[iclelland]

They're derived. You can tell that they're derived information by the fact that you can compute them for old commits, long after commit time, which is exactly what part of this proposal is to do. You can derive them simply by counting the maximum number of steps between a commit and any of its roots. The essential information isn't the generation numbers; it's the structure of the commit history -- the actual chains of commits, with all of the branches and merges. Generation numbers are just an artifact of counting.

On the other hand, this information is very handy, once you have it, for certain algorithms, and it could be expensive to re-compute all the time, which is why the proposal is to generate and store them explicitly. (This is also the reason that timestamps have been used before, even if they were a bit of a hack -- they're readily available, and way faster than recomputing generation numbers all the time)