/*
 * Licensed to the Apache Software Foundation (ASF) under one or more
 * contributor license agreements.  See the NOTICE file distributed with
 * this work for additional information regarding copyright ownership.
 * The ASF licenses this file to You under the Apache License, Version 2.0
 * (the "License"); you may not use this file except in compliance with
 * the License.  You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */
package org.apache.solr.search;

import java.io.Closeable;
import java.io.IOException;
import java.lang.invoke.MethodHandles;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.Collection;
import java.util.Collections;
import java.util.Comparator;
import java.util.Date;
import java.util.HashMap;
import java.util.List;
import java.util.Map;
import java.util.Objects;
import java.util.Set;
import java.util.concurrent.ConcurrentHashMap;
import java.util.concurrent.TimeUnit;
import java.util.concurrent.atomic.AtomicLong;
import java.util.concurrent.atomic.AtomicReference;

import com.codahale.metrics.Gauge;
import com.google.common.collect.Iterables;

import org.apache.lucene.document.Document;
import org.apache.lucene.index.DirectoryReader;
import org.apache.lucene.index.ExitableDirectoryReader;
import org.apache.lucene.index.FieldInfos;
import org.apache.lucene.index.IndexReader;
import org.apache.lucene.index.LeafReader;
import org.apache.lucene.index.LeafReaderContext;
import org.apache.lucene.index.MultiPostingsEnum;
import org.apache.lucene.index.PostingsEnum;
import org.apache.lucene.index.StoredFieldVisitor;
import org.apache.lucene.index.Term;
import org.apache.lucene.index.Terms;
import org.apache.lucene.index.TermsEnum;
import org.apache.lucene.search.*;
import org.apache.lucene.search.TotalHits.Relation;
import org.apache.lucene.store.AlreadyClosedException;
import org.apache.lucene.store.Directory;
import org.apache.lucene.util.Bits;
import org.apache.lucene.util.BytesRef;
import org.apache.lucene.util.FixedBitSet;
import org.apache.solr.common.SolrException;
import org.apache.solr.common.SolrException.ErrorCode;
import org.apache.solr.common.params.ModifiableSolrParams;
import org.apache.solr.common.util.ObjectReleaseTracker;
import org.apache.solr.core.DirectoryFactory;
import org.apache.solr.core.DirectoryFactory.DirContext;
import org.apache.solr.core.SolrConfig;
import org.apache.solr.core.SolrCore;
import org.apache.solr.core.SolrInfoBean;
import org.apache.solr.index.SlowCompositeReaderWrapper;
import org.apache.solr.metrics.MetricsMap;
import org.apache.solr.metrics.SolrMetricManager;
import org.apache.solr.metrics.SolrMetricsContext;
import org.apache.solr.request.LocalSolrQueryRequest;
import org.apache.solr.request.SolrQueryRequest;
import org.apache.solr.request.SolrRequestInfo;
import org.apache.solr.response.SolrQueryResponse;
import org.apache.solr.schema.IndexSchema;
import org.apache.solr.schema.SchemaField;
import org.apache.solr.search.facet.UnInvertedField;
import org.apache.solr.search.stats.StatsCache;
import org.apache.solr.search.stats.StatsSource;
import org.apache.solr.uninverting.UninvertingReader;
import org.apache.solr.update.IndexFingerprint;
import org.apache.solr.update.SolrIndexConfig;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;

/**
 * SolrIndexSearcher adds schema awareness and caching functionality over {@link IndexSearcher}.
 *
 * @since solr 0.9
 */
public class SolrIndexSearcher extends IndexSearcher implements Closeable, SolrInfoBean {

  private static final Logger log = LoggerFactory.getLogger(MethodHandles.lookup().lookupClass());

  public static final String STATS_SOURCE = "org.apache.solr.stats_source";
  public static final String STATISTICS_KEY = "searcher";
  // These should *only* be used for debugging or monitoring purposes
  public static final AtomicLong numOpens = new AtomicLong();
  public static final AtomicLong numCloses = new AtomicLong();
  @SuppressWarnings({"rawtypes"})
  private static final Map<String,SolrCache> NO_GENERIC_CACHES = Collections.emptyMap();
  @SuppressWarnings({"rawtypes"})
  private static final SolrCache[] NO_CACHES = new SolrCache[0];

  private final SolrCore core;
  private final IndexSchema schema;
  private final SolrDocumentFetcher docFetcher;

  private final String name;
  private final Date openTime = new Date();
  private final long openNanoTime = System.nanoTime();
  private Date registerTime;
  private long warmupTime = 0;
  private final DirectoryReader reader;
  private final boolean closeReader;

  private final int queryResultWindowSize;
  private final int queryResultMaxDocsCached;
  private final boolean useFilterForSortedQuery;

  private final boolean cachingEnabled;
  private final SolrCache<Query,DocSet> filterCache;
  private final SolrCache<QueryResultKey,DocList> queryResultCache;
  private final SolrCache<String,UnInvertedField> fieldValueCache;

  // map of generic caches - not synchronized since it's read-only after the constructor.
  @SuppressWarnings({"rawtypes"})
  private final Map<String,SolrCache> cacheMap;

  // list of all caches associated with this searcher.
  @SuppressWarnings({"rawtypes"})
  private final SolrCache[] cacheList;

  private DirectoryFactory directoryFactory;

  private final LeafReader leafReader;
  // only for addIndexes etc (no fieldcache)
  private final DirectoryReader rawReader;

  private final String path;
  private boolean releaseDirectory;

  private final StatsCache statsCache;

  private Set<String> metricNames = ConcurrentHashMap.newKeySet();
  private SolrMetricsContext solrMetricsContext;

  private static DirectoryReader getReader(SolrCore core, SolrIndexConfig config, DirectoryFactory directoryFactory,
                                           String path) throws IOException {
    final Directory dir = directoryFactory.get(path, DirContext.DEFAULT, config.lockType);
    try {
      return core.getIndexReaderFactory().newReader(dir, core);
    } catch (Exception e) {
      directoryFactory.release(dir);
      throw new SolrException(ErrorCode.SERVER_ERROR, "Error opening Reader", e);
    }
  }

  // TODO: wrap elsewhere and return a "map" from the schema that overrides get() ?
  // this reader supports reopen
  private static DirectoryReader wrapReader(SolrCore core, DirectoryReader reader) throws IOException {
    assert reader != null;
    return ExitableDirectoryReader.wrap(
        UninvertingReader.wrap(reader, core.getLatestSchema().getUninversionMapper()),
        SolrQueryTimeoutImpl.getInstance());
  }
  
  /**
   * Builds the necessary collector chain (via delegate wrapping) and executes the query against it. This method takes
   * into consideration both the explicitly provided collector and postFilter as well as any needed collector wrappers
   * for dealing with options specified in the QueryCommand.
   * @return The collector used for search
   */
  private Collector buildAndRunCollectorChain(QueryResult qr, Query query, Collector collector, QueryCommand cmd,
      DelegatingCollector postFilter) throws IOException {

