compare week-year not calendar year in compareWeeks

src/main/java/org/apache/commons/validator/routines/AbstractCalendarValidator.java

@@ -117,17 +117,23 @@ protected int compare(final Calendar value, final Calendar compare, final int fi
 
         int result;
 
+        // Week of Year is relative to its own week-year, which at the year boundary can differ
+        // from the calendar year (for example 31 December may fall in week 1 of the following
+        // week-year), so the week-year is compared rather than YEAR for this field.
+        if (field == Calendar.WEEK_OF_YEAR) {
+            result = Integer.compare(weekYear(value), weekYear(compare));
+            if (result != 0) {
+                return result;
+            }
+            return calculateCompareResult(value, compare, Calendar.WEEK_OF_YEAR);
+        }
+
         // Compare Year
         result = calculateCompareResult(value, compare, Calendar.YEAR);
         if (result != 0 || field == Calendar.YEAR) {
             return result;
         }
 
-        // Compare Week of Year
-        if (field == Calendar.WEEK_OF_YEAR) {
-            return calculateCompareResult(value, compare, Calendar.WEEK_OF_YEAR);
-        }
-
         // Compare Day of the Year
         if (field == Calendar.DAY_OF_YEAR) {
             return calculateCompareResult(value, compare, Calendar.DAY_OF_YEAR);
@@ -416,4 +422,14 @@ protected Object parse(String value, final String pattern, final Locale locale,
      */
     @Override
     protected abstract Object processParsedValue(Object value, Format formatter);
+
+    /**
+     * Returns the week-year of a calendar, which a {@code WEEK_OF_YEAR} value is relative to.
+     *
+     * @param calendar The calendar to read the week-year from.
+     * @return the week-year, or the calendar year if week dates are not supported.
+     */
+    private int weekYear(final Calendar calendar) {
+        return calendar.isWeekDateSupported() ? calendar.getWeekYear() : calendar.get(Calendar.YEAR);
+    }
 }

src/test/java/org/apache/commons/validator/routines/CalendarValidatorTest.java

@@ -232,6 +232,27 @@ void testCompare() {
         assertEquals("Invalid field: -1", e.getMessage(), "check message");
     }
 
+    /**
+     * Test compareWeeks() at the week-year boundary, where WEEK_OF_YEAR is relative to a
+     * week-year that differs from the calendar year.
+     */
+    @Test
+    @DefaultLocale(country = "US", language = "en")
+    void testCompareWeeksAcrossYearBoundary() {
+        final int noon = 120000;
+        // 31 Dec 2018 (Monday) is in week 1 of week-year 2019 (US: first day Sunday, minimal days 1)
+        final Calendar dec31y2018 = createCalendar(TimeZones.GMT, 20181231, noon);
+        final Calendar jan01y2018 = createCalendar(TimeZones.GMT, 20180101, noon);
+        final Calendar jan01y2019 = createCalendar(TimeZones.GMT, 20190101, noon);
+
+        // both are calendar year 2018 and week-of-year 1, but lie ~52 weeks apart
+        assertEquals(1, calValidator.compareWeeks(dec31y2018, jan01y2018), "Dec 31 2018 is weeks after Jan 1 2018");
+        assertEquals(-1, calValidator.compareWeeks(jan01y2018, dec31y2018), "Jan 1 2018 is weeks before Dec 31 2018");
+
+        // 31 Dec 2018 and 1 Jan 2019 share week 1 of week-year 2019, so they are the same week
+        assertEquals(0, calValidator.compareWeeks(dec31y2018, jan01y2019), "Dec 31 2018 is the same week as Jan 1 2019");
+    }
+
     /**
      * Test Date/Time style Validator (there isn't an implementation for this)
      */