What's a function you've used/wrote that brought you joy?

If you want to create permutations, most algorithms are recursive. That means if you only want the 10,000th permutation you have to calculate the 9999 permutations before it. I was interested in calculating random-access permutations so every permutation calculation takes the same amount of time. So I wrote an algorithm and wrote up the process here.

The first time I used recursion in production code and then reused it for something slightly different. It's basically a tree traversal that gives users a sequence of different choices that depend on the previous choice.