    EarlyTerminatingSortingCollector earlyTerminatingSortingCollector = null;
    if (cmd.getSegmentTerminateEarly()) {
      final Sort cmdSort = cmd.getSort();
      final int cmdLen = cmd.getLen();
      final Sort mergeSort = core.getSolrCoreState().getMergePolicySort();

      if (cmdSort == null || cmdLen <= 0 || mergeSort == null ||
          !EarlyTerminatingSortingCollector.canEarlyTerminate(cmdSort, mergeSort)) {
        log.warn("unsupported combination: segmentTerminateEarly=true cmdSort={} cmdLen={} mergeSort={}", cmdSort, cmdLen, mergeSort);
      } else {
        collector = earlyTerminatingSortingCollector = new EarlyTerminatingSortingCollector(collector, cmdSort, cmd.getLen());
      }
    }

    final boolean terminateEarly = cmd.getTerminateEarly();
    if (terminateEarly) {
      collector = new EarlyTerminatingCollector(collector, cmd.getLen());
    }

    final long timeAllowed = cmd.getTimeAllowed();
    if (timeAllowed > 0) {
      collector = new TimeLimitingCollector(collector, TimeLimitingCollector.getGlobalCounter(), timeAllowed);
    }

    if (postFilter != null) {
      postFilter.setLastDelegate(collector);
      collector = postFilter;
    }

    try {
      super.search(query, collector);
    } catch (TimeLimitingCollector.TimeExceededException | ExitableDirectoryReader.ExitingReaderException x) {
      log.warn("Query: [{}]; ", query, x);
      qr.setPartialResults(true);
    } catch (EarlyTerminatingCollectorException etce) {
      if (collector instanceof DelegatingCollector) {
        ((DelegatingCollector) collector).finish();
      }
      throw etce;
    } finally {
      if (earlyTerminatingSortingCollector != null) {
        qr.setSegmentTerminatedEarly(earlyTerminatingSortingCollector.terminatedEarly());
      }
    }
    if (collector instanceof DelegatingCollector) {
      ((DelegatingCollector) collector).finish();
    }
    return collector;
  }

  public SolrIndexSearcher(SolrCore core, String path, IndexSchema schema, SolrIndexConfig config, String name,
      boolean enableCache, DirectoryFactory directoryFactory) throws IOException {
    // We don't need to reserve the directory because we get it from the factory
    this(core, path, schema, name, getReader(core, config, directoryFactory, path), true, enableCache, false,
        directoryFactory);
    // Release the directory at close.
    this.releaseDirectory = true;
  }

  @SuppressWarnings({"unchecked", "rawtypes"})
  public SolrIndexSearcher(SolrCore core, String path, IndexSchema schema, String name, DirectoryReader r,
      boolean closeReader, boolean enableCache, boolean reserveDirectory, DirectoryFactory directoryFactory)
          throws IOException {
    super(wrapReader(core, r));

    this.path = path;
    this.directoryFactory = directoryFactory;
    this.reader = (DirectoryReader) super.readerContext.reader();
    this.rawReader = r;
    this.leafReader = SlowCompositeReaderWrapper.wrap(this.reader);
    this.core = core;
    this.statsCache = core.createStatsCache();
    this.schema = schema;
    this.name = "Searcher@" + Integer.toHexString(hashCode()) + "[" + core.getName() + "]"
        + (name != null ? " " + name : "");
    log.debug("Opening [{}]", this.name);

    if (directoryFactory.searchersReserveCommitPoints()) {
      // reserve commit point for life of searcher
      // TODO: This may not be safe w/softCommit, see SOLR-13908
      core.getDeletionPolicy().saveCommitPoint(reader.getIndexCommit().getGeneration());
    }

    if (reserveDirectory) {
      // Keep the directory from being released while we use it.
      directoryFactory.incRef(getIndexReader().directory());
      // Make sure to release it when closing.
      this.releaseDirectory = true;
    }

    this.closeReader = closeReader;
    setSimilarity(schema.getSimilarity());

    final SolrConfig solrConfig = core.getSolrConfig();
    this.queryResultWindowSize = solrConfig.queryResultWindowSize;
    this.queryResultMaxDocsCached = solrConfig.queryResultMaxDocsCached;
    this.useFilterForSortedQuery = solrConfig.useFilterForSortedQuery;

    this.docFetcher = new SolrDocumentFetcher(this, solrConfig, enableCache);

    this.cachingEnabled = enableCache;
    if (cachingEnabled) {
      final ArrayList<SolrCache> clist = new ArrayList<>();
      fieldValueCache = solrConfig.fieldValueCacheConfig == null ? null
          : solrConfig.fieldValueCacheConfig.newInstance();
      if (fieldValueCache != null) clist.add(fieldValueCache);
      filterCache = solrConfig.filterCacheConfig == null ? null : solrConfig.filterCacheConfig.newInstance();
      if (filterCache != null) clist.add(filterCache);
      queryResultCache = solrConfig.queryResultCacheConfig == null ? null
          : solrConfig.queryResultCacheConfig.newInstance();
      if (queryResultCache != null) clist.add(queryResultCache);
      SolrCache<Integer, Document> documentCache = docFetcher.getDocumentCache();
      if (documentCache != null) clist.add(documentCache);

      if (solrConfig.userCacheConfigs.isEmpty()) {
        cacheMap = NO_GENERIC_CACHES;
      } else {
        cacheMap = new HashMap<>(solrConfig.userCacheConfigs.size());
        for (Map.Entry<String,CacheConfig> e : solrConfig.userCacheConfigs.entrySet()) {
          SolrCache cache = e.getValue().newInstance();
          if (cache != null) {
            cacheMap.put(cache.name(), cache);
            clist.add(cache);
          }
        }
      }

      cacheList = clist.toArray(new SolrCache[clist.size()]);
    } else {
      this.filterCache = null;
      this.queryResultCache = null;
      this.fieldValueCache = null;
      this.cacheMap = NO_GENERIC_CACHES;
      this.cacheList = NO_CACHES;
    }

    // We already have our own filter cache
    setQueryCache(null);

    // do this at the end since an exception in the constructor means we won't close
    numOpens.incrementAndGet();
    assert ObjectReleaseTracker.track(this);
  }

  public SolrDocumentFetcher getDocFetcher() {
    return docFetcher;
  }

  List<LeafReaderContext> getLeafContexts() {
    return super.leafContexts;
  }

  public StatsCache getStatsCache() {
    return statsCache;
  }

  public FieldInfos getFieldInfos() {
    return leafReader.getFieldInfos();
  }

  /*
     * Override these two methods to provide a way to use global collection stats.
     */
  @Override
  public TermStatistics termStatistics(Term term, int docFreq, long totalTermFreq) throws IOException {
    final SolrRequestInfo reqInfo = SolrRequestInfo.getRequestInfo();
    if (reqInfo != null) {
      final StatsSource statsSrc = (StatsSource) reqInfo.getReq().getContext().get(STATS_SOURCE);
      if (statsSrc != null) {
        return statsSrc.termStatistics(this, term, docFreq, totalTermFreq);
      }
    }
    return localTermStatistics(term, docFreq, totalTermFreq);
  }

  @Override
  public CollectionStatistics collectionStatistics(String field) throws IOException {
    final SolrRequestInfo reqInfo = SolrRequestInfo.getRequestInfo();
    if (reqInfo != null) {
      final StatsSource statsSrc = (StatsSource) reqInfo.getReq().getContext().get(STATS_SOURCE);
      if (statsSrc != null) {
        return statsSrc.collectionStatistics(this, field);
      }
    }
    return localCollectionStatistics(field);
  }

  public TermStatistics localTermStatistics(Term term, int docFreq, long totalTermFreq) throws IOException {
    return super.termStatistics(term, docFreq, totalTermFreq);
  }

  public CollectionStatistics localCollectionStatistics(String field) throws IOException {
    // Could call super.collectionStatistics(field); but we can use a cached MultiTerms
    assert field != null;
    // SlowAtomicReader has a cache of MultiTerms
    Terms terms = getSlowAtomicReader().terms(field);
    if (terms == null) {
      return null;
    }
    return new CollectionStatistics(field, reader.maxDoc(),
        terms.getDocCount(), terms.getSumTotalTermFreq(), terms.getSumDocFreq());
  }

  public boolean isCachingEnabled() {
    return cachingEnabled;
  }

  public String getPath() {
    return path;
  }

  @Override
  public String toString() {
    return name + "{" + reader + "}";
  }

  public SolrCore getCore() {
    return core;
  }

  public final int maxDoc() {
    return reader.maxDoc();
  }

  public final int numDocs() {
    return reader.numDocs();
  }

  public final int docFreq(Term term) throws IOException {
    return reader.docFreq(term);
  }

