gist.rb

require 'cgi'
require 'open-uri'

# Require the ActiveSupport::Cache module
require 'active_support/cache'

module Fiftyfive
  module Liquid
    class GistTag < ::Liquid::Tag
      def initialize(tag_name, gist_ref, tokens)
        @gist_ref = gist_ref.strip
        @gist_id, @filename = @gist_ref.split('/')
        super
      end
      
      # A simple file-based cache in ./_tmp shared by all GistTag instances
      def cache
        @@cache ||= ActiveSupport::Cache::FileStore.new("_tmp/gist")
      end
      
      # Return from cache if possible; otherwise fetch and add it to cache
      def fetch_gist(raw_gist_uri)
        cache.fetch(raw_gist_uri) do
          open(raw_gist_uri).read.chomp
        end
      end
    
      def render(context)
        raw_uri = "https://gist.github.com/raw/#{@gist_id}/#{@filename}"
        script_uri = "https://gist.github.com/#{@gist_id}.js?file=#{@filename}"

        <<MARKUP.strip
<div id="gist-#{@gist_ref.gsub(/[^a-z0-9]/i,'-')}">
<script src="#{script_uri}"></script>
<noscript>
<pre>#{CGI.escapeHTML(fetch_gist(raw_uri))}</pre>
</noscript>
</div>
MARKUP
      end
    end
  end
end

Liquid::Template.register_tag('gist', Fiftyfive::Liquid::GistTag)

gist_markup.html

<div>
  <!-- For browsers that know JavaScript -->
  <script src="https://gist.github.com/12345.js?file=abc.txt"></script>

  <!-- For browsers or RSS readers that don't deal with JavaScript -->
  <noscript>
    <pre>
Raw content of a Gist file.
    </pre>
  </noscript>
</div>

gist_tag_output.html

<div id="gist-12345-abc-txt">
<script src="https://gist.github.com/12345.js?file=abc.txt"></script>
<noscript>
<pre>Raw content of a Gist file.</pre>
</noscript>
</div>

gist_take1.rb

require 'cgi'
require 'open-uri'

module Fiftyfive
  module Liquid
    class GistTag < ::Liquid::Tag
      def initialize(tag_name, gist_ref, tokens)
        @gist_ref = gist_ref.strip

        # gist_ref is in the form of `gist_id` or `gist_id/filename`
        @gist_id, @filename = @gist_ref.split('/')
        super
      end
      
      def render(context)
        raw_uri = "https://gist.github.com/raw/#{@gist_id}/#{@filename}"
        script_uri = "https://gist.github.com/#{@gist_id}.js?file=#{@filename}"

        <<MARKUP.strip
<div id="gist-#{@gist_ref.gsub(/[^a-z0-9]/i,'-')}">
<script src="#{script_uri}"></script>
<noscript>
<pre>#{CGI.escapeHTML(open(raw_gist_uri).read.chomp)}</pre>
</noscript>
</div>
MARKUP
      end
    end
  end
end

Liquid::Template.register_tag('gist', Fiftyfive::Liquid::GistTag)