Lucene.Net.Search.DuplicateFilter.FastBits C# (CSharp) Method

        private OpenBitSet FastBits(IndexReader reader)
        {
            OpenBitSet bits = new OpenBitSet(reader.MaxDoc);
            bits.Set(0, reader.MaxDoc); //assume all are valid
            Term startTerm = new Term(fieldName);
            TermEnum te = reader.Terms(startTerm);
            if (te != null)
            {
                Term currTerm = te.Term;

                while ((currTerm != null) && (currTerm.Field == startTerm.Field)) //term fieldnames are interned
                {
                    if (te.DocFreq() > 1)
                    {
                        int lastDoc = -1;
                        //unset potential duplicates
                        TermDocs td = reader.TermDocs(currTerm);
                        td.Next();
                        if (keepMode == KM_USE_FIRST_OCCURRENCE)
                        {
                            td.Next();
                        }
                        do
                        {
                            lastDoc = td.Doc;
                            bits.Clear(lastDoc);
                        } while (td.Next());
                        if (keepMode == KM_USE_LAST_OCCURRENCE)
                        {
                            //restore the last bit
                            bits.Set(lastDoc);
                        }
                    }
                    if (!te.Next())
                    {
                        break;
                    }
                    currTerm = te.Term;
                }
            }
            return bits;
        }