  /**
   * Not recommended to call this method unless there is some particular reason due to internally calling {@link SlowCompositeReaderWrapper}.
   * Use {@link IndexSearcher#leafContexts} to get the sub readers instead of using this method.
   */
  public final LeafReader getSlowAtomicReader() {
    return leafReader;
  }

  /** Raw reader (no fieldcaches etc). Useful for operations like addIndexes */
  public final DirectoryReader getRawReader() {
    return rawReader;
  }

  @Override
  public final DirectoryReader getIndexReader() {
    assert reader == super.getIndexReader();
    return reader;
  }

  /**
   * Register sub-objects such as caches and our own metrics
   */
  public void register() {
    final Map<String,SolrInfoBean> infoRegistry = core.getInfoRegistry();
    // register self
    infoRegistry.put(STATISTICS_KEY, this);
    infoRegistry.put(name, this);
    for (@SuppressWarnings({"rawtypes"})SolrCache cache : cacheList) {
      cache.setState(SolrCache.State.LIVE);
      infoRegistry.put(cache.name(), cache);
    }
    this.solrMetricsContext = core.getSolrMetricsContext().getChildContext(this);
    for (@SuppressWarnings({"rawtypes"})SolrCache cache : cacheList) {
      cache.initializeMetrics(solrMetricsContext, SolrMetricManager.mkName(cache.name(), STATISTICS_KEY));
    }
    initializeMetrics(solrMetricsContext, STATISTICS_KEY);
    registerTime = new Date();
  }

  /**
   * Free's resources associated with this searcher.
   *
   * In particular, the underlying reader and any cache's in use are closed.
   */
  @Override
  public void close() throws IOException {
    if (log.isDebugEnabled()) {
      if (cachingEnabled) {
        final StringBuilder sb = new StringBuilder();
        sb.append("Closing ").append(name);
        for (@SuppressWarnings({"rawtypes"})SolrCache cache : cacheList) {
          sb.append("\n\t");
          sb.append(cache);
        }
        log.debug("{}", sb);
      } else {
        log.debug("Closing [{}]", name);
      }
    }

    core.getInfoRegistry().remove(name);

    // super.close();
    // can't use super.close() since it just calls reader.close() and that may only be called once
    // per reader (even if incRef() was previously called).

    long cpg = reader.getIndexCommit().getGeneration();
    try {
      if (closeReader) rawReader.decRef();
    } catch (Exception e) {
      SolrException.log(log, "Problem dec ref'ing reader", e);
    }

    if (directoryFactory.searchersReserveCommitPoints()) {
      core.getDeletionPolicy().releaseCommitPoint(cpg);
    }

    for (@SuppressWarnings({"rawtypes"})SolrCache cache : cacheList) {
      try {
        cache.close();
      } catch (Exception e) {
        SolrException.log(log, "Exception closing cache " + cache.name(), e);
      }
    }

    if (releaseDirectory) {
      directoryFactory.release(getIndexReader().directory());
    }

    try {
      SolrInfoBean.super.close();
    } catch (Exception e) {
      log.warn("Exception closing", e);
    }

    // do this at the end so it only gets done if there are no exceptions
    numCloses.incrementAndGet();
    assert ObjectReleaseTracker.release(this);
  }

  /** Direct access to the IndexSchema for use with this searcher */
  public IndexSchema getSchema() {
    return schema;
  }

  /**
   * Returns a collection of all field names the index reader knows about.
   */
  public Iterable<String> getFieldNames() {
    return Iterables.transform(getFieldInfos(), fieldInfo -> fieldInfo.name);
  }

  public SolrCache<Query,DocSet> getFilterCache() {
    return filterCache;
  }

  //
  // Set default regenerators on filter and query caches if they don't have any
  //
  public static void initRegenerators(SolrConfig solrConfig) {
    if (solrConfig.fieldValueCacheConfig != null && solrConfig.fieldValueCacheConfig.getRegenerator() == null) {
      solrConfig.fieldValueCacheConfig.setRegenerator(new CacheRegenerator() {
        @Override
        public boolean regenerateItem(SolrIndexSearcher newSearcher,
                                      @SuppressWarnings({"rawtypes"})SolrCache newCache,
                                      @SuppressWarnings({"rawtypes"})SolrCache oldCache,
            Object oldKey, Object oldVal) throws IOException {
          if (oldVal instanceof UnInvertedField) {
            UnInvertedField.getUnInvertedField((String) oldKey, newSearcher);
          }
          return true;
        }
      });
    }

    if (solrConfig.filterCacheConfig != null && solrConfig.filterCacheConfig.getRegenerator() == null) {
      solrConfig.filterCacheConfig.setRegenerator(new CacheRegenerator() {
        @Override
        @SuppressWarnings({"rawtypes"})public boolean regenerateItem(SolrIndexSearcher newSearcher
                , @SuppressWarnings({"rawtypes"})SolrCache newCache
                , @SuppressWarnings({"rawtypes"})SolrCache oldCache,
            Object oldKey, Object oldVal) throws IOException {
          newSearcher.cacheDocSet((Query) oldKey, null, false);
          return true;
        }
      });
    }

    if (solrConfig.queryResultCacheConfig != null && solrConfig.queryResultCacheConfig.getRegenerator() == null) {
      final int queryResultWindowSize = solrConfig.queryResultWindowSize;
      solrConfig.queryResultCacheConfig.setRegenerator(new CacheRegenerator() {
        @Override
        @SuppressWarnings({"rawtypes"})
        public boolean regenerateItem(SolrIndexSearcher newSearcher, SolrCache newCache, SolrCache oldCache,
            Object oldKey, Object oldVal) throws IOException {
          QueryResultKey key = (QueryResultKey) oldKey;
          int nDocs = 1;
          // request 1 doc and let caching round up to the next window size...
          // unless the window size is <=1, in which case we will pick
          // the minimum of the number of documents requested last time and
          // a reasonable number such as 40.
          // TODO: make more configurable later...

          if (queryResultWindowSize <= 1) {
            DocList oldList = (DocList) oldVal;
            int oldnDocs = oldList.offset() + oldList.size();
            // 40 has factors of 2,4,5,10,20
            nDocs = Math.min(oldnDocs, 40);
          }

          int flags = NO_CHECK_QCACHE | key.nc_flags;
          QueryCommand qc = new QueryCommand();
          qc.setQuery(key.query)
              .setFilterList(key.filters)
              .setSort(key.sort)
              .setLen(nDocs)
              .setSupersetMaxDoc(nDocs)
              .setFlags(flags);
          QueryResult qr = new QueryResult();
          newSearcher.getDocListC(qr, qc);
          return true;
        }
      });
    }
  }

  public QueryResult search(QueryResult qr, QueryCommand cmd) throws IOException {
    getDocListC(qr, cmd);
    return qr;
  }

  // FIXME: This option has been dead/noop since 3.1, should we re-enable or remove it?
  // public Hits search(Query query, Filter filter, Sort sort) throws IOException {
  // // todo - when Solr starts accepting filters, need to
  // // change this conditional check (filter!=null) and create a new filter
  // // that ANDs them together if it already exists.
  //
  // if (optimizer==null || filter!=null || !(query instanceof BooleanQuery)
  // ) {
  // return super.search(query,filter,sort);
  // } else {
  // Query[] newQuery = new Query[1];
  // Filter[] newFilter = new Filter[1];
  // optimizer.optimize((BooleanQuery)query, this, 0, newQuery, newFilter);
  //
  // return super.search(newQuery[0], newFilter[0], sort);
  // }
  // }

