[icu-design] Islamic calendar customization of monthStart

[Lutter, C. <chr...@sa...>]

Hello, 

The Islamic calendar of the ICU currently supports civil and non civil
calendar by calculation of the month start. 
There are a few ways to calculate the non civil (religious) Islamic
calendar, which was already discussed here. 
However in our case a customization of the month start is needed
(although rarely). 

In case this requirement exists for other developers, here is how the
calendar can be customized: 

class ICUIslamicCalendar extends com.ibm.icu.util.IslamicCalendar {

	// for own customization of month start
	private CalendarCache customizedCache = new CalendarCache();

	@Override
	protected void handleComputeFields(int julianDay) {
		int year, month, dayOfMonth, dayOfYear;
		long days = julianDay - 1948440;
		{
			// guess the month start
			int months = (int) Math.floor(days /
CalendarAstronomer.SYNODIC_MONTH);

			/*
			 * Always also check the next month, since
customization can
			 * differ. It can differ for not more than 3
days. so that
			 * checking the next month is enough.
			 */
			months++;

			/*
			 * Check the true month start for the given
month. If it is
			 * later, check the previous month, until a
suitable is found.
			 */
			while (getCustomMonthStart(months) > days) {
				months--;
			}

			year = months / 12 + 1;
			month = months % 12;
		}

		dayOfMonth = (int) (days - getCustomMonthStart(12 *
(year - 1) + month)) + 1;

		// Now figure out the day of the year.
		dayOfYear = (int) (days - getCustomMonthStart(12 * (year
- 1)) + 1);

		internalSet(ERA, 0);
		internalSet(YEAR, year);
		internalSet(EXTENDED_YEAR, year);
		internalSet(MONTH, month);
		internalSet(DAY_OF_MONTH, dayOfMonth);
		internalSet(DAY_OF_YEAR, dayOfYear);
	}

	@Override
	protected int handleComputeMonthStart(int eyear, int month,
			boolean useMonth) {
		long months = 12 * (eyear - 1) + month;
		int daysFromHijraTo1970 = 1948439; // Hijra = 7/16/622
		return (int) getCustomMonthStart(months) +
daysFromHijraTo1970;
	}

	@Override
	protected int handleGetMonthLength(int extendedYear, int month)
{
		int months = 12 * (extendedYear - 1) + month;
		int length = (int) (getCustomMonthStart(months + 1) -
getCustomMonthStart(months));
		return length;
	}

	@Override
	protected int handleGetYearLength(int extendedYear) {
		int months = 12*(extendedYear-1);
            return (int)(getCustomMonthStart(months + 12) -
getCustomMonthStart(months));
	} 

	private long getCustomMonthStart(long months) {
		long start = customizedCache.get(months);
		if (start == CalendarCache.EMPTY) {
			start = // whatever
			customizedCache.put(months, start);
		}
		return start;
	}
}

Here the customizedCache object is not static, because different
customizations might exist. 
A flag to set customization instead of civil calendar is not used. I
assume that in case no customization exists, the civil calculation is
correct. Finally civil flag is ignored. 

Best regards
Christian