{-# LANGUAGE TupleSections #-}
module Day2B where
import Data.Maybe (mapMaybe)
import Data.Foldable (toList)
import qualified Data.Sequence as Seq

distance' :: Int -> (Seq.Seq Char) -> String -> String -> Maybe String
distance' n common (a:x) (b:y)
  | a == b    = distance' n (common Seq.|> a) x y
  | otherwise = distance' (n + 1) common x y
distance' 1 common "" "" = Just(toList common)
distance' _ _ _ _        = Nothing

distance :: (String, String) -> Maybe String
distance (a, b) = distance' 0 Seq.empty a b

distances :: [(String, String)] -> String
distances xs = head $ mapMaybe distance xs

combinations :: [String] -> [(String, String)]
combinations (x:xs) = map (x,) xs ++ combinations xs
combinations [] = []

main :: IO ()
main = interact $ distances . combinations . lines