Confident Ruby: Traffic Light Refactoring

traffic_light.rb

class TrafficLight
  class Lamp
    %w(name color).each do |attr|
      define_singleton_method("#{attr}=") do |value|
        define_method(attr) { value }
      end
    end

    def to_s
      name
    end

    def turn_on
      puts "Turning on #{color} lamp"
    end
  end
  
  class RedLamp < Lamp
    self.name  = 'stop'
    self.color = 'red'
  end

  class GreenLamp < Lamp
    self.name  = 'proceed'
    self.color = 'green'
  end

  class YellowLamp < Lamp
    self.name  = 'caution'
    self.color = 'yellow'

    def turn_on
      super
      puts 'Ring ring ring!'
    end
  end

  class Lamps
    COLLECTION = [
      RedLamp.new,
      GreenLamp.new,
      YellowLamp.new
    ]

    class << self
      @current = 0

      def select_by_name(name)
        index = COLLECTION.index{ |l| l.name == name.to_s }

        if index
          @current = index
        else
          raise(ArgumentError, "Could not find Lamp with name: #{name}")
        end
      end

      def signal
        current_lamp.turn_on
      end

      def current_lamp
        COLLECTION[@current]
      end

      def next
        index = if (@current + 1) >= COLLECTION.length
          0
        else
          @current + 1
        end

        COLLECTION[index]
      end
    end
  end


  def change_to(state)
    Lamps.select_by_name(state)
  end

  def signal
    Lamps.signal
  end

  def next_state
    Lamps.next.to_s
  end
end

traffic_light_test.rb

require 'minitest/autorun'
require_relative 'traffic_light'

describe TrafficLight do
  subject { TrafficLight.new }

  describe "when current state is stop" do
    before do
      subject.change_to 'stop'
    end

    it "turns on red light" do
      proc { subject.signal }.must_output "Turning on red lamp\n"
    end

    it "has next state of proceed" do
      subject.next_state.must_equal 'proceed'
    end
  end

  describe "when current state is caution" do
    before do
      subject.change_to :caution
    end

    it "turns on yellow light and rings warning bell" do
      proc{ subject.signal }.must_output "Turning on yellow lamp\nRing ring ring!\n"
    end

    it "has next state of stop" do
      subject.next_state.must_equal 'stop'
    end
  end

  describe "when current state is proceed" do
    before do
      subject.change_to :proceed
    end

    it "turns on green light" do
      proc{ subject.signal }.must_output "Turning on green lamp\n"
    end

    it "has next state of caution" do
      subject.next_state.must_equal 'caution'
    end
  end

  describe "when changing to an invalid state" do
    it "raises an ArgumentError exception" do
      proc { subject.change_to 'lolwut' }.must_raise ArgumentError
    end
  end
end