Patching Files

For updating files WoW uses bsdiff. Have a look at "http://www.daemonology.net/bsdiff/" or "http://www.pokorra.de/coding/bsdiff.html"

Depending on version, this might not be BSDIFF4 but ZBSDIFF1, which is a variant of BSDIFF4 with seemingly no differences than exchanging BZ2 library calls with their libz inflate equivalents.