# Haskell Quiz/The Solitaire Cipher/Solution Dolio

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```module Main where
import Data.List
import Data.Char
import Data.Ix
import Control.Monad
import Control.Monad.State
import System
import System.Random
import Data.Ord (comparing)

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])

-- Scrubs/pads a string to turn it into the format expected by the cipher
scrub = join . intersperse " " . unfoldr split . pad . filter isAlpha . map toUpper
where
pad l = l ++ replicate ((5 - length l) `mod` 5) 'X'
split [] = Nothing
split l  = Just \$ 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
(t,b) = splitAt (n + 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 . 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 . 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."
```