public DateTime GetNextOccurrence(DateTime baseTime, DateTime endTime)
{
const int nil = -1;
var baseYear = baseTime.Year;
var baseMonth = baseTime.Month;
var baseDay = baseTime.Day;
var baseHour = baseTime.Hour;
var baseMinute = baseTime.Minute;
var endYear = endTime.Year;
var endMonth = endTime.Month;
var endDay = endTime.Day;
var year = baseYear;
var month = baseMonth;
var day = baseDay;
var hour = baseHour;
var minute = baseMinute + 1;
//
// Minute
//
minute = _minutes.Next(minute);
if (minute == nil)
{
minute = _minutes.GetFirst();
hour++;
}
//
// Hour
//
hour = _hours.Next(hour);
if (hour == nil)
{
minute = _minutes.GetFirst();
hour = _hours.GetFirst();
day++;
}
else if (hour > baseHour)
{
minute = _minutes.GetFirst();
}
//
// Day
//
day = _days.Next(day);
RetryDayMonth:
if (day == nil)
{
minute = _minutes.GetFirst();
hour = _hours.GetFirst();
day = _days.GetFirst();
month++;
}
else if (day > baseDay)
{
minute = _minutes.GetFirst();
hour = _hours.GetFirst();
}
//
// Month
//
month = _months.Next(month);
if (month == nil)
{
minute = _minutes.GetFirst();
hour = _hours.GetFirst();
day = _days.GetFirst();
month = _months.GetFirst();
year++;
}
else if (month > baseMonth)
{
minute = _minutes.GetFirst();
hour = _hours.GetFirst();
day = _days.GetFirst();
}
//
// The day field in a cron expression spans the entire range of days
// in a month, which is from 1 to 31. However, the number of days in
// a month tend to be variable depending on the month (and the year
// in case of February). So a check is needed here to see if the
// date is a border case. If the day happens to be beyond 28
// (meaning that we're dealing with the suspicious range of 29-31)
// and the date part has changed then we need to determine whether
// the day still makes sense for the given year and month. If the
// day is beyond the last possible value, then the day/month part
// for the schedule is re-evaluated. So an expression like "0 0
// 15,31 * *" will yield the following sequence starting on midnight
// of Jan 1, 2000:
//
// Jan 15, Jan 31, Feb 15, Mar 15, Apr 15, Apr 31, ...
//
var dateChanged = day != baseDay || month != baseMonth || year != baseYear;
if (day > 28 && dateChanged && day > Calendar.GetDaysInMonth(year, month))
{
if (year >= endYear && month >= endMonth && day >= endDay)
{
return(endTime);
}
day = nil;
goto RetryDayMonth;
}
var nextTime = new DateTime(year, month, day, hour, minute, 0, 0, baseTime.Kind);
if (nextTime >= endTime)
{
return(endTime);
}
//
// Day of week
//
if (_daysOfWeek.Contains((int)nextTime.DayOfWeek))
{
return(nextTime);
}
return(GetNextOccurrence(new DateTime(year, month, day, 23, 59, 0, 0, baseTime.Kind), endTime));
}