title: Autocomplete based on popularity
body: <p>For a general autocomplete suggestion (lexicographically sorted keys) one could use tries or TSTs.  However I would like to do autocomplete to return results based on popularity of results.  So searching for "clinton" would show "bill clinton", "hillary clinton" and "chelsea clinton" in that order.  I believe this can be done using ngram approach within Lucene.  Are there any other ways to do it?  </p>
  
  <p>On a similar note, what's the backend to return a list of results for the tags autocomplete on stackoverflow?  Is it lucene based?</p>