Recurrence: Reproduce and fix issues if BYWEEKNO and INTERVAL are used together

Ical.Net.Tests/Calendars/Recurrence/RecurrenceTestCases.txt

@@ -6,3 +6,24 @@ INSTANCES:20241024,20241028,20241031
 # Illegal rule part with multiple '='
 RRULE:FREQ=WEEKLY;BYDAY=MO=;COUNT=3
 EXCEPTION:System.ArgumentException
+
+# The first date belongs to 2009, so after 6 years it will be 2015, which has a week 53.
+RRULE:FREQ=YEARLY;BYWEEKNO=53;BYDAY=TU,SA;INTERVAL=6;UNTIL=20170101T000000Z
+DTSTART:20100102T000000
+INSTANCES:20100102T000000,20151229T000000,20160102T000000
+
+# The first date belongs to 2009, so after 5 years it will be 2014, which has NO week 53.
+RRULE:FREQ=YEARLY;BYWEEKNO=53;BYDAY=TU,SA;INTERVAL=5;UNTIL=20170101T000000Z
+DTSTART:20100102T000000
+INSTANCES:20100102T000000
+
+# The first date belongs to 2009, so after 7 years it will be 2014, which has NO week 53.
+RRULE:FREQ=YEARLY;BYWEEKNO=53;BYDAY=TU,SA;INTERVAL=7;UNTIL=20170101T000000Z
+DTSTART:20100102T000000
+INSTANCES:20100102T000000
+
+# DTSTART is in 2024 but the week belongs to 2025, so after 3 years it will be 2028.
+RRULE:FREQ=YEARLY;BYWEEKNO=1;BYDAY=MO,TU;INTERVAL=3;UNTIL=20320101
+DTSTART:20241231
+INSTANCES:20241231,20280103,20280104,20301230,20301231
+

Ical.Net/Evaluation/Evaluator.cs

@@ -55,11 +55,8 @@ protected IDateTime ConvertToIDateTime(DateTime dt, IDateTime referenceDate)
 
     protected void IncrementDate(ref DateTime dt, RecurrencePattern pattern, int interval)
     {
-        // FIXME: use a more specific exception.
         if (interval == 0)
-        {
-            throw new Exception("Cannot evaluate with an interval of zero.  Please use an interval other than zero.");
-        }
+            return;
 
         var old = dt;
         switch (pattern.Frequency)

Ical.Net/Evaluation/RecurrencePatternEvaluator.cs

@@ -211,6 +211,14 @@
         var originalDate = DateUtil.GetSimpleDateTimeData(seed);
         var seedCopy = DateUtil.GetSimpleDateTimeData(seed);
 
+        if ((pattern.Frequency == FrequencyType.Yearly) && (pattern.ByWeekNo.Count != 0))
+        {
+            // Dates in the first or last week of the year could belong weeks that belong to
+            // the prev/next year, in which case we must adjust that year. This is necessary
+            // to get the invervals right.
+            IncrementDate(ref seedCopy, pattern, Calendar.GetIso8601YearOfWeek(seedCopy, pattern.FirstDayOfWeek) - seedCopy.Year);
+        }
+
         // optimize the start time for selecting candidates
         // (only applicable where a COUNT is not specified)
         if (pattern.Count == int.MinValue)
@@ -398,34 +406,39 @@
             foreach (var weekNo in GetByWeekNoForYearNormalized(pattern, t.Year))
             {
                 var date = t;
+
+                // Make sure we start from a reference date that is in a week that belongs to the current year.
+                // Its not important that the date lies in a certain week, but that the week belongs to the
+                // current year and that the week day is preserved.
+                if (date.Month == 1)
+                    date = date.AddDays(7);
+                else if (date.Month >= 12)
+                    date = date.AddDays(-7);
+
                 // Determine our current week number
                 var currWeekNo = Calendar.GetIso8601WeekOfYear(date, pattern.FirstDayOfWeek);
-                while (currWeekNo > weekNo)
-                {
-                    // If currWeekNo > weekNo, then we're likely at the start of a year
-                    // where currWeekNo could be 52 or 53.  If we simply step ahead 7 days
-                    // we should be back to week 1, where we can easily make the calculation
-                    // to move to weekNo.
-                    date = date.AddDays(7);
-                    currWeekNo = Calendar.GetIso8601WeekOfYear(date, pattern.FirstDayOfWeek);
-                }
 
                 // Move ahead to the correct week of the year
                 date = date.AddDays((weekNo - currWeekNo) * 7);
 
-                // Step backward single days until we're at the correct DayOfWeek
-                while (date.DayOfWeek != pattern.FirstDayOfWeek)
+                // Ignore the week if it doesn't belong to the current year.
+                if (Calendar.GetIso8601YearOfWeek(date, pattern.FirstDayOfWeek) == t.Year)
                 {
-                    date = date.AddDays(-1);
-                }
+                    // Step backward single days until we're at the correct DayOfWeek
+                    while (date.DayOfWeek != pattern.FirstDayOfWeek)
+                    {
+                        date = date.AddDays(-1);
+                    }
 
-                for (var k = 0; k < 7; k++)
-                {
-                    weekNoDates.Add(date);
-                    date = date.AddDays(1);
+                    for (var k = 0; k < 7; k++)
+                    {
+                        weekNoDates.Add(date);
+                        date = date.AddDays(1);
+                    }
                 }
             }
         }
+
         return weekNoDates;
     }