Question

First off, I marked this question as language agnostic, but I’m using PHP and MySQL. It shouldn’t affect the question itself very much tho.

I’m creating an application which shows times of certain shows throughout the week. Every single show is recurring (on weekly basis) and there might be shows which are airing through 2 days – eg. starting on Sunday at 23:30, ending on Monday at 00:30. I’m storing start of the show (day of the week – Monday, Tuesday… – it’s never exact date; time) and duration. There are never shows that would take more than 24 hours.

My problem is with validation if newly added shows aren’t overlapping some old ones. Especially if it comes to Sunday-Monday shows.

How are such recurring events usually handled on both DB side and server side?

tl;dr version with stuff I considered

My first idea was to create some custom validation algorithm, but it seemed too cumbersome and complicated. Not that I’d whine about complicated hand-made solutions, but I’m interested if there isn’t something more basic that I’m missing.

Other alternative that came to mind was to change table structure to use datetime (instead of “day of week” and “time”), and use a fake fixed date range to store the data. For example all Mondays would be set to 5th Jan 1970, Sundays would use 11th Jan 1970. There would be one exception to this rule – if there would be some show which starts on Sunday and ends on Monday, it would be stored as 12th Jan 1970. This solution would allow more flexible quering of the DB than the original one, and it would also simplify queries for shows which overlap between individual weeks (since we can do the comparison directly in the query). There are some disadvantages to this solution as well (for one, using fake dates might make it confusing).

Both solutions smell of wrong algorithms to me and would love to hear some opinions from more experienced fellow developers.

Answer 1

Sounds like you could just store the starting minute of each show as an integer number of minutes since the start of the week (10,080 possible values).

Then a show starting at minute $a with duration $dur_a will overlap $b if and only if

(10080 + $b - $a) % 10080 < $dur_a

For example consider a show starting at 11pm Sunday and another starting at 12.30am Monday. Here $a == 10020 and $dur_a == 120 and $b == 30. (10080 + $b - $a) % 10080 == 90. This is less than $dur_a and hence the shows overlap.