  /**
   * Retrieve the {@link Document} instance corresponding to the document id.
   *
   * @see SolrDocumentFetcher
   */
  @Override
  public Document doc(int docId) throws IOException {
    return doc(docId, (Set<String>) null);
  }

  /**
   * Visit a document's fields using a {@link StoredFieldVisitor}.
   * This method does not currently add to the Solr document cache.
   * 
   * @see IndexReader#document(int, StoredFieldVisitor)
   * @see SolrDocumentFetcher
   */
  @Override
  public final void doc(int docId, StoredFieldVisitor visitor) throws IOException {
    getDocFetcher().doc(docId, visitor);
  }

  /**
   * Retrieve the {@link Document} instance corresponding to the document id.
   * <p>
   * <b>NOTE</b>: the document will have all fields accessible, but if a field filter is provided, only the provided
   * fields will be loaded (the remainder will be available lazily).
   *
   * @see SolrDocumentFetcher
   */
  @Override
  public final Document doc(int i, Set<String> fields) throws IOException {
    return getDocFetcher().doc(i, fields);
  }

  /** expert: internal API, subject to change */
  public SolrCache<String,UnInvertedField> getFieldValueCache() {
    return fieldValueCache;
  }

  /** Returns a weighted sort according to this searcher */
  public Sort weightSort(Sort sort) throws IOException {
    return (sort != null) ? sort.rewrite(this) : null;
  }

  /** Returns a weighted sort spec according to this searcher */
  public SortSpec weightSortSpec(SortSpec originalSortSpec, Sort nullEquivalent) throws IOException {
    return implWeightSortSpec(
        originalSortSpec.getSort(),
        originalSortSpec.getCount(),
        originalSortSpec.getOffset(),
        nullEquivalent);
  }

  /** Returns a weighted sort spec according to this searcher */
  private SortSpec implWeightSortSpec(Sort originalSort, int num, int offset, Sort nullEquivalent) throws IOException {
    Sort rewrittenSort = weightSort(originalSort);
    if (rewrittenSort == null) {
      rewrittenSort = nullEquivalent;
    }

    final SortField[] rewrittenSortFields = rewrittenSort.getSort();
    final SchemaField[] rewrittenSchemaFields = new SchemaField[rewrittenSortFields.length];
    for (int ii = 0; ii < rewrittenSortFields.length; ++ii) {
      final String fieldName = rewrittenSortFields[ii].getField();
      rewrittenSchemaFields[ii] = (fieldName == null ? null : schema.getFieldOrNull(fieldName));
    }

    return new SortSpec(rewrittenSort, rewrittenSchemaFields, num, offset);
  }

  /**
   * Returns the first document number containing the term <code>t</code> Returns -1 if no document was found. This
   * method is primarily intended for clients that want to fetch documents using a unique identifier."
   * 
   * @return the first document number containing the term
   */
  public int getFirstMatch(Term t) throws IOException {
    long pair = lookupId(t.field(), t.bytes());
    if (pair == -1) {
      return -1;
    } else {
      final int segIndex = (int) (pair >> 32);
      final int segDocId = (int) pair;
      return leafContexts.get(segIndex).docBase + segDocId;
    }
  }

  /**
   * lookup the docid by the unique key field, and return the id *within* the leaf reader in the low 32 bits, and the
   * index of the leaf reader in the high 32 bits. -1 is returned if not found.
   * 
   * @lucene.internal
   */
  public long lookupId(BytesRef idBytes) throws IOException {
    return lookupId(schema.getUniqueKeyField().getName(), idBytes);
  }

  private long lookupId(String field, BytesRef idBytes) throws IOException {
    for (int i = 0, c = leafContexts.size(); i < c; i++) {
      final LeafReaderContext leaf = leafContexts.get(i);
      final LeafReader reader = leaf.reader();

      final Terms terms = reader.terms(field);
      if (terms == null) continue;

      TermsEnum te = terms.iterator();
      if (te.seekExact(idBytes)) {
        PostingsEnum docs = te.postings(null, PostingsEnum.NONE);
        docs = BitsFilteredPostingsEnum.wrap(docs, reader.getLiveDocs());
        int id = docs.nextDoc();
        if (id == DocIdSetIterator.NO_MORE_DOCS) continue;
        assert docs.nextDoc() == DocIdSetIterator.NO_MORE_DOCS;

        return (((long) i) << 32) | id;
      }
    }

    return -1;
  }

  /**
   * Compute and cache the DocSet that matches a query. The normal usage is expected to be cacheDocSet(myQuery,
   * null,false) meaning that Solr will determine if the Query warrants caching, and if so, will compute the DocSet that
   * matches the Query and cache it. If the answer to the query is already cached, nothing further will be done.
   * <p>
   * If the optionalAnswer DocSet is provided, it should *not* be modified after this call.
   *
   * @param query
   *          the lucene query that will act as the key
   * @param optionalAnswer
   *          the DocSet to be cached - if null, it will be computed.
   * @param mustCache
   *          if true, a best effort will be made to cache this entry. if false, heuristics may be used to determine if
   *          it should be cached.
   */
  public void cacheDocSet(Query query, DocSet optionalAnswer, boolean mustCache) throws IOException {
    // Even if the cache is null, still compute the DocSet as it may serve to warm the Lucene
    // or OS disk cache.
    if (optionalAnswer != null) {
      if (filterCache != null) {
        filterCache.put(query, optionalAnswer);
      }
      return;
    }

    // Throw away the result, relying on the fact that getDocSet
    // will currently always cache what it found. If getDocSet() starts
    // using heuristics about what to cache, and mustCache==true, (or if we
    // want this method to start using heuristics too) then
    // this needs to change.
    getDocSet(query);
  }

  private BitDocSet makeBitDocSet(DocSet answer) {
    // TODO: this should be implemented in DocSet, most likely with a getBits method that takes a maxDoc argument
    // or make DocSet instances remember maxDoc
    if (answer instanceof BitDocSet) {
      return (BitDocSet) answer;
    }
    FixedBitSet bs = new FixedBitSet(maxDoc());
    DocIterator iter = answer.iterator();
    while (iter.hasNext()) {
      bs.set(iter.nextDoc());
    }

    return new BitDocSet(bs, answer.size());
  }

  public BitDocSet getDocSetBits(Query q) throws IOException {
    DocSet answer = getDocSet(q);
    BitDocSet answerBits = makeBitDocSet(answer);
    if (answerBits != answer && filterCache != null) {
      filterCache.put(q, answerBits);
    }
    return answerBits;
  }

  /**
   * Returns the set of document ids matching a query. This method is cache-aware and attempts to retrieve the answer
   * from the cache if possible. If the answer was not cached, it may have been inserted into the cache as a result of
   * this call. This method can handle negative queries.
   * <p>
   * The DocSet returned should <b>not</b> be modified.
   */
  public DocSet getDocSet(Query query) throws IOException {
    if (query instanceof ExtendedQuery) {
      ExtendedQuery eq = (ExtendedQuery) query;
      if (!eq.getCache()) {
        if (query instanceof WrappedQuery) {
          query = ((WrappedQuery) query).getWrappedQuery();
        }
        query = QueryUtils.makeQueryable(query);
        return getDocSetNC(query, null);
      }
    }

