require "commonmarker"

class HeaderWithIdRender < CommonMarker::HtmlRenderer
  def header(node)
    block do
      old_stream = @stream
      @stream = StringIO.new(String.new.force_encoding("utf-8"))
      out(:children)
      content = @stream.string
      @stream = old_stream

      id = generate_id(content)

      out("<h", node.header_level, " id=\"", id, "\">",
               content, "</h", node.header_level, ">")
    end
  end

  def generate_id(content)
    content.downcase.strip.gsub(' ', '-').gsub(/[^\w-]/, '')
  end
end

markdown = "## My nice long header"
doc = CommonMarker.render_doc(markdown, :DEFAULT)
puts HeaderWithIdRender.new.render(doc)