convert_utf.cpp

/* <editor-fold desc="MIT License">

Copyright(c) 2022-2025 Robert Osfield, Chris Djali

Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the "Software"), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:

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</editor-fold> */

#include <vsg/io/convert_utf.h>

#include <cstdint>
#include <limits>

using namespace vsg;

////////////////////////////////////////////////////////////////////////////////////////////////////
//
// copy between UTF8 <-> UTF16
//
// https://en.wikipedia.org/wiki/UTF-8
// https://en.wikipedia.org/wiki/UTF-16
// OpenSceneGraph/src/osgText/String.cpp

// use count to avoid erroneous utf8 encodings causing reading beyond size of buffer.
template<typename Iterator, class Func>
bool decode_utf8(Iterator itr, size_t count, Func op)
{
    while (count > 0)
    {
        auto c0 = *itr++;
        --count;

        if ((c0 & 0x80) == 0) // 1-byte ascii character
        {
            op(c0);
            continue;
        }

        if (count == 0) return false;

        auto c1 = *itr++;
        --count;
        if ((c0 & 0xe0) == 0xc0) // 2-byte character
        {
            op(((c0 & 0x1f) << 6) | (c1 & 0x3f));
            continue;
        }

        if (count == 0) return false;

        auto c2 = *itr++;
        --count;
        if ((c0 & 0xf0) == 0xe0) // 3-byte character
        {
            op(((c0 & 0xf) << 12) | ((c1 & 0x3f) << 6) | (c2 & 0x3f));
            continue;
        }

        if (count == 0) return false;

        auto c3 = *itr++;
        --count;

        // 4-byte character. Could check for char0 < 0xf8 to ensure encoding is valid?
        op(((c0 & 0x7) << 18) | ((c1 & 0x3f) << 12) | ((c2 & 0x3f) << 6) | (c3 & 0x3f));
    }

    return true;
}

template<typename Iterator, class Func>
bool encode_utf8(Iterator itr, Iterator end, Func op)
{
    while (itr != end)
    {
        uint32_t c = *itr++;
        if (c < 0x80) // 1-byte ascii character
        {
            op(c);
            continue;
        }

        if (c < 0x800) // 2-byte character
        {
            op(0xc0 | ((c >> 6) & 0x1f)); // byte 1
            op(0x80 | (c & 0x3f));        // byte 2
            continue;
        }

        if (c < 0x10000) // 3-byte character
        {
            op(0xe0 | ((c >> 12) & 0x0f)); // byte 1
            op(0x80 | ((c >> 6) & 0x3f));  // byte 2
            op(0x80 | (c & 0x3f));         // byte 3
            continue;
        }

        // 4 byte character
        op(0xf0 | ((c >> 18) & 0x07)); // byte 1
        op(0x80 | ((c >> 12) & 0x3f)); // byte 2
        op(0x80 | ((c >> 6) & 0x3f));  // byte 3
        op(0x80 | (c & 0x3f));         // byte 4
    }

    return true;
}

template<typename Iterator, class Func>
bool decode_utf16(Iterator itr, size_t count, Func op)
{
    while (count > 0)
    {
        auto c0 = *itr++;
        --count;

        if ((c0 >= 0x0000 && c0 <= 0xD7FF) || (c0 >= 0xE000 && c0 <= 0xFFFF )) // 2-byte UCS2 character
        {
            op(c0);
            continue;
        }

        // unpaired surrogate
        if (count == 0 || c0 >= 0xDC00) return false;

        auto c1 = *itr++;
        --count;
        if (c1 >= 0xDC00 && c1 <= 0xDFFF) // 4-byte surrogate pair
        {
            op((((c0 - 0xD800) << 10) | (c1 - 0xDC00)) + 0x10000);
            continue;
        }
        else return false; // unpaired surrogate
    }

    return true;
}

template<typename Iterator, class Func>
bool encode_utf16(Iterator itr, Iterator end, Func op)
{
    while (itr != end)
    {
        uint32_t c = *itr++;
        if ((c >= 0x0000 && c <= 0xD7FF) || (c >= 0xE000 && c <= 0xFFFF )) // 2-byte UCS2 character
        {
            op(c);
            continue;
        }

        // unpaired surrogate
        if (c < 0x10000) return false;
        else // 4-byte surrogate pair
        {
            op(0xD800 + (((c - 0x10000) >> 10) & 0x3FF)); // high surrogate
            op(0xDC00 + ((c - 0x10000) & 0x3FF));  // low surrogate
            continue;
        }
    }

    return true;
}

void vsg::convert_utf(const std::string& utf8, std::wstring& dst)
{
    dst.clear();
    if constexpr (std::numeric_limits<wchar_t>::max() == 0xFFFF)
        decode_utf8(utf8.begin(), utf8.size(), [&dst](uint32_t c) { encode_utf16(&c, (&c) + 1, [&dst](uint32_t cu) { dst.push_back(cu); }); });
    else
        decode_utf8(utf8.begin(), utf8.size(), [&dst](uint32_t c) { dst.push_back(c); });
}

void vsg::convert_utf(const std::wstring& src, std::string& utf8)
{
    utf8.clear();
    if constexpr (std::numeric_limits<wchar_t>::max() == 0xFFFF)
    {
        std::u32string intermediate;
        decode_utf16(src.begin(), src.size(), [&intermediate](char32_t c) { intermediate.push_back(c); });
        encode_utf8(intermediate.begin(), intermediate.end(), [&utf8](char32_t c) { utf8.push_back(static_cast<char>(c)); });
    }
    else
        encode_utf8(src.begin(), src.end(), [&utf8](uint32_t c) { utf8.push_back(static_cast<char>(c)); });
}