CIS194 Ex. 1

cis194-ex1.hs

module CIS194 where

-- Exercise 1
toDigits :: Integer -> [Integer]
toDigits 0 = []
toDigits x = map read [[c] | c <- show x, x > 0]

toDigitsRev :: Integer -> [Integer]
toDigitsRev = reverse . toDigits

doubleEveryOther :: [Integer] -> [Integer]
doubleEveryOther [] = []
doubleEveryOther (x:[]) = [x]
doubleEveryOther (x:y:[]) = [x, y + y]
doubleEveryOther (x:y:ls) = [x, y + y] ++ doubleEveryOther ls

sumDigits :: [Integer] -> Integer
sumDigits xs = sum $ concatMap toDigits xs

validate :: Integer -> Bool
validate x = 0 == mod sum 10
         where sum = sumDigits . doubleEveryOther . toDigitsRev $ x

-- validate 4012888888881881 = True
-- validate 4012888888881882 = False