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Checking a Credit Card Checksum
CodeExpectedActual
require 'rubygems'
require 'creditcard'
'5276 4400 6542 1319'.creditcard?
 true true 
'5276440065421313'.creditcard?
 false false 
1276440065421319.creditcard?
 false false 
5276440065421313.creditcard_type
 "mastercard" "mastercard" 
module CreditCard
  def creditcard?
    numbers = self.to_s.gsub(/[^\d]+/, '').split(//)
    checksum = 0
    0.upto numbers.length do |i|
      weight = numbers[-1*(i+2)].to_i * (2 - (i%2))
      checksum += weight % 9
    end
    return numbers[-1].to_i == 10 - checksum % 10
  end
end
class String
  include CreditCard
end
class Integer
  include CreditCard
end
'5276 4400 6542 1319'.creditcard?
 true true 
numbers = '5276 4400 6542 1319'.gsub(/[^\d]+/, '').split(//)
 ["5", "2", "7", "6", "4", "4", "0", "0", ["5", "2", "7", "6", "4", "4", "0", "0", "6", "5", "4", "2", "1", "3", "1", "9"] 
checksum = 0
0.upto numbers.length do |i|
  weight = numbers[-1*(i+2)].to_i * (2 - (i%2))
  checksum += weight % 9
end
checksum
 51 51 
numbers[-1].to_i == 10 - checksum % 10
 true true