Advent of Code 2021

These are my Advent of Code 2021 solutions. I generally try to focus on making clean and documented Haskell solutions to each puzzle.

We'll be chatting about AoC on IRC all December. You can find AoC discussion on Libera Chat's #adventofcode and #haskell (webchat)

Building

I recommend installing ghc with ghcup.

$ curl --proto '=https' --tlsv1.2 -sSf https://get-ghcup.haskell.org | sh

I'm using GHC 9.0.1 this year

$ ghcup install ghc 9.0.1
$ ghcup set     ghc 9.0.1

cabal-install is the best way to get things built, and is provided by ghcup

$ cabal update
$ cabal build

Running solutions

All the solutions take an optional command line argument that can be an input file name or - to read the input from stdin. The default input filename is inputs/input##.txt

$ cabal run Day01
...
1681
1704

Common libraries used

I try to avoid using too many dependencies for these problems.

• containers - Almost all of the solutions will benefit from having access to Map and Set types from this package.
• array - Standard mutable and immutable array library
• doctest - Having checked examples in the documentation makes it easier to understand what code does and easy to add unit tests.
• alex - Lexer generator package I use for my input format DSL
• happy - Parser generator package I use for my input format DSL

Input file DSL

To speed up the process of writing input parsers, I use a quasiquoter that generates ReadP parsers.

Day 13 used in input format of pairs of numbers and fold instructions as seen below.

6,10
0,14
9,10
0,3
10,4
4,11
6,0
6,12
4,1
0,13
10,12
3,4
3,0
8,4
1,10
2,14
8,10
9,0

fold along y=7
fold along x=5

This can be parsed using the following pattern string from Day13.hs

data A = Ax | Ay
[format|13 (%u,%u%n)*%n(fold along @A=%u%n)*|]

which processes an output of

([(6,10),(0,14),(9,10),(0,3),(10,4),(4,11),(6,0),(6,12),(4,1),(0,13),(10,12),(3,4),(3,0),(8,4),(1,10),(2,14),(8,10),(9,0)],[(Ay,7),(Ax,5)])

Getting the data into Haskell datatypes quickly allows me to focus on the problem and not the input file, and it keeps the solution files focused, as well.