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