1 files changed, 55 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..a7120fbd46e
--- /dev/null
+++ b/lib/banzai/filter/footnote_filter.rb
@@ -0,0 +1,55 @@
+# 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
+
+      def call
+        return doc unless first_footnote = doc.at_css('ol > li[id=fn1]')
+
+        # Sanitization stripped off the section wrapper - add it back in
+        first_footnote.parent.wrap('<section class="footnotes">')
+
+        doc.css('sup > a[id]').each do |link_node|
+          ref_num       = link_node[:id].delete_prefix('fnref')
+          footnote_node = doc.at_css("li[id=fn#{ref_num}]")
+          backref_node  = doc.at_css("li[id=fn#{ref_num}] a[href=\"#fnref#{ref_num}\"]")
+
+          if ref_num =~ INTEGER_PATTERN && footnote_node && backref_node
+            rand_ref_num        = "#{ref_num}-#{random_number}"
+            link_node[:href]    = "#fn#{rand_ref_num}"
+            link_node[:id]      = "fnref#{rand_ref_num}"
+            footnote_node[:id]  = "fn#{rand_ref_num}"
+            backref_node[:href] = "#fnref#{rand_ref_num}"
+
+            # 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
+    end
+  end
+end