ruby/lib

Before leaving for work, I'm spending an hour or two reading 3 random ruby/lib files:

observer.rb
benchmark.rb
optparse.rb

One hour is plenty of time to skip around a file, following ctags, getting a sense of its structure. There's even time to dive into hairy methods like #summarize and tweak out their logic.

While reading the code in vim (it is great for reading code, especially with NERDtree), I've got an emacs open to experiment with, comment on oddities, and run tests.

Try it. While reading the code, yank a method into its own file, and try to break it. Create your own unit tests, and see how hard it is to change the method without regression. Ideas that come to you while puttering about breaking things? Pop open a .org buffer and scribble them for later.

The important thing is not to approach it like a Google whiteboard interview. You don't need to memorize the entire damn file. Just figure out how it works, where it hides its superpowers, and what its weaknesses are. Once that's done, you've gotten halfway to being able to improve it and push your contributions to the project.