I have a perl app that spies for me. It looks regularly at certain web pages and tells me if there has been a change. This is done in a fairly easily by taking an MD5 hash of the text of a web page and comparing it with a shared hash saved in a DBM file. (Minus a certain amount of fudging to make sure things like revolving web ads don't mess me up.) Within those constraints, it's a boolean: was there a change? Y/N.
What I'm now thinking of is quantitative change. I'm hoping to find a way to say how much or how little things have changed, preferrably without saving the whole text as a value in a DBM file. Yes, I know, disk is cheap, but it seems like I'm being insufficiently clever if I accept that that's the only way to do it.