Diff tools

Does anyone know of a good resource for learning how a diff tool works?

The Python language (http://www.python.org) has a diff module as part of its standard library. The module is implemented in Python so you can read the source and experiment with it. The module provides functions for standard text diffing, but also low-level control to allow you to create more specialised diffs.

Python is an open source dynamically typed language with a very clear, regular syntax. An experienced programmer could learn enough to start programming in a few hours.

Dave - The Developers' Coach

I wrote one in VB a few years ago. It was pretty easy to figure out.

You start with pointers to each file. The pointers indicate the location of the last match. (they may not be the same, so you need one for each file)

When the pointers stop matching, you advance one only, until it starts matching again. Then do the same for the other pointer. If Neither find a match, advance the pointer by one and repeat. (the line has changed)

Your results will be:

match
Addition/removal (one side has data that doesn't match the other)
Moved (pointers match somewhere, but not each other)
Change (nothing matches on either side for a while)

If you want to get a little more complex, you can store blocks of matching line numbers into a list and find the largest sets of matches, in case you're comparing text that repeats a lot.

I hope that helps.