XMLEscape.hh

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#ifndef XMLESCAPE_HH
#define XMLESCAPE_HH
 
#include <array>
#include <string>
#include <string_view>
 
// The following routines do the following substitutions:
//  & -> &amp;   must always be done
//  < -> &lt;    must always be done
//  > -> &gt;    always allowed, but must be done when it appears as ]]>
//  ' -> &apos;  always allowed, but must be done inside quoted attribute
//  " -> &quot;  always allowed, but must be done inside quoted attribute
//  all chars less than 32 -> &#xnn;
// So to simplify things we always do these 5+32 substitutions.
 
// This is a low-level version. It takes an input string and an output-functor.
// That functor is called (possibly) multiple times, each with a (small) chunk
// for the final output (chunk are either parts of the input or xml escape
// sequences).
template<typename Output>
inline void XMLEscape(std::string_view s, Output output)
{
        auto normal = [&](auto f, auto l) {
                // c++20 std::string_view(f, l);
                return std::string_view(&*f, l - f);
        };
 
        auto chunk = s.begin();
        auto last = s.end();
        auto it = chunk;
        while (it != last) {
                char c = *it;
                if (auto uc = static_cast<unsigned char>(c); uc < 32) {
                        output(normal(chunk, it));
                        std::array<char, 6> buf = {'&', '#', 'x', '0', '0', ';'};
                        auto hex = [](unsigned x) { return (x < 10) ? char(x + '0') : char(x - 10 + 'a'); };
                        buf[3] = hex(uc / 16);
                        buf[4] = hex(uc % 16);
                        output(std::string_view(buf.data(), sizeof(buf)));
                        chunk = ++it;
                } else if (c == '<') {
                        output(normal(chunk, it));
                        output("&lt;");
                        chunk = ++it;
                } else if (c == '>') {
                        output(normal(chunk, it));
                        output("&gt;");
                        chunk = ++it;
                } else if (c == '&') {
                        output(normal(chunk, it));
                        output("&amp;");
                        chunk = ++it;
                } else if (c == '"') {
                        output(normal(chunk, it));
                        output("&quot;");
                        chunk = ++it;
                } else if (c == '\'') {
                        output(normal(chunk, it));
                        output("&apos;");
                        chunk = ++it;
                } else {
                        // no output yet
                        ++it;
                }
        }
        output(normal(chunk, last));
}
 
// Like above, but produces the output as a (single) std::string.
inline std::string XMLEscape(std::string_view s)
{
        std::string result;
        XMLEscape(s, [&](std::string_view chunk) { result += chunk; });
        return result;
}
 
#endif