gistfile1.rb

# This FSM recognizes the sequence 1 2 3 anywhere in the  
# input. Note that 1 2 1 2 3  and 1 1 2 3 and both valid.  

class FSM
  
  class State
    def initialize(name)
      @name = name
      @hash = {}
    end
    
    def transition(trigger_and_new_state)
      @hash.merge!(trigger_and_new_state)
    end
    
    def otherwise(new_state)
      @hash.default = new_state
    end
    
    def next_state(event)
      new_state = @hash[event]
      puts "#{event} => #{new_state}"
      new_state
    end
  end
  
  def initialize
    @transitions = {}
  end
  
  def state(name, &transitions)
    state = State.new(name)
    state.instance_eval(&transitions)
    @transitions[name] = state
  end
  
  def recognize(inputs)
    current_state = :start
    inputs.each do |event|
      current_state = @transitions[current_state].next_state(event)
      break if current_state == :done
    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 ]