Teach SyntaxHighlightFilter about a :highlight context attribute

This allows any code blocks without a language specified (indented
blocks or fences with no language) to be highlighted with a default.

For extra fun, add an additional class to the "highlight" div
specifying the highlighted language.

Author: jbarnette

lib/html/pipeline/syntax_highlight_filter.rb

@@ -11,14 +11,19 @@ class Pipeline
     class SyntaxHighlightFilter < Filter
       def call
         doc.search('pre').each do |node|
-          next unless lang = node['lang']
+          default = context[:highlight] && context[:highlight].to_s
+          next unless lang = node['lang'] || default
           next unless lexer = Pygments::Lexer[lang]
           text = node.inner_text
 
           html = highlight_with_timeout_handling(lexer, text)
           next if html.nil?
 
-          node.replace(html)
+          node = node.replace(html).first
+          klass = node["class"]
+          klass = [klass, "highlight-#{lang}"].compact.join " "
+
+          node["class"] = klass
         end
         doc
       end
@@ -30,4 +35,4 @@ def highlight_with_timeout_handling(lexer, text)
       end
     end
   end
-end
+end

test/html/pipeline/syntax_highlight_filter_test.rb

@@ -0,0 +1,22 @@
+require "test_helper"
+
+SyntaxHighlightFilter = HTML::Pipeline::SyntaxHighlightFilter
+
+class HTML::Pipeline::SyntaxHighlightFilterTest < Test::Unit::TestCase
+  def test_highlight_default
+    filter = SyntaxHighlightFilter.new \
+      "<pre>hello</pre>", :highlight => "coffeescript"
+
+    doc = filter.call
+    assert_not_empty doc.css ".highlight-coffeescript"
+  end
+
+  def test_highlight_default_will_not_override
+    filter = SyntaxHighlightFilter.new \
+      "<pre lang='c'>hello</pre>", :highlight => "coffeescript"
+
+    doc = filter.call
+    assert_empty doc.css ".highlight-coffeescript"
+    assert_not_empty doc.css ".highlight-c"
+  end
+end