[XMLWriter] Add a new class for writing XML.

Should have a much lower memory footprint than the ptree based writer.

H2Codez/util/XMLWriter.h

@@ -0,0 +1,81 @@
+#pragma once
+#include <ostream>
+#include <functional>
+#include <optional>
+#include <variant>
+#include <map>
+#include "FastString.h"
+
+class XMLWriter {
+public:
+	XMLWriter() = delete;
+	XMLWriter(std::ostream& stream) : _stream(stream)
+	{};
+	~XMLWriter() {
+		_stream.get().flush();
+	}
+
+	inline void write_self_closing(FastString name, FastString contents, std::optional<std::map<FastString, FastString>> attributes = std::nullopt) {
+		start_element(name, attributes, true);
+		_stream.get() << contents.get() << " >\n";
+	}
+
+	inline void write(FastString name, std::function<void(std::map<FastString, FastString>&)> setup, std::function<bool(XMLWriter&)> element) {
+		std::map<FastString, FastString> attributes;
+		setup(attributes);
+
+		start_element(name, attributes, false);
+
+		indent_level++;
+		while (element(*this)) {};
+		indent_level--;
+
+		write_end_element(name);
+	}
+
+	inline void write(FastString name, std::function<bool(XMLWriter&)> element, std::optional<std::map<FastString, FastString>> attributes = std::nullopt) {
+		start_element(name, attributes, false);
+
+		indent_level++;
+		while (element(*this)) {};
+		indent_level--;
+
+		write_end_element(name);
+	}
+
+
+private:
+
+	inline void write_indent(int additional = 0) {
+		std::fill_n(std::ostream_iterator<char>(_stream), indent_level + additional, '\t');
+	}
+
+	inline void start_element(const FastString &element, std::optional <std::map<FastString, FastString>> attributes, bool self_closing) {
+
+		write_indent();
+
+		_stream.get() << "<" << element.get();
+
+		if (attributes)
+			write_attributes(attributes.value());
+
+		if (!self_closing)
+			_stream.get() << ">\n";
+		else
+			_stream.get() << " ";
+	}
+
+	inline void write_end_element(const FastString& element) {
+		write_indent();
+		_stream.get() << "<\\" << element.get() << ">\n";
+	}
+
+	template <typename T>
+	inline void write_attributes(T attributes) {
+		for (std::pair<FastString, FastString> atpair : attributes)
+			_stream.get() << " " << atpair.first.get() << " = \"" << atpair.second.get() << "\"";
+	}
+
+	int indent_level = 0;
+	std::reference_wrapper<std::ostream> _stream;
+};