state_pattern.rb

# State interface
class TrafficLightState
  def change_state(_traffic_light)
    raise 'This method should be implemented in a subclass'
  end

  def display_color
    raise 'This method should be implemented in a subclass'
  end
end

# Concrete states
class RedState < TrafficLightState
  def change_state(traffic_light)
    traffic_light.state = YellowState.new
  end

  def display_color
    'Red'
  end
end

class GreenState < TrafficLightState
  def change_state(traffic_light)
    traffic_light.state = YellowState.new
  end

  def display_color
    'Green'
  end
end

class YellowState < TrafficLightState
  def change_state(traffic_light)
    if traffic_light.previous_state.is_a?(RedState)
      traffic_light.state = GreenState.new
    else
      traffic_light.state = RedState.new
    end
  end

  def display_color
    'Yellow'
  end
end

# Context
class TrafficLight
  attr_accessor :state, :previous_state

  def initialize(state = GreenState.new)
    @state = state
    @previous_state = state
  end

  def change_state
    @previous_state = @state
    @state.change_state(self)
  end

  def display_color
    @state.display_color
  end
end

# Usage
traffic_light = TrafficLight.new

puts traffic_light.display_color
traffic_light.change_state
puts traffic_light.display_color
traffic_light.change_state
puts traffic_light.display_color
traffic_light.change_state