extras/markdowner.rb

class Markdowner
  # opts[:allow_images] allows <img> tags

  def self.to_html(text, opts = {})
    if text.blank?
      return ""
    end

    exts = [:tagfilter, :autolink, :strikethrough]
    root = CommonMarker.render_doc(text.to_s, [:SMART], exts)

    walk_text_nodes(root) {|n| postprocess_text_node(n) }

    ng = Nokogiri::HTML(root.to_html([:DEFAULT], exts))

    # change <h1>, <h2>, etc. headings to just bold tags
    ng.css("h1, h2, h3, h4, h5, h6").each do |h|
      h.name = "strong"
    end

    # This should happen before adding rel=ugc to all links
    convert_images_to_links(ng) unless opts[:allow_images]

    # make links have rel=ugc
    ng.css("a").each do |h|
      h[:rel] = "ugc" unless (URI.parse(h[:href]).host.nil? rescue false)
    end

    if ng.at_css("body")
      ng.at_css("body").inner_html
    else
      ""
    end
  end

  def self.walk_text_nodes(node, &block)
    return if node.type == :link
    return block.call(node) if node.type == :text

    node.each do |child|
      walk_text_nodes(child, &block)
    end
  end

  def self.postprocess_text_node(node)
    while node
      return unless node.string_content =~ /\B(@#{User::VALID_USERNAME})/
      before, user, after = $`, $1, $'

      node.string_content = before

      if User.exists?(:username => user[1..-1])
        link = CommonMarker::Node.new(:link)
        link.url = Rails.application.root_url + "u/#{user[1..-1]}"
        node.insert_after(link)

        link_text = CommonMarker::Node.new(:text)
        link_text.string_content = user
        link.append_child(link_text)

        node = link
      else
        node.string_content += user
      end

      if after.length > 0
        remainder = CommonMarker::Node.new(:text)
        remainder.string_content = after
        node.insert_after(remainder)

        node = remainder
      else
        node = nil
      end
    end
  end

  def self.convert_images_to_links(node)
    node.css("img").each do |img|
      link = node.create_element('a')

      link['href'], title, alt = img.attributes
        .values_at('src', 'title', 'alt')
        .map(&:to_s)

      link.content = [title, alt, link['href']].find(&:present?)

      img.replace link
    end
  end
end