1st solution to csi 194 hw1 probl

hw-csi194.hs

main = putStrLn "Hello, World!"

doubleSecondNum :: [Int] -> [Int]
doubleSecondNum numbers
  | length numbers == 0 = numbers
  | otherwise = reverse (map doubleEven (zip (reverse numbers) [0..]))

doubleEven :: (Int, Int) -> Int
doubleEven pair
  | (succ (snd pair)) `mod` 2 == 0 = (fst pair) * 2
  | otherwise = fst pair

addListDigits :: [Int] -> Int
addListDigits ints
  | length ints == 0 = 0
  | otherwise = sum (map combineDigits ints)

combineDigits :: Int -> Int
combineDigits 0 = 0
combineDigits n
  | n < 10 = n `mod` 10
  | otherwise = (n `mod` 10) + (combineDigits (n `quot` 10))

doubleAndCombineInts :: [Int] -> Int
doubleAndCombineInts ints
 | otherwise = addListDigits (doubleSecondNum ints)

isValidCCNum :: [Int] -> Bool
isValidCCNum ints
  | otherwise = (doubleAndCombineInts ints) `mod` 10 == 0