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Redcarpet Is _awesome_!
2011-12-21 11:00

It's true.

If you haven't used it yet, Redcarpet is the Markdown parser that GitHub uses to work all that magic on their site. So of course, it's awesome.

You can use it and abuse it

What makes it really awesome is the custom renderers feature. Here's the one from the documentation:

# create a custom renderer that allows highlighting of code blocks
class HTMLwithAlbino < Redcarpet::Render::HTML
  def block_code(code, language)
    Albino.safe_colorize(code, language)

markdown =

You can guess what that does: uses some library called Albino to render code blocks. There are a whole bunch of hooks you can use to make a custom renderer. They make it super easy to do something neat with markdown.

So I did.

Check it: outline generation

I'm working on... this project. And it needs to render a bunch of articles that are Markdown files. There will be a sidebar. I want to have links to each section. But I'm a good programmer, which means I'm lazy. So why bother making my own sidebar? Especially when I can abuse Redcarpet to do it.

Oh, and disclaimer: normally, I'm all about super clean code. Last night, that wasn't the mood I was in. This code is probably terrible. That's part of the fun! Please feel free to {clean up,obfuscate,golf,unit test} this code, email me, and tell me how awesome you are.

With that disclaimer out of the way, round one:

class OutlineRenderer < Redcarpet::Render::HTML
  attr_accessor :outline

  def initialize
    @outline = []

  def header(text, header_level)
    text_slug = text.gsub(/\W/, "_").downcase
    self.outline << [header_level, "<a href='##{text_slug}'>#{text}</a>"]

    "<h#{header_level} id='#{text_slug}'>#{text}</h#{header_level}>"

Every time we hit a header, Redcarpet gives us the text and the header level. We grab the text, turn it into a slug, and then append a two-element array to our outline. It keeps track of the level of this header, and makes a link from the slug. Then, we spit out a header tag, adding on an id element that we linked to in our link.

Next up, rendering:

renderer =
r =
content = r.render(some_content)
outline = renderer.outline

This instantiates our Renderer, creates our Markdown parser, and renders the content. We also grab the outline we made. Sweet.

Finally, rendering the outline:

def format_outline(outline)
  prev_level = 2

  "<ul>" + outline.inject("") do |html, data|
    level, link = data
    if prev_level < level
      html += "<ul>"
    elsif prev_level > level
      html += "</ul>"
    prev_level = level
    html += "<li>#{link}</li>"
  end + "</ul>"

This... is amazing. And terrible. Don't drink scotch and code, kids. Or do, whatever. This takes our Array of Arrays that we made with our renderer and runs inject over it. If our previous level is less than the new level, we indent with a <ul>, and if it's greater, we outdent with a </ul>. Then we render our link, and wrap the whole thing in its own set of <ul>s. Done!

We can call this helper method in our layout, and bam! We get automatic outlines generated for our Markdown files. Like I said, this is quick and dirty, but for a first pass, I don't feel too bad about it. Get it done, and then get it done right.