From 566bdfc66ce8a4036e981a32c3af59108e75b725 Mon Sep 17 00:00:00 2001
From: Michael Mandl <mandlm@molez.net>
Date: Thu, 22 Sep 2016 23:50:31 +0200
Subject: [PATCH] Credit card number validation

---
 Lecture 01/CreditCardValidation/CreditCard.hs | 32 +++++++++++++++++++
 Lecture 01/scratch/isEven.hs                  |  3 ++
 Lecture 01/scratch/sumFun.hs                  | 10 ++++++
 3 files changed, 45 insertions(+)
 create mode 100644 Lecture 01/CreditCardValidation/CreditCard.hs
 create mode 100644 Lecture 01/scratch/isEven.hs
 create mode 100644 Lecture 01/scratch/sumFun.hs

diff --git a/Lecture 01/CreditCardValidation/CreditCard.hs b/Lecture 01/CreditCardValidation/CreditCard.hs
new file mode 100644
index 0000000..cb399ad
--- /dev/null
+++ b/Lecture 01/CreditCardValidation/CreditCard.hs	
@@ -0,0 +1,32 @@
+
+
+lastDigit :: Integer -> Integer
+lastDigit n = n `mod` 10
+
+dropLastDigit :: Integer -> Integer
+dropLastDigit n = n `div` 10
+
+toDigits :: Integer -> [Integer]
+toDigits n
+  | n <= 0 = []
+  | otherwise = toDigits(dropLastDigit(n)) ++ [lastDigit(n)]
+
+doubleEveryOtherLeft :: [Integer] -> [Integer]
+doubleEveryOtherLeft [] = []
+doubleEveryOtherLeft (x:[]) = [x]
+doubleEveryOtherLeft (x:y:zs) = x : (y * 2) : doubleEveryOtherLeft(zs)
+
+doubleEveryOther :: [Integer] -> [Integer]
+doubleEveryOther x = reverse(doubleEveryOtherLeft(reverse(x)))
+
+crossSum :: Integer -> Integer
+crossSum x
+  | x < 10 = x
+  | otherwise = lastDigit(x) + crossSum(dropLastDigit(x))
+
+sumDigits :: [Integer] -> Integer
+sumDigits [] = 0
+sumDigits (x:xs) = crossSum(x) + sumDigits(xs)
+
+validate :: Integer -> Bool
+validate x = lastDigit(sumDigits(doubleEveryOther(toDigits(x)))) == 0
diff --git a/Lecture 01/scratch/isEven.hs b/Lecture 01/scratch/isEven.hs
new file mode 100644
index 0000000..9b84a8e
--- /dev/null
+++ b/Lecture 01/scratch/isEven.hs	
@@ -0,0 +1,3 @@
+isEven :: Integer -> Bool
+isEven n = n `mod` 2 == 0
+
diff --git a/Lecture 01/scratch/sumFun.hs b/Lecture 01/scratch/sumFun.hs
new file mode 100644
index 0000000..5ad1033
--- /dev/null
+++ b/Lecture 01/scratch/sumFun.hs	
@@ -0,0 +1,10 @@
+sumFun :: Integer -> Integer
+sumFun 0 = 0
+sumFun n
+  | n < 0 = 0
+  | otherwise = sumFun(n - 1) + n
+
+sumList :: Integer -> [Integer]
+sumList 0 = []
+sumList n = sumFun(n) : sumList(n - 1)
+