Parse recurrence rules with BYDAY entries with frequency

For example, `-1SU`.
Unfortunately this commit contains a manual parsing implementation,
because the `icalendar` package does not support these right now.
(Though it will at the next update)

integreat_cms/cms/utils/external_calendar_utils.py

@@ -7,6 +7,7 @@
 import dataclasses
 import datetime
 import logging
+import re
 from typing import Any, Self
 
 import icalendar.cal
@@ -27,6 +28,11 @@
 from integreat_cms.cms.models import EventTranslation, ExternalCalendar, RecurrenceRule
 from integreat_cms.cms.utils.content_utils import clean_content
 
+# Copied from https://github.com/collective/icalendar/blob/4725c1bd57ca3afe61e7d45805a2b337842fbd29/src/icalendar/prop.py#L68-L69
+WEEKDAY_RULE = re.compile(
+    r"(?P<signal>[+-]?)(?P<relative>[\d]{0,2})(?P<weekday>[\w]{2})$"
+)
+
 
 @dataclasses.dataclass(frozen=True, kw_only=True)
 class IcalEventData:
@@ -175,12 +181,13 @@ class RecurrenceRuleData:
     interval: vInt
     until: vDDDTypes | None
     by_day: list[vWeekday] | None
-    by_set_pos: vInt | None
+    by_set_pos: int | None
 
     @classmethod
     def from_ical_rrule(
         cls, recurrence_rule: vRecur, start: datetime.date, logger: logging.Logger
     ) -> Self:
+        # pylint: disable=too-many-locals
         """
         Constructs this class from an ical recurrence rule.
         :return: An instance of this class
@@ -208,6 +215,26 @@ def pop_single_value(name: str, *, required: bool = False) -> Any:
         by_day = recurrence_rule.pop("BYDAY")
         by_set_pos = pop_single_value("BYSETPOS")
 
+        # by_set_pos can also be specified in `by_day`. We don't support multiple days with `by_set_pos` right now, though.
+        if by_day and len(by_day) == 1:
+            set_pos, weekday = _parse_weekday(by_day[0])
+            if by_set_pos is not None and set_pos is not None:
+                raise ValueError(
+                    f"Conflicting `BYSETPOS` and `BYDAY`: {by_set_pos} and {by_day}"
+                )
+            by_day[0] = weekday
+            by_set_pos = set_pos or by_set_pos
+        elif by_day:
+            updated_days = []
+            for day in by_day:
+                set_pos, weekday = _parse_weekday(day)
+                updated_days.append(weekday)
+                if set_pos is not None:
+                    raise ValueError(
+                        f"Cannot support multiple days with frequency right now: {by_day}"
+                    )
+            by_day = updated_days
+
         # WKST currently always has to be monday (or unset, because it defaults do monday)
         if (wkst := pop_single_value("WKST")) and wkst.lower() != "mo":
             raise ValueError(
@@ -302,6 +329,30 @@ def decode_by_day(self) -> list[int]:
         return weekdays
 
 
+def _parse_weekday(weekdaynum: str) -> tuple[int | None, str]:
+    """
+    Parses a weekday according to https://www.rfc-editor.org/rfc/rfc5545#section-3.3.10 (see `weekdaynum`)
+    TODO: This should be handled by our icalendar package. Remove this code when a new version of icalendar is released.
+    :param weekdaynum: The weekday + frequency number
+    :return: A tuple of frequency and weekday, where the frequency is None if the weekdaynum only contained a weekday.
+    """
+    match = WEEKDAY_RULE.match(weekdaynum)
+    assert match is not None
+    sign = match["signal"]
+    relative = match["relative"]
+    weekday = match["weekday"]
+    if not relative:
+        return None, weekday
+    relative = int(relative)
+    match sign:
+        case "-":
+            return -relative, weekday
+        case "+" | "":
+            return relative, weekday
+        case _:
+            return None, weekday
+
+
 def import_events(calendar: ExternalCalendar, logger: logging.Logger) -> None:
     """
     Imports events from this calendar and sets or clears the errors field of the calendar
@@ -442,7 +493,11 @@ def import_event(
     try:
         recurrence_rule_form_data = event_data.to_recurrence_rule_form_data()
     except ValueError as e:
-        logger.error("Could not import event due to unsupported recurrence rule: %r", e)
+        logger.error(
+            "Could not import event due to unsupported recurrence rule: %r\n%s",
+            e,
+            event_data,
+        )
         errors.append(
             _("Could not import '{}': Unsupported recurrence rule").format(
                 event_data.title, e

tests/cms/utils/assets/calendars/invalid_recurrence_rule_conflicting_byday_and_bysetpos.ics

