1 files changed, 68 insertions, 0 deletions
diff --git a/lib/banzai/filter/footnote_filter.rb b/lib/banzai/filter/footnote_filter.rb
new file mode 100644
index 00000000000..97527976437
--- /dev/null
+++ b/lib/banzai/filter/footnote_filter.rb
@@ -0,0 +1,68 @@
+# frozen_string_literal: true
+
+module Banzai
+  module Filter
+    # HTML Filter for footnotes
+    #
+    # Footnotes are supported in CommonMark.  However we were stripping
+    # the ids during sanitization.  Those are now allowed.
+    #
+    # Footnotes are numbered the same - the first one has `id=fn1`, the
+    # second is `id=fn2`, etc.  In order to allow footnotes when rendering
+    # multiple markdown blocks on a page, we need to make each footnote
+    # reference unique.
+    #
+    # This filter adds a random number to each footnote (the same number
+    # can be used for a single render). So you get `id=fn1-4335` and `id=fn2-4335`.
+    #
+    class FootnoteFilter < HTML::Pipeline::Filter
+      INTEGER_PATTERN                 = /\A\d+\z/.freeze
+      FOOTNOTE_ID_PREFIX              = 'fn'.freeze
+      FOOTNOTE_LINK_ID_PREFIX         = 'fnref'.freeze
+      FOOTNOTE_LI_REFERENCE_PATTERN   = /\A#{FOOTNOTE_ID_PREFIX}\d+\z/.freeze
+      FOOTNOTE_LINK_REFERENCE_PATTERN = /\A#{FOOTNOTE_LINK_ID_PREFIX}\d+\z/.freeze
+      FOOTNOTE_START_NUMBER           = 1
+
+      def call
+        return doc unless first_footnote = doc.at_css("ol > li[id=#{fn_id(FOOTNOTE_START_NUMBER)}]")
+
+        # Sanitization stripped off the section wrapper - add it back in
+        first_footnote.parent.wrap('<section class="footnotes">')
+        rand_suffix = "-#{random_number}"
+
+        doc.css('sup > a[id]').each do |link_node|
+          ref_num       = link_node[:id].delete_prefix(FOOTNOTE_LINK_ID_PREFIX)
+          footnote_node = doc.at_css("li[id=#{fn_id(ref_num)}]")
+          backref_node  = footnote_node.at_css("a[href=\"##{fnref_id(ref_num)}\"]")
+
+          if ref_num =~ INTEGER_PATTERN && footnote_node && backref_node
+            link_node[:href]    += rand_suffix
+            link_node[:id]      += rand_suffix
+            footnote_node[:id]  += rand_suffix
+            backref_node[:href] += rand_suffix
+
+            # Sanitization stripped off class - add it back in
+            link_node.parent.append_class('footnote-ref')
+            backref_node.append_class('footnote-backref')
+          end
+        end
+
+        doc
+      end
+
+      private
+
+      def random_number
+        @random_number ||= rand(10000)
+      end
+
+      def fn_id(num)
+        "#{FOOTNOTE_ID_PREFIX}#{num}"
+      end
+
+      def fnref_id(num)
+        "#{FOOTNOTE_LINK_ID_PREFIX}#{num}"
+      end
+    end
+  end
+end