@Override
 protected void doSetNextReader(LeafReaderContext context) throws IOException {
   FunctionValues values = groupSource.getValues(vsContext, context);
   groupFiller = values.getValueFiller();
   groupMval = groupFiller.getValue();
   values = countSource.getValues(vsContext, context);
   countFiller = values.getValueFiller();
   countMval = countFiller.getValue();
 }
 @Override
 public void collect(int doc) throws IOException {
   groupFiller.fillValue(doc);
   GroupCount groupCount = groupMap.get(groupMval);
   if (groupCount != null) {
     countFiller.fillValue(doc);
     groupCount.uniqueValues.add(countMval.duplicate());
   }
 }