Reading old code slowly

#Begin at the boundary

The public entry point usually tells a clearer story than the helpers beneath it. Follow one ordinary input through the program before collecting suspicious functions or unusual branches.

#Keep an evidence list

I write down three kinds of observations:

• behavior that callers visibly depend on;
• complexity caused by a requirement that still exists;
• complexity whose original requirement can no longer be found.

The third list is where simplification begins, but only after the first two are understood.