I decided to use this year's Advent of Code to learn Scheme. I chose to use the Guile implementation and tried use SRFIs and not Guile modules whenever possible.
There is an extensive write-up and description of an algorithm in the day 8 readme, which I may one day turn into a blog post.
Activate the nix development shell via direnv, or manually with nix develop in the root directory. (You can also run nix develop .#clojure or nix develop .#guile. The default development shell has both.)
To run a Guile program, say day1, run guile -s src/day1.scm data/day1.txt.
To run a Clojure program, say day9, run clj -M -m day9 data/day9.txt.