Don't skip anchors with strict matching in Calendar.RecurrenceRule. Resolve #881

In recurrence rules that expand days or weekdays in a month, we first use a base
recurrence to calculate "anchors" in the month, and then change the day of month
or weekday to find results. Because the base recurrence used to match the day of
month of the start date, we could miss anchors if matching was set to `.strict`.
This change makes sure that if we know that the day of month is known to change,
we reset it to 1 in the base recurrence.

Author: hristost

Sources/FoundationEssentials/Calendar/Calendar_Recurrence.swift

@@ -230,9 +230,23 @@ extension Calendar {
                     case .monthly:  [.second, .minute, .hour, .day]
                     case .yearly:   [.second, .minute, .hour, .day, .month]
                 }
-                let componentsForEnumerating = recurrence.calendar._dateComponents(components, from: start) 
+                var componentsForEnumerating = recurrence.calendar._dateComponents(components, from: start) 
 
                 let rangeForBaseRecurrence: Range<Date>? = nil
+                var baseRecurrenceStartDate = start
+                if componentsForEnumerating.day != nil, dayOfMonthAction == .expand || weekdayAction == .expand {
+                    // If we expand either the day of the month or weekday, then
+                    // the day of month is likely to not match that of the start
+                    // date. Reset it to 1 in the base recurrence as to not skip
+                    // "invalid" anchors, such as February 30
+                    componentsForEnumerating.day = 1
+                }
+                if componentsForEnumerating.month != nil, monthAction == .expand {
+                    // Likewise, if we will be changing the month, reset it to 1 
+                    // in case the start date falls on a leap month
+                    componentsForEnumerating.month = 1
+                    componentsForEnumerating.isLeapMonth = nil
+                }
                 baseRecurrence = Calendar.DatesByMatching(calendar: recurrence.calendar,
                                                           start: start,
                                                           range: rangeForBaseRecurrence,

Tests/FoundationEssentialsTests/GregorianCalendarRecurrenceRuleTests.swift

@@ -676,4 +676,30 @@ final class GregorianCalendarRecurrenceRuleTests: XCTestCase {
 
         XCTAssertEqual(results, [])
     }
+    
+    func testFirstMondaysStrictMatching() {
+        let startDate = Date(timeIntervalSince1970: 1706659200.0) // 2024-01-31T00:00:00-0000
+        
+        var rule = Calendar.RecurrenceRule(calendar: gregorian, frequency: .monthly, matchingPolicy: .strict)
+        rule.weekdays = [.nth(1, .monday)]
+        
+        var dates = rule.recurrences(of: startDate).makeIterator()
+        XCTAssertEqual(dates.next(), Date(timeIntervalSince1970: 1707091200.0)) // 2024-02-05T00:00:00-0000
+        XCTAssertEqual(dates.next(), Date(timeIntervalSince1970: 1709510400.0)) // 2024-03-04T00:00:00-0000
+        XCTAssertEqual(dates.next(), Date(timeIntervalSince1970: 1711929600.0)) // 2024-04-01T00:00:00-0000
+        XCTAssertEqual(dates.next(), Date(timeIntervalSince1970: 1714953600.0)) // 2024-05-06T00:00:00-0000
+    }
+    
+    func testFifthFridaysStrictMatching() {
+        let startDate = Date(timeIntervalSince1970: 1706659200.0) // 2024-01-31T00:00:00-0000
+        
+        var rule = Calendar.RecurrenceRule(calendar: gregorian, frequency: .monthly, matchingPolicy: .strict)
+        rule.weekdays = [.nth(5, .friday)]
+        
+        var dates = rule.recurrences(of: startDate).makeIterator()
+        XCTAssertEqual(dates.next(), Date(timeIntervalSince1970: 1711670400.0)) // 2024-03-29T00:00:00-0000
+        XCTAssertEqual(dates.next(), Date(timeIntervalSince1970: 1717113600.0)) // 2024-05-31T00:00:00-0000
+        XCTAssertEqual(dates.next(), Date(timeIntervalSince1970: 1724976000.0)) // 2024-08-30T00:00:00-0000
+        XCTAssertEqual(dates.next(), Date(timeIntervalSince1970: 1732838400.0)) // 2024-11-29T00:00:00-0000
+    }
 }