Using natural language processing and supervised learning, it automatically tagged news with 9 categories.
Our implementation and algorithm detail is followings.
Its categorizing process is written in Python.
Using nltk, it makes corpus with TFIDF model from HN topics and comments. And it generates classifiers from this corpus with SVM algorithm using scipy and numpy.
FYI, its web interface is written in Clojure and ClojureScript.
I trained classifiers with hand annotations (about 1000 contents or so)