Let's see how far I get with Elixir.
I chose to use an umbrella project. The apps folder contains has seperate Elixir applications for each day. Additionally there is a Utilities project where commonly used functions are made available to all days.
In the lib folder of each day my solution can be found. In the test folder the solver is run with the input.
Run mix test in the days project folder, or in the root dir to run the tests for all days.