"The source code for Doom 3 has been promised for a while now,
but got a final hold up in the form of nervous lawyers at Zenimax.
Carmack's Reverse was the sticking point, as Creative Labs owns a
patent relating to it. That could have meant months of delays
through talks and agreements, but Carmack decided to solve the
problem by rewriting some of the code.
"He decided to do that last week, and he must of had a busy
weekend because late yesterday the source code appeared on GitHub
ready to be downloaded and dissected by many an eager programmer.
In a tweet Carmack said that thanks must go to Timothee Besset for
doing "most of the work" that made the release possible.
"Let's be clear here. This isn't the code for the game, it's the
code for the engine named id Tech 4 that made the game possible.
Zenimax decided to release it under the General Public License v3
(GPL3) with detailed information about copying and modifications
contained in the Copying.txt file."