HW01.hs

{-# OPTIONS_GHC -Wall #-}
module HW01 where

-- Exercise 1 -----------------------------------------

-- Get the last digit from a number
-- Note : casting (last (show x)) to array to make String from char. -- Ger
lastDigit :: Integer -> Integer
lastDigit x = read [(last (show x))]

-- Drop the last digit from a number
dropLastDigit :: Integer -> Integer
dropLastDigit x = read (replaceEmptyStringByZero (init (show x)))

replaceEmptyStringByZero :: String -> String
replaceEmptyStringByZero x = if (length x) > 0 then x
                                               else "0"

-- Exercise 2 -----------------------------------------

toRevDigits :: Integer -> [Integer]
toRevDigits x = if x > 0 then lastDigit x : toRevDigits (dropLastDigit x)
                         else []

-- Exercise 3 -----------------------------------------

-- Double every second number in a list starting on the left.
doubleEveryOther :: [Integer] -> [Integer]
doubleEveryOther [] = []
doubleEveryOther xs = head xs : doubleNow (tail xs)

doubleNow :: [Integer] -> [Integer]
doubleNow [] = []
doubleNow xs = 2 * (head xs) : doubleEveryOther (tail xs)

-- Exercise 4 -----------------------------------------

-- Calculate the sum of all the digits in every Integer.
sumDigits :: [Integer] -> Integer
sumDigits xs = sumLeft (fixLeftmostDigit xs)
								 			
-- Splits the starting digit so it can be used to correctly calculate sum
-- if starting value of that digit >= 10											
fixLeftmostDigit :: [Integer] -> [Integer]
fixLeftmostDigit xs = if (head xs > 9) then (head xs) - 9 : tail xs
                                       else xs
														 
-- Sums every digit seperately (substracting 9 is the same as adding the digits seperately for values under 20)
sumLeft :: [Integer] -> Integer
sumLeft xs = if (length xs) == 1 then head xs
                                 else if (head (tail xs)) > 9 then sumLeft (head xs + ((head (tail xs)) - 9)  : tail (tail xs))
								                              else sumLeft (head xs + head (tail xs) : tail (tail xs))

-- Exercise 5 -----------------------------------------

-- Validate a credit card number using the above functions.
luhn :: Integer -> Bool
luhn x = (lastDigit x) == 10 - lastDigit((sumDigits (tail (doubleEveryOther (toRevDigits x)))))