fsm.rb

class FSM

  def initialize
    @states = {}
  end

  def state(name, &block)
    s = State.new
    s.instance_eval(&block)
    @states[name] = s
  end

  def recognize(list)
    state = @states[:start]
    next_state = nil
    raise "no start state!" unless state
    list.each do |item|
      print "#{item} "
      next_state = state.execute(item)
      if next_state == :done
        puts "found it!"
        break
      end
      state = @states[next_state]
    end
  end

  class State
    def initialize
      @transitions = {}
      @otherwise = :start
    end

    def transition(trans)
      @transitions.merge!(trans)
    end

    def otherwise(other)
      @otherwise = other
    end

    def execute(item)
      @transitions[item] || @otherwise
    end
  end

end

fsm = FSM.new
fsm.state :start do
  transition "1" => :one
  otherwise :start
end

fsm.state :one do
  transition "2" => :two
  transition "1" => :one
  otherwise :start
end

fsm.state :two do
  transition "3" => :done
  transition "1" => :one
  otherwise :start
end

fsm.recognize %w[1 4 2 1 1 2 1 2 3 2 3 1]