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 double 3 and 1. For the number 98765, we double 6 and 8.

  • After doubling a digit, sum the digits if the result is greater than 9. So doubling 7 becomes 14 which becomes 1 + 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.

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