caesar-cipher.hs

import Data.Char
import Language.Haskell.TH.Syntax (Strict)
import Data.List

lowers :: String -> Int
lowers xs = length [ x | x <- xs, x >= 'a' && x <= 'z']

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

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]

table :: [Float]
table = [8.1, 1.5, 2.8, 4.2, 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, 9.0, 2.8, 1.0, 2.4, 0.2, 2.0, 0.1]

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..], x == x']
positions x xs = [i | (x',i) <- zip xs [0..], x == x']

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

-- Chapter 5 exercises
-- 1.
a = sum [ x^2 | x <- [1..100]]

grid :: Int -> Int -> [(Int, Int)]
grid m n = [(x,y) | x <- [0..m], y <- [0..n]]

square :: Int -> [(Int,Int)]
square n = [(x, y) | (x, y) <- grid n n, x /= y]

replicate' :: Int -> a -> [a]
replicate' n a = [ a | _ <- [1..n] ]

pyths :: Int -> [(Int, Int, Int)]
pyths n = [ (x,y,z) | x <- [1..n],
                      y <- [1..n],
                      z <- [1..n], 
                      x^2 + y^2 == z^2]

factors :: Int -> [Int]
factors n = [x | x <- [1..n], n `mod` x == 0]

perfects :: Int -> [Int]
perfects n = [x | x <- [1..n], (sum (factors x)) - x == x]

my = [(x,y) | x <- [1,2], y <- [3,4]]
-- my' = [(x,y) | x <- concat [1,2] [3,4]]