tags sorted by their depth, *committerdate, and taggerdate

[clime <clime7@gmail.com>]

Hello,

I have some very specific requirements to get annotated tags sorted in
a certain way. I would like them to be sorted:
- primarily by their depth (i.e. how far they are from current HEAD
and if they are not reachable then they should not be displayed).
- secondarily by committerdate of the commit they are associated with
- ternarily by taggerdate

Is something like that possible?
Is something like that possible even with git 1.7.1?

If this isn't possible, I am thinking I could write some simple
utility in C to do it but can I rely on certain binary format of
commit and tag objects? Has the format changed at some point since git
1.7.1? Is there a git library I could use for it? Or is the format
documented somewhere?

Thank you very much and sorry for so many questions
clime