PlantUML support for Markdown

Allow rendering of PlantUML diagrams in Markdown documents using fenced blocks:

    ```plantuml
    Bob -> Sara : Hello
    Sara -> Bob : Go away
    ```

Closes: #4048

1 files changed, 39 insertions, 0 deletions

diff --git a/lib/banzai/filter/plantuml_filter.rb b/lib/banzai/filter/plantuml_filter.rb
new file mode 100644
index 00000000000..e194cf59275
--- /dev/null
+++ b/lib/banzai/filter/plantuml_filter.rb
@@ -0,0 +1,39 @@
+require "nokogiri"
+require "asciidoctor-plantuml/plantuml"
+
+module Banzai
+  module Filter
+    # HTML that replaces all `code plantuml` tags with PlantUML img tags.
+    #
+    class PlantumlFilter < HTML::Pipeline::Filter
+      def call
+        return doc unless doc.at('pre.plantuml') and settings.plantuml_enabled
+
+        plantuml_setup
+
+        doc.css('pre.plantuml').each do |el|
+          img_tag = Nokogiri::HTML::DocumentFragment.parse(
+            Asciidoctor::PlantUml::Processor.plantuml_content(el.content, {}))
+          el.replace img_tag
+        end
+
+        doc
+      end
+
+      private
+
+      def settings
+        ApplicationSetting.current || ApplicationSetting.create_from_defaults
+      end
+
+      def plantuml_setup
+        Asciidoctor::PlantUml.configure do |conf|
+          conf.url = settings.plantuml_url
+          conf.png_enable = settings.plantuml_enabled
+          conf.svg_enable = false
+          conf.txt_enable = false
+        end
+      end
+    end
+  end
+end