luhn

luhn.rb

def luhn(id, check=0)
  id = id.to_s.split("").reverse.each_with_index.map do |digit, index|  
    digit = digit.to_i
    if (index + 1) % 2 == 0
      if digit * 2 > 9
        (digit * 2) - 9
      else
        digit * 2
      end
    else
      digit
    end
  end

  id = id.inject{ |total, num| total + num }
  puts id
  ((id + check) % 10 == 0) ? true : false
end

luhn_spec.rb

require 'rspec'
require 'luhn'

describe 'luhn' do
  it 'checks if a number passes the luhn algorithm' do
    luhn(8763).should be_true
    luhn(1111).should be_false
    luhn(4473_0504_0429_1622).should be_true
  end

  it 'checks against a passed check number' do
    luhn(1111, 4).should be_true
  end
end