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            return 4;
        }
        CATCH_INTERNAL_ERROR("Invalid multibyte utf-8 start byte encountered");
    }

    uint32_t headerValue(unsigned char c) {
        if ((c & 0xE0) == 0xC0) {
            return c & 0x1F;
        }
        if ((c & 0xF0) == 0xE0) {
            return c & 0x0F;
        }
        if ((c & 0xF8) == 0xF0) {
            return c & 0x07;
        }
        CATCH_INTERNAL_ERROR("Invalid multibyte utf-8 start byte encountered");
    }

    void hexEscapeChar(std::ostream& os, unsigned char c) {
        os << "\\x"
            << std::uppercase << std::hex << std::setfill('0') << std::setw(2)
            << static_cast<int>(c);
    }

} // anonymous namespace

    XmlEncode::XmlEncode( std::string const& str, ForWhat forWhat )
    :   m_str( str ),
        m_forWhat( forWhat )
    {}

    void XmlEncode::encodeTo( std::ostream& os ) const {
        // Apostrophe escaping not necessary if we always use " to write attributes
        // (see: http://www.w3.org/TR/xml/#syntax)

        for( std::size_t idx = 0; idx < m_str.size(); ++ idx ) {
            uchar c = m_str[idx];
            switch (c) {
            case '<':   os << "&lt;"; break;
            case '&':   os << "&amp;"; break;

            case '>':
                // See: http://www.w3.org/TR/xml/#syntax
                if (idx > 2 && m_str[idx - 1] == ']' && m_str[idx - 2] == ']')
                    os << "&gt;";
                else
                    os << c;
                break;

            case '\"':
                if (m_forWhat == ForAttributes)
                    os << "&quot;";
                else
                    os << c;
                break;

            default:
                // Check for control characters and invalid utf-8

                // Escape control characters in standard ascii
                // see http://stackoverflow.com/questions/404107/why-are-control-characters-illegal-in-xml-1-0
                if (c < 0x09 || (c > 0x0D && c < 0x20) || c == 0x7F) {
                    hexEscapeChar(os, c);
                    break;
                }

                // Plain ASCII: Write it to stream
                if (c < 0x7F) {
                    os << c;
                    break;
                }

                // UTF-8 territory
                // Check if the encoding is valid and if it is not, hex escape bytes.
                // Important: We do not check the exact decoded values for validity, only the encoding format
                // First check that this bytes is a valid lead byte:
                // This means that it is not encoded as 1111 1XXX
                // Or as 10XX XXXX
                if (c <  0xC0 ||
                    c >= 0xF8) {
                    hexEscapeChar(os, c);
                    break;
                }

                auto encBytes = trailingBytes(c);
                // Are there enough bytes left to avoid accessing out-of-bounds memory?
                if (idx + encBytes - 1 >= m_str.size()) {
                    hexEscapeChar(os, c);
                    break;
                }
                // The header is valid, check data
                // The next encBytes bytes must together be a valid utf-8
                // This means: bitpattern 10XX XXXX and the extracted value is sane (ish)
                bool valid = true;
                uint32_t value = headerValue(c);
                for (std::size_t n = 1; n < encBytes; ++n) {
                    uchar nc = m_str[idx + n];
                    valid &= ((nc & 0xC0) == 0x80);
                    value = (value << 6) | (nc & 0x3F);
                }

                if (
                    // Wrong bit pattern of following bytes
                    (!valid) ||
                    // Overlong encodings
                    (value < 0x80) ||
                    (0x80 <= value && value < 0x800   && encBytes > 2) ||
                    (0x800 < value && value < 0x10000 && encBytes > 3) ||
                    // Encoded value out of range
                    (value >= 0x110000)
                    ) {
                    hexEscapeChar(os, c);
                    break;
                }

                // If we got here, this is in fact a valid(ish) utf-8 sequence
                for (std::size_t n = 0; n < encBytes; ++n) {
                    os << m_str[idx + n];
                }
                idx += encBytes - 1;
                break;