CIS194/Homework 1/luhn.lhs

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2016-03-27 10:55:25 +00:00
This is an implementation of the Luhn Algorithm. It's used to validate
credit card numbers.
Return the last digit of an integer number
> lastDigit :: Integer -> Integer
> lastDigit n = mod n 10
Remove the last digit of an integer number and return the rest
> dropLastDigit :: Integer -> Integer
> dropLastDigit n = div n 10
Put all digits of an integer number in reverse order into a list
> toRevDigits :: Integer -> [Integer]
> toRevDigits n
> | n < 1 = []
> | otherwise = lastDigit(n) : toRevDigits(dropLastDigit(n))
Put all digits of an integer number into a list
> toDigits :: Integer -> [Integer]
> toDigits n = reverse(toRevDigits(n))
Double every second value in the input list.
> doubleEveryOther :: [Integer] -> [Integer]
> doubleEveryOther([]) = []
> doubleEveryOther(x:[]) = [x]
> doubleEveryOther(a:(b:xs)) = a : 2*b : doubleEveryOther(xs)
Sum up all digits from all integers.
> sumDigits :: [Integer] -> Integer
> sumDigits([]) = 0
> sumDigits(0:xs) = sumDigits(xs)
> sumDigits(x:xs) = lastDigit(x) + sumDigits(dropLastDigit(x) : xs)
Evaluate if a credit card number is 'valid'.
> luhn :: Integer -> Bool
> luhn x = lastDigit(sumDigits(doubleEveryOther(toRevDigits(x)))) == 0