1 files changed, 45 insertions, 0 deletions
diff --git a/lib/banzai/filter/syntax_highlight_filter.rb b/lib/banzai/filter/syntax_highlight_filter.rb
new file mode 100644
index 00000000000..c889cc1e97c
--- /dev/null
+++ b/lib/banzai/filter/syntax_highlight_filter.rb
@@ -0,0 +1,45 @@
+require 'banzai'
+require 'html/pipeline/filter'
+require 'rouge/plugins/redcarpet'
+
+module Banzai
+  module Filter
+    # HTML Filter to highlight fenced code blocks
+    #
+    class SyntaxHighlightFilter < HTML::Pipeline::Filter
+      include Rouge::Plugins::Redcarpet
+
+      def call
+        doc.search('pre > code').each do |node|
+          highlight_node(node)
+        end
+
+        doc
+      end
+
+      def highlight_node(node)
+        language = node.attr('class')
+        code     = node.text
+
+        begin
+          highlighted = block_code(code, language)
+        rescue
+          # Gracefully handle syntax highlighter bugs/errors to ensure
+          # users can still access an issue/comment/etc.
+          highlighted = "<pre>#{code}</pre>"
+        end
+
+        # Replace the parent `pre` element with the entire highlighted block
+        node.parent.replace(highlighted)
+      end
+
+      private
+
+      # Override Rouge::Plugins::Redcarpet#rouge_formatter
+      def rouge_formatter(lexer)
+        Rouge::Formatters::HTMLGitlab.new(
+          cssclass: "code highlight js-syntax-highlight #{lexer.tag}")
+      end
+    end
+  end
+end