How to "select" sth which takes place every n days?

Question

Hi there,
I'm developing a JSP/Servlet-based event calendar for young people in a big German city. I recently ran into a problem while implementing a new feature requested by the team which is maintaining the website. Editors can enter events over a web-based form; currently supported are events which take place on a single day or spawn over multiple days.
I've been asked to provide means of entering something like events which take place every n days, starting at a given date.
I thought about storing the start date and an integer which holds the number of days after that the event should take place again. This would be very easy to implement on the frontend: A textfield/dropdowns for the date and a textfield/dropdown for the "repeat every n days" thing.
But I can't think of any reasonably fast way to get those events from the database when my servlet should display all events between, let's say, December 1 and December 24.
The only thing which came to my mind was selecting every event's start date, taking the "repeat every n days" field an check if the current date is one of those "repeat days" by repeatedly adding the "repeat every n days" value. But this would seriously affect the servlet's performance. Let me explain the above idea with some pseudo code:
ResultSet rs = stmt.executeQuery("SELECT * FROM events ...");
while (rs.next())
Date startDate = rs.getDate(...);
// don't care about the single-day and multiple-day events here
int repeatEveryNDays = rs.getInt(...);
while (startDate <= currentDate)
if (startDate.equals(currentDate))
// show event on search results page
else
// increase date by "repeatEveryNDays"
startDate.day += repeatEveryNDays
}As I knew this forum very well, let me emphasize that this is not some kind of homework assignment. If you want prove, go to http://www.under18.de and look at the site. But I doubt that will keep the trolls away from this topic :(
Thanks for your ideas,
Philipp
PS: For those not so capable of understanding what some call German g, here's a Googled translation into English (ahem... ignore the text, it's colloquial German and Google doesn't seem to like it): http://translate.google.com/translate?u=http%3A%2F%2Fwww.under18.de&langpair=de%7Cen&hl=en&ie=UTF-8&oe=UTF-8&safe=off&prev=%2Flanguage_tools

Answer 1

I just thought about this one: Assume I have an event
starting on Nov 2 taking place every 7 days. Now I'd
search for events between Nov 3 and Nov 11. The
difference of start date - STARTDATE would be
1, divided by 7 would be 1/7 (about 0.142857).
Adding 1 to 0.142857 would be rounded to 1 (using
integers), resulting in Nov 4 if I'm right. Well, Nov
4 falls between start date and end date
but there actually is no event on Nov 4 according to
the 7-day period defined above. I don't get any
further with your "calculate the next event based on
that number plus one".Taking your example, the difference between start date and STARTDATE is 1 day, which means that the first event is after start date. Now the first occurrence that is after start date is (0+1)*7+STARTDATE, which is (Nov 2 + 7).
All you have to do with this number is to make sure it isn't after the ENDDATE value (to verify it is in your repeating schedule), and doesn't fall after the end date that you are comparing for.
Some math major can probably explain the calculation better than I have, but the basic idea is figure out what the first occurrence is that is after your start date and then make sure it's not greater than end date.