So there's nearly as many answers to your question as there are gamedevs. I've seen quite a few use an OO modeling approach. That's fallen out of favor for goo

So there's nearly as many answers to your question as there are gamedevs. I've seen quite a few use an OO modeling approach. That's fallen out of favor for good reason. Another approach is Entity Component Systems. I suggest googling for one of the better blog posts on that. Another approach is to build the equivalent of a datalog engine. An approach I favor is to have a stack per entity, and as you parse input events things get stacked up. Then a processing pass iterates across popping things off the stack. If you know how MTG rules work it's roughly the same idea. None of these are a panacea, because you can have logical contradictions in your ruleset that are subtle and only emerge in very niche situations. It's definitely very difficult problem. What you suggest is similar to what I termed the datalog style. If you've ever tried to debug a prolog or datalog program, you'll know that it all being in one file is a trivial concern compared to the emergent complexity and potential for you to have misstated your logic or included contradictions.