http://qiita.com/items/9d80de41903775296ca6 の問題を haskell で解いてみた

turn.hs

import Data.Char
import Test.HUnit
import System.Environment

data Image = Image Int String

instance Show Image where
  show (Image l d) = "Image " ++ [(intToDigit l)] ++ ":" ++ (apply $ appendZero d)

appendZero :: String -> String
appendZero d = if m == 0
           then d
           else d ++ (take (4 - m) $ repeat '0')
  where
    m = (length d) `mod` 4

apply :: String -> String
apply [] = []
apply (a:b:c:d:xs) = bin2hex [a,b,c,d] : apply xs

initializeImage :: String -> Image
initializeImage string = Image l dt
  where
    [(l,':':d)] = reads string :: [(Int, String)]
    dt = take (l * l) $ foldr (\x acc -> (hex2bin x) ++ acc) "" d


bin2hex :: String -> Char
bin2hex = intToDigit . foldl (\acc x -> (digitToInt x) + acc * 2) 0

hex2bin :: Char -> String
hex2bin x = if (length bin) == 4
            then bin
            else (take (4 - length bin) (repeat '0')) ++ bin
  where
    bin = cal $ digitToInt x

cal :: Int -> String
cal 0 = []
cal x = cal d ++ [intToDigit m]
  where (d, m)  =  x `divMod` 2

turnImage :: Image -> Image
turnImage (Image l d) = Image l newData
  where
    indexes = turnIndexes l
    newData = foldr (\x acc -> (d !! x) : acc) [] indexes


turnIndexes :: Int -> [Int]
turnIndexes x = gv x (x * x - x)
gv x t 
  | t < x * x = calTurnIndexes x t ++ gv x (t + 1)
  | otherwise = []

calTurnIndexes :: Int -> Int -> [Int]
calTurnIndexes x t
  | t < 0 = []
  | otherwise = t : calTurnIndexes x (t - x) 

tests = test [[6,3,0,7,4,1,8,5,2] ~=? turnIndexes 3]

main = do
  args <- getArgs
  let im = initializeImage . head $ args
      tim = turnImage im
  print tim