Unexpected results with Calendar.RecurrenceRule #881
Description
I tried to use the new Calendar.Recurrence objects and functionality, and I've bumped into some surprising (wrong?) output.
Given the following date range: 2024-01-01...2024-12-31, I used the .monthly recurrence rule to find every 5th Friday in the given range, using the strict matching rule.
The result was the (correct) following:
2024-03-29
2024-05-31
2024-08-30
2024-11-29
Now if I change the lower bound (start date) of the range to 2024-01-31 the result is unexpectedly the following:
2024-03-29
2024-05-31
// Missing date here
2024-08-30
The 2024-11-29 is missing from the output.
Output becomes even stranger if the calendar's firstWeekday is changed. If firstWeekday == 2, and the lower bound (start day) is 2024-01-01 then the output is the following:
2024-03-29
2024-05-31
2024-08-30
2024-09-27 // Month with only 4 Fridays
2024-11-29
2024-12-27 // Month with only 4 Fridays
If the lower bound (start date) is 2024-01-31 then the result is the following.
2024-03-29
2024-05-31
2024-08-30
// Missing date here
2024-12-27 // Month with only 4 Fridays
I believe that this is a bug, unless I am missing something in the way the algorithm works.
Some code if someone wants to quickly run some tests:
var calendar = Calendar(identifier: .gregorian)
// calendar.firstWeekday = 2
// let startComponents = DateComponents(year: 2024, month: 1, day: 1)
let startComponents = DateComponents(year: 2024, month: 1, day: 31)
let startDate = calendar.date(from: startComponents)!
let endComponents = DateComponents(year: 2024, month: 12, day: 31)
let endDate = calendar.date(from: endComponents)!
let rule = Calendar.RecurrenceRule.monthly(
calendar: calendar,
interval: 1,
matchingPolicy: .strict,
weekdays: [.nth(5, .friday)]
)
let output = rule.recurrences(of: startDate, in: startDate..<endDate)