Add to the original start date, not the iterative result

Author: parkera

Sources/FoundationEssentials/Calendar/Calendar.swift

@@ -554,7 +554,11 @@ public struct Calendar : Hashable, Equatable, Sendable {
                         byAdding component: Calendar.Component,
                         value: Int = 1,
                         wrappingComponents: Bool = false) -> some (Sequence<Date> & Sendable) {
-        DatesByAdding(calendar: self, start: start, range: range, components: DateComponents(component: component, value: value), wrappingComponents: wrappingComponents)
+        guard let components = DateComponents(component: component, value: value) else {
+            preconditionFailure("Attempt to add with an invalid Calendar.Component argument")
+        }
+        
+        return DatesByAdding(calendar: self, start: start, range: range, components: components, wrappingComponents: wrappingComponents)
     }
 
     /// Returns a sequence of `Date`s, calculated by repeatedly adding an amount of `Calendar.Component`s to a starting `Date` and then to each subsequent result.

Sources/FoundationEssentials/Calendar/Calendar_Enumerate.swift

@@ -284,13 +284,13 @@ extension Calendar {
     struct DatesByMatching : Sendable, Sequence {
         typealias Element = Date
 
-        var calendar: Calendar
-        var start: Date
-        var range: Range<Date>?
-        var matchingComponents: DateComponents
-        var matchingPolicy: Calendar.MatchingPolicy
-        var repeatedTimePolicy: Calendar.RepeatedTimePolicy
-        var direction: Calendar.SearchDirection
+        let calendar: Calendar
+        let start: Date
+        let range: Range<Date>?
+        let matchingComponents: DateComponents
+        let matchingPolicy: Calendar.MatchingPolicy
+        let repeatedTimePolicy: Calendar.RepeatedTimePolicy
+        let direction: Calendar.SearchDirection
 
         init(calendar: Calendar, start: Date, range: Range<Date>?, matchingComponents: DateComponents, matchingPolicy: Calendar.MatchingPolicy = .nextTime, repeatedTimePolicy: Calendar.RepeatedTimePolicy = .first, direction: Calendar.SearchDirection = .forward) {
             self.calendar = calendar
@@ -391,14 +391,14 @@ extension Calendar {
     struct DatesByAdding : Sendable, Sequence {
         typealias Element = Date
 
-        var calendar: Calendar
-        var start: Date
-        var range: Range<Date>?
-        var components: DateComponents?
-        var wrappingComponents: Bool
+        let calendar: Calendar
+        let start: Date
+        let range: Range<Date>?
+        let components: DateComponents
+        let wrappingComponents: Bool
 
         /// If `components` is `nil`, the result is an empty `Sequence`.
-        internal init(calendar: Calendar, start: Date, range: Range<Date>?, components: DateComponents?, wrappingComponents: Bool) {
+        internal init(calendar: Calendar, start: Date, range: Range<Date>?, components: DateComponents, wrappingComponents: Bool) {
             self.calendar = calendar
             self.start = start
             self.range = range
@@ -407,47 +407,43 @@ extension Calendar {
         }
 
         struct Iterator: Sendable, IteratorProtocol {
-            var calendar: Calendar
-            var previousResult: Date?
+            let calendar: Calendar
+            let start: Date
             let range: Range<Date>?
-            var components: DateComponents?
-            var wrappingComponents: Bool
+            let components: DateComponents
+            let wrappingComponents: Bool
+            var finished = false
+            var iteration = 1
 
             /// If `components` is `nil`, the result is an empty `Sequence`.
-            init(calendar: Calendar, start: Date, range: Range<Date>?, components: DateComponents?, wrappingComponents: Bool) {
+            init(calendar: Calendar, start: Date, range: Range<Date>?, components: DateComponents, wrappingComponents: Bool) {
+                self.start = start
                 self.calendar = calendar
-                previousResult = start
                 self.components = components
                 self.range = range
                 self.wrappingComponents = wrappingComponents
             }
 
             mutating func next() -> Element? {
-                guard let components else {
-                    // We have nothing to add
+                guard !finished else {
                     return nil
                 }
 
-                guard let startAt = previousResult else {
-                    // We have finished
-                    return nil
-                }
-                                
-                let next = calendar.date(byAdding: components, to: startAt, wrappingComponents: wrappingComponents)
+                let scaled = components.scaled(by: iteration)
+                let next = calendar.date(byAdding: scaled, to: start, wrappingComponents: wrappingComponents)
                 guard let next else {
                     // We have finished
-                    previousResult = nil
+                    finished = false
                     return nil
                 }
 
                 if let range, !range.contains(next) {
                     // We are out of the range
-                    previousResult = nil
+                    finished = true
                     return nil
                 }
 
-                // Next addition is based on this new result
-                previousResult = next
+                iteration += 1
                 return next
             }
         }

Sources/FoundationEssentials/Calendar/DateComponents.swift

@@ -393,6 +393,27 @@ public struct DateComponents : Hashable, Equatable, Sendable {
         }
         return true
     }
+    
+    // MARK: -
+    
+    /// Returns a new `DateComponents` where the subset of fields that can be scaled have been mulitplied by `value`.
+    internal func scaled(by value: Int) -> DateComponents {
+        var dc = self
+        if let era = _era { dc.era = era * value }
+        if let year = _year { dc.year = year * value }
+        if let month = _month { dc.month = month * value }
+        if let day = _day { dc.day = day * value }
+        if let hour = _hour { dc.hour = hour * value }
+        if let minute = _minute { dc.minute = minute * value }
+        if let second = _second { dc.second = second * value }
+        if let nanosecond = _nanosecond { dc.nanosecond = nanosecond * value }
+        if let quarter = _quarter { dc.quarter = quarter * value }
+        if let week = _week { dc.week = week * value }
+        if let weekOfMonth = _weekOfMonth { dc.weekOfMonth = weekOfMonth * value }
+        if let weekOfYear = _weekOfYear { dc.weekOfYear = weekOfYear * value }
+        if let yearForWeekOfYear = _yearForWeekOfYear { dc.yearForWeekOfYear = yearForWeekOfYear * value }
+        return dc
+    }
 
     // MARK: -
 

Tests/FoundationInternationalizationTests/CalendarTests.swift

@@ -659,6 +659,19 @@ final class CalendarTests : XCTestCase {
         XCTAssertEqual(numberOfDays, 3)
     }
 
+    @available(FoundationPreview 0.4, *)
+    func test_datesAdding_year() {
+        // Verify that adding 12 months once is the same as adding 1 month 12 times
+        let startDate = Date(timeIntervalSinceReferenceDate: 688946558.712307) // 2022-10-31 22:02:38 UTC
+        var cal = Calendar(identifier: .gregorian)
+        cal.timeZone = TimeZone.gmt
+        
+        let oneYearOnce = cal.date(byAdding: .month, value: 12, to: startDate)
+        let oneYearTwelve = Array(cal.dates(startingAt: startDate, byAdding: .month, value: 1).prefix(12)).last!
+        
+        XCTAssertEqual(oneYearOnce, oneYearTwelve)
+    }
+    
     @available(FoundationPreview 0.4, *)
     func test_datesMatching_simpleExample() {
         let cal = Calendar(identifier: .gregorian)