Revenge of the Pancakes (Google Code Jam 2016, Qualification Round, Problem B)

pancakes.hs

import Data.List

-- Code Jam boilerplate

main :: IO ()
main = getContents >>= putStr . showCases . tail . lines

showCase :: (Int, String) -> String
showCase (i, c) = "Case #" ++ show i ++ ": " ++ show (solve c)

showCases :: [String] -> String
showCases cs = unlines $ map showCase $ zip [1..] cs

-- Solution

type Pancake = Bool
type Stack   = [Pancake]
type Flips   = Int

parse :: String -> Stack
parse xs = [c == '+' | c <- xs]

flip' :: Stack -> Int -> Stack
flip' xs n = reverse (map not $ take n xs) ++ (drop n xs)

flipTop :: Stack -> Stack
flipTop s = case elemIndex (not $ head s) s of
  Just index -> flip' s index
  Nothing    -> flip' s $ length s

solve' :: Stack -> Flips -> Flips
solve' s i | all id s  = i
           | otherwise = solve' (flipTop s) (i + 1)

solve :: String -> Flips
solve s = solve' (parse s) 0