Yeah I found the same. It seems that there hasn't really been much research in this area, and somewhat annoyingly there isn't a widely agreed term for the problem, though for some reason most of the algorithms are described in terms of diffing XML so if you search for "XML difference" you can find some papers.

There's a few algorithms like XDiff, XyDiff, XChange etc. but be prepared to find very old code on sourceforge or more likely no code at all.

I couldn't find anything with a decent complexity that either had code or was simple/well described enough that I could implement it so I gave up.