Exercise: Luhn Algorithm
The Luhn algorithm is used to validate credit card numbers. The algorithm takes a string as input and does the following to validate the credit card number:
-
Ignore all spaces. Reject numbers with fewer than two digits.
-
Moving from right to left, double every second digit: for the number
1234
, we double3
and1
. For the number98765
, we double6
and8
. -
After doubling a digit, sum the digits if the result is greater than 9. So doubling
7
becomes14
which becomes1 + 4 = 5
. -
Sum all the undoubled and doubled digits.
-
The credit card number is valid if the sum ends with
0
.
The provided code provides a buggy implementation of the luhn algorithm, along with two basic unit tests that confirm that most of the algorithm is implemented correctly.
Copy the code below to https://play.rust-lang.org/ and write additional tests to uncover bugs in the provided implementation, fixing any bugs you find.