Thoughts on Advent of Code + Rust

Diego wrote about his dislike for Advent of Code and that reminded me I hadn’t written up my experience from 2023. Mostly because, spoiler, I never actually completed it and always intended to do so and then write it up. I think it’s time to accept I’m not going to do that, and write down some thoughts before I forget all of them. These are somewhat vague, given the time that’s elapsed, but I think still relevant. You might also find Roger’s problem write up interesting.

I’ve tried AoC a couple of times before; I think I had a very brief attempt back in 2021, and I got 4 days in for 2022. For Advent of Code 2023 I tried much harder to actually complete the challenges, and got most of the way there. I didn’t allow myself to move on to the next day until fully completing the previous day, and didn’t end up doing the second half of December 24th, or any of December 25th.

Rust

First I want to talk about Rust, which is the language I chose to use for the problems. I’ve dabbled a little in it, but I’d like more familiarity with the basic language, and some programming problems seemed like a good way to get that. It’s a language I want to like; I’ve spent a lot of my career writing C, do more in Go these days, and generally think Rust promises a low level, run-time light environment like C but with the rough edges taken off.

I set myself the challenge of using just bare Rust; no external crates, no use of cargo. I was accused of playing on hard mode by doing this, but it really wasn’t the intention – I figured that I should be able to do what I needed without recourse to anything outside the core language, and didn’t want what seemed like the extra complexity of dealing with cargo.

That caused problems, however. I’m used to by-default generic error handling in Go through the error type, but Rust seems to have much more tightly typed errors. I was pointed at anyhow as the right way to do this in Rust. I still find this surprising; I ended up using unwrap() a lot when I think with more generic error handling I could have used ?.

The other thing I discovered is that by default rustc is heavy on the debug output. I got significantly better results on some of the solutions with rustc -O -C target-cpu=native source.rs. I probably shouldn’t be surprised by this, but worth noting.

Rust, to me, has a syntax only a C++ programmer could love. I am not a C++ programmer. Coming from C I found Go to be a nice, simple syntax to learn. Rust has not been the same. There’s a lot more punctuation, and it’s not always clear to me what it’s doing. This applies more when reading other people’s code than when writing it myself, obviously, but I see a lot of Rust code that could give Perl a run for its money in terms of looking like line noise.

The borrow checker didn’t bug me too much, but did add overhead to my thinking. The Rust compiler is generally very good at outputting helpful error messages when the programmer is an idiot. I ended up having to use a RefCell for one solution, and using .iter() for loops rather than explicit iterators (why, why is this different?). I also kept forgetting to explicitly mark variables as mutable when declaring them.

Things I liked? There’s a rich set of first class data types. Look, I’m a C programmer, I’m easily pleased. You give me some sort of hash array and I’ll be happy. Rust manages that, tuples, strings, all the standard bits any modern language can provide. The whole impl thing for adding methods to structures I like as a way of providing some abstraction, though I think Go has a nicer syntax for it. The compiler, as mentioned, is great at spitting out useful errors for the most part. Also although I wasn’t using external crates for AoC I do appreciate there’s a decent ecosystem there now (though that brings up another gripe: rust seems to still be a fairly fast moving target, to the extent I can no longer rely on the compiler in Debian stable to be able to compile random projects I find).

Advent of Code

Let’s talk about the advent of code bit now. Hopefully it’s long enough since it came out that this won’t be spoilers for anyone, but if you haven’t attempted the 2023 AoC and might, you might want to stop reading here.

First, a refresher on the format for those who might not be aware of it. Problems are posted daily from December 1st until the 25th. Each is in 2 parts; the second part is not viewable until you have provided the correct answer for the first part. There’s a whole leaderboard thing going on, but the puzzle opens at midnight UTC-5 so generally by the time I wake up and have time to look the problem has been solved many times over; no chance of getting listed.

Credit to AoC creator, Eric Wastl, for writing up the set of problems in an entertaining fashion. I quite enjoyed seeing how the puzzle would be phrased each day, and the whole thing obviously brings a lot of joy to folk I know.

I always start AoC thinking it’ll be a fun set of puzzles to solve. Then something happens and I miss a day or two, and all of a sudden I’ve a bunch of catching up to do and it’s all a bit more of a chore. I hit that at some points this time, but made a concerted effort to try and power through it.

That perseverance was required up front, because I found the second part of Day 1 to be ill specified, and had to iterate a few times to actually calculate the desired solution (IIRC, issues about whether sevenone at the end of a line ended up as 7 or 1 really tripped me up). I don’t recall any other problems that bit me as hard on the specification as this one, but it happening up front was unfortunate.

The short example input doesn’t always help with this either; either it’s not enough to be able to extrapolate patterns, or it doesn’t show all the variations you need to account for (that aren’t fully specified in the text), or in a few cases it turned out I needed to understand the shape of the actual data to produce a solution that could actually complete in a reasonable time.

Which brings me to another matter, sometimes brute force doesn’t actually work. This is fine, but the second part of the day’s problem can change the approach you’d take. So sometimes I got lucky in the way I handled the first half, and doing the second half was a simple 5 minute tweak, and sometimes I had to entirely change the way I was storing data.

You might claim that if I was a better programmer I’d have always produced a first half solution that was amenable to extension for the second half. First, I dispute that; I think there are always situations where the problem domain can change in enough directions that you can’t handle all of them without a lot of effort. Secondly, I didn’t find AoC an environment that encouraged me to optimise for generic solutions. Maybe some of the puzzles in isolation would allow for that, but a month of daily problems to solve while still engaging in regular life meant I hacked things up, took short cuts based on the knowledge I had of the input data, etc, etc.

Overall I can see the appeal, but the sheer quantity and the fact I write code as part of my day job just made it feel too much like a chore, rather than a fun mental exercise. I did wonder how they’d look as a set of interview puzzles (obviously a subset, rather than all of them), but I’m not sure how you’d actually use them for that – I wouldn’t want anyone to have to solve them in a live interview.

So, in case it’s not obvious, I’m not planning to engage in AoC again this yet. But I’m continuing to persevere with Rust (though most of my work stuff is thankfully still Go).