1 files changed, 51 insertions, 0 deletions
diff --git a/lib/banzai/filter/math_filter.rb b/lib/banzai/filter/math_filter.rb
new file mode 100644
index 00000000000..cb037f89337
--- /dev/null
+++ b/lib/banzai/filter/math_filter.rb
@@ -0,0 +1,51 @@
+require 'uri'
+
+module Banzai
+  module Filter
+    # HTML filter that adds class="code math" and removes the dollar sign in $`2+2`$.
+    #
+    class MathFilter < HTML::Pipeline::Filter
+      # This picks out <code>...</code>.
+      INLINE_MATH = 'descendant-or-self::code'.freeze
+
+      # Pick out a code block which is declared math
+      DISPLAY_MATH = "descendant-or-self::pre[contains(@class, 'math') and contains(@class, 'code')]".freeze
+
+      # Attribute indicating inline or display math.
+      STYLE_ATTRIBUTE = 'data-math-style'.freeze
+
+      # Class used for tagging elements that should be rendered
+      TAG_CLASS = 'js-render-math'.freeze
+
+      INLINE_CLASSES = "code math #{TAG_CLASS}".freeze
+
+      DOLLAR_SIGN = '$'.freeze
+
+      def call
+        doc.xpath(INLINE_MATH).each do |code|
+          closing = code.next
+          opening = code.previous
+
+          # We need a sibling before and after.
+          # They should end and start with $ respectively.
+          if closing && opening &&
+              closing.content.first == DOLLAR_SIGN &&
+              opening.content.last == DOLLAR_SIGN
+
+            code[:class] = INLINE_CLASSES
+            code[STYLE_ATTRIBUTE] = 'inline'
+            closing.content = closing.content[1..-1]
+            opening.content = opening.content[0..-2]
+          end
+        end
+
+        doc.xpath(DISPLAY_MATH).each do |el|
+          el[STYLE_ATTRIBUTE] = 'display'
+          el[:class] += " #{TAG_CLASS}"
+        end
+
+        doc
+      end
+    end
+  end
+end