-- 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