    // Get the absolute value (positive version) of this query. If we
    // get back the same reference, we know it's positive.
    Query absQ = QueryUtils.getAbs(query);
    boolean positive = query == absQ;

    if (filterCache != null) {
      DocSet absAnswer = filterCache.get(absQ);
      if (absAnswer != null) {
        if (positive) return absAnswer;
        else return getLiveDocSet().andNot(absAnswer);
      }
    }

    DocSet absAnswer = getDocSetNC(absQ, null);
    DocSet answer = positive ? absAnswer : getLiveDocSet().andNot(absAnswer);

    if (filterCache != null) {
      // cache negative queries as positive
      filterCache.put(absQ, absAnswer);
    }

    return answer;
  }

  // only handle positive (non negative) queries
  DocSet getPositiveDocSet(Query q) throws IOException {
    DocSet answer;
    if (filterCache != null) {
      answer = filterCache.get(q);
      if (answer != null) return answer;
    }
    answer = getDocSetNC(q, null);
    if (filterCache != null) filterCache.put(q, answer);
    return answer;
  }

  private static Query matchAllDocsQuery = new MatchAllDocsQuery();
  private volatile BitDocSet liveDocs;

  @Deprecated // TODO remove for 8.0
  public BitDocSet getLiveDocs() throws IOException {
    return getLiveDocSet();
  }

  /**
   * Returns an efficient random-access {@link DocSet} of the live docs.  It's cached.  Never null.
   * @lucene.internal the type of DocSet returned may change in the future
   */
  public BitDocSet getLiveDocSet() throws IOException {
    // Going through the filter cache will provide thread safety here if we only had getLiveDocs,
    // but the addition of setLiveDocs means we needed to add volatile to "liveDocs".
    BitDocSet docs = liveDocs;
    if (docs == null) {
      //note: maybe should instead calc manually by segment, using FixedBitSet.copyOf(segLiveDocs); avoid filter cache?
      liveDocs = docs = getDocSetBits(matchAllDocsQuery);
    }
    assert docs.size() == numDocs();
    return docs;
  }

  /**
   * Returns an efficient random-access {@link Bits} of the live docs.  It's cached.  Null means all docs are live.
   * Use this like {@link LeafReader#getLiveDocs()}.
   * @lucene.internal
   */
  //TODO rename to getLiveDocs in 8.0
  public Bits getLiveDocsBits() throws IOException {
    return getIndexReader().hasDeletions() ? getLiveDocSet().getBits() : null;
  }

  /** @lucene.internal */
  public boolean isLiveDocsInstantiated() {
    return liveDocs != null;
  }

  /** @lucene.internal */
  public void setLiveDocs(DocSet docs) {
    // a few places currently expect BitDocSet
    assert docs.size() == numDocs();
    this.liveDocs = makeBitDocSet(docs);
  }

  private static Comparator<Query> sortByCost = (q1, q2) -> ((ExtendedQuery) q1).getCost() - ((ExtendedQuery) q2).getCost();

  /**
   * Returns the set of document ids matching all queries. This method is cache-aware and attempts to retrieve the
   * answer from the cache if possible. If the answer was not cached, it may have been inserted into the cache as a
   * result of this call. This method can handle negative queries.
   * A null/empty list results in {@link #getLiveDocSet()}.
   * <p>
   * The DocSet returned should <b>not</b> be modified.
   */
  public DocSet getDocSet(List<Query> queries) throws IOException {

    ProcessedFilter pf = getProcessedFilter(null, queries);

    if (pf.postFilter == null) {
      if (pf.answer != null) {
        return pf.answer;
      } else if (pf.filter == null) {
        return getLiveDocSet(); // note: this is what happens when queries is an empty list
      }
    }

    DocSetCollector setCollector = new DocSetCollector(maxDoc());
    Collector collector = setCollector;
    if (pf.postFilter != null) {
      pf.postFilter.setLastDelegate(collector);
      collector = pf.postFilter;
    }

    Query query = pf.filter != null ? pf.filter : matchAllDocsQuery;

    search(query, collector);

    if (collector instanceof DelegatingCollector) {
      ((DelegatingCollector) collector).finish();
    }

    return DocSetUtil.getDocSet(setCollector, this);
  }

  /**
   * INTERNAL: The response object from {@link #getProcessedFilter(DocSet, List)}.
   * Holds a filter and postFilter pair that together match a set of documents.
   * Either of them may be null, in which case the semantics are to match everything.
   * @see #getProcessedFilter(DocSet, List)
   */
  public static class ProcessedFilter {
    public DocSet answer; // maybe null. Sometimes we have a docSet answer that represents the complete answer / result.
    public Filter filter; // maybe null
    public DelegatingCollector postFilter; // maybe null
  }

  /**
   * INTERNAL: Processes conjunction (AND) of both args into a {@link ProcessedFilter} result.
   * Either arg may be null/empty thus doesn't restrict the matching docs.
   * Queries typically are resolved against the filter cache, and populate it.
   */
  public ProcessedFilter getProcessedFilter(DocSet setFilter, List<Query> queries) throws IOException {
    ProcessedFilter pf = new ProcessedFilter();
    if (queries == null || queries.size() == 0) {
      if (setFilter != null) {
        pf.answer = setFilter;
        pf.filter = setFilter.getTopFilter();
      }
      return pf;
    }

    // We combine all the filter queries that come from the filter cache & setFilter into "answer".
    // This might become pf.filterAsDocSet but not if there are any non-cached filters
    DocSet answer = null;

    boolean[] neg = new boolean[queries.size() + 1];
    DocSet[] sets = new DocSet[queries.size() + 1];
    List<Query> notCached = null;
    List<Query> postFilters = null;

    int end = 0;
    int smallestIndex = -1;

    if (setFilter != null) {
      answer = sets[end++] = setFilter;
      smallestIndex = end;
    } // we are done with setFilter at this point

    int smallestCount = Integer.MAX_VALUE;
    for (Query q : queries) {
      if (q instanceof ExtendedQuery) {
        ExtendedQuery eq = (ExtendedQuery) q;
        if (!eq.getCache()) {
          if (eq.getCost() >= 100 && eq instanceof PostFilter) {
            if (postFilters == null) postFilters = new ArrayList<>(sets.length - end);
            postFilters.add(q);
          } else {
            if (notCached == null) notCached = new ArrayList<>(sets.length - end);
            notCached.add(q);
          }
          continue;
        }
      }

      if (filterCache == null) {
        // there is no cache: don't pull bitsets
        if (notCached == null) notCached = new ArrayList<>(sets.length - end);
        WrappedQuery uncached = new WrappedQuery(q);
        uncached.setCache(false);
        notCached.add(uncached);
        continue;
      }

      Query posQuery = QueryUtils.getAbs(q);
      sets[end] = getPositiveDocSet(posQuery);
      // Negative query if absolute value different from original
      if (q == posQuery) {
        neg[end] = false;
        // keep track of the smallest positive set.
        // This optimization is only worth it if size() is cached, which it would
        // be if we don't do any set operations.
        int sz = sets[end].size();
        if (sz < smallestCount) {
          smallestCount = sz;
          smallestIndex = end;
          answer = sets[end];
        }
      } else {
        neg[end] = true;
      }

      end++;
    }

    // Are all of our normal cached filters negative?
    if (end > 0 && answer == null) {
      answer = getLiveDocSet();
    }

    // do negative queries first to shrink set size
    for (int i = 0; i < end; i++) {
      if (neg[i]) answer = answer.andNot(sets[i]);
    }

    for (int i = 0; i < end; i++) {
      if (!neg[i] && i != smallestIndex) answer = answer.intersection(sets[i]);
    }

    // ignore "answer" if it simply matches all docs
    if (answer != null && answer.size() == numDocs()) {
      answer = null;
    }

    // answer is done.

