33 lines
850 B
Haskell
33 lines
850 B
Haskell
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lastDigit :: Integer -> Integer
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lastDigit n = n `mod` 10
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dropLastDigit :: Integer -> Integer
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dropLastDigit n = n `div` 10
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toDigits :: Integer -> [Integer]
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toDigits n
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| n <= 0 = []
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| otherwise = toDigits(dropLastDigit(n)) ++ [lastDigit(n)]
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doubleEveryOtherLeft :: [Integer] -> [Integer]
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doubleEveryOtherLeft [] = []
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doubleEveryOtherLeft (x:[]) = [x]
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doubleEveryOtherLeft (x:y:zs) = x : (y * 2) : doubleEveryOtherLeft(zs)
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doubleEveryOther :: [Integer] -> [Integer]
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doubleEveryOther x = reverse(doubleEveryOtherLeft(reverse(x)))
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crossSum :: Integer -> Integer
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crossSum x
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| x < 10 = x
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| otherwise = lastDigit(x) + crossSum(dropLastDigit(x))
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sumDigits :: [Integer] -> Integer
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sumDigits [] = 0
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sumDigits (x:xs) = crossSum(x) + sumDigits(xs)
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validate :: Integer -> Bool
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validate x = lastDigit(sumDigits(doubleEveryOther(toDigits(x)))) == 0
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