module Day1

import System
import System.File
import Data.String
import Text.Parser

parsePair : String -> Maybe (Int, Int)
parsePair pair = do
  num1 <- parseInteger (fst p1)
  num2 <- parseInteger (snd p2)
  pure (num1, num2) where
  p1 = span (/= ' ') pair
  p2 = span (== ' ') (snd p1)

parseStr : String -> List (Int, Int)
parseStr str = do
  nums <- lines str
  maybe [] pure (parsePair nums)
  
transposePairs : List (a, b) -> (List a, List b)
transposePairs ls = (map fst ls, map snd ls)

part1 : (List Int, List Int) -> Int
part1 inp = sum $ zipWith (\x => \y => abs (x - y)) (sort $ fst inp) (sort $ snd inp)

occur : Eq a => Num b => a -> List a -> b
occur x fs = foldr (\y => \r =>  if x == y then r + 1 else r) 0 fs

part2 : (List Int, List Int) -> Int
part2 (l1, l2) = sum $ map (\x => x * (occur x l2)) l1

main : IO ()
main = do
  estr <- readFile "data/day1.txt"
  str <- either (die . show) pure estr
  putStrLn $ show $ part1 $ transposePairs $ parseStr str
  putStrLn $ show $ part2 $ transposePairs $ parseStr str