About the "terrible" aspect of it, to quote: "Granted it’s not very readable and log / pow probably makes it less efficient than other solutions. But there were no loops and almost no branching which I thought was pretty neat." No need to insult the author over it.

About your "proper simple solution": I don't think that's a good idea either. Based on your next paragraph, the version you suggest with the handwritten binary search and "likely" annotations is for the case where the code isn't performance critical: for where the code is performance critical, you suggest a different solution. If the code isn't performance critical, please do not turn it into an unreadable mess over what would become a negligible overall performance gain. Write it in a simple, obviously correct way, keep it boring, and you'll keep it stable; you can use the time you save on fixing bugs in your super optimised version on improving more critical parts of your program.