2018/Day05/Day5A.hs

#!/usr/bin/env stack
-- stack --resolver lts-12.5 script

module Day5A where
import Data.Bits (xor)
import Data.Char
import qualified Data.Foldable as F

invertCase :: Char -> Char
invertCase = chr . xor 0x20 . ord

calcLength :: String -> String -> Int
calcLength ys (x:y:xs)
  | x == invertCase y = if null ys
      then calcLength ys xs
      else calcLength (tail ys) (head ys:xs)
  | otherwise        = calcLength (x:ys) (y:xs)
calcLength ys [x]    = calcLength (x:ys) []
calcLength ys []     = length ys

main :: IO ()
main = interact $ show . calcLength "" . filter (/= '\n')