Haskell Solution for Store Credit (Google Code Jam)

store-credit.hs

type Input = [String]

data Unsolved = Unsolved { unsolvedNumber :: Int, input :: Input }
data Solved = Solved { solvedNumber :: Int, answer :: String }

data Item = Item { index :: Int, price :: Int }

instance Show Solved where
    show (Solved n ans) = "Case #" ++ (show n) ++ ": " ++ ans

instance Show Item where
    show (Item index price) = show index

main :: IO ()
main = do
    input <- fmap (tail.lines) getContents
    let cases = zipWith Unsolved [1..] (splitIntoCases input)
    mapM_ putStrLn $ map (show.solve) cases

splitIntoCases :: [String] -> [Input]
splitIntoCases input
    | post == [] = [pre]
    | otherwise = pre:(splitIntoCases post)
    where (pre,post) = splitAt 3 input

solve :: Unsolved -> Solved
solve (Unsolved n input) = Solved n ans
    where (l1:l2:l3:_) = input
          credit = read l1 :: Int
          prices = map read $ words l3 :: [Int]
          items = zipWith Item [1..] prices
          (res1,res2) = head [(item1,item2) |
                              item1 <- items, item2 <- items,
                              (index item1) < (index item2),
                              (price item1) + (price item2) == credit]
          ans = (show res1) ++ " " ++ (show res2)