class Enumerator
  class Lazy
    def initialize(coll)

      @lazy = Yid.new do |yielder|
        if block_given?
          coll.each do |x|
            yield yielder, x
          end
        else
          coll.each do |x|
            yielder.yield x
          end
        end
      end
    end

    def each
      loop { yield @lazy.next }
    rescue StopIteration
    end

    def select
      Lazy.new(self) do |yielder, *vals|
        result = yield(*vals)
        yielder.yield(*vals) if result
      end
    end

    def map
      Lazy.new(self) do |yielder, *vals|
        yielder.yield yield(*vals)
      end
    end

    def take(n)
      i = 0
      Lazy.new(self) do |yielder, *vals|
        if i < n
          i += 1
          yielder.yield(*vals)
        else
          raise StopIteration
        end
      end
    end

    def drop(n)
      i = 0
      Lazy.new(self) do |yielder, *vals|
        if i < n
          i += 1
        else
          yielder.yield(*vals)
        end
      end
    end

    def drop_while
      to_drop = true
      Lazy.new(self) do |yielder, *vals|
        if to_drop
          result = yield(*vals)
          if !result
            to_drop = false
          end
        end

        if !to_drop
          yielder.yield(*vals)
        end
      end
    end

    def next
      @lazy.next
    end

    def force
      ret = []
      while val = @lazy.next
        ret << val
      end
    rescue StopIteration
      ret
    end

    private
    # kudos http://stackoverflow.com/a/19660333/482757
    class Yid
      def initialize(&block)
        # creates a new Fiber to be used as an Yielder
        @yielder  = Fiber.new do
          yield Fiber
          raise StopIteration # raise an error if there is no more calls
        end
      end

      def next
        # return the value and wait until the next call
        @yielder.resume
      end
    end
  end
end