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Lucene’s docFreq Got You Down? Replace It With a Custom Collector
A Counting Collector

When you do searches in Lucene you can give the searcher a Collector which feels a bit like the visitor pattern as the search calls your collector once for each document that matches your query.

public class CounterCollector : Collector

{

    public int Count { get; private set; }

    public void Reset()

    {

        Count = 0;

    }

    public override void Collect(int docID)

    {

        Count = Count + 1;

    }

    public override void SetScorer(Scorer scorer) { }

    public override void SetNextReader(IndexReader reader, int docBase) { }

    public override bool AcceptsDocsOutOfOrder()

    {

        return true;

    }

}

How Do I Use One Of Those?

public int GetNumberOfDocumentsForTerm(Term term)

{

    return searchIndex(searcher =>

    {

        //replacing this

        //return searcher.DocFreq(term);

        

        //with this

        var counterCollector = new CounterCollector();

        searcher.Search(new TermQuery(term), counterCollector);

        return counterCollector.Count;

    });

}

Take Away
Doing searches with custom collectors is quite easy. You just give an instance of one to the search method and interrogate it afterwards for the information you require.

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