ = static_cast<uchar>(ch);
                else {
                    // For other codepoints we can't consume our input as we may find ourselves in a state
                    // where all input is consumed but not all output written, i.e. only the high surrogate is written.
                    //
                    // So we write only the high surrogate and mark this in the state.
                    // We also set the from pointer to the previous position, i.e. don't consume the input, so this
                    // codepoint will be read again and then we will consume our input together with writing the low
                    // surrogate.
                    ch -= 0x10000;
                    const std::uint16_t w1 = static_cast<std::uint16_t>(0xD800 | (ch >> 10));
                    const std::uint16_t w2 = static_cast<std::uint16_t>(0xDC00 | (ch & 0x3FF));
                    if(!low_surrogate_pending) {
                        from = from_saved;
                        *to++ = w1;
                    } else
                        *to++ = w2;
                    low_surrogate_pending = !low_surrogate_pending;
                }
            }
            from_next = from;
            to_next = to;
            if(r == std::codecvt_base::ok && (from != from_end || low_surrogate_pending))
                r = std::codecvt_base::partial;
            *reinterpret_cast<char*>(&std_state) = low_surrogate_pending;
            return r;
        }
 
        std::codecvt_base::result do_out(std::mbstate_t& std_state,
                                         const uchar* from,
                                         const uchar* from_end,
                                         const uchar*& from_next,
                                         char* to,
                                         char* to_end,
                                         char*& to_next) const override
        {
            std::codecvt_base::result r = std::codecvt_base::ok;
            // mbstate_t is POD type and should be initialized to 0 (i.a. state = stateT())
            // according to standard. We assume that sizeof(mbstate_t) >=2 in order
            // to be able to store first observed surrogate pair
            //
            // State: state!=0 - a first surrogate pair was observed (state = first pair),
            // we expect the second one to come and then zero the state
            std::uint16_t state = detail::read_state(std_state);
            auto cvt_state = implementation().initial_state(from_unicode_state);
            while(to < to_end && from < from_end) {
                utf::code_point ch = 0;
                if(state != 0) {
                    // if the state indicates that 1st surrogate pair was written
                    // we should make sure that the second one that comes is actually
                    // second surrogate
                    std::uint16_t w1 = state;
                    std::uint16_t w2 = *from;
                    // we don't forward from as writing may fail to incomplete or
                    // partial conversion
                    if(0xDC00 <= w2 && w2 <= 0xDFFF) {
                        std::uint16_t vh = w1 - 0xD800;
                        std::uint16_t vl = w2 - 0xDC00;
                        ch = ((uint32_t(vh) << 10) | vl) + 0x10000;
                    } else {
                        // Invalid surrogate
                        r = std::codecvt_base::error;
                        break;
                    }
                } else {
                    ch = *from;
                    if(0xD800 <= ch && ch <= 0xDBFF) {
                        // if this is a first surrogate pair we put
                        // it into the state and consume it, note we don't
                        // go forward as it should be illegal so we increase
                        // the from pointer manually
                        state = static_cast<uint16_t>(ch);
                        from++;
                        continue;
                    } else if(0xDC00 <= ch && ch <= 0xDFFF) {
                        // if we observe second surrogate pair and
                        // first only may be expected we should break from the loop with error
                        // as it is illegal input
                        r = std::codecvt_base::error;
                        break;
                    }
                }
                if(!boost::locale::utf::is_valid_codepoint(ch)) {
                    r = std::codecvt_base::error;
                    break;
                }
                const utf::code_point len = implementation().from_unicode(cvt_state, ch, to, to_end);
                if(len == boost::locale::utf::incomplete) {
                    r = std::codecvt_base::partial;
                    break;
                } else if(len == boost::locale::utf::illegal) {
                    r = std::codecvt_base::error;
                    break;
                } else
                    to += len;
                state = 0;
                from++;
            }
            from_next = from;
            to_next = to;
            if(r == std::codecvt_base::ok && (from != from_end || state != 0))
                r = std::codecvt_base::partial;
            detail::write_state(std_state, state);
            return r;
        }
    };

