This year's competition is at this url.
I've learnt from last year and am using a simple LaTeX3 file for the core code with a document wrapper for the nice typesetting.
I'm using the same function paradigm whereby a "function" defines a
scope and so to return a value then there are certain output
registers (one of each type) that get set globally, and which can then
be read for the solution.
Part way through last time I implemented a caching system to save the
answers to a file and then read them back in at the start.
I've copied that across to this year.
The format of the cache is a LaTeX3 prop, so reading it back in
populates the prop from which the cached answers can be read (or
deleted if I need to regenerate one).