Fix for dates belonging to previous year (#34)

* Update year

* Fix for dates belonging to previous year

* Update year

LICENSE.txt

@@ -1,4 +1,4 @@
-Copyright (c) 2021-2023 Stephen F. Booth <[email protected]>
+Copyright (c) 2021-2024 Stephen F. Booth <[email protected]>
 
 Permission is hereby granted, free of charge, to any person obtaining a copy
 of this software and associated documentation files (the "Software"), to deal

Sources/JulianDayNumber/ISOCalendar.swift

@@ -1,5 +1,5 @@
 //
-// Copyright © 2021-2023 Stephen F. Booth <[email protected]>
+// Copyright © 2021-2024 Stephen F. Booth <[email protected]>
 // Part of https://github.com/sbooth/JulianDayNumber
 // MIT license
 //
@@ -64,7 +64,7 @@ public struct ISOCalendar {
 		let weekday = isoWeekdayFrom(year: Y, month: M, day: D)
 		let w = (10 + N - weekday) / 7
 		if w == 0 {
-			return (Y - 1, isoWeeksInYear(Y), weekday)
+			return (Y - 1, isoWeeksInYear(Y - 1), weekday)
 		} else if w > isoWeeksInYear(Y) {
 			return (Y + 1, 1, weekday)
 		}

Tests/JulianDayNumberTests/ISOCalendarTests.swift

@@ -1,5 +1,5 @@
 //
-// Copyright © 2021-2023 Stephen F. Booth <[email protected]>
+// Copyright © 2021-2024 Stephen F. Booth <[email protected]>
 // Part of https://github.com/sbooth/JulianDayNumber
 // MIT license
 //
@@ -27,6 +27,26 @@ final class ISOCalendarTests: XCTestCase {
 	func testDateFrom() {
 		XCTAssertTrue(ISOCalendar.isoDateFrom(year: 2023, month: 11, day: 16) == (2023, 46, 4))
 		XCTAssertTrue(ISOCalendar.dateFromISO(year: 2023, week: 46, weekday: 4) == (2023, 11, 16))
+
+		XCTAssertTrue(ISOCalendar.isoDateFrom(year: 2020, month: 12, day: 27) == (2020, 52, 7))
+		XCTAssertTrue(ISOCalendar.isoDateFrom(year: 2020, month: 12, day: 28) == (2020, 53, 1))
+		XCTAssertTrue(ISOCalendar.isoDateFrom(year: 2020, month: 12, day: 29) == (2020, 53, 2))
+		XCTAssertTrue(ISOCalendar.isoDateFrom(year: 2020, month: 12, day: 30) == (2020, 53, 3))
+		XCTAssertTrue(ISOCalendar.isoDateFrom(year: 2020, month: 12, day: 31) == (2020, 53, 4))
+		XCTAssertTrue(ISOCalendar.isoDateFrom(year: 2021, month: 1, day: 1) == (2020, 53, 5))
+		XCTAssertTrue(ISOCalendar.isoDateFrom(year: 2021, month: 1, day: 2) == (2020, 53, 6))
+		XCTAssertTrue(ISOCalendar.isoDateFrom(year: 2021, month: 1, day: 3) == (2020, 53, 7))
+		XCTAssertTrue(ISOCalendar.isoDateFrom(year: 2021, month: 1, day: 4) == (2021, 1, 1))
+
+		XCTAssertTrue(ISOCalendar.dateFromISO(year: 2020, week: 52, weekday: 7) == (2020, 12, 27))
+		XCTAssertTrue(ISOCalendar.dateFromISO(year: 2020, week: 53, weekday: 1) == (2020, 12, 28))
+		XCTAssertTrue(ISOCalendar.dateFromISO(year: 2020, week: 53, weekday: 2) == (2020, 12, 29))
+		XCTAssertTrue(ISOCalendar.dateFromISO(year: 2020, week: 53, weekday: 3) == (2020, 12, 30))
+		XCTAssertTrue(ISOCalendar.dateFromISO(year: 2020, week: 53, weekday: 4) == (2020, 12, 31))
+		XCTAssertTrue(ISOCalendar.dateFromISO(year: 2020, week: 53, weekday: 5) == (2021, 1, 1))
+		XCTAssertTrue(ISOCalendar.dateFromISO(year: 2020, week: 53, weekday: 6) == (2021, 1, 2))
+		XCTAssertTrue(ISOCalendar.dateFromISO(year: 2020, week: 53, weekday: 7) == (2021, 1, 3))
+		XCTAssertTrue(ISOCalendar.dateFromISO(year: 2021, week: 1, weekday: 1) == (2021, 1, 4))
 	}
 
 	func testJulianDayNumber() {