    // If no notCached nor postFilters, we can return now.
    if (notCached == null && postFilters == null) {
      // "answer" is the only part of the filter, so set it.
      if (answer != null) {
        pf.answer = answer;
        pf.filter = answer.getTopFilter();
      }
      return pf;
    }
    // pf.answer will remain null ...  (our local "answer" var is not the complete answer)

    // Set pf.filter based on combining "answer" and "notCached"
    if (notCached == null) {
      if (answer != null) {
        pf.filter = answer.getTopFilter();
      }
    } else {
      Collections.sort(notCached, sortByCost);
      List<Weight> weights = new ArrayList<>(notCached.size());
      for (Query q : notCached) {
        Query qq = QueryUtils.makeQueryable(q);
        weights.add(createWeight(rewrite(qq), ScoreMode.COMPLETE_NO_SCORES, 1));
      }
      pf.filter = new FilterImpl(answer, weights);
    }

    // Set pf.postFilter
    if (postFilters != null) {
      Collections.sort(postFilters, sortByCost);
      for (int i = postFilters.size() - 1; i >= 0; i--) {
        DelegatingCollector prev = pf.postFilter;
        pf.postFilter = ((PostFilter) postFilters.get(i)).getFilterCollector(this);
        if (prev != null) pf.postFilter.setDelegate(prev);
      }
    }

    return pf;
  }

  /** @lucene.internal */
  public DocSet getDocSet(DocsEnumState deState) throws IOException {
    int largestPossible = deState.termsEnum.docFreq();
    boolean useCache = filterCache != null && largestPossible >= deState.minSetSizeCached;
    TermQuery key = null;

    if (useCache) {
      key = new TermQuery(new Term(deState.fieldName, deState.termsEnum.term()));
      DocSet result = filterCache.get(key);
      if (result != null) return result;
    }

    int smallSetSize = DocSetUtil.smallSetSize(maxDoc());
    int scratchSize = Math.min(smallSetSize, largestPossible);
    if (deState.scratch == null || deState.scratch.length < scratchSize) deState.scratch = new int[scratchSize];

    final int[] docs = deState.scratch;
    int upto = 0;
    int bitsSet = 0;
    FixedBitSet fbs = null;

    PostingsEnum postingsEnum = deState.termsEnum.postings(deState.postingsEnum, PostingsEnum.NONE);
    postingsEnum = BitsFilteredPostingsEnum.wrap(postingsEnum, deState.liveDocs);
    if (deState.postingsEnum == null) {
      deState.postingsEnum = postingsEnum;
    }

    if (postingsEnum instanceof MultiPostingsEnum) {
      MultiPostingsEnum.EnumWithSlice[] subs = ((MultiPostingsEnum) postingsEnum).getSubs();
      int numSubs = ((MultiPostingsEnum) postingsEnum).getNumSubs();
      for (int subindex = 0; subindex < numSubs; subindex++) {
        MultiPostingsEnum.EnumWithSlice sub = subs[subindex];
        if (sub.postingsEnum == null) continue;
        int base = sub.slice.start;
        int docid;

        if (largestPossible > docs.length) {
          if (fbs == null) fbs = new FixedBitSet(maxDoc());
          while ((docid = sub.postingsEnum.nextDoc()) != DocIdSetIterator.NO_MORE_DOCS) {
            fbs.set(docid + base);
            bitsSet++;
          }
        } else {
          while ((docid = sub.postingsEnum.nextDoc()) != DocIdSetIterator.NO_MORE_DOCS) {
            docs[upto++] = docid + base;
          }
        }
      }
    } else {
      int docid;
      if (largestPossible > docs.length) {
        fbs = new FixedBitSet(maxDoc());
        while ((docid = postingsEnum.nextDoc()) != DocIdSetIterator.NO_MORE_DOCS) {
          fbs.set(docid);
          bitsSet++;
        }
      } else {
        while ((docid = postingsEnum.nextDoc()) != DocIdSetIterator.NO_MORE_DOCS) {
          docs[upto++] = docid;
        }
      }
    }

    DocSet result;
    if (fbs != null) {
      for (int i = 0; i < upto; i++) {
        fbs.set(docs[i]);
      }
      bitsSet += upto;
      result = new BitDocSet(fbs, bitsSet);
    } else {
      result = upto == 0 ? DocSet.empty() : new SortedIntDocSet(Arrays.copyOf(docs, upto));
    }

    if (useCache) {
      filterCache.put(key, result);
    }

    return result;
  }

  // query must be positive
  protected DocSet getDocSetNC(Query query, DocSet filter) throws IOException {
    return DocSetUtil.createDocSet(this, query, filter);
  }

  /**
   * Returns the set of document ids matching both the query and the filter. This method is cache-aware and attempts to
   * retrieve the answer from the cache if possible. If the answer was not cached, it may have been inserted into the
   * cache as a result of this call.
   * <p>
   *
   * @param filter
   *          may be null
   * @return DocSet meeting the specified criteria, should <b>not</b> be modified by the caller.
   */
  public DocSet getDocSet(Query query, DocSet filter) throws IOException {
    if (filter == null) return getDocSet(query);

    if (query instanceof ExtendedQuery) {
      ExtendedQuery eq = (ExtendedQuery) query;
      if (!eq.getCache()) {
        if (query instanceof WrappedQuery) {
          query = ((WrappedQuery) query).getWrappedQuery();
        }
        query = QueryUtils.makeQueryable(query);
        return getDocSetNC(query, filter);
      }
    }

    // Negative query if absolute value different from original
    Query absQ = QueryUtils.getAbs(query);
    boolean positive = absQ == query;

    DocSet first;
    if (filterCache != null) {
      first = filterCache.get(absQ);
      if (first == null) {
        first = getDocSetNC(absQ, null);
        filterCache.put(absQ, first);
      }
      return positive ? first.intersection(filter) : filter.andNot(first);
    }

    // If there isn't a cache, then do a single filtered query if positive.
    return positive ? getDocSetNC(absQ, filter) : filter.andNot(getPositiveDocSet(absQ));
  }

  /**
   * Returns documents matching both <code>query</code> and <code>filter</code> and sorted by <code>sort</code>.
   * <p>
   * This method is cache aware and may retrieve <code>filter</code> from the cache or make an insertion into the cache
   * as a result of this call.
   * <p>
   * FUTURE: The returned DocList may be retrieved from a cache.
   *
   * @param filter
   *          may be null
   * @param lsort
   *          criteria by which to sort (if null, query relevance is used)
   * @param offset
   *          offset into the list of documents to return
   * @param len
   *          maximum number of documents to return
   * @return DocList meeting the specified criteria, should <b>not</b> be modified by the caller.
   * @throws IOException
   *           If there is a low-level I/O error.
   */
  public DocList getDocList(Query query, Query filter, Sort lsort, int offset, int len) throws IOException {
    QueryCommand qc = new QueryCommand();
    qc.setQuery(query)
        .setFilterList(filter)
        .setSort(lsort)
        .setOffset(offset)
        .setLen(len);
    QueryResult qr = new QueryResult();
    search(qr, qc);
    return qr.getDocList();
  }

