Implement logic described in src/lib.rs.
Make sure to find all places with TODO to not miss any assignments :)
Note that this assignment was tested on Linux only, where I assume that you are using
the epoll syscall. You can also implement it on other OSes, but you'll need to use
the corresponding OS interface for that (e.g. kqueue on macOS or I/O completion ports on Windows).
I would suggest using the mio crate for that.