# Difference between revisions of "Haskell Quiz/The Solitaire Cipher/Solution Dolio"

From HaskellWiki

(A first try at a haskell solution.) |
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Line 27: | Line 27: | ||

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

− | where |
+ | where comparing f a b = compare (f a) (f b) |

− | shuffle' g a [] = a |
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− | shuffle' g a l = shuffle' g' (i:a) l' |
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− | where |
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− | (n, g') = randomR (0, length l - 1) g |
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− | (h, (i:t)) = splitAt n l |
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− | l' = h ++ t |
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-- Scrubs/pads a string to turn it into the format expected by the cipher |
-- Scrubs/pads a string to turn it into the format expected by the cipher |

## Revision as of 06:22, 26 October 2006

```
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 A = 53
value B = 53
value (C i) = i
-- 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."
```