Please find attached my solution to your credit card validation problem.

While working on this problem I adopted a classical TDD work flow, and this is my preferred method of development.

To run the Credit Card Validator, please first run bundle install to install the required dependencies.

To check the validity of a credit card please run bin/card 4111111111111111

To use the class directly do the following:

card = CreditCard.new '4111111111111111'

card.issuer?
# => 'Visa'

card.valid?
# => true

card.number
# => 4111111111111111

There tests are written in RSpec and Cucumber for this problem. They run with the rspec and cucumber commands respectivly.

The RSpec tests are written as class unit tests. When I use RSpec, I generally follow the guidelines set out at http://betterspecs.org however I am very adaptable and can quickly learn and apply your chosen methodology.

The Cucumber tests check the functionality of the command line interface.

I have left the class to allow a card to be reported as valid even if we are given 'Unknown' as the credit card issuer. We could add in an extra conditions so that cards of an 'Unknown' issuer are reported as invalid however this would require clarification and a specification from the product owner.