add method to from resource path to request path

incorporealcms/pages.py

@@ -2,6 +2,7 @@
 import datetime
 import logging
 import os
+import re
 
 from flask import Blueprint, Markup, abort
 from flask import current_app as app
@@ -110,6 +111,15 @@ def request_path_to_instance_resource_path(path):
     return absolute_resource.replace(f'{app.instance_path}{os.path.sep}', '')
 
 
+def instance_resource_path_to_request_path(path):
+    """Reverse a (presumed to exist) disk path to the canonical path that would show up in a Flask route.
+
+    This does not include the leading /, so aside from the root index case, this should be
+    bidirectional.
+    """
+    return re.sub(r'^pages/', '', re.sub(r'.md$', '', re.sub(r'index.md$', '', path)))
+
+
 def resolve_page_file(path):
     """Manipulate the request path to find appropriate page file.