/**
* Obtains an instance of {@code WeekFields} from the first day-of-week and minimal days.
* <p>
* The first day-of-week defines the ISO {@code DayOfWeek} that is day 1 of the week.
* The minimal number of days in the first week defines how many days must be present
* in a month or year, starting from the first day-of-week, before the week is counted
* as the first week. A value of 1 will count the first day of the month or year as part
* of the first week, whereas a value of 7 will require the whole seven days to be in
* the new month or year.
* <p>
* WeekFields instances are singletons; for each unique combination
* of {@code firstDayOfWeek} and {@code minimalDaysInFirstWeek} the
* the same instance will be returned.
*
* @param DayOfWeek $firstDayOfWeek the first day of the week, not null
* @param int $minimalDaysInFirstWeek the minimal number of days in the first week, from 1 to 7
* @return WeekFields the week-definition, not null
* @throws IllegalArgumentException if the minimal days value is less than one
* or greater than 7
*/
public static function of(DayOfWeek $firstDayOfWeek, $minimalDaysInFirstWeek)
{
$key = $firstDayOfWeek->__toString() . $minimalDaysInFirstWeek;
$rules = @self::$CACHE[$key];
if ($rules === null) {
$rules = new WeekFields($firstDayOfWeek, $minimalDaysInFirstWeek);
self::$CACHE[$key] = $rules;
$rules = self::$CACHE[$key];
}
return $rules;
}