Oven

oven.rb

class Oven
  attr_reader :state

  def initialize
    @state = InactiveState.new
  end

  def temp_up
    @state = state.temp_up
  end

  def bake
    @state = state.bake
  end
end

class InactiveState
  def temp_up
    self
  end

  def bake
    SetTemperatureState300.new
  end
end

temps = (300...500).select{|t| t % 5 == 0}
temps.each do |temp|
  class_name = "SetTemperatureState#{temp}"
  Object.const_set(class_name,
    Class.new do
      def current_temp
        self.class.to_s[-3..-1].to_i
      end

      def temp_up
        next_state = "SetTemperatureState#{current_temp + 5}"
        next_class = Object.const_get(next_state)
        return next_class.new
      end
    end
  )
end

class SetTemperatureState500
  def temp_up
    self
  end
end

oven_test.rb

require 'minitest/autorun'
require './oven'

class OvenTest < Minitest::Test
  def test_it_starts_inactive
    oven = Oven.new
    assert oven.state.kind_of?(InactiveState)
  end

  def test_temp_up_is_inert_when_inactive
    oven = Oven.new
    oven.temp_up
    assert oven.state.kind_of?(InactiveState)
  end

  def test_bake_activates_temp_setter_when_inactive
    oven = Oven.new
    oven.bake
    assert oven.state.kind_of?(SetTemperatureState300)
  end
end

class SetTemperatureStateTest < Minitest::Test
  def test_temp_up_increases_temperature
    state = SetTemperatureState300.new
    state = state.temp_up
    assert state.kind_of?(SetTemperatureState305)
    state = state.temp_up
    assert state.kind_of?(SetTemperatureState310)
  end

  def test_temp_maxes_out_at_500
    state = SetTemperatureState300.new
    50.times{ state = state.temp_up }
    assert state.kind_of?(SetTemperatureState500)
  end
end