Solutions and Write-Ups for my Advent Of Code adventures (mainly in Haskell)
Let’s talk a little bit about me first, shall we?
This is my fourth time doing this in Haskell. Last year was the first time I managed to solve all days, so I thought I might stop doing those in Haskell as I felt like I had accomplished everything I ought to do with this language.
But after thinking for a while, looking for a challenge that would make me happy, I realised that this small period of the year where I do Haskell is probably my favorite time of the year. Thus, I’ve decided once again to solve thoses in Haskell!
TL;DR: Hello, my name is Sheinxy and I’m that weird Haskell guy! 🐈⬛
Anyway, let’s start with the challenge. For this year, I’ve decided to make a small shell script to generate some boilerplate code to make things faster (let’s see how long I use this until I find it not specific enough for each problem):
module Main where
import Data.Char
import Data.List
import System.Environment
type Input = [String]
type Output = Int
parseInput :: String -> Input
parseInput = lines
partOne :: Input -> Output
partOne = sum . map (\s -> read $ [head s, last s] ) . map (filter isDigit)
partTwo :: Input -> Output
partTwo = partOne . map go
where go "" = ""
go line@(c : xs)
| "one" `isPrefixOf` line = '1' : go xs
| "two" `isPrefixOf` line = '2' : go xs
| "three" `isPrefixOf` line = '3' : go xs
| "four" `isPrefixOf` line = '4' : go xs
| "five" `isPrefixOf` line = '5' : go xs
| "six" `isPrefixOf` line = '6' : go xs
| "seven" `isPrefixOf` line = '7' : go xs
| "eight" `isPrefixOf` line = '8' : go xs
| "nine" `isPrefixOf` line = '9' : go xs
| otherwise = c : go xs
compute :: Input -> String -> IO ()
compute input "parse" = print input
compute input "one" = print . partOne $ input
compute input "two" = print . partTwo $ input
compute input _ = error "Unknown part"
main = do
args <- getArgs
input <- parseInput <$> (readFile $ last args)
mapM (compute input) $ init args
Okay, that makes the code quite long actually, so from now on I’m only going to focus on the parseInput, partOne and partTwo functions.
type Input = [String]
type Output = Int
parseInput :: String -> Input
parseInput = lines
partOne :: Input -> Output
partOne = sum . map (\s -> read $ [head s, last s] ) . map (filter isDigit)
partTwo :: Input -> Output
partTwo = partOne . map go
where go "" = ""
go line@(c : xs)
| "one" `isPrefixOf` line = '1' : go xs
| "two" `isPrefixOf` line = '2' : go xs
| "three" `isPrefixOf` line = '3' : go xs
| "four" `isPrefixOf` line = '4' : go xs
| "five" `isPrefixOf` line = '5' : go xs
| "six" `isPrefixOf` line = '6' : go xs
| "seven" `isPrefixOf` line = '7' : go xs
| "eight" `isPrefixOf` line = '8' : go xs
| "nine" `isPrefixOf` line = '9' : go xs
| otherwise = c : go xs
Okay that is already less scary :3c
Parsing the input is just spliting the file line by line, nothing too fancy. My input is therefore a list of lines, and my output is a number.
Let’s focus on part one first:
partOne = sum . map (\s -> read $ [head s, last s] ) . map (filter isDigit)
I start by removing anything that is not a digit from the line, then I keep the first and last digit and convert them to an int. I sum every line result together, and I’m done! That is not the most optimised (nor probably the most elegant), but it’s simple enough in my opinion.
Then, part two:
partTwo = partOne . map go
where go "" = ""
go line@(c : xs)
| "one" `isPrefixOf` line = '1' : go xs
| "two" `isPrefixOf` line = '2' : go xs
| "three" `isPrefixOf` line = '3' : go xs
| "four" `isPrefixOf` line = '4' : go xs
| "five" `isPrefixOf` line = '5' : go xs
| "six" `isPrefixOf` line = '6' : go xs
| "seven" `isPrefixOf` line = '7' : go xs
| "eight" `isPrefixOf` line = '8' : go xs
| "nine" `isPrefixOf` line = '9' : go xs
| otherwise = c : go xs
Basically part two starts by “reparsing” the input before doing exactly what part one does. The reparsing simply changes every number written in letter form as the same number in digit form.
The only small problem that I had with part 2 was that “oneight” is actually “18” and not “1ight”, so I cannot drop the whole “one” from the string (I was too lazy to check what can be a prefix of what to be optimise, so I simply drop the first letter)
That’s all for today folks!
The Tiger programming language is a toy language used in Andrew Appel’s book “Modern compiler implementation in ML”
It also happens that a project at my school is to implement a compiler for that language.
It also so happens that I’m a Tiger maintainer, which means that I’m working of the reference implementation and subject for that project.
Anyway, all of that to say that I did day 1 in tiger lol 😻