flatten.cr

class Flatten(T)
  include Iterator(T)

  def initialize(@iterator : Iterator(Iterator(T)))
    @current = @iterator.next
  end

  def next_
    if (current = @current).is_a?(Iterator::Stop)
      stop
    else
      next_val = current.next
      if next_val.is_a?(Iterator::Stop)
        @current = @iterator.next
        next_
      else
        next_val
      end
    end
  end

  def next
    next_
  end
end