```Let's try this again. List ate my post. Maybe it's the S/MIME sig?

----

OK, this is the first Quiz contribution I have made (publicly). I
think it is a good balance between terse and readable.

JEG2, the luhn_valid? check is what I mean by Functional Programming
in Ruby, or as you say, the Power of Iterators. :-)

--be

####
#!/usr/bin/env ruby -wKU

# Some utility functions first
require 'enumerator'
module Enumerable
# Maps n-at-a-time (n = arity of given block) and collects the
results
def mapn(&b)
r = []
each_slice(b.arity) {|*args| r << b.call(*args) }
r
end

def sum; inject(0){|s, i| s + i} end
end

class CreditCardNumber < String
TYPES = {"3[47]\\d{13}"       => "Amex",
"6011\\d{12}"        => "Discover",
"5[1-5]\\d{14}"      => "Mastercard",
"4(\\d{12}|\\d{15})" => "Visa"}

# Returns the type of the given card, or nil if the card does not
match a pattern
def card_type
(t = TYPES.detect{|re, t| /^#{re}\$/ === self}) && t.last
end

# Returns true iff Luhn check passes for this number
def luhn_valid?
# a trick: double_and_sum[8] == sum_digits(8*2) == sum_digits
(16) == 1 + 6 == 7
double_and_sum = [0, 2, 4, 6, 8, 1, 3, 5, 7, 9]
split(//).reverse.mapn{|a,b| a.to_i + double_and_sum
[b.to_i]}.sum % 10 == 0
end

# Returns true iff card matches a known type and Luhn check passes.
def valid?
luhn_valid? && !card_type.nil?
end
end

if (arg = ARGV.join.gsub(/[^0-9]/, '')) and !arg.empty?
number = CreditCardNumber.new(arg)

if number.valid?
puts "Valid #{number.card_type}"
else
puts "Invalid card"
end
end

```