Haskell Quiz/The Solitaire Cipher/Solution JFoutz

This took longer than expected. I didn't read the specification closely enough to realize both jokers count as 53 rather than A == 53 and B == 54

Other than that, looking through Data.List for shortcuts was educational. I'm really like zipWith.

This could probably use a lot more orginization, and a bit more commentary. Fortunately the function names match the spec closely, which helps.

import Data.Char
import Data.List
import System

-- discard any non a to z characers, uppercase the rest
-- split into groups of 5, padded with 'X'

prep ls = loop $ map toUpper $ filter (\x -> and [isAscii x, isLetter x]) ls
    where loop [] = []
          loop ls 
              | length ls < 5 = [take 5 (ls ++ "XXXX")]
              | otherwise = (take 5 ls) : loop (drop 5 ls)


drawMsg msg = concat $ intersperse " " (loop msg)
    where loop msg
              | length msg > 5 = (take 5 msg) : loop (drop 5 msg)
              | otherwise = [msg]

churn f msg = drawMsg $ toChr $ zipWith f (toNum $ concat $ prep msg) (keyStream [1..54])
crypt msg = churn solAdd msg
decrypt msg = churn solSub msg

main = do { x <- getArgs
          ; case (head x) of
                 "c" -> putStrLn $ crypt $ concat $ tail x
                 "d" -> putStrLn $ decrypt $ concat $ tail x
                 _ -> putStrLn "Try solitare c my message, or d my message"}

-- letters to numbers and back
toNum = map (\x -> ord x - 65)
toChr = map (\x -> chr (x + 65))

-- add and subtract base solitare style
solAdd x y = mod (x + y) 26
solSub x y = mod (x + 26 - y) 26


--keyStream
down1 x ls = move $ break (==x) ls
    where move (x:xs, t:[]) = x:t:xs
          move (xs, c:n:cx) = xs ++ n : c : cx

down2 x ls = down1 x $ down1 x ls


notEither a b = (\x y -> x /= a && y /= a && x /= b && y /= b)
tripleCut a b ls = swap a b $ concat $ reverse $ groupBy (notEither a b) ls

swap a b [] = []
swap a b (x:xs)
     | a == x = b : swap a b xs
     | b == x = a : swap a b xs
     | otherwise = x : swap a b xs
                   
cardVal c = if c == 54 then 53 else c
countCut ls = glue $ splitAt (cardVal $ last ls) (init ls)
    where glue (f,b) = b ++ f ++ [last ls]

jokerA = 53
jokerB = 54

keyStep deck = countCut $ tripleCut jokerA jokerB $ down2 jokerB $ down1 jokerA deck

getCard deck = deck !! (cardVal $ head deck)

keyStream deck = let d2 = keyStep deck 
                     out = getCard d2
                 in if out == jokerA || out == jokerB
                    then keyStream d2
                    else out : keyStream d2