CIS194-hw1.hs

-- CIS 194: Homework 1 (http://www.seas.upenn.edu/~cis194/hw/01-intro.pdf)


-- toDigits 1234 == [1,2,3,4]
-- toDigitsRev 1234 == [4,3,2,1]
-- toDigits 0 == []
-- toDigits (-17) == []

toDigitsRev :: Integer -> [Integer]
toDigitsRev x = if x <= 0
				then []
				else (x `mod` 10) : toDigitsRev (x `div` 10)

-- Alternative declaration using guards
toDigitsRev' x
	| x <= 0 	= []
	| otherwise 	= (x `mod` 10) : toDigitsRev' (x `div` 10)

toDigits :: Integer -> [Integer]
toDigits x = reverse (toDigitsRev x)

-- Example: doubleEveryOther [8,7,6,5] == [16,7,12,5] 
-- Example: doubleEveryOther [1,2,3] == [1,4,3]

doubleEveryOther' :: [Integer] -> [Integer]
doubleEveryOther' [] = []
doubleEveryOther' [x] = [x]
doubleEveryOther' (x:y:xs) = x : (y * 2) : doubleEveryOther' xs

doubleEveryOther xs = reverse (doubleEveryOther' (reverse xs))

-- Example: sumDigits [16,7,12,5] = 1 + 6 + 7 + 1 + 2 + 5 = 22
sumDigits :: [Integer] -> Integer
sumDigits [] = 0
sumDigits (x:xs) 
	| x < 10 	= x + sumDigits xs
	| otherwise 	= sumDigits ((toDigits x) ++ xs)
	

-- Example: validate 4012888888881881 = True 
-- Example: validate 4012888888881882 = False

validate :: Integer -> Bool
validate cc = ((sumDigits (doubleEveryOther (toDigits cc))) `mod` 10) == 0