Saveload: Add helper to conditionally save template functor

src/sl/saveload.cpp

@@ -2073,6 +2073,26 @@ std::pair<byte *, size_t> SlSaveToTempBufferRestore(uint8 state)
 	return result;
 }
 
+SlConditionallySaveState SlConditionallySaveSetup()
+{
+	assert(_sl.action == SLA_SAVE);
+	if (_sl.dumper->IsAutoLengthActive()) {
+		return { (size_t)(_sl.dumper->buf - _sl.dumper->autolen_buf), 0, true };
+	} else {
+		return { 0, SlSaveToTempBufferSetup(), false };
+	}
+}
+
+extern void SlConditionallySaveCompletion(const SlConditionallySaveState &state, bool save)
+{
+	if (state.nested) {
+		if (!save) _sl.dumper->buf = _sl.dumper->autolen_buf + state.current_len;
+	} else {
+		auto result = SlSaveToTempBufferRestore(state.need_length);
+		if (save) _sl.dumper->CopyBytes(result.first, result.second);
+	}
+}
+
 SlLoadFromBufferState SlLoadFromBufferSetup(const byte *buffer, size_t length)
 {
 	assert(_sl.action == SLA_LOAD || _sl.action == SLA_LOAD_CHECK);

src/sl/saveload.h

@@ -747,6 +747,29 @@ std::vector<uint8> SlSaveToVector(F proc)
 	return std::vector<uint8>(result.begin(), result.end());
 }
 
+struct SlConditionallySaveState {
+	size_t current_len;
+	uint8 need_length;
+	bool nested;
+};
+
+/**
+ * Run proc, saving as normal if proc returns true, otherwise the saved data is discarded
+ * @param proc The callback procedure that is called, this should return true to save, false to discard
+ * @return Whether the callback procedure returned true to save
+ */
+template <typename F>
+bool SlConditionallySave(F proc)
+{
+	extern SlConditionallySaveState SlConditionallySaveSetup();
+	extern void SlConditionallySaveCompletion(const SlConditionallySaveState &state, bool save);
+
+	SlConditionallySaveState state = SlConditionallySaveSetup();
+	bool save = proc();
+	SlConditionallySaveCompletion(state, save);
+	return save;
+}
+
 struct SlLoadFromBufferState {
 	size_t old_obj_len;
 	byte *old_bufp;

src/sl/saveload_buffer.h

@@ -259,6 +259,7 @@ struct MemoryDumper {
 	size_t GetSize() const;
 	void StartAutoLength();
 	std::pair<byte *, size_t> StopAutoLength();
+	bool IsAutoLengthActive() const { return this->saved_buf != nullptr; }
 };
 
 #endif /* SL_SAVELOAD_BUFFER_H */