Captcha

import           Data.Char
import           Data.Monoid

zipRot :: Int -> [a] -> [(a, a)]
zipRot n xs =
  let rotXs = (drop n xs) ++ (take n xs)
  in zip xs rotXs

captcha :: (Monoid a, Eq a) => Int -> [a] -> a
captcha n xs =
  foldr
    (\(x, y) acc ->
       if x == y
         then acc <> x
         else acc)
    mempty $
  zipRot n xs

toSum :: Char -> Sum Int
toSum c = Sum $ read [c]

main :: IO ()
main = do
  file <- readFile "data.txt"
  let trimFile = takeWhile isDigit file
  let sums = toSum <$> trimFile
  --putStrLn . show . captcha 1 $ sums
  putStrLn . show . captcha (length trimFile `div` 2) $ sums