AoC in rust

December 25, 2021


For me rust was pleasant to write small code snippets like AoC solutions. Rust has a decent REPL in form of evcxr.

Story mode

Following Bill’s suggestion I gave Advent of Code a try this year.

I did not know how much time it would take me. I hoped it would not be more than 30 minutes per morning for each of the problems. It was true for most problems, but some took a lot more than that.


Problems 1 to 7 I solved in haskell as I’m most familiar with it compared to other languages when it comes to one liners and throw-away scripts. Here is an example of solution for Problem 5 part 1 to get the feeling of how I would write things:

{-# OPTIONS_GHC -Wall #-}
import qualified Control.Monad as CM
import qualified Data.Char as DC
import qualified Data.List as DL

type I = Integer

data L = H I I I
       | V I I I
       | Unsupported

solve :: [L] -> Integer
solve = DL.genericLength
      . filter (\g -> length g > 1)
      . DL.sort
      . concatMap expand
    where expand (H y x1 x2) = [ (x,y) | x <- [x1..x2]]
          expand (V x y1 y2) = [ (x,y) | y <- [y1..y2]]
          expand Unsupported = []

readInput :: String -> [L]
readInput inp = parseL <$> lines inp
    where parseL s = norm ((x1, y1), (x2, y2))
              where [x1,y1,x2,y2] = read <$> words (map (\c -> if DC.isDigit c then c else ' ') s)
          norm ((x1, y1), (x2, y2)) = case () of
            _ | x1 == x2 -> V x1 (min y1 y2) (max y1 y2)
            _ | y1 == y2 -> H y1 (min x1 x2) (max x1 x2)
            -- _            -> error $ "Unexpected line: " ++ show l
            _            -> Unsupported

main :: IO ()
main = CM.forM_ ["example", "input"] $ \fn -> do
    f <- readFile fn
    let inp = readInput f
        ans = solve inp
    print $ (fn, ans)

Error handling is virtually non-existent. Efficiency (list sorting instead of using something like Data.Set) is probably crippled in favour of smaller code length.


The 1-7 problems looked too simple and I gave rust a try starting from problem 8.

My rust background is tiny: I read a few rust books before but never tried writing anything more than a hello world. AoC set of challenges looked like a good opportunity to get used to syntax and some basics of it’s standard library.

Here is my solution for Problem 8 part 1 and my first “non-trivial” rust program:

use std::{*, fs::*, io::*};

fn get_input(input_file: &str) -> Vec<Vec<String>> {
    let r = BufReader::new(File::open(input_file).unwrap());

    return r.lines().map(|l|
         .split(' ')
         .map(|s| String::from(s))
         .skip_while(|e| e != "|").skip(1)

fn main() {
    for input_file in ["example", "input"] {
        let input = get_input(input_file);

        let ans = input.into_iter()
        println!("{}: {}", input_file, ans);

Rust certainly has a feel of haskell at least when it comes to chaining operations on sequences. It’s non-idiomatic in absent error handling, inefficient in terms of excessive lifetimes for temporaries. But look quite readable.

rust resources

To get more familiarity with the language I reread and used extensively to look up useful helper in a standard library.

rust REPL

As a playground I was initially using and rustc in the command line. But it was a bit tedious when it comes down to exploration of simple operations. I needed a ghci equivalent (a haskell REPL environment).

Looks like evcxr ( is a popular rust REPL out there. Luckily nixpkgs providesit in the standard repository:

$ nix run nixpkgs#evcxr
Welcome to evcxr. For help, type :help
>> 'a'.to_up<TAB>
>> 'a'.to_uppercase()
>> String::from_iter('a'.to_uppercase())
       ^^^^^^^^^ function or associated item not found in `String`
no function or associated item named `from_iter` found for struct `String` in the current scope
help: items from traits can only be used if the trait is in scope
help: the following trait is implemented but not in scope; perhaps add a `use` for it:

use std::iter::FromIterator;
help: there is an associated function with a similar name

>> use std::iter::FromIterator;
>> String::from_iter('a'.to_uppercase())

It supports type inspection for bound variables:

>> let v = [1,2,3];
>> :vars
v: [i32; 3]

And even supports use of external crates:

>> :dep "quickcheck"
>> fn is_ok(b: bool)->bool { b }
>> quickcheck::quickcheck(is_ok as fn(bool)->bool)
thread '<unnamed>' panicked at '[quickcheck] TEST FAILED. Arguments: (false)', .../quickcheck-1.0.3/src/

I spent a few hours poking at various containers, iterators and available methods.

Prepared with these basics I managed to solve the rest of AoC challenges.

Nice AoC challenges

Most AoC problems were straightforward and did not really pose a real challenge.

But some of them were not so simple. Here is my list of the nicest ones:

“Extended Polymerization” is a susprisingly concise problem that managed to trick me more than once. First, I did not notice exponential growth. Then it took me a while to reformulate correctly it in a form suitable for dynamic programming solution. And then I missed a few subtleties.

“Beacon Scanner” looked like a tedious problem, but it has many fun angles to explore. For some reason on my input not all expected pairs in the solution had 12+ points overlap. That wedged me for a day of debugging exploration of data.

“Amphipod” reminded me of a Norvig’s online AI class I took almost 10 years ago. Second best problem of this year’s AoC I’d say.

“Arithmetic Logic Unit” is the only problem I managed to solve using pen and paper. Best problem of this year’s AoC on my rating :) It has a few hidden layers that transform a problem statement in leasantly surprising ways. It remindedme of challenges. These are the best :)

Parting words

While coding the solutions I made many mistakes like use-after-free, index out-of-bounds and similar. Borrow checker errors are great at explaining the lifetimes and sources of references. Integer overflow runtime checks are also nice.

It feels like rust is very explicit about value moves and copies. It’s a nice feeling of confidence where you create (potentially expensive) object copies and when you can just cheaply move them around.

Compiler warnings are also superb: superfluous mut annotations, dead code, even accidental camel case!

If you are thinking of trying rust but did not yet you certainly should.

Have fun!