I tend to find that for things like this that are really math heavy, it's usually better to create a DSL (or create easily readable function calls, etc) that you can easily write yourself instead of relying on AI to understand math heavy rules. Bonus points, if the rules are in an easily editable format, you can change them easily when they need to. It seems that was the path the author took...

And yes this kind of use-case is exactly where unit tests shine...

> create a DSL (or create easily readable function calls, etc)

These aren't really that different. Consider the history of the earliest (non-assembly) programming languages, particularly https://en.wikipedia.org/wiki/Speedcoding , as well as the ideas expressed by Lisp.

I do the opposite, set up everything myself in terms of architecture/design of the software, so the AI can do the boring boilerplate like "math heavy rules". Always interesting to see how differently we all use LLMs.