Ruby Quiz #122 Solution: Checking Credit Cards using meta-programming

So this is the first time I actually did a RubyQuiz for real. I spent probably 3 or 4 hours on it. Not too shabby. And, I got to do a little bit of meta-programming! It's basic meta-programming, but I liked the solution. Brief intro to the quiz:

Before a credit card is submitted to a financial institution, it generally makes sense to run some simple reality checks on the number. The numbers are a good length and it's common to make minor transcription errors when the card is not scanned directly.

The first check people often do is to validate that the card matches a known pattern from one of the accepted card providers. Some of these patterns are:

      +============+=============+===============+
     | Card Type  | Begins With | Number Length |
     +============+=============+===============+
     | AMEX       | 34 or 37    | 15            |
     +------------+-------------+---------------+
     | Discover   | 6011        | 16            |
     +------------+-------------+---------------+
     | MasterCard | 51-55       | 16            |
     +------------+-------------+---------------+
     | Visa       | 4           | 13 or 16      |
     +------------+-------------+---------------+ 

There's more rules for each credit card at wikipedia. So normally, how would you do this with OO design? First thing that came to mind was creating a general CreditCard base class, and use polymorphism to implement the rule for each type of card, which is a subclass of CreditCard (i.e. Mastercard extends CreditCard). The problem with this, I've always found is that there's a proliferation of classes when you do something like this. People have solved this problem with other patterns, such as Factories, to build families of classes.

But that's a lot of structure that I didn't want to write for a little RubyQuiz. So I opted for case statements at first:

def type(cc_num)
  case cc_num
  when /^6011.*/
    return :discover if cc_num.length == 15
  when /^5[1-5].*/
    return :mastercard if cc_num.length == 16
  ...other card rules...blah blah blah
  end
  return :unknown
end

But as we all learned from having to maintain a proprietary server program written in C nested 11 or 12 layers deep all in one main file, case statements suck and don't scale (guess who had to do that?). So what's a better solution? I'd like to think I came up with a nice one.

With dynamic programming languages, I find that a lot of the problems that design patterns solve simply go away. And with meta-programming, it can be a much more flexible tool to solve design problems, rather than with design patterns. In a way, I created a very very tiny domain specific language for checking credit card type and validity. All you need to do to use it is define the rules in the table above in your class which subclasses credit card checker:

require 'credit_card_checker'

class MyCreditCardChecker < CreditCardChecker
  credit_card(:amex) { |cc| (cc =~ /^34.*/ or cc =~ /^37.*/) and (cc.length == 15) }
  credit_card(:discover) { |cc| (cc =~ /^6011.*/) and (cc.length == 16) }
  credit_card(:mastercard) { |cc| cc =~ /^5[1-5].*/ and (cc.length == 16) }
  credit_card(:visa) { |cc| (cc =~ /^4.*/) and (cc.length == 13 or cc.length == 16) }
end

CCnum = "4408041234567893"
cccheck = MyCreditCardChecker.new
puts cccheck.type(CCnum) # => :visa 
puts cccheck.valid?(CCnum) # => true

Neat! So this way, you can have any type of credit card checker you want, in any combination. And if suddenly there was a proliferation of new credit card companies, you can add them pretty easily. How is this done? Well, let me show you:

require 'enumerator'

class CreditCardChecker
  def self.metaclass; class << self; self; end; end

  class << self
    attr_reader :cards

    def credit_card(card_name, &rules)
      @cards ||= []
      @cards << card_name

      metaclass.instance_eval do 
        define_method("#{card_name}?") do |cc_num|
          return rules.call(cc_num) ? true : false
        end
      end
    end

  end

  def cctype(cc_num)
    self.class.cards.each do |card_name|
      return card_name if self.class.send("#{card_name}?", normalize(cc_num))
    end
    return :unknown
  end
  
  def valid?(cc_num)
    rev_num = []
    normalize(cc_num).split('').reverse.each_slice(2) do |pair|
      rev_num << pair.first.to_i << pair.last.to_i * 2
    end
    rev_num = rev_num.to_s.split('')
    sum = rev_num.inject(0) { |t, digit| t += digit.to_i }
    (sum % 10) == 0 ? true : false
  end
  
  private
  def normalize(cc_num)
    cc_num.gsub(/\s+/, '')
  end
end

If you don't know much about meta-programming yet, you might want to try _why's take on seeing metaclasses clearly along with Idiomatic Dynamic Ruby. Don't worry if it takes a while...I was stumped for a while also.

Anyway, the magic is in the method credit_card. Notice it's between "class << self" and "end", which means that this method is defined in the singleton class of the class CreditCardChecker. But you can just think of it as a class method. Same thing with the method metaclass(), it is a class function that returns the singleton class of the caller.

Now, the thing is, this isn't very exciting in itself. However, notice that credit_card() is executed in the subclass MyCreditChecker. This means that when inside credit_card(), metaclass returns NOT the singleton class of CreditCardChecker, but the singleton class of MyCreditCardChecker! Then when we proceed to do an instance_eval() and a define_method(), we are defining a new method in the singleton class of the subclass MyCreditChecker. Inside the method, it will call the block that evaluates the rule given for that card. If true, it returns true and false if false. The only reason I did it that way, is so in case the block returns an object, it'll return true instead of the object.

Therefore, to any instance of MyCreditChecker, it will look like there's a class method with the name of the credit card. So if you did:

require 'credit_card_checker'

class MyCreditCardChecker < CreditCardChecker
  credit_card(:amex) { |cc| (cc =~ /^34.*/ or cc =~ /^37.*/) and (cc.length == 15) }
end

MyCreditCardChecker.amex?(cc_num) would be a valid method that checks if the credit card number is an American Express Card. And what cctype() method does is that it cycles through all the known credit cards and returns the first one that's valid. The rest is standard fare, so I won't go through it.

And oh, btw, each_slice() and each_cons() got moved to the standard library, so you have to include enumerator in order to use it--even though the official ruby docs say that it's still in the Enumerables class in the language.