Calculate Luhn (http://en.wikipedia.org/wiki/Luhn_algorithm)

luhn_number.rb

  class LuhnNumber

    attr_accessor :full_number

    def initialize(full_number)
      self.full_number = full_number
    end

    def full_number=(full_number)
      @full_number = full_number.gsub(/[^0-9]/, '').to_s
    end

    def number_part
      full_number[0, full_number.length-1] if full_number
    end

    def check_digit_part
      full_number[-1, 1] if full_number
    end

    def valid?
      return false unless full_number and full_number.length > 2
      parts   = full_number.split(//).map(&:to_i)

      double  = parts.length % 2 == 0
      parts   = parts.collect do |part|
        number  = double ? part*2 : part
        double  = !double
        number
      end

      sum     = parts.join("").split(//).map(&:to_i).inject(:+)
      sum % 10 == 0
    end

    def invalid?
      !valid?
    end

  end

luhn_number_spec.rb

require 'spec_helper'

describe LuhnNumber do

  subject { described_class.new("4485071359608368") }

  describe "#initialize" do
    it "sets full_number" do
      described_class.any_instance.should_receive(:full_number=).with('123')
      described_class.new('123')
    end
  end

  describe "#full_number" do
    it "returns only number part of set value" do
      subject.full_number = '123 abc-'
      subject.full_number.should == '123'
    end
  end

  describe "#number_part" do
    it "returns number part of full_number" do
      subject.number_part.should == "448507135960836"
    end
  end

  describe "#check_digit_part" do
    it "returns number part of full_number" do
      subject.check_digit_part.should == "8"
    end
  end

  describe "#valid?" do
    it "returns if luhn algorytm % 10 is true" do
      subject.should be_valid
    end
  end

  describe "#valid?" do
    context "valid luhn" do
      it "returns true" do
        described_class.new("5500005555555559").should be_valid
        described_class.new("5555555555555557").should be_valid
        described_class.new("5454545454545454").should be_valid
        described_class.new("5555515555555551").should be_valid
        described_class.new("5405222222222226").should be_valid
        described_class.new("5478050000000007").should be_valid
      end
    end
    context "invalid luhn" do
      it "returns false" do
        described_class.new("5500005555555558").should be_invalid
        described_class.new("5555555555555556").should be_invalid
        described_class.new("5454545454545453").should be_invalid
        described_class.new("5555515555555550").should be_invalid
        described_class.new("5405222222222225").should be_invalid
        described_class.new("5478050000000006").should be_invalid
      end
    end
  end

end