 # [QUIZ] Checking Credit Cards (#122)

I was trying to go for “most compact and obfuscated version of the
Luhn algorithm”, but unfortunately the complications of the algorithm
meant that I had to debug my implementation, which left it slightly
more readable. Oh well. It’s still pretty dense and while my type
method might be readable it’s as dense as I care to make it.

#!ruby -x
def type(s)
case s.gsub(/\D/,’’)
when /^(?=34).{15}\$/; “AMEX”
when /^(?=37).{15}\$/; “AMEX”
when /^(?=6011).{16}\$/; “Discover”
when /^(?=5[1-5]).{16}\$/; “MasterCard”
when /^(?=4).{13}(…)?\$/; “Visa”
else ; “Unknown”
end
end

def luhn(s)
s.scan(/\d/).inject([0,0]){|(a,b),c|[b+c.to_i, a+c.to_i*2%9+(c==‘9’ ? 9 : 0)]}%10 == 0
end

s = ARGV.join
puts “#{type(s)} #{luhn(s)?‘V’:‘Inv’}alid”

END

My submission is pastied here:

http://pastie.caboo.se/57536

and pasted below. I tried to come up with a solution that would allow
easy manipulation of the list of credit card providers.

• donald

# version 1.0

require ‘enumerator’

class Integer
def digits
self.to_s.scan(/\d/).map{|n| n.to_i}
end

def sum_digits
digits.inject{|sum, n| sum + n}
end

def luhn
digits.reverse.enum_slice(2).inject(0) do |sum, sl|
sum + sl.sum_digits + (sl.length == 2 ? (2*sl).sum_digits :
0)
end
end

def luhn?
luhn % 10 == 0
end
end

module Credit
class Provider

``````def initialize(name, pattern)
@name = name
@pattern = pattern
end

def valid?(number)
@pattern.match(number)
end

def to_s
@name
end
``````

end

Providers = []
Providers << Provider.new(‘AMEX’, /^(34|37)\d{13}\$/)
Providers << Provider.new(‘Discover’, /^6011\d{12}\$/)
Providers << Provider.new(‘MasterCard’, /^5(1|2|3|4|5)\d{14}\$/)
Providers << Provider.new(‘Visa’, /^4(\d{12}|\d{15})\$/)

class Card

``````def initialize(number)
if number.is_a? Integer
@number = number
elsif number.is_a? String
@number = number.gsub(/\s/, '').to_i
else
raise InvalidArgument, number
end
end
def provider
Providers.each do |provider|
return provider if provider.valid?(@number.to_s)
end
return 'Unknown'
end
def valid?
@number.luhn?
end
def to_s
@number.to_s
end
``````

end
end

card = Credit::Card.new(ARGV)
puts card.provider.to_s << ’ ’ << (card.valid? ? ‘valid’ : ‘invalid’)

On Thursday 30 May 2002, Jeff Gray wrote:

to the ‘method=’ style of method naming. For example, you could define a
(Simply removing the ‘=’ from the method name allows the second case to
work.) I couldn’t find anything in the pickaxe book or in ruby-talk
behavior changed in later versions, or is it a permanent trait of Ruby
syntax?

Thanks,

• jeff

Also consider the test case below, where a method with = in its name
always returns the value passed to it. This is really frustrating, as I
want to use = in method names as syntactic sugar but can’t because the
methods don’t behave like they’re supposed to.
Is this a bug or a “feature”?

This script:

class TestClass
def test=(number)
# 42 is the answer to life the universe and everything.
42
end
def test(number)
# 42 is the answer to life the universe and everything.
42
end
end

test = TestClass.new

puts ‘With the = in the method name:’
puts test.test=7
puts test.test=(“Doesn’t work with parentheses either.”)
puts test.test=[1,2,3,“Arrays are cool!”]

puts ‘And without:’
puts test.test(7)
puts test.test(“Doesn’t work with parentheses either.”)
puts test.test([1,2,3,“Arrays are cool!”])

Produces output:

With the = in the method name:
7
Doesn’t work with parentheses either.
1
2
3
Arrays are cool!
And without:
42
42
42

Daniel M. [email protected] writes:

I was trying to go for “most compact and obfuscated version of the
Luhn algorithm”, but unfortunately the complications of the algorithm
meant that I had to debug my implementation, which left it slightly
more readable. Oh well. It’s still pretty dense and while my type
method might be readable it’s as dense as I care to make it.

You know, it’s longer, but I think I like this definition of the luhn
algorithm better, in terms of its conceptual conciseness:

def luhn(s)
s.scan(/\d/).map{|x|x.to_i}.inject([0,0]){
|(a,b),c|[b+c%9,a+2*c%9]}%10 == s.scan(/9/).size%10
end

(Now - the challenge is to figure out why that works)

An interesting observation based on this representation is that the
Luhn algorithm will fail to catch the transposition of an adjacent “0”
and “9”:

both ‘446-667-6097’ and ‘446-667-6907’ pass.

The only reason I’m submitting this is because I added a
inject_with_index method to Array. I liked Daniel M.'s use of the
inject([0,0]), I didn’t even think of doing that.

class Array
def inject_with_index(injected)
each_with_index{|obj, index| injected = yield(injected, obj, index)
}
injected
end
end

class CreditCard
@@types = {
‘AMEX’ => { :start => [34, 37], :length =>  },
‘Discover’ => { :start => , :length =>  },
‘MasterCard’ => { :start => (51…55).to_a, :length =>  },
‘Vista’ => { :start => , :length => [13, 16] }
}
attr :type, true
attr :valid, true
def initialize(number)
@type = (@@types.find do |card_type, card_st|
card_st[:start].any?{|st| /^#{st}/ =~ number } and
card_st[:length].any?{|le| number.length == le }
end || [‘Unknown’]).first
@valid = number.reverse.split(’’).inject_with_index(0) do
|acc,num,ind|
acc + ( ind%2 == 0 ? num.to_i :
(num.to_i*2).to_s.split(’’).inject(0){|a,e|a.to_i+e.to_i} )
end % 10 == 0
puts " Type: #{@type}"
puts “Valid: #{@valid}”
end
end

This was my first rubyquiz. I took a meta programming approach, since
it seemed like an easy enough example for it, and I haven’t done
enough meta programming yet. The only one of the solutions above that
I saw using some type of dynamic programming was the one using
method_missing above. Critiques welcome. For those interested, I did
manage to write a more details description on my blog:
http://webjazz.blogspot.com/2007/04/ruby-quiz-122-solution-checking-credit.html

Metaprogramming is probably overkill for this, since you don’t get a
new type of credit card company all the time, but it’s probably a nice
technique for when you have an object that needs lots of different
combinations of things each time.

Here is my solution:

require ‘enumerator’

class CreditCardChecker

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

class << self

``````# writes a method with the card_name? as a method name.  The
``````

method created would
# check what type of credit card a number is, based on the rules
given in the block.
# Use this function in the subclass
#
# class MyChecker < CreditCardChecker
# credit_card(:amex) { |cc| (cc =~ /^34./ or cc =~ /^37./)
and (cc.length == 15) }
# end
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

class CreditCard < 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 = ARGV

cccheck = CreditCard.new
puts cccheck.cctype(CCnum)
puts cccheck.valid?(CCnum)

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