Hi @justyy, thanks a lot for your review and very detailed suggestions.
Re 1: Of course, I actually thought about this (especially for hacking prevention, but it isn't that simple. I mean without changing the original code too much. Since I'm an external contributor, so I've decided not to change too much the code. But I guess now I got some trust from Busy team :) so maybe more substantial change will come :)
Re 2: You're right, at that time, I forgot to add the test code.
Re 3: Again you're right. The reason why I just hardcoded it was the original code was with such magic numbers in some cases (of course when it's very simple and only used there once like in this case), so I followed their practice. As you can see, the entire class and file is quite short. But in general I totally agree with you. This is actually the difficulty of contributing to the already established open-source project as external contributor :) sometimes I don't agree with the style, but I think I should follow their style unless the change is absolutely needed.

Many thanks again!

Thank you for your review, @justyy! Keep up the good work!