Calculating IDF value more efficiently

[Kasun Perera]

This is my program to calculate TF-IDF value for a document in a collection
of documents. This is working fine, but takes lot of time when calculating
the "IDF" values (finding the no of documents which contains particular
term).

Is there a more efficient way of finding the no of documents which contains
a particular term?

freq = termsFreq.getTermFrequencies();

terms = termsFreq.getTerms();

int noOfTerms = terms.length;

score = new float[noOfTerms];
DefaultSimilarity simi = new DefaultSimilarity();

        for (i = 0; i < noOfTerms; i++) {

            int noofDocsContainTerm = noOfDocsContainTerm(terms[i]);

            float tf = simi.tf(freq[i]);

            float idf = simi.idf(noofDocsContainTerm, noOfDocs);

            score[i] = tf * idf ;

        }

////

public int noOfDocsContainTerm(String querystr) throws
CorruptIndexException, IOException, ParseException{

QueryParser qp=new QueryParser(Version.LUCENE_35, "docuemnt", new
StandardAnalyzer(Version.LUCENE_35));

Query q=qp.parse(querystr);

int hitsPerPage = docNames.length; //minumum number or search results
IndexSearcher searcher = new IndexSearcher(ramMemDir, true);
TopScoreDocCollector collector = TopScoreDocCollector.create(hitsPerPage, true);

searcher.search(q, collector);

ScoreDoc[] hits = collector.topDocs().scoreDocs;

    return hits.length;
}


-- 
Regards

Kasun Perera