  /**
   * Returns documents matching both <code>query</code> and the intersection of the <code>filterList</code>, sorted by
   * <code>sort</code>.
   * <p>
   * This method is cache aware and may retrieve <code>filter</code> from the cache or make an insertion into the cache
   * as a result of this call.
   * <p>
   * FUTURE: The returned DocList may be retrieved from a cache.
   *
   * @param filterList
   *          may be null
   * @param lsort
   *          criteria by which to sort (if null, query relevance is used)
   * @param offset
   *          offset into the list of documents to return
   * @param len
   *          maximum number of documents to return
   * @return DocList meeting the specified criteria, should <b>not</b> be modified by the caller.
   * @throws IOException
   *           If there is a low-level I/O error.
   */
  public DocList getDocList(Query query, List<Query> filterList, Sort lsort, int offset, int len, int flags)
      throws IOException {
    QueryCommand qc = new QueryCommand();
    qc.setQuery(query)
        .setFilterList(filterList)
        .setSort(lsort)
        .setOffset(offset)
        .setLen(len)
        .setFlags(flags);
    QueryResult qr = new QueryResult();
    search(qr, qc);
    return qr.getDocList();
  }

  public static final int NO_CHECK_QCACHE = 0x80000000;
  public static final int GET_DOCSET = 0x40000000;
  static final int NO_CHECK_FILTERCACHE = 0x20000000;
  static final int NO_SET_QCACHE = 0x10000000;
  static final int SEGMENT_TERMINATE_EARLY = 0x08;
  public static final int TERMINATE_EARLY = 0x04;
  public static final int GET_DOCLIST = 0x02; // get the documents actually returned in a response
  public static final int GET_SCORES = 0x01;

  /**
   * getDocList version that uses+populates query and filter caches. In the event of a timeout, the cache is not
   * populated.
   */
  private void getDocListC(QueryResult qr, QueryCommand cmd) throws IOException {
    DocListAndSet out = new DocListAndSet();
    qr.setDocListAndSet(out);
    QueryResultKey key = null;
    int maxDocRequested = cmd.getOffset() + cmd.getLen();
    // check for overflow, and check for # docs in index
    if (maxDocRequested < 0 || maxDocRequested > maxDoc()) maxDocRequested = maxDoc();
    int supersetMaxDoc = maxDocRequested;
    DocList superset = null;

    int flags = cmd.getFlags();
    Query q = cmd.getQuery();
    if (q instanceof ExtendedQuery) {
      ExtendedQuery eq = (ExtendedQuery) q;
      if (!eq.getCache()) {
        flags |= (NO_CHECK_QCACHE | NO_SET_QCACHE | NO_CHECK_FILTERCACHE);
      }
    }

    // we can try and look up the complete query in the cache.
    // we can't do that if filter!=null though (we don't want to
    // do hashCode() and equals() for a big DocSet).
    if (queryResultCache != null && cmd.getFilter() == null
        && (flags & (NO_CHECK_QCACHE | NO_SET_QCACHE)) != ((NO_CHECK_QCACHE | NO_SET_QCACHE))) {
      // all of the current flags can be reused during warming,
      // so set all of them on the cache key.
      key = new QueryResultKey(q, cmd.getFilterList(), cmd.getSort(), flags, cmd.getMinExactCount());
      if ((flags & NO_CHECK_QCACHE) == 0) {
        superset = queryResultCache.get(key);

        if (superset != null) {
          // check that the cache entry has scores recorded if we need them
          if ((flags & GET_SCORES) == 0 || superset.hasScores()) {
            // NOTE: subset() returns null if the DocList has fewer docs than
            // requested
            out.docList = superset.subset(cmd.getOffset(), cmd.getLen());
          }
        }
        if (out.docList != null) {
          // found the docList in the cache... now check if we need the docset too.
          // OPT: possible future optimization - if the doclist contains all the matches,
          // use it to make the docset instead of rerunning the query.
          if (out.docSet == null && ((flags & GET_DOCSET) != 0)) {
            if (cmd.getFilterList() == null) {
              out.docSet = getDocSet(cmd.getQuery());
            } else {
              List<Query> newList = new ArrayList<>(cmd.getFilterList().size() + 1);
              newList.add(cmd.getQuery());
              newList.addAll(cmd.getFilterList());
              out.docSet = getDocSet(newList);
            }
          }
          return;
        }
      }

      // If we are going to generate the result, bump up to the
      // next resultWindowSize for better caching.

      if ((flags & NO_SET_QCACHE) == 0) {
        // handle 0 special case as well as avoid idiv in the common case.
        if (maxDocRequested < queryResultWindowSize) {
          supersetMaxDoc = queryResultWindowSize;
        } else {
          supersetMaxDoc = ((maxDocRequested - 1) / queryResultWindowSize + 1) * queryResultWindowSize;
          if (supersetMaxDoc < 0) supersetMaxDoc = maxDocRequested;
        }
      } else {
        key = null; // we won't be caching the result
      }
    }
    cmd.setSupersetMaxDoc(supersetMaxDoc);

    // OK, so now we need to generate an answer.
    // One way to do that would be to check if we have an unordered list
    // of results for the base query. If so, we can apply the filters and then
    // sort by the resulting set. This can only be used if:
    // - the sort doesn't contain score
    // - we don't want score returned.

    // check if we should try and use the filter cache
    boolean useFilterCache = false;
    if ((flags & (GET_SCORES | NO_CHECK_FILTERCACHE)) == 0 && useFilterForSortedQuery && cmd.getSort() != null
        && filterCache != null) {
      useFilterCache = true;
      SortField[] sfields = cmd.getSort().getSort();
      for (SortField sf : sfields) {
        if (sf.getType() == SortField.Type.SCORE) {
          useFilterCache = false;
          break;
        }
      }
    }

    if (useFilterCache) {
      // now actually use the filter cache.
      // for large filters that match few documents, this may be
      // slower than simply re-executing the query.
      if (out.docSet == null) {
        out.docSet = getDocSet(cmd.getQuery(), cmd.getFilter());
        List<Query> filterList = cmd.getFilterList();
        if (filterList != null && !filterList.isEmpty()) {
          out.docSet = out.docSet.intersection(getDocSet(cmd.getFilterList()));
        }
      }
      // todo: there could be a sortDocSet that could take a list of
      // the filters instead of anding them first...
      // perhaps there should be a multi-docset-iterator
      sortDocSet(qr, cmd);
    } else {
      // do it the normal way...
      if ((flags & GET_DOCSET) != 0) {
        // this currently conflates returning the docset for the base query vs
        // the base query and all filters.
        DocSet qDocSet = getDocListAndSetNC(qr, cmd);
        // cache the docSet matching the query w/o filtering
        if (qDocSet != null && filterCache != null && !qr.isPartialResults()) filterCache.put(cmd.getQuery(), qDocSet);
      } else {
        getDocListNC(qr, cmd);
      }
      assert null != out.docList : "docList is null";
    }

    if (null == cmd.getCursorMark()) {
      // Kludge...
      // we can't use DocSlice.subset, even though it should be an identity op
      // because it gets confused by situations where there are lots of matches, but
      // less docs in the slice then were requested, (due to the cursor)
      // so we have to short circuit the call.
      // None of which is really a problem since we can't use caching with
      // cursors anyway, but it still looks weird to have to special case this
      // behavior based on this condition - hence the long explanation.
      superset = out.docList;
      out.docList = superset.subset(cmd.getOffset(), cmd.getLen());
    } else {
      // sanity check our cursor assumptions
      assert null == superset : "cursor: superset isn't null";
      assert 0 == cmd.getOffset() : "cursor: command offset mismatch";
      assert 0 == out.docList.offset() : "cursor: docList offset mismatch";
      assert cmd.getLen() >= supersetMaxDoc : "cursor: superset len mismatch: " + cmd.getLen() + " vs "
          + supersetMaxDoc;
    }

    // lastly, put the superset in the cache if the size is less than or equal
    // to queryResultMaxDocsCached
    if (key != null && superset.size() <= queryResultMaxDocsCached && !qr.isPartialResults()) {
      queryResultCache.put(key, superset);
    }
  }

