Testing for similar string content

I'm writing a bot that will analys开发者_开发问答e posts and reply with a vaguely related strings from a database. I'm not aiming for coherence, just for vague similarity that could pass as someone ignorant to the topic (but knowledgeable enough to try to reply). What are some methods that would help me to choose the right reply?

One thing I've come up with is to create a vocabulary list, check which elements of the list are in the post, and get a reply from the database based on these results. This crude method has been successful about 10% of the time (based on 100 replies to random posts). I might expand the list by more words, but this method has its limit. Any better ones?

(P. S. The database is sizeable -- about 500 000 replies)

First of all, I think the best you can hope for will be about a 50% answer rate, unless you're prepared to write a lot of code.

If you're willing to get your hands dirty with some statistics, check out term frequency–inverse document frequency. Basically, you will use the frequency of uncommon words to determine what keywords are critical to the document, and use this as the input into the tf-idf algorithm to pull out other replies with those same keywords.

You can then combine this further with whitelisting and blacklisting techniques to ignore common words and prioritize certain keywords. You can then keep tuning those lists to enhance the algorithm as you see it work.

There are also simpler string metrics you can use to test basic similarity. Take a look at this list of string metrics.

You might want to look into vector-space mapping and resemblance. The "vaguely related" problem could be handled by resemblance statistical analysis most likely.

Check out this novel use of resemblance:

http://www.cromwell-intl.com/security/attack-study/

There is a PHP function called "similar_text()", (e.g.: $percent_similar = similar_text($str1, $str2);) This works fairly well but I didn't come up with anything similar in C#. If you could get hold of the source for the PHP function you might try to translate it. I think there may be a Java version also.