Checking Credit Card Numbers in Ruby

This is not meant to be an exhaustive list of all possible numbers, nor the only or best method to verify that they pass the "checksum" test, but here's what I came up with.

I wrote this mostly to link a Ruby version of the code to Wikipedia's article on Luhn checksum validation, since nearly every other language in use was listed, but Ruby was sadly missing.

#!/usr/bin/env ruby

#
# Copyright (c) 2008 Michael Graff.  All rights reserved.
#
# Redistribution and use in source and binary forms, with or
# without modification, are permitted provided that the following
# conditions are met:
# 1. Redistributions of source code must retain the above copyright
#    notice, this list of conditions and the following disclaimer.
# 2. Redistributions in binary form must reproduce the above
#    copyright notice, this list of conditions and the following
#    disclaimer in the documentation and/or other materials provided
#    with the distribution.
# 3. The name of Michael Graff may not be used to endorse or promote
#    products derived from this software without specific prior
#    written permission.
#
# THIS SOFTWARE IS PROVIDED BY Michael Graff ``AS IS'' AND ANY
# EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO,
# THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A
# PARTICULAR PURPOSE ARE DISCLAIMED.  IN NO EVENT SHALL Micahel Graff
# BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL,
# EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED
# TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
# DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND
# ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
# OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
# OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY
# OF SUCH DAMAGE.
#

class Luhn
  public
  def self.check_luhn(s)
    s.gsub!(/[^0-9]/, "")
    ss = s.reverse.split(//)

    alternate = false
    total = 0
    ss.each do |c|
      if alternate
        total += double_it(c.to_i)
      else
        total += c.to_i
      end
      alternate = !alternate
    end
    (total % 10) == 0
  end

  private
  def self.double_it(i)
    i = i * 2
    if i > 9
      i = i % 10 + 1
    end
    i
  end

end

if $0 == __FILE__
  def test_valid(s)
    result = Luhn::check_luhn(s)
    if result
      puts "VALID: #{s}"
    else
      puts "INVALID: #{s} (should be valid)"
    end
  end

  test_valid('5105 1051 0510 5100') # Mastercard
  test_valid('5555 5555 5555 4444') # Mastercard

  test_valid('4222 2222 2222 2')    # Visa
  test_valid('4111 1111 1111 1111') # Visa
  test_valid('4012 8888 8888 1881') # Visa

  test_valid('3782 8224 6310 005')  # American Express
  test_valid('3714 4963 5398 431')  # American Express
  test_valid('3787 3449 3671 000')  # American Express Corporate
  test_valid('3782 8224 6310 005')  # Amex
  test_valid('3400 0000 0000 009')  # Amex
  test_valid('3700 0000 0000 002')  # Amex

  test_valid('38520000023237')      # Diners Club (14 digits)
  test_valid('30569309025904')      # Diners Club (14 digits)

  test_valid('6011111111111117')    # Discover (16 digits)
  test_valid('6011 0000 0000 0004') # Discover
  test_valid('6011 0000 0000 0012') # Discover
  test_valid('6011000990139424')    # Discover (16 digits)
  test_valid('6011601160116611')    # Discover (16 digits)

  test_valid('3530111333300000')    # JCB (16 digits)
  test_valid('3566002020360505')    # JCB (16 digits)

  test_valid('5431111111111111')    # Mastercard (16 digits)
end