my_enumerator.rb

class MyEnumerator
  include Enumerable

  attr_writer :args

  attr_writer :size

  attr_reader :generator

  attr_reader :lookahead

  def initialize(&block)
    receiver = Generator.new(&block)
    size = nil

    initialize_enumerator(receiver, size)
  end

  def initialize_enumerator(receiver, size, method_name=:each)
    @object = receiver
    @size = size
    @iter = method_name
    @generator = nil
    @lookahead = []
    @feedvalue = nil

    self
  end

  def next
    return @lookahead.shift unless @lookahead.empty?

    @generator ||= Iterator.new self

    begin
      return @generator.next if @generator.next?
    rescue StopIteration
    end

    exception = StopIteration.new "iteration reached end"

    raise exception
  end

  def each(*args)
    enumerator = self
    new_args = @args

    unless args.empty?
      enumerator = dup
      new_args = @args.empty? ? args : (@args + args)
    end

    enumerator.args = new_args

    if block_given?
      enumerator.each_with_block do |*yield_args|
        yield(*yield_args)
      end
    else
      enumerator
    end
  end

  def each_with_block
    @object.__send__ @iter, *@args do |*args|
      ret = yield(*args)
      unless @feedvalue.nil?
        ret = @feedvalue
        @feedvalue = nil
      end
      ret
    end
  end

  class Generator
    include Enumerable

    def initialize(&block)
      raise LocalJumpError, "Expected a block to be given" unless block_given?

      @proc = block

      self
    end
    private :initialize

    def each(*args)
      enclosed_yield = Proc.new { |*enclosed_args| yield *enclosed_args }

      @proc.call Yielder.new(&enclosed_yield), *args
    end
  end

  class Yielder
    def initialize(&block)
      raise LocalJumpError, "Expected a block to be given" unless block_given?

      @proc = block

      self
    end
    private :initialize

    def yield(*args)
      @proc.call *args
    end

    def <<(*args)
      self.yield(*args)

      self
    end
  end

  class Iterator
    attr_reader :result

    def initialize(obj)
      @object = obj
      rewind
    end

    private :initialize

    def next?
      !@done
    end

    def next
      reset unless @fiber

      val = @fiber.resume

      raise StopIteration, "iteration has ended" if @done

      return val
    end

    def rewind
      @fiber = nil
      @done = false
    end

    def reset
      @done = false
      @fiber = Fiber.new do
        obj = @object
        @result = obj.each { |*val| Fiber.yield *val }
        @done = true
      end
    end
  end
end