cis194 w01 solution

README.md

What is this?

Solutions to the week 1 homework exercises in Brent Yorgey's 2013 offering of cis194.

Links

• the course
• the homework

CreditCardValidator.hs

module Week01.CreditCardValidator
  ( toDigits
  , toDigitsRev
  , doubleEveryOther
  , sumDigits
  , validate
  ) where

reverseList :: [Integer] -> [Integer]
reverseList [] = []
reverseList (x:xs) = reverseList xs ++ [x]

toDigits :: Integer -> [Integer]
toDigits n = reverseList (toDigitsRev n)

toDigitsRev :: Integer -> [Integer]
toDigitsRev n
  | n <= 0 = []
  | otherwise = n `mod` 10 : toDigitsRev (n `div` 10)

doubleEveryOtherFromLeft :: [Integer] -> [Integer]
doubleEveryOtherFromLeft [] = []
doubleEveryOtherFromLeft (x:y:zs) = x : 2*y : doubleEveryOtherFromLeft zs
doubleEveryOtherFromLeft (x:[]) = [x]

doubleEveryOther :: [Integer] -> [Integer]
doubleEveryOther xs = reverseList (doubleEveryOtherFromLeft (reverseList xs))

sumInts :: [Integer] -> Integer
sumInts [] = 0
sumInts (x:xs) = x + sumInts xs

sumDigits :: [Integer] -> Integer
sumDigits [] = 0
sumDigits (x:xs) = sumInts (toDigits x) + (sumDigits xs)

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