Understanding someone else's code: Analysis of experiences

[Friend] What kind of research do you do these days? [AL] I am initiating some work on program understanding. [F] What is that? [AL] Well.. you recover the design of a software system from its source code so you know how it does whatever it does. [F] You mean the design of a whole system? [AL] Yes. [AL] Interesting.. why would you want to do that? [M] So that you can understand the system when you are modifying it. [AL] Hmmm.. I can’t see how it will help me in my work. [M] Well.. then what would you like? [F] I’d like something that helps me get the job done ... fast. [AL] Which is? [F] Help me in finding what needs to be changed, make the change, and get the hell out. [AL] Well wouldn’t the design of the system help then. [F] It will ... but you don’t need the design of the whole system. Only the part that needs to be changed.