Programming Praxis – Bifid Cipher

Today’s Programming Praxis problem is another cipher, specifically Bifid’s cipher. Let’s get started, shall we?

Some imports:

import Data.Char
import Data.List
import Data.List.HT hiding (unzip)
import Data.Map ((!), fromList)

There’s no J in the Bifid alphabet.

alphabet :: String
alphabet = delete 'J' ['A'..'Z']

Next we need some functions to get the index from a character and vice versa. The second line in fromIndex is only to get rid of a compiler warning; you can leave it out if you want to.

indices :: [(Int, Int)]
indices = zip (concatMap (replicate 5) [1..5]) (cycle [1..5])

toIndex :: Char -> (Int, Int)
toIndex c = fromList (zip alphabet indices) ! c

fromIndex :: [Int] -> Char
fromIndex [x, y] = fromList (zip indices alphabet) ! (x, y)
fromIndex _      = undefined

Next, we need a way to prepare the strings for the algorithm.

prepare :: String -> String
prepare = filter (`elem` alphabet) . replace "J" "I" . map toUpper

The basic structure of encode and decode is the same, only the way of getting the intermediate data in the right order differs.

encode :: String -> String
encode = map fromIndex . sliceVertical 2 . uncurry (++) .
         unzip . map toIndex . prepare

decode :: String -> String
decode xs = map fromIndex . sliceHorizontal (length xs) .
            concatMap ((\(x, y) -> [x, y]) . toIndex) $ prepare xs

And, as usual, some tests to see if everything’s working properly:

main :: IO ()
main = do print $ encode "BONSAICODE"
          print . decode $ encode "BONSAICODE"

Looks like it is. Another one down.

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