import Data.Char

main :: IO ()
main = print $ crack "kdvnhoo lv ixq"

lowers :: String -> Int
lowers xs = length [x | x <- xs, isLower x]

count :: Char -> String -> Int
count x xs = length [x' | x' <- xs, x == x']

let2int :: Char -> Int
let2int c = ord c - ord 'a'

int2let :: Int -> Char
int2let n = chr (ord 'a' + n)

shift :: Int -> Char -> Char
shift n c
  | isLower c = int2let ((let2int c + n) `mod` 26)
  | otherwise = c
  
encode :: Int -> String -> String
encode n xs = [shift n x | x <- xs]

percent :: Int -> Int -> Float
percent n m = (fromIntegral n / fromIntegral m) * 100

freqs :: String -> [Float]
freqs xs = [percent (count x xs) n | x <- ['a'..'z']]
  where n = lowers xs

chisqr :: [Float] -> [Float] -> Float
chisqr os es = sum [((o - e)^2) / e | (o,e) <- zip os es]

rotate :: Int -> [a] -> [a]
rotate n xs = drop n xs ++ take n xs

positions :: Eq a => a -> [a] -> [Int]
positions x xs =
  [i | (x',i) <- zip xs [0..n], x == x']
  where n = length xs - 1

crack :: String -> String
crack xs = encode (-factor) xs
  where
    factor = head (positions (minimum chitab) chitab)
    chitab = [chisqr (rotate n table') table | n <- [0..25]]
    table' = freqs xs

table :: [Float]
table =
  [ 8.2
  , 1.5
  , 2.8
  , 4.3
  , 12.7
  , 2.2
  , 2.0
  , 6.1
  , 7.0
  , 0.2
  , 0.8
  , 4.0
  , 2.4
  , 6.7
  , 7.5
  , 1.9
  , 0.1
  , 6.0
  , 6.3
  , 9.1
  , 2.8
  , 1.0
  , 2.4
  , 0.2
  , 2.0
  , 0.1
  ]