Ruby LUN checker

lun_check.rb

# each_with_index is useful, so why not inject_with_index?
module Enumerable
  def inject_with_index(memo)
    each_with_index {|obj, idx| memo = yield(memo, obj, idx)}
    memo
  end
end

def passes_lun?(cardnum)
  digits = cardnum.gsub(/\D/,'').reverse.split('').map(&:to_i)
  digits.inject_with_index(0) do |memo, digit, idx|
    memo += (idx.even?) ? digit : ((digit*2 > 9) ? (digit*2 - 9) : (digit*2))
  end % 10 == 0
end

# Examples from http://en.wikipedia.org/wiki/Luhn_algorithm 
# 79927398713 should pass the test, as should 16 "8"s
("79927398710".."79927398719").each {|num| puts "#{num}: #{passes_lun?(num)}" }
puts passes_lun?("79927398713")
puts passes_lun?("8888888888888888")