@@ -0,0 +1,23 @@
+BEGIN:VEVENT
+RRULE:FREQ=MONTHLY;UNTIL=20271231T230000Z;INTERVAL=1;BYDAY=2WE;BYSETPOS=3
+UID:2ecade0f-3f6c-4094-96a5-fd73fce5c773
+SUMMARY:Monthly?
+DTSTART;VALUE=DATE:20250101
+DTEND;VALUE=DATE:20250102
+CLASS:PUBLIC
+PRIORITY:5
+DTSTAMP:20241119T134045Z
+TRANSP:OPAQUE
+STATUS:CONFIRMED
+SEQUENCE:2
+X-MICROSOFT-CDO-APPT-SEQUENCE:2
+X-MICROSOFT-CDO-BUSYSTATUS:BUSY
+X-MICROSOFT-CDO-INTENDEDSTATUS:BUSY
+X-MICROSOFT-CDO-ALLDAYEVENT:TRUE
+X-MICROSOFT-CDO-IMPORTANCE:1
+X-MICROSOFT-CDO-INSTTYPE:1
+X-MICROSOFT-DONOTFORWARDMEETING:FALSE
+X-MICROSOFT-DISALLOW-COUNTER:FALSE
+X-MICROSOFT-REQUESTEDATTENDANCEMODE:DEFAULT
+X-MICROSOFT-ISRESPONSEREQUESTED:FALSE
+END:VEVENT

tests/cms/utils/assets/calendars/invalid_recurrence_rule_multiple_byday_frequencies.ics

@@ -0,0 +1,23 @@
+BEGIN:VEVENT
+RRULE:FREQ=MONTHLY;UNTIL=20271231T230000Z;BYDAY=2WE,3TH
+UID:2ecade0f-3f6c-4094-96a5-fd73fce5c773
+SUMMARY:Monthly every second Wednesday and third Thursday
+DTSTART;VALUE=DATE:20250101
+DTEND;VALUE=DATE:20250102
+CLASS:PUBLIC
+PRIORITY:5
+DTSTAMP:20241119T134045Z
+TRANSP:OPAQUE
+STATUS:CONFIRMED
+SEQUENCE:2
+X-MICROSOFT-CDO-APPT-SEQUENCE:2
+X-MICROSOFT-CDO-BUSYSTATUS:BUSY
+X-MICROSOFT-CDO-INTENDEDSTATUS:BUSY
+X-MICROSOFT-CDO-ALLDAYEVENT:TRUE
+X-MICROSOFT-CDO-IMPORTANCE:1
+X-MICROSOFT-CDO-INSTTYPE:1
+X-MICROSOFT-DONOTFORWARDMEETING:FALSE
+X-MICROSOFT-DISALLOW-COUNTER:FALSE
+X-MICROSOFT-REQUESTEDATTENDANCEMODE:DEFAULT
+X-MICROSOFT-ISRESPONSEREQUESTED:FALSE
+END:VEVENT

tests/cms/utils/test_external_calendar_utils.py

@@ -22,6 +22,14 @@
         "tests/cms/utils/assets/calendars/invalid_recurrence_rule_bymonthday.ics",
         r"Month day of recurrence rule does not match month day of event: 3 and 1",
     ),
+    (
+        "tests/cms/utils/assets/calendars/invalid_recurrence_rule_conflicting_byday_and_bysetpos.ics",
+        r"Conflicting `BYSETPOS` and `BYDAY`: 3 and \['2WE'\]",
+    ),
+    (
+        "tests/cms/utils/assets/calendars/invalid_recurrence_rule_multiple_byday_frequencies.ics",
+        r"Cannot support multiple days with frequency right now: \['2WE', '3TH'\]",
+    ),
 ]
 
 

tests/core/management/commands/assets/calendars/recurrence_rules.ics

@@ -83,4 +83,16 @@ STATUS:CONFIRMED
 SUMMARY:Every year until 2034
 RRULE:FREQ=YEARLY;BYMONTH=11;UNTIL=20341112T230000Z
 END:VEVENT
+BEGIN:VEVENT
+CREATED:20241113T130016Z
+DTSTAMP:20241113T130159Z
+LAST-MODIFIED:20241113T130159Z
+SEQUENCE:2
+UID:6d6abd00-27af-4eff-868f-396ba9cd7013
+DTSTART;VALUE=DATE:20241113
+DTEND;VALUE=DATE:20241114
+STATUS:CONFIRMED
+SUMMARY:Every 3rd Wednesday
+RRULE:FREQ=MONTHLY;BYDAY=3WE
+END:VEVENT
 END:VCALENDAR

tests/core/management/commands/test_import_events.py

@@ -55,12 +55,14 @@
             "Every second day",
             "Weekly event",
             "Every year until 2034",
+            "Every 3rd Wednesday",
         ],
         {
             "DTSTART:20241112T230000\nRRULE:FREQ=MONTHLY;BYDAY=+1MO",
             "DTSTART:20241113T130002\nRRULE:FREQ=DAILY;INTERVAL=2",
             "DTSTART:20241113T190000\nRRULE:FREQ=WEEKLY;BYDAY=MO,WE,FR",
             "DTSTART:20241112T230000\nRRULE:FREQ=YEARLY;UNTIL=20341112T235959",
+            "DTSTART:20241112T230000\nRRULE:FREQ=MONTHLY;BYDAY=+3WE",
         },
     ),
     (