I had a hard time convincing myself that the new logic in this PR is correct, so eventually I rewrote it in 00026c6, hopefully more clearly. I did use your tests though; thanks!

Clarity is in the eye of the beholder apparently haha
My brain seems to be wired in a more functional programming way than is usually considered Pythonic I guess.
I tend to favour complex list comprehensions over for loops that touch 4 variables.
Glad it's fixed either way :)

Clarity is in the eye of the beholder apparently haha
My brain seems to be wired in a more functional programming way than is usually considered Pythonic I guess.

It's mostly the use of arithmetic operations on bools (which most functional languages I know don't allow). The original code that I wrote did it, and that was pretty confusing (I think you actually got confused at first). I had to read it a few times to convince myself that it's correct, so I thought I might as well rewrite it.