import Data.Char
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 = toUpper (int2let ((let2int (toLower c) + n) `mod` 26))
encode :: Int -> String -> String
encode n xs = [shift n x | x <- 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]
percent :: Int -> Int -> Float
percent n m = (fromIntegral n / fromIntegral m) * 100
count :: Eq a => a -> [a] -> Int
count x xs = length [x'| x' <- xs, x' == x]
lowers :: String -> Int
lowers xs = length [x | x <- xs, isLower x]
freqs :: String -> [Float]
freqs xs = [percent (count x (map toLower xs)) n | x <- ['a' .. 'z']]
where
n = length 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 | (i, x') <- zip [0 .. n] xs, 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
