moved to Homework 1 folder

luhn.lhs

@@ -1,43 +0,0 @@
-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