Advent of Nix 2021

This is my attempt to solve all the Advent of Code 2021 problems in pure nix.

I wasn't trying to make efficient solutions, just efficient enough to complete and get me a successful answer. This was primarily to just play around with nix, not to code golf or do anything fancy.

Running solutions

In general, nix eval '.#dayX' (where 'X' is the number of the day, padded to length 2, such as nix eval '.#day03') will display the answer to a given day.

Specific day's notes

day 05

This one is the first one where the performance is bad. Really bad.

Bad enough that you need to run it specially:

$ ulimit -s unlimited
$ nix eval '.#day05.part1'
$ nix eval '.#day05.part2'

Running the parts separately helps, and removing the stack size limit is needed for part 2.

On my computer, it takes around 4 minutes to complete. It takes more than 15GiB of memory as well.

I may come back to this one to optimize the solution further since having to increase the stack size seems far from ideal.

day 13 - part 2

This is the first part where the answer needs specific formatting.

Using --raw or --json | jq '.' -r will provide correct formatting. For example:

$ nix eval '.#day13.part2' --raw

Also, you need a fixed width terminal, but that's kinda a given.

day 15 - part 2

This one takes on the order of 8 minutes on my machine, with ~40GiB of memory usage.

day 19

This one's another real slow one. It takes 13 minutes per part on my machine.

day 20 - part 2

Slow. 8 minutes on my machine.

day 21 - part 2

We're on a roll with slow implementations that run just quick enough to complete. 5 minutes on my machine, and it took ~60GiB of RSS.

From here on, I'll stop noting slow runtimes unless they're really big.

day 24

Each part takes over 30 minutes to run. Really big runtime.