private void returnFields(ResponseBuilder rb, ShardRequest sreq) {
    // Keep in mind that this could also be a shard in a multi-tiered system.
    // TODO: if a multi-tiered system, it seems like some requests
    // could/should bypass middlemen (like retrieving stored fields)
    // TODO: merge fsv to if requested

    if ((sreq.purpose & ShardRequest.PURPOSE_GET_FIELDS) != 0) {
      boolean returnScores = (rb.getFieldFlags() & SolrIndexSearcher.GET_SCORES) != 0;

      assert (sreq.responses.size() == 1);
      ShardResponse srsp = sreq.responses.get(0);
      SolrDocumentList docs =
          (SolrDocumentList) srsp.getSolrResponse().getResponse().get("response");

      String keyFieldName = rb.req.getSchema().getUniqueKeyField().getName();

      for (SolrDocument doc : docs) {
        Object id = doc.getFieldValue(keyFieldName);
        ShardDoc sdoc = rb.resultIds.get(id.toString());
        if (sdoc != null) {
          if (returnScores && sdoc.score != null) {
            doc.setField("score", sdoc.score);
          }
          rb._responseDocs.set(sdoc.positionInResponse, doc);
        }
      }
    }
  }
  private void mergeIds(ResponseBuilder rb, ShardRequest sreq) {
    SortSpec ss = rb.getSortSpec();
    Sort sort = ss.getSort();

    SortField[] sortFields = null;
    if (sort != null) sortFields = sort.getSort();
    else {
      sortFields = new SortField[] {SortField.FIELD_SCORE};
    }

    SchemaField uniqueKeyField = rb.req.getSchema().getUniqueKeyField();

    // id to shard mapping, to eliminate any accidental dups
    HashMap<Object, String> uniqueDoc = new HashMap<Object, String>();

    // Merge the docs via a priority queue so we don't have to sort *all* of the
    // documents... we only need to order the top (rows+start)
    ShardFieldSortedHitQueue queue;
    queue = new ShardFieldSortedHitQueue(sortFields, ss.getOffset() + ss.getCount());

    long numFound = 0;
    Float maxScore = null;
    for (ShardResponse srsp : sreq.responses) {
      SolrDocumentList docs =
          (SolrDocumentList) srsp.getSolrResponse().getResponse().get("response");

      // calculate global maxScore and numDocsFound
      if (docs.getMaxScore() != null) {
        maxScore = maxScore == null ? docs.getMaxScore() : Math.max(maxScore, docs.getMaxScore());
      }
      numFound += docs.getNumFound();

      NamedList sortFieldValues =
          (NamedList) (srsp.getSolrResponse().getResponse().get("sort_values"));

      // go through every doc in this response, construct a ShardDoc, and
      // put it in the priority queue so it can be ordered.
      for (int i = 0; i < docs.size(); i++) {
        SolrDocument doc = docs.get(i);
        Object id = doc.getFieldValue(uniqueKeyField.getName());

        String prevShard = uniqueDoc.put(id, srsp.getShard());
        if (prevShard != null) {
          // duplicate detected
          numFound--;

          // For now, just always use the first encountered since we can't currently
          // remove the previous one added to the priority queue.  If we switched
          // to the Java5 PriorityQueue, this would be easier.
          continue;
          // make which duplicate is used deterministic based on shard
          // if (prevShard.compareTo(srsp.shard) >= 0) {
          //  TODO: remove previous from priority queue
          //  continue;
          // }
        }

        ShardDoc shardDoc = new ShardDoc();
        shardDoc.id = id;
        shardDoc.shard = srsp.getShard();
        shardDoc.orderInShard = i;
        Object scoreObj = doc.getFieldValue("score");
        if (scoreObj != null) {
          if (scoreObj instanceof String) {
            shardDoc.score = Float.parseFloat((String) scoreObj);
          } else {
            shardDoc.score = (Float) scoreObj;
          }
        }

        shardDoc.sortFieldValues = sortFieldValues;

        queue.insertWithOverflow(shardDoc);
      } // end for-each-doc-in-response
    } // end for-each-response

    // The queue now has 0 -> queuesize docs, where queuesize <= start + rows
    // So we want to pop the last documents off the queue to get
    // the docs offset -> queuesize
    int resultSize = queue.size() - ss.getOffset();
    resultSize = Math.max(0, resultSize); // there may not be any docs in range

    Map<Object, ShardDoc> resultIds = new HashMap<Object, ShardDoc>();
    for (int i = resultSize - 1; i >= 0; i--) {
      ShardDoc shardDoc = (ShardDoc) queue.pop();
      shardDoc.positionInResponse = i;
      // Need the toString() for correlation with other lists that must
      // be strings (like keys in highlighting, explain, etc)
      resultIds.put(shardDoc.id.toString(), shardDoc);
    }

    SolrDocumentList responseDocs = new SolrDocumentList();
    if (maxScore != null) responseDocs.setMaxScore(maxScore);
    responseDocs.setNumFound(numFound);
    responseDocs.setStart(ss.getOffset());
    // size appropriately
    for (int i = 0; i < resultSize; i++) responseDocs.add(null);

    // save these results in a private area so we can access them
    // again when retrieving stored fields.
    // TODO: use ResponseBuilder (w/ comments) or the request context?
    rb.resultIds = resultIds;
    rb._responseDocs = responseDocs;
  }