From: Paul Novak <novakps@...>
Date: 2007-05-03T10:50:04+09:00
Subject: Re: Checking Credit Cards (#122)

Thank you James for another fun quiz.

My solution is a more verbose version of the metaprogramming-based
routing used in why's excellent camping.  But instead of regexen
matching urls to controller classes, here they are matching card
numbers to the card classes.

What I like about this approach is that you can add more card patterns
just by adding more classes with the appropriate regexen, and you can
override validation or other functionality as needed.

Regards,

Paul.

# cardvalidator.rb
#
require 'metaid'

module Card
  @cards=[]

  def Card.Base *u
    c = @cards
    Class.new {
      meta_def(:patterns){u}
      meta_def(:validity){|x|Card.validity(x)}
      meta_def(:inherited){|x|c<<x}
    }
  end
  def Card.validate cardnum
    @cards.map { |k|
      k.patterns.map { |x|
        if cardnum =~ /^#{x}\/?$/
          return [k.name.upcase, k.validity(cardnum)].join( " ")
        end
      }
    }
    raise "Unexpected Card Number pattern:  '#{cardnum}'"
  end
  def Card.sum_of_digits s
    s.split("").inject(0){|sum,x|sum + Integer(x)}
  end
  def Card.luhnSum s
    temp = ""
    r = 0..s.size
    a = s.split("").reverse
    r.each do |i|
      if i%2==1
        x = (Integer(a[i])*2).to_s
      else
        x = a[i]
      end
      temp << x.to_s
    end
    sum_of_digits temp
  end
  def Card.validity cardnum
    if (Card.luhnSum(cardnum) % 10)==0
      return "Valid"
    else
      return "Invalid"
    end
  end
end

# patterns will be tested for match in order defined
class Visa < Card.Base /4[0-9]{12,15}/
end
class Amex < Card.Base /3[4,7][0-9]{13}/
end
class Mastercard < Card.Base /5[1-5][0-9]{13}/
end
class Discover < Card.Base /6011[0-9]{12}/
end
# catch-all
class Unknown < Card.Base /.+/
end

p Card.validate(ARGV[0])