Quick sort comes to mind. It’s beautiful. Bisecting stuff just feels like magic; an universal algorithm to look for problems and zero in on them.

I’ll nominate the following way to generate all permutations:

def permutations([]), do: [[]]
def permutations(list), do: for x <- list, rest <- permutations(list -- [x]), do: [x | rest]

I expected it to be so much more complicated than it was!

It’s one of those things that sounds simple and has a solution that looks simple, but actually coming up with the solution is less obvious.

Yes! This is actually 100% analogous to a problem that I deal with frequently at work: do two time ranges overlap? So our codebase is littered with this kind of check.

This is also (partially) why I wrote CompareChain. This kind of bounds checking is so much more annoying when you can’t use the infix structural operators like >=.

While I’m at it, have you come across the Allen's interval algebra - Wikipedia? It’s a generalization of the Law of trichotomy - Wikipedia to 1D intervals. In interval-land, two intervals fall into one of 13 specific cases (or 26 if you allow 0-width intervals) instead of just 3. This isn’t from The Book per se, but the number 13 always struck me as surprising for some reason.