Function GetISOWeekNumber

source/util.cpp:43–77

int GetISOWeekNumber(LPTSTR aBuf, int aYear, int aYDay, int aWDay)
// Caller must ensure that aBuf is of size 7 or greater, that aYear is a valid year (e.g. 2005),
// that aYDay is between 1 and 366, and that aWDay is between 0 and 6 (day of the week).
// Produces the week number in YYYYNN format, e.g. 200501.
// Note that year is also returned because it isn't necessarily the same as aTime's calendar year.
// Based on Linux glibc source code (GPL).
{
	--aYDay;  // Convert to zero based.
	#define ISO_WEEK_START_WDAY 1 // Monday
	#define ISO_WEEK1_WDAY 4      // Thursday
	#define ISO_WEEK_DAYS(yday, wday) (yday - (yday - wday + ISO_WEEK1_WDAY + ((366 / 7 + 2) * 7)) % 7 \
		+ ISO_WEEK1_WDAY - ISO_WEEK_START_WDAY);

	int year = aYear;
	int days = ISO_WEEK_DAYS(aYDay, aWDay);

	if (days < 0) // This ISO week belongs to the previous year.
	{
		--year;
		days = ISO_WEEK_DAYS(aYDay + (365 + IS_LEAP_YEAR(year)), aWDay);
	}
	else
	{
		int d = ISO_WEEK_DAYS(aYDay - (365 + IS_LEAP_YEAR(year)), aWDay);
		if (0 <= d) // This ISO week belongs to the next year.
		{
			++year;
			days = d;
		}
	}

	// Use sntprintf() for safety; that is, in case year contains a value longer than 4 digits.
	// This also adds the leading zeros in front of year and week number, if needed.
	return sntprintf(aBuf, 7, _T("%04d%02d"), year, (days / 7) + 1); // Return the length of the string produced.
}