Yeah JFC. I was so completely lost until I just ran a sample function logging the arguments and what was expected and it instantly made sense. What a convoluted mess of a description

why even mention books? Try reading it again substituting "medical conditions", "pokemon", or "opinions". Makes about the same amount of sense to me.

why are the codes 3,4,5 or more letters long?

I'd probably solve the problem quicker if just given the inputs and outputs and "good luck"

This problem is terribly written. It reads like one of those high-school level math problems that takes a very simple concept and makes it seem arcane and obscure through its poor use of language and symbolic abstraction.

I think the authors who write these sorts of "math textbook" problems think they're making it clear via symbolic abstraction, but they are in fact completely obfuscating it. This problem is pretty much "creating a running list of books by category" but somehow it manages to blather for paragraphs about c, M, L, and Clojure.

and the inputs don't even match up the ones given in the problem description! Jeebus, this needs to be rewritten. Shame, because it's actually a fun problem.

This isn't a very performanent solution...n^2