Supporting a corner case, last day of a month

[yangyud-cn]

It would be great if we could expand the cron expression to support one corner case, last day of a month.

https://stackoverflow.com/questions/6139189/cron-job-to-run-on-the-last-day-of-the-month

Also, instead of using "L" for it, an alternative is to use "0" for this special day, such that it works easier with the existing integer valued parser.

[atifaziz]

I'd like to avoid supporting extensions and corner cases if it can be helped unless someone helps to maintain and support those. That said, I think some extensions can be easily supported and built on top of NCrontab without changing the core implementation. I'm definitely planning to ship the feature developed in PR #37 that allows building occurrences out of multiple schedules. If you checkout that branch then something like L in your application could be done like so:

static IEnumerable<DateTime> EvaluateCrontabExpression(string expression, DateTime baseTime)
{
    if (expression.Split(' ', 5, StringSplitOptions.RemoveEmptyEntries)
        is not [var min, var hour, var day, var mon, var dow])
    {
        throw new FormatException("Invalid crontab expression.");
    }

    if (day.IndexOfAny(['L', 'l']) < 0) // short-circuit if no "L"
    {
        return CrontabSchedule.Parse(expression)
                              .GetNextOccurrences(baseTime, DateTime.MaxValue);
    }
    else
    {
        var days = day.Split(',', StringSplitOptions.TrimEntries).ToList();
        _ = days.RemoveAll(d => d is "L" or "l");

        // Schedule without "L"
        var schedule1 = CrontabSchedule.Parse(string.Join(' ', [min, hour, string.Join(",", days), mon, dow]));

        // Schedule with all possible "L" days
        var schedule2 = CrontabSchedule.Parse(string.Join(' ', [min, hour, "28-31", mon, dow]));

        // Prepare to combine the schedules
        var schedules = ImmutableArray.Create(schedule1, schedule2);

        return
            from e in schedules.GetNextOccurrences(baseTime, DateTime.MaxValue,
                                                   (s, dt) => (Schedule: s, Occurrence: dt))
            where e.Schedule == schedule1 // all of first schedule, or...
               || e.Occurrence.AddDays(1).Day == 1 // ...just where next day is 1st of next month
            select e.Occurrence;
    }
}

[stevehansen]

@atifaziz do you have an idea when a new version might be expected?

[atifaziz]

@atifaziz do you have an idea when a new version might be expected?

@stevehansen If you're asking a general question, please post to the general discussions area.