build_packet.cpp

template <typename T>
concept requires_checksum = requires (T a, void *b, ptrdiff_t c, size_t d) {
	{ a.do_checksum(b, c, d) } -> std::same_as<void>;
};

template <typename ...Args>
mem::buffer build_packet(Args &&...args) {
	size_t total_size;
	auto parts = frg::make_tuple(std::forward<Args>(args)...);

	// collect size
	total_size = [&parts]<size_t ...I>(std::index_sequence<I...>) {
		return (parts.template get<I>().get_size() + ...);
	}(std::make_index_sequence<sizeof...(Args)>{});

	mem::buffer buf{total_size};

	// generate data
	void *dest = buf.data();
	[&parts, &dest]<size_t ...I>(std::index_sequence<I...>) {
		((dest = parts.template get<I>().to_bytes(dest)), ...);
	}(std::make_index_sequence<sizeof...(Args)>{});

	// generate do checksums
	dest = buf.data();
	[&]<size_t ...I>(std::index_sequence<I...>) {
		auto do_csum = [&buf, dest, total_size]<typename T>(T &item) {
			if constexpr (requires_checksum<T>) {
				ptrdiff_t off = reinterpret_cast<uintptr_t>(dest) - reinterpret_cast<uintptr_t>(buf.data());
				item.do_checksum(buf.data(), off, total_size);
			}
		};
		((do_csum(parts.template get<I>()), (dest = static_cast<uint8_t *>(dest) + parts.template get<I>().get_size())), ...);
	}(std::make_index_sequence<sizeof...(Args)>{});

	return buf;
}