automata.rb

class Automata
  class Error < StandardError; end
  class InvalidTransition < Error; end

  attr_reader :current_state

  def initialize(initial_state:, &block)
    @initial_state = initial_state
    @current_state = nil
    @events = {}

    instance_exec(&block) if block_given?
  end

  def define_event(event_name, from: [], to:)
    @events[event_name.to_s] = { from: from, to: to }
  end

  def start
    @current_state = @initial_state
  end

  def fire(event_name)
    event = @events.fetch(event_name.to_s)
    raise InvalidTransition, "'#{event_name}' can not transition from #{@current_state}" unless event[:from].include?(current_state)

    @current_state = event[:to]
  end
end

automata = Automata.new(initial_state: '寝ている') do
  define_event('起きる', from: ['寝ている', '休んでいる'], to: '起きている')
  define_event('走る', from: ['休んでいる', '起きている', '泣いている'], to: '走っている')
  define_event('休む', from: ['走っている', '泣いている'], to: '起きている')
  define_event('泣く', from: ['走っている', '起きている'], to: '泣いている')
  define_event('寝る', from: ['起きている', '泣いている'], to: '寝ている')
end

automata.start

event_names = ['起きる', '走る', '休む', '寝る']
event_names.each { |event_name| automata.fire(event_name) }

puts automata.current_state #=> 寝ている

# ---

automata.start

event_names = ['起きる', '走る', '寝る']
event_names.each { |event_name| automata.fire(event_name) } #=> raise Automata::InvalidTransition