internal int GetWeekOfYearFullDays(DateTime time, CalendarWeekRule rule, int firstDayOfWeek, int fullDays) {
int dayForJan1;
int offset;
int day;
int dayOfYear = GetDayOfYear(time) - 1; // Make the day of year to be 0-based, so that 1/1 is day 0.
//
// Calculate the number of days between the first day of year (1/1) and the first day of the week.
// This value will be a positive value from 0 ~ 6. We call this value as "offset".
//
// If offset is 0, it means that the 1/1 is the start of the first week.
// Assume the first day of the week is Monday, it will look like this:
// Sun Mon Tue Wed Thu Fri Sat
// 12/31 1/1 1/2 1/3 1/4 1/5 1/6
// +--> First week starts here.
//
// If offset is 1, it means that the first day of the week is 1 day ahead of 1/1.
// Assume the first day of the week is Monday, it will look like this:
// Sun Mon Tue Wed Thu Fri Sat
// 1/1 1/2 1/3 1/4 1/5 1/6 1/7
// +--> First week starts here.
//
// If offset is 2, it means that the first day of the week is 2 days ahead of 1/1.
// Assume the first day of the week is Monday, it will look like this:
// Sat Sun Mon Tue Wed Thu Fri Sat
// 1/1 1/2 1/3 1/4 1/5 1/6 1/7 1/8
// +--> First week starts here.
// Day of week is 0-based.
// Get the day of week for 1/1. This can be derived from the day of week of the target day.
// Note that we can get a negative value. It's ok since we are going to make it a positive value when calculating the offset.
dayForJan1 = (int)GetDayOfWeek(time) - (dayOfYear % 7);
// Now, calucalte the offset. Substract the first day of week from the dayForJan1. And make it a positive value.
offset = (firstDayOfWeek - dayForJan1 + 14) % 7;
if (offset != 0 && offset >= fullDays)
{
//
// If the offset is greater than the value of fullDays, it means that
// the first week of the year starts on the week where Jan/1 falls on.
//
offset -= 7;
}
//
// Calculate the day of year for specified time by taking offset into account.
//
day = dayOfYear - offset;
if (day >= 0) {
//
// If the day of year value is greater than zero, get the week of year.
//
return (day/7 + 1);
}
//
// Otherwise, the specified time falls on the week of previous year.
// Call this method again by passing the last day of previous year.
//
return (GetWeekOfYearFullDays(time.AddDays(-(dayOfYear+1)), rule, firstDayOfWeek, fullDays));
}
