fizzbuzz.rb

class MessageDispatcher
  def initialize
    @observers = []
  end

  def register(observer)
    @observers << observer
  end

  def dispatch(message)
    @observers.each do |observer|
      observer.notify(message)
    end
  end
end

class ObserverFactoryFactory
  def build(&block)
    Class.new do
      define_method :notify do |message|
        block.call(message)
      end
    end
  end
end

class OutputBuffer
  def initialize(stream)
    @messages = []
    @stream = stream
  end

  def <<(message)
    @messages << message
  end

  def flush
    @messages.each do |message|
      @stream.print(message)
    end

    @messages = []
  end
end

buffer = OutputBuffer.new(File.new("output", "w"))

fizz_observer_factory = ObserverFactoryFactory.new.build do |message|
  if message % 3 == 0
    buffer << "fizz"
  end
end

buzz_observer_factory = ObserverFactoryFactory.new.build do |message|
  if message % 5 == 0
    buffer << "buzz"
  end
end

number_observer_factory = ObserverFactoryFactory.new.build do |message|
  if message % 3 != 0 && message % 5 != 0
    buffer << message
  end
end

null_observer_factory = ObserverFactoryFactory.new.build do |message|
  buffer << "\n"
end

dispatcher = MessageDispatcher.new
dispatcher.register(fizz_observer_factory.new)
dispatcher.register(buzz_observer_factory.new)
dispatcher.register(number_observer_factory.new)
dispatcher.register(null_observer_factory.new)

1.upto(100).each do |i|
  dispatcher.dispatch(i)
end

buffer.flush