Optimize CSS expressions produced by Nokogiri

Nokogiri produces inefficient XPath expressions when given CSS
expressions such as "a.gfm". Luckily these expressions can be optimized
quite easily while still achieving the same results.

In the two cases where this optimization is applied the run time has
been reduced from around 170 ms to around 15 ms.

lib/banzai/filter/redactor_filter.rb

@@ -10,7 +10,7 @@ module Banzai
     #
     class RedactorFilter < HTML::Pipeline::Filter
       def call
-        doc.css('a.gfm').each do |node|
+        Querying.css(doc, 'a.gfm').each do |node|
           unless user_can_see_reference?(node)
             # The reference should be replaced by the original text,
             # which is not always the same as the rendered text.

lib/banzai/filter/reference_gatherer_filter.rb

@@ -16,7 +16,7 @@ module Banzai
       end
 
       def call
-        doc.css('a.gfm').each do |node|
+        Querying.css(doc, 'a.gfm').each do |node|
           gather_references(node)
         end
 

lib/banzai/querying.rb

@@ -0,0 +1,18 @@
+module Banzai
+  module Querying
+    # Searches a Nokogiri document using a CSS query, optionally optimizing it
+    # whenever possible.
+    #
+    # document - A document/element to search.
+    # query    - The CSS query to use.
+    #
+    # Returns a Nokogiri::XML::NodeSet.
+    def self.css(document, query)
+      # When using "a.foo" Nokogiri compiles this to "//a[...]" but
+      # "descendant::a[...]" is quite a bit faster and achieves the same result.
+      xpath = Nokogiri::CSS.xpath_for(query)[0].gsub(%r{^//}, 'descendant::')
+
+      document.xpath(xpath)
+    end
+  end
+end

spec/lib/banzai/querying_spec.rb

@@ -0,0 +1,13 @@
+require 'spec_helper'
+
+describe Banzai::Querying do
+  describe '.css' do
+    it 'optimizes queries for elements with classes' do
+      document = double(:document)
+
+      expect(document).to receive(:xpath).with(/^descendant::a/)
+
+      described_class.css(document, 'a.gfm')
+    end
+  end
+end