http://en.wikipedia.org/wiki/Luhn_algorithm

LUHN_formula

def creditcard_number_luhn_validator(card_num, check_num)
  s1 = ""
  card_num.split(//).map(&:to_i).reverse.each_with_index do |n, i|
    s1 << ((i%2 == 1)? (n*2) : n)
  end

  s2 = s1.split(//).inject(0){|sum,c| sum + c.to_i}
  
  s3 = (s2.divmod(10)[0]+1)*10 - s2

  s4 = (s3 == check_num)
end

puts creditcard_number_luhn_validator("4579730322257044", 10)? "valid" : "invalid"