This library implements tf-idf as described by http://en.wikipedia.org/wiki/Tf-idf

The library constructs an IDF corpus and stopword list either from documents specified by the client, or by reading from input files. It computes IDF for a specified term based on the corpus, or generates keywords ordered by tf-idf for a specified document.

If you find this useful, send an email to niniane (at) gmail (dot) com to let me know!