Add StringBuilder class from upstream

src/strings_internal.h

@@ -248,4 +248,104 @@ static auto MakeParameters(const Args&... args)
 	return parameters;
 }
 
+/**
+ * Equivalent to the std::back_insert_iterator in function, with some
+ * convenience helpers for string concatenation.
+ */
+class StringBuilder {
+	std::string *string;
+
+public:
+	/* Required type for this to be an output_iterator; mimics std::back_insert_iterator. */
+	using value_type = void;
+	using difference_type = void;
+	using iterator_category = std::output_iterator_tag;
+	using pointer = void;
+	using reference = void;
+
+	/**
+	 * Create the builder of an external buffer.
+	 * @param start The start location to write to.
+	 * @param last  The last location to write to.
+	 */
+	StringBuilder(std::string &string) : string(&string) {}
+
+	/* Required operators for this to be an output_iterator; mimics std::back_insert_iterator, which has no-ops. */
+	StringBuilder &operator++() { return *this; }
+	StringBuilder operator++(int) { return *this; }
+	StringBuilder &operator*() { return *this; }
+
+	/**
+	 * Operator to add a character to the end of the buffer. Like the back
+	 * insert iterators this also increases the position of the end of the
+	 * buffer.
+	 * @param value The character to add.
+	 * @return Reference to this inserter.
+	 */
+	StringBuilder &operator=(const char value)
+	{
+		return this->operator+=(value);
+	}
+
+	/**
+	 * Operator to add a character to the end of the buffer.
+	 * @param value The character to add.
+	 * @return Reference to this inserter.
+	 */
+	StringBuilder &operator+=(const char value)
+	{
+		this->string->push_back(value);
+		return *this;
+	}
+
+	/**
+	 * Operator to append the given string to the output buffer.
+	 * @param str The string to add.
+	 * @return Reference to this inserter.
+	 */
+	StringBuilder &operator+=(std::string_view str)
+	{
+		*this->string += str;
+		return *this;
+	}
+
+	/**
+	 * Encode the given Utf8 character into the output buffer.
+	 * @param c The character to encode.
+	 */
+	void Utf8Encode(char32_t c)
+	{
+		auto iterator = std::back_inserter(*this->string);
+		::Utf8Encode(iterator, c);
+	}
+
+	/**
+	 * Remove the given amount of characters from the back of the string.
+	 * @param amount The amount of characters to remove.
+	 * @return true iff there was enough space and the character got added.
+	 */
+	void RemoveElementsFromBack(size_t amount)
+	{
+		this->string->erase(this->string->size() - std::min(amount, this->string->size()));
+	}
+
+	/**
+	 * Get the current index in the string.
+	 * @return The index.
+	 */
+	size_t CurrentIndex()
+	{
+		return this->string->size();
+	}
+
+	/**
+	 * Get the reference to the character at the given index.
+	 * @return The reference to the character.
+	 */
+	char &operator[](size_t index)
+	{
+		return (*this->string)[index];
+	}
+};
+
 #endif /* STRINGS_INTERNAL_H */