[OSM-dev] tilenumbers

[Jonas Svensson]

How do I calculate tilenumbers when retrieving from tileserver? Ie 
how do I transform from lat,lon and zoomlevel to 71212 and 39189 in
http://dev.openstreetmap.org/~ojw/Tiles/tile.php/17/71212/39189.png?
 I assume that calculation is available all over in the codebase 
but so far I have been unable to locate it, so if someone could 
give some help on where to read, please?

/Jonas S