CIS194 HW1

ccValidate.hs

-- Lamest answer to http://www.seas.upenn.edu/~cis194/lectures.html HW1
toDigits :: Integer -> [Integer]
toDigits x
  | x <= 0    = []
  | otherwise = toDigits a ++ [b]
      where
        a = div x 10
        b = mod x 10

double2nd :: [Integer] -> [Integer]
double2nd (x:y:xs) = x : 2 * y : double2nd xs
double2nd a = a

doubleEveryOther :: [Integer] -> [Integer]
doubleEveryOther x = reverse $ double2nd $ reverse x

sumDigits :: [Integer] -> Integer
sumDigits (x:[]) = a+b
  where
    a = mod x 10
    b = div x 10
sumDigits (x:xs) = a + b + sumDigits xs
  where
    a = mod x 10
    b = div x 10

validate :: Integer -> Bool
validate x = mod ( sumDigits $ double2ndRev $ toDigits x ) 10 == 0