    template<typename CharType, typename CodecvtImpl>
    class generic_codecvt<CharType, CodecvtImpl, 4> : public std::codecvt<CharType, char, std::mbstate_t>,
                                                      public generic_codecvt_base {
    public:
        typedef CharType uchar;
 
        generic_codecvt(size_t refs = 0) : std::codecvt<CharType, char, std::mbstate_t>(refs) {}
 
        const CodecvtImpl& implementation() const { return *static_cast<const CodecvtImpl*>(this); }
 
    protected:
        std::codecvt_base::result
        do_unshift(std::mbstate_t& /*s*/, char* from, char* /*to*/, char*& next) const override
        {
            next = from;
            return std::codecvt_base::ok;
        }
        int do_encoding() const noexcept override { return 0; }
        int do_max_length() const noexcept override { return implementation().max_encoding_length(); }
        bool do_always_noconv() const noexcept override { return false; }
 
        int do_length(std::mbstate_t& /*state*/, const char* from, const char* from_end, size_t max) const override
        {
            const char* start_from = from;
            auto cvt_state = implementation().initial_state(to_unicode_state);
            while(max > 0 && from < from_end) {
                const char* save_from = from;
                const utf::code_point ch = implementation().to_unicode(cvt_state, from, from_end);
                if(ch == boost::locale::utf::incomplete || ch == boost::locale::utf::illegal) {
                    from = save_from;
                    break;
                }
                max--;
            }
 
            return static_cast<int>(from - start_from);
        }
 
        std::codecvt_base::result do_in(std::mbstate_t& /*state*/,
                                        const char* from,
                                        const char* from_end,
                                        const char*& from_next,
                                        uchar* to,
                                        uchar* to_end,
                                        uchar*& to_next) const override
        {
            std::codecvt_base::result r = std::codecvt_base::ok;
 
            auto cvt_state = implementation().initial_state(to_unicode_state);
            while(to < to_end && from < from_end) {
                const char* from_saved = from;
 
                const utf::code_point ch = implementation().to_unicode(cvt_state, from, from_end);
 
                if(ch == boost::locale::utf::illegal) {
                    r = std::codecvt_base::error;
                    from = from_saved;
                    break;
                }
                if(ch == boost::locale::utf::incomplete) {
                    r = std::codecvt_base::partial;
                    from = from_saved;
                    break;
                }
                *to++ = ch;
            }
            from_next = from;
            to_next = to;
            if(r == std::codecvt_base::ok && from != from_end)
                r = std::codecvt_base::partial;
            return r;
        }
 
        std::codecvt_base::result do_out(std::mbstate_t& /*std_state*/,
                                         const uchar* from,
                                         const uchar* from_end,
                                         const uchar*& from_next,
                                         char* to,
                                         char* to_end,
                                         char*& to_next) const override
        {
            std::codecvt_base::result r = std::codecvt_base::ok;
            auto cvt_state = implementation().initial_state(from_unicode_state);
            while(to < to_end && from < from_end) {
                const std::uint32_t ch = *from;
                if(!boost::locale::utf::is_valid_codepoint(ch)) {
                    r = std::codecvt_base::error;
                    break;
                }
                const utf::code_point len = implementation().from_unicode(cvt_state, ch, to, to_end);
                if(len == boost::locale::utf::incomplete) {
                    r = std::codecvt_base::partial;
                    break;
                } else if(len == boost::locale::utf::illegal) {
                    r = std::codecvt_base::error;
                    break;
                }
                to += len;
                from++;
            }
            from_next = from;
            to_next = to;
            if(r == std::codecvt_base::ok && from != from_end)
                r = std::codecvt_base::partial;
            return r;
        }
    };
 
    template<typename CodecvtImpl>
    class generic_codecvt<char, CodecvtImpl, 1> : public std::codecvt<char, char, std::mbstate_t>,
                                                  public generic_codecvt_base {
    public:
        typedef char uchar;
 
        const CodecvtImpl& implementation() const { return *static_cast<const CodecvtImpl*>(this); }
 
        generic_codecvt(size_t refs = 0) : std::codecvt<char, char, std::mbstate_t>(refs) {}
    };
 
}} // namespace boost::locale
 
#endif