-- this is my first attempt at part one of the optional programming assignment 
-- for the stanford Introduction to Artificial Intelligence class
-- it solves a rotating or caesar cypher assuming the most common 
-- letter in english is 'e'
-- PAC 9th January, 2012
 
import Data.Char (ord, chr, isUpper, isLower, toLower)
import Data.List (sortBy, elemIndices)
import Data.Ord (comparing)
import qualified Data.Map as M
import System.Environment (getArgs)
 
-- the code to break
defaultEncodedMsg = "Esp qtcde nzyqpcpynp zy esp ezatn zq Lcetqtntlw Tyepwwtrpynp hld spwo le Olcexzfes Nzwwprp ty estd jplc."
 
-- Shifts a character to the right if positive, left if negative. Wraps around.
shift :: Int -> Char -> Char -- Modulus handles the wraparound(shift 1 'z' = 'a')
shift n c | isUpper c = chr $ ord 'A' + ((ord c + n - ord 'A') `mod` 26)
          | isLower c = chr $ ord 'a' + ((ord c + n - ord 'a') `mod` 26)
          | otherwise = c 
 
-- rotate input string by n
shiftString:: Int -> String -> String
shiftString n = map (shift n)
 
--count number of instances of a letter in a string
instances:: Char -> String -> Int
instances x ys = length $ elemIndices (toLower x) (map toLower ys)
 
--calculate frequency of a letter
frequency:: Char -> String -> Float
frequency x string = fromIntegral (instances x string) / fromIntegral (length string)
 
-- creates a sorted descending list list of tuples of letter and its frequency
frequencyAll:: String -> [(Char, Float)]
frequencyAll string = reverse $ sortBy (comparing snd) [ (x , frequency x string) | x <- ['a' .. 'z' ] ]
 
-- assuming 'e' is the most common letter in english
decode:: String -> String
decode string = shiftString (ord 'e' - ord (fst $ head $ frequencyAll string)) string
 
-- parse arguments, if no supplied use default
parseArgs:: [String] -> String
parseArgs args = if null args then defaultEncodedMsg else head args
 
-- entry point
main = do
 args <- getArgs
 let msg = parseArgs args
 putStrLn "Caesar Cypher Solver (ai-class) - naive approach\n"
 putStrLn "encoded message:"
 putStrLn msg
 putStrLn "\nThe top candidate based on the frequency of the letter 'e' in english is:\n"
 putStrLn $ decode msg
 

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