The Setup

At my job one of the products we offer is a radio service. There are a bunch of channels and each channel has a bunch of playlists. I was tasked with writing something to automatically switch the active playlist for each channel. My initial thought was to make a flag in the database marking the active playlist and then write a script that would be run from cron.

The Solution

After a bit of messing around I came up with something better. It didn’t need the database or cron and it was one line:

$active = round(time() / PLAYLIST_ROTATE_PERIOD) % count($playlists);

Why It Works

Dividing the current unixtime by the amount of time you want each playlist to be active for leads to a grouping of time chunks. Say you want each playlist to run for 3 hours (10800 seconds): round(1179685800 / 10800) = 109230.
Now 15 minutes later: round(1179686700 / 10800) = 109230. Same result.
Now 3 hours later: round(1179696600 / 10800) = 102931. The result increases one, which when modded by the number of playlists gives you the id of the next one in the list.

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