Advent of Nix 2024

This is my go at Advent of Code (2024) in pure nix.

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.

Note, many solutions require increasing the max call depth and increasing the stack size. For all solutions, consider running them as:

$ ulimit -s unlimited
$ nix eval --option max-call-depth 4294967295 ...

You also likely want to run parts one at a time, such as nix eval '.#day01.part2' in order to reduce total resource usage; I only ever run them like such, so some solutions may OOM if both parts are run together.

I'm also running this on a machine with 64GiB of memory, so solutions may take up to that amount of memory.

I also have experimental-features = ca-derivations dynamic-derivations flakes nix-command recursive-nix set in my nix show-config, though I think only flakes nix-command are used in practice in this repo.

Running solutions with tvix

If for some reason you want to see how slow tvix is on something, you can do that too:

$ time ./run-tvix.sh day06.part1

19.77s
max memory:                2879 MB

# as compared to nix
$ time nix eval --option max-call-depth 4294967295 '.#day06.part1'
3.07s
max memory:                2139 MB

In general tvix seems to be significantly slower, but it's interesting to compare!

Since tvix doesn't support fetchGit yet, run-tvix will populate a 'nixpkgs' symlink in the repository to use automatically.

This is done using nix, so you need nix installed as well.

Runtime of all solutions

On my machine, running all solutions serially takes about 2 hours:

$ time nix run --option max-call-depth 4294967295 '.#check.all'

6537.53s  user 55.99s system 99% cpu 1:50:16.46 total
max memory:                40045 MB

This is on a decently fast desktop cpu (Ryzen 7 7700X), so YMMV.

I'll update these numbers if I ever bother to make them much faster, but it's all dominated by running reasonably efficient graph algorithms (dijkstra's) just being really slow in nix, so I feel like I'd need a breakthrough on how to implemnet Dijkstra's in a way that doesn't tank nix's performance in order to get the runtime under an hour.

Day specific notes

Day 06

Part 2 takes an hour and 20GiB of memory to run on my machine.

I know there's a more optimal solution, but you know, it's fine.

Day 09

Both parts take around 12GiB of memory and 1.5 minutes.

Day 11

~10 minutes, but not much memory!

Thank gosh for builtins.deepSeq to avoid 60+ GiBs of thunks!

Day 12

Part1 is 5 minutes.

Day 16

Part 1 is ~5 minutes.

Day 18

Part 1 took 37GiB of memory on my machine

Day 20

Both parts take ~7.5 minutes (with 20GiB of memory usage).

Performance charts

Day	CPU Time (s)	Wall Time (s)	Peak Memory (GB)
day01	.04	0.1	.04
day02	.07	0.07	.05
day03	.03	0.04	.04
day04	.13	0.14	.07
day05	.36	0.37	.23
day06	3657.90	2383.72	4.88
day07	.15	0.16	.08
day08	.09	0.1	.05
day09	180.70	143.12	8.44
day10	.07	0.08	.06
day11	561.84	562.52	.45
day12	703.25	540.01	2.74
day13	.12	0.12	.07
day14	42.51	32.9	2.05
day15	370.38	332.77	11.75
day16	605.60	529.05	19.95
day17	.06	0.07	.05
day18	244.87	223.38	25.93
day19	4.37	4.24	.44
day20	930.12	819.36	24.76
day21	.04	0.05	.05
day22	170.40	67.83	.54
day23	.63	0.63	.32
day24	7.40	7.34	.44
day25	.16	0.17	.11

xychart-beta
    title "CPU Time (User + System)"
    x-axis [day01, day02, day03, day04, day05, day06, day07, day08, day09, day10, day11, day12, day13, day14, day15, day16, day17, day18, day19, day20, day21, day22, day23, day24, day25]
    y-axis "Time (seconds)" 0 --> 3667
    bar [.04,.07,.03,.13,.36,3657.90,.15,.09,180.70,.07,561.84,703.25,.12,42.51,370.38,605.60,.06,244.87,4.37,930.12,.04,170.40,.63,7.40,.16]

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xychart-beta
    title "Wall Clock Time"
    x-axis [day01, day02, day03, day04, day05, day06, day07, day08, day09, day10, day11, day12, day13, day14, day15, day16, day17, day18, day19, day20, day21, day22, day23, day24, day25]
    y-axis "Time (seconds)" 0 --> 2393
    bar [0.1,0.07,0.04,0.14,0.37,2383.72,0.16,0.1,143.12,0.08,562.52,540.01,0.12,32.9,332.77,529.05,0.07,223.38,4.24,819.36,0.05,67.83,0.63,7.34,0.17]

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xychart-beta
    title "Peak Memory Usage"
    x-axis [day01, day02, day03, day04, day05, day06, day07, day08, day09, day10, day11, day12, day13, day14, day15, day16, day17, day18, day19, day20, day21, day22, day23, day24, day25]
    y-axis "Memory (GB)" 0 --> 26
    bar [.04,.05,.04,.07,.23,4.88,.08,.05,8.44,.06,.45,2.74,.07,2.05,11.75,19.95,.05,25.93,.44,24.76,.05,.54,.32,.44,.11]

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