My code curiosity recently turned to my favorite (D)VCSes, Mercurial, and Bazaar. (I've tried reading Subversion, but the C is just too much.) Much of the world (including me) likes to battle over their merits, but I'd like to talk about what I saw in the source. (
On the superficial side, Bazaar has a lot more code than Mercurial does: About 1 MB with 2.5 MB of tests for Mercurial and 6MB with 6 MB of tests for Bazaar. I'm not going to make anything of it, though, for fear of condemnation. Mercurial's non-capitalized class names also drive me a bit crazy...
Overall, Mercurial appears to be a much simpler system, true to the wisdom of "do one thing well". The one repository format is beautifully simple. Bazaar, on the other hand, has to have layers of abstraction in order to make access over many protocols and different repository, branch, and working tree formats possible. For this complexity, it gains much flexibility allowing it to be a hybrid distributed and central VCS. Bazaar's source code also has many more comments and docstrings. Mercurial's looks rather bare in comparison.
Bazaar and Mercurial both seem to implement dirstate the same way. They also both have C extensions targeted to speed themselves up.
In the end, the both work well and are a pleasure to use, so I'm not going to complain.