Haskell Quiz/The Solitaire Cipher/Solution Dolio

module Main where
import Data.List
import Data.Char
import Data.Ix
import Control.Monad
import Control.Monad.State
import System
import System.Random

data Card = A | B | C Int deriving (Eq, Show)
type Deck = [Card]
type Cipher a = State Deck a

unkeyed = map C [1..52] ++ [A,B]

isJoker c = c == A || c == B

value (C i) = i
value _     = 53

-- en/decodes upper case characters into a 0 - 25 Int representation
-- (for easier arithmetic than 1 - 26)
decode n = chr (n + 65)
encode n = ord n - 65

-- Shuffles a given deck using n as the seed for the random generator
shuffle n = map snd . sortBy (comparing fst) . zip (randoms $ mkStdGen n :: [Int])
 where comparing f a b = compare (f a) (f b)

-- Scrubs/pads a string to turn it into the format expected by the cipher
scrub = break5 . pad . filter isAlpha . map toUpper
 where
 pad l = l ++ replicate ((5 - length l) `mod` 5) 'X'
 break5 l 
    | null t    = h
    | otherwise = h ++ " " ++ break5 t
  where (h, t) = splitAt 5 l
  
-- Moves element e in l forward n places using the appropriate rules
push n e l = t ++ [e] ++ b
 where
 (Just i) = elemIndex e l
 l' = delete e l
 i' = if i + n >= length l then 1 + i + n else i + n
 (t,b) = splitAt (i' `mod` (length l)) l'

-- Performs the triple cut
tcut l = bottom ++ middle ++ top
 where
 [i,j] = findIndices isJoker l
 (top, m) = splitAt i l
 (middle, bottom) = splitAt (1 + j - i) m

-- Performs the counting cut
ccut l = init bottom ++ top ++ [last bottom]
 where
 n = value (last l)
 (top, bottom) = splitAt n l

-- Extracts a code from a given deck according to the appropriate rules.
-- Returns Nothing in the event that a joker is picked
extract l@(h:_) = if isJoker c then Nothing else Just (value c)
 where
 n = value h
 c = l !! n

-- Gets the next code in the key stream
getCode = do modify (ccut . tcut . push 2 B . push 1 A)
             deck <- get
             maybe getCode return (extract deck)

-- Uses the function f and initial deck d to en/decrypt a message
crypt f d = map decode . flip evalState d . mapM cipher . map encode
 where
 cipher a 
    | inRange (0,25) a = getCode >>= return . flip mod 26 . f a
    | otherwise        = return a

decrypt = crypt (-)
encrypt = crypt (+)

crypto f = unlines . map f . map scrub . lines

main = do (o:l) <- getArgs
          let deck = if null l then unkeyed else shuffle (read (head l)) unkeyed
          case o of
            "d" -> interact (crypto $ decrypt deck)
            "e" -> interact (crypto $ encrypt deck)
            _   -> putStrLn "Unrecognized option."