code: @Override
    public int nextDoc() throws IOException {
      IndexDoc indexDoc = mtasCodecInfo.getNextDoc(field, docId);
      if (indexDoc != null) {
        docId = indexDoc.docId;
        minPosition = indexDoc.minPosition;
        maxPosition = indexDoc.maxPosition;
        currentStartPosition = -1;
        currentEndPosition = -1;
      } else {
        docId = NO_MORE_DOCS;
        minPosition = NO_MORE_POSITIONS;
        maxPosition = NO_MORE_POSITIONS;
        currentStartPosition = NO_MORE_POSITIONS;
        currentEndPosition = NO_MORE_POSITIONS;
      }
      return docId;
    }
docs: /*
  (non-Javadoc)
  
  @see org.apache.lucene.search.DocIdSetIterator#nextDoc()