Update plugin for SIP redirects (#3243)

* Fix permalink due to SIP changes

* Update plugin for SIP redirects

This goes hand in hand with scala/improvement-proposals#128

Author: bracevac

_plugins/sip_number_permalink.rb

@@ -1,6 +1,6 @@
 # frozen_string_literal: true
 
-# This plugin allows using front matter variables in permalinks.
+# This plugin allows using front matter variables in permalinks and redirect_from.
 # For example: permalink: /sips/:number will use the 'number' front matter field.
 
 Jekyll::Hooks.register :site, :after_init do
@@ -10,3 +10,18 @@ def number
     end
   end
 end
+
+# Expand :number and :title in redirect_from values
+Jekyll::Hooks.register :documents, :pre_render do |doc|
+  next unless doc.data["redirect_from"]
+
+  redirects = doc.data["redirect_from"]
+  redirects = [redirects] unless redirects.is_a?(Array)
+
+  doc.data["redirect_from"] = redirects.map do |redirect|
+    result = redirect
+    result = result.gsub(":number", doc.data["number"].to_s) if doc.data["number"]
+    result = result.gsub(":title", Jekyll::Utils.slugify(doc.data["title"])) if doc.data["title"]
+    result
+  end
+end