Add polyfill changes for weekOfYear() and yearOfWeek()

polyfill/lib/calendar.mjs

@@ -230,15 +230,66 @@ export class Calendar {
     date = ES.ToTemporalDate(date);
     return impl[GetSlot(this, CALENDAR_ID)].dayOfYear(date);
   }
-  weekOfYear(date) {
+  calendarDateWeekOfYear(date) {
+    // Supports only Gregorian and ISO8601 calendar; can be updated to add support for other calendars.
+    // Returns undefined for calendars without a well-defined week calendar system.
+    // eslint-disable-next-line max-len
+    // Also see: https://github.com/unicode-org/icu/blob/ab72ab1d4a3c3f9beeb7d92b0c7817ca93dfdb04/icu4c/source/i18n/calendar.cpp#L1606
     if (!ES.IsTemporalCalendar(this)) throw new TypeError('invalid receiver');
     date = ES.ToTemporalDate(date);
-    return impl[GetSlot(this, CALENDAR_ID)].weekOfYear(date);
+    const id = GetSlot(this, CALENDAR_ID);
+    if (id !== 'gregory' && id !== 'iso8601') {
+      return { week: undefined, year: undefined };
+    }
+    const calendar = impl[id];
+    let yow = GetSlot(date, ISO_YEAR);
+    const dayOfWeek = this.dayOfWeek(date);
+    const dayOfYear = this.dayOfYear(date);
+    const fdow = id === 'iso8601' ? 1 : calendar.helper.getFirstDayOfWeek();
+    const mdow = id === 'iso8601' ? 4 : calendar.helper.getMinimalDaysInFirstWeek();
+
+    // For both the input date and the first day of its calendar year, calculate the day of week
+    // relative to first day of week in the relevant calendar (e.g., in iso8601, relative to Monday).
+    var relDow = (dayOfWeek + 7 - fdow) % 7;
+    // Assuming the year length is less than 7000 days.
+    var relDowJan1 = (dayOfWeek - dayOfYear + 7001 - fdow) % 7;
+
+    var woy = MathFloor((dayOfYear - 1 + relDowJan1) / 7);
+    if (7 - relDowJan1 >= mdow) {
+      ++woy;
+    }
+
+    // Adjust for weeks at the year end that overlap into the previous or next calendar year.
+    if (woy == 0) {
+      // Check for last week of previous year; if true, handle the case for
+      // first week of next year
+      var prevDoy = dayOfYear + this.daysInYear(this.dateAdd(date, { years: -1 }));
+      woy = weekNumber(fdow, mdow, prevDoy, dayOfWeek);
+      yow--;
+    } else {
+      // For it to be week 1 of the next year, dayOfYear must be >= lastDoy - 5
+      //          L-5                  L
+      // doy: 359 360 361 362 363 364 365 001
+      // dow:      1   2   3   4   5   6   7
+      var lastDoy = this.daysInYear(date);
+      if (dayOfYear >= lastDoy - 5) {
+        var lastRelDow = (relDow + lastDoy - dayOfYear) % 7;
+        if (lastRelDow < 0) {
+          lastRelDow += 7;
+        }
+        if (6 - lastRelDow >= mdow && dayOfYear + 7 - relDow > lastDoy) {
+          woy = 1;
+          yow++;
+        }
+      }
+    }
+    return { week: woy, year: yow };
+  }
+  weekOfYear(date) {
+    return this.calendarDateWeekOfYear(date).week;
   }
   yearOfWeek(date) {
-    if (!ES.IsTemporalCalendar(this)) throw new TypeError('invalid receiver');
-    date = ES.ToTemporalDate(date);
-    return impl[GetSlot(this, CALENDAR_ID)].yearOfWeek(date);
+    return this.calendarDateWeekOfYear(date).year;
   }
   daysInWeek(date) {
     if (!ES.IsTemporalCalendar(this)) throw new TypeError('invalid receiver');
@@ -388,12 +439,6 @@ impl['iso8601'] = {
   dayOfYear(date) {
     return ES.DayOfYear(GetSlot(date, ISO_YEAR), GetSlot(date, ISO_MONTH), GetSlot(date, ISO_DAY));
   },
-  weekOfYear(date) {
-    return ES.WeekOfYear(GetSlot(date, ISO_YEAR), GetSlot(date, ISO_MONTH), GetSlot(date, ISO_DAY)).week;
-  },
-  yearOfWeek(date) {
-    return ES.WeekOfYear(GetSlot(date, ISO_YEAR), GetSlot(date, ISO_MONTH), GetSlot(date, ISO_DAY)).year;
-  },
   daysInWeek() {
     return 7;
   },
@@ -459,6 +504,16 @@ function resolveNonLunisolarMonth(calendarDate, overflow = undefined, monthsPerY
   return { ...calendarDate, month, monthCode };
 }
 
+function weekNumber(firstDayOfWeek, minimalDaysInFirstWeek, desiredDay, dayOfWeek) {
+  var periodStartDayOfWeek = (dayOfWeek - firstDayOfWeek - desiredDay + 1) % 7;
+  if (periodStartDayOfWeek < 0) periodStartDayOfWeek += 7;
+  var weekNo = MathFloor((desiredDay + periodStartDayOfWeek - 1) / 7);
+  if (7 - periodStartDayOfWeek >= minimalDaysInFirstWeek) {
+    ++weekNo;
+  }
+  return weekNo;
+}
+
 // Note: other built-in calendars than iso8601 are not part of the Temporal
 // proposal for ECMA-262. An implementation of these calendars is present in
 // this polyfill in order to validate the Temporal API and to get early feedback
@@ -1702,6 +1757,12 @@ const helperGregory = ObjectAssign(
       if (era === 'bc' || era === 'b') era = 'bce';
       if (era === 'ad' || era === 'a') era = 'ce';
       return { era, eraYear };
+    },
+    getFirstDayOfWeek() {
+      return 1;
+    },
+    getMinimalDaysInFirstWeek() {
+      return 1;
     }
   }
 );
@@ -2091,12 +2152,6 @@ const nonIsoGeneralImpl = {
     const diffDays = this.helper.calendarDaysUntil(startOfYear, calendarDate, cache);
     return diffDays + 1;
   },
-  weekOfYear(date) {
-    return impl['iso8601'].weekOfYear(date);
-  },
-  yearOfWeek(date) {
-    return impl['iso8601'].yearOfWeek(date);
-  },
   daysInWeek(date) {
     return impl['iso8601'].daysInWeek(date);
   },

polyfill/lib/ecmascript.mjs

@@ -3089,29 +3089,6 @@ export function DayOfYear(year, month, day) {
   return days;
 }
 
-export function WeekOfYear(year, month, day) {
-  let doy = DayOfYear(year, month, day);
-  let dow = DayOfWeek(year, month, day) || 7;
-  let doj = DayOfWeek(year, 1, 1);
-
-  const week = MathFloor((doy - dow + 10) / 7);
-
-  if (week < 1) {
-    if (doj === 5 || (doj === 6 && LeapYear(year - 1))) {
-      return { week: 53, year: year - 1 };
-    } else {
-      return { week: 52, year: year - 1 };
-    }
-  }
-  if (week === 53) {
-    if ((LeapYear(year) ? 366 : 365) - doy < 4 - dow) {
-      return { week: 1, year: year + 1 };
-    }
-  }
-
-  return { week, year };
-}
-
 export function DurationSign(y, mon, w, d, h, min, s, ms, µs, ns) {
   const fields = [y, mon, w, d, h, min, s, ms, µs, ns];
   for (let index = 0; index < fields.length; index++) {