CIS194/Lecture 01/CreditCardValidation/CreditCard.hs

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2016-09-24 21:10:37 +00:00
module CreditCard where
2016-09-22 21:50:31 +00:00
lastDigit :: Integer -> Integer
lastDigit n = n `mod` 10
dropLastDigit :: Integer -> Integer
dropLastDigit n = n `div` 10
toDigits :: Integer -> [Integer]
toDigits n
| n <= 0 = []
| otherwise = toDigits(dropLastDigit(n)) ++ [lastDigit(n)]
doubleEveryOtherLeft :: [Integer] -> [Integer]
doubleEveryOtherLeft [] = []
doubleEveryOtherLeft (x:[]) = [x]
doubleEveryOtherLeft (x:y:zs) = x : (y * 2) : doubleEveryOtherLeft(zs)
doubleEveryOther :: [Integer] -> [Integer]
doubleEveryOther x = reverse(doubleEveryOtherLeft(reverse(x)))
crossSum :: Integer -> Integer
crossSum x
| x < 10 = x
| otherwise = lastDigit(x) + crossSum(dropLastDigit(x))
sumDigits :: [Integer] -> Integer
sumDigits [] = 0
sumDigits (x:xs) = crossSum(x) + sumDigits(xs)
validate :: Integer -> Bool
validate x = lastDigit(sumDigits(doubleEveryOther(toDigits(x)))) == 0