Measuring similarity between document sets

Question

For illustration purposes, let's assume this is a forum service. I need to calculate the "similarity" among each users' posts, so that the result would be something like:

among posts by user A, similarity 60%
among posts by user B, similarity 20%
...

I'm dealing with multibyte strings, so I guess I'm stuck with search engines here. We already use Solr, already have moreLikeThis implemented, but I'm not quite sure how to construct the query. Any help appreciated!

Answer 1

Possibly Carrot2 will interest you (and this blog related to it)