2024 Day One

DayOne.hs

{-# LANGUAGE OverloadedStrings #-}

module DayOne (partOne, partTwo, testLines) where

import qualified Text.Read as TR
import qualified Data.Text as T
import qualified Data.List as L
import qualified Test.LeanCheck.Stats as TS
import qualified Data.Map as DM


testLines :: [T.Text]
testLines = [ "3   4", "4   3", "2   5", "1   3", "3   9", "3   3" ]


parseLine :: T.Text -> (Int, Int)
parseLine line = (TR.read (T.unpack a), TR.read (T.unpack b))
  where (a:(b:_)) = T.words line

partOne :: [T.Text] -> Int
partOne inputLines = sum scores
  where tuples = fmap parseLine inputLines
        firsts = L.sort $ fmap fst tuples
        seconds = L.sort $ fmap snd tuples
        pairs = zip firsts seconds
        scores = fmap scorePair pairs
        scorePair (a,b) = abs $ a - b


partTwo :: [T.Text] -> Int
partTwo inputLines = sum scores
  where tuples = fmap parseLine inputLines
        firsts = fmap fst tuples
        seconds = fmap snd tuples
        countLookup = DM.fromList $ TS.counts seconds
        score n = n * DM.findWithDefault 0 n countLookup
        scores = fmap score firsts