fix: label > 26 sessions per group (#7599)

* fix: label > 26 sessions correctly

* test: test new helper

---------

Co-authored-by: Robert Sparks <rjsparks@nostrum.com>

ietf/meeting/models.py

@@ -1205,19 +1205,30 @@ class Session(models.Model):
         else:
             return ""
 
+    @staticmethod
+    def _alpha_str(n: int):
+        """Convert integer to string of a-z characters (a, b, c, ..., aa, ab, ...)"""
+        chars = []
+        while True:
+            chars.append(string.ascii_lowercase[n % 26])
+            n //= 26
+            # for 2nd letter and beyond, 0 means end the string
+            if n == 0:
+                break
+            # beyond the first letter, no need to represent a 0, so decrement
+            n -= 1
+        return "".join(chars[::-1])
+
     def docname_token(self):
         sess_mtg = Session.objects.filter(meeting=self.meeting, group=self.group).order_by('pk')
         index = list(sess_mtg).index(self)
-        return 'sess%s' % (string.ascii_lowercase[index])
+        return f"sess{self._alpha_str(index)}"
 
     def docname_token_only_for_multiple(self):
         sess_mtg = Session.objects.filter(meeting=self.meeting, group=self.group).order_by('pk')
         if len(list(sess_mtg)) > 1:
             index = list(sess_mtg).index(self)
-            if index < 26:
-                token = 'sess%s' % (string.ascii_lowercase[index])
-            else:
-                token = 'sess%s%s' % (string.ascii_lowercase[index//26],string.ascii_lowercase[index%26])
+            token = f"sess{self._alpha_str(index)}"
             return token
         return None
         

ietf/meeting/tests_models.py

@@ -10,6 +10,7 @@ from django.test import override_settings
 
 from ietf.group.factories import GroupFactory, GroupHistoryFactory
 from ietf.meeting.factories import MeetingFactory, SessionFactory, AttendedFactory, SessionPresentationFactory
+from ietf.meeting.models import Session
 from ietf.stats.factories import MeetingRegistrationFactory
 from ietf.utils.test_utils import TestCase
 from ietf.utils.timezone import date_today, datetime_today
@@ -147,6 +148,14 @@ class SessionTests(TestCase):
         session.chat_room = 'fnord'
         self.assertEqual(session.chat_room_name(), 'fnord')
 
+    def test_alpha_str(self):
+        self.assertEqual(Session._alpha_str(0), "a")
+        self.assertEqual(Session._alpha_str(1), "b")
+        self.assertEqual(Session._alpha_str(25), "z")
+        self.assertEqual(Session._alpha_str(26), "aa")
+        self.assertEqual(Session._alpha_str(27 * 26 - 1), "zz")
+        self.assertEqual(Session._alpha_str(27 * 26), "aaa")
+
     def test_session_recording_url(self):
         group_acronym = "foobar"
         meeting_date = datetime.date.today()