It requires the user to understand callbacks, depth-first vs. width first, How to get the current directory name

Ah, but if you are going to roll it by hand, you should probably understand depth vs. breadth first searching. And certainly how to get the current directory name. You might also want to know a thing or two about symlinks and symlink loops, and think out your error handling. And, really, if you are going to do all that, there's a good chance you understand callbacks already. (BTW, I don't know how much you really need to understand about callbacks to follow an example and use one; not that I advocate using something that you don't understand... but you gotta learn somehow.)