  /**
   * Helper method for extracting the {@link FieldDoc} sort values from a {@link TopFieldDocs} when available and making
   * the appropriate call to {@link QueryResult#setNextCursorMark} when applicable.
   *
   * @param qr
   *          <code>QueryResult</code> to modify
   * @param qc
   *          <code>QueryCommand</code> for context of method
   * @param topDocs
   *          May or may not be a <code>TopFieldDocs</code>
   */
  private void populateNextCursorMarkFromTopDocs(QueryResult qr, QueryCommand qc, TopDocs topDocs) {
    // TODO: would be nice to rename & generalize this method for non-cursor cases...
    // ...would be handy to reuse the ScoreDoc/FieldDoc sort vals directly in distrib sort
    // ...but that has non-trivial queryResultCache implications
    // See: SOLR-5595

    if (null == qc.getCursorMark()) {
      // nothing to do, short circuit out
      return;
    }

    final CursorMark lastCursorMark = qc.getCursorMark();

    // if we have a cursor, then we have a sort that at minimum involves uniqueKey..
    // so we must have a TopFieldDocs containing FieldDoc[]
    assert topDocs instanceof TopFieldDocs : "TopFieldDocs cursor constraint violated";
    final TopFieldDocs topFieldDocs = (TopFieldDocs) topDocs;
    final ScoreDoc[] scoreDocs = topFieldDocs.scoreDocs;

    if (0 == scoreDocs.length) {
      // no docs on this page, re-use existing cursor mark
      qr.setNextCursorMark(lastCursorMark);
    } else {
      ScoreDoc lastDoc = scoreDocs[scoreDocs.length - 1];
      assert lastDoc instanceof FieldDoc : "FieldDoc cursor constraint violated";

      List<Object> lastFields = Arrays.<Object> asList(((FieldDoc) lastDoc).fields);
      CursorMark nextCursorMark = lastCursorMark.createNext(lastFields);
      assert null != nextCursorMark : "null nextCursorMark";
      qr.setNextCursorMark(nextCursorMark);
    }
  }

  /**
   * Helper method for inspecting QueryCommand and creating the appropriate {@link TopDocsCollector}
   *
   * @param len
   *          the number of docs to return
   * @param cmd
   *          The Command whose properties should determine the type of TopDocsCollector to use.
   */
  @SuppressWarnings({"rawtypes"})
  private TopDocsCollector buildTopDocsCollector(int len, QueryCommand cmd) throws IOException {
    int minNumFound = cmd.getMinExactCount();
    Query q = cmd.getQuery();
    if (q instanceof RankQuery) {
      RankQuery rq = (RankQuery) q;
      return rq.getTopDocsCollector(len, cmd, this);
    }

    if (null == cmd.getSort()) {
      assert null == cmd.getCursorMark() : "have cursor but no sort";
      return TopScoreDocCollector.create(len, minNumFound);
    } else {
      // we have a sort
      final Sort weightedSort = weightSort(cmd.getSort());
      final CursorMark cursor = cmd.getCursorMark();

      final FieldDoc searchAfter = (null != cursor ? cursor.getSearchAfterFieldDoc() : null);
      return TopFieldCollector.create(weightedSort, len, searchAfter, minNumFound);
    }
  }

  private void getDocListNC(QueryResult qr, QueryCommand cmd) throws IOException {
    int len = cmd.getSupersetMaxDoc();
    int last = len;
    if (last < 0 || last > maxDoc()) last = maxDoc();
    final int lastDocRequested = last;
    int nDocsReturned;
    int totalHits;
    float maxScore;
    int[] ids;
    float[] scores;

    boolean needScores = (cmd.getFlags() & GET_SCORES) != 0;

    ProcessedFilter pf = getProcessedFilter(cmd.getFilter(), cmd.getFilterList());
    final Query query = QueryUtils.combineQueryAndFilter(QueryUtils.makeQueryable(cmd.getQuery()), pf.filter);
    Relation hitsRelation;

    // handle zero case...
    if (lastDocRequested <= 0) {
      final float[] topscore = new float[] {Float.NEGATIVE_INFINITY};
      final int[] numHits = new int[1];

      Collector collector;

      if (!needScores) {
        collector = new SimpleCollector() {
          @Override
          public void collect(int doc) {
            numHits[0]++;
          }

          @Override
          public ScoreMode scoreMode() {
            return ScoreMode.COMPLETE_NO_SCORES;
          }
        };
      } else {
        collector = new SimpleCollector() {
          Scorable scorer;

          @Override
          public void setScorer(Scorable scorer) {
            this.scorer = scorer;
          }

          @Override
          public void collect(int doc) throws IOException {
            numHits[0]++;
            float score = scorer.score();
            if (score > topscore[0]) topscore[0] = score;
          }

          @Override
          public ScoreMode scoreMode() {
            return ScoreMode.COMPLETE;
          }
        };
      }

      buildAndRunCollectorChain(qr, query, collector, cmd, pf.postFilter);

      nDocsReturned = 0;
      ids = new int[nDocsReturned];
      scores = new float[nDocsReturned];
      totalHits = numHits[0];
      maxScore = totalHits > 0 ? topscore[0] : 0.0f;
      // no docs on this page, so cursor doesn't change
      qr.setNextCursorMark(cmd.getCursorMark());
      hitsRelation = Relation.EQUAL_TO;
    } else {
      final TopDocsCollector<?> topCollector = buildTopDocsCollector(len, cmd);
      MaxScoreCollector maxScoreCollector = null;
      Collector collector = topCollector;
      if ((cmd.getFlags() & GET_SCORES) != 0) {
        maxScoreCollector = new MaxScoreCollector();
        collector = MultiCollector.wrap(topCollector, maxScoreCollector);
      }
      ScoreMode scoreModeUsed = buildAndRunCollectorChain(qr, query, collector, cmd, pf.postFilter).scoreMode();

      totalHits = topCollector.getTotalHits();
      TopDocs topDocs = topCollector.topDocs(0, len);
      if (scoreModeUsed == ScoreMode.COMPLETE || scoreModeUsed == ScoreMode.COMPLETE_NO_SCORES) {
        hitsRelation = TotalHits.Relation.EQUAL_TO;
      } else {
        hitsRelation = topDocs.totalHits.relation;
      }
      if (cmd.getSort() != null && cmd.getQuery() instanceof RankQuery == false && (cmd.getFlags() & GET_SCORES) != 0) {
        TopFieldCollector.populateScores(topDocs.scoreDocs, this, query);
      }
      populateNextCursorMarkFromTopDocs(qr, cmd, topDocs);

      maxScore = totalHits > 0 ? (maxScoreCollector == null ? Float.NaN : maxScoreCollector.getMaxScore()) : 0.0f;
      nDocsReturned = topDocs.scoreDocs.length;
      ids = new int[nDocsReturned];
      scores = (cmd.getFlags() & GET_SCORES) != 0 ? new float[nDocsReturned] : null;
      for (int i = 0; i < nDocsReturned; i++) {
        ScoreDoc scoreDoc = topDocs.scoreDocs[i];
        ids[i] = scoreDoc.doc;
        if (scores != null) scores[i] = scoreDoc.score;
      }
    }

    int sliceLen = Math.min(lastDocRequested, nDocsReturned);
    if (sliceLen < 0) sliceLen = 0;
    qr.setDocList(new DocSlice(0, sliceLen, ids, scores, totalHits, maxScore, hitsRelation));
  }

  // any DocSet returned is for the query only, without any filtering... that way it may
  // be cached if desired.
  private DocSet getDocListAndSetNC(QueryResult qr, QueryCommand cmd) throws IOException {
    int len = cmd.getSupersetMaxDoc();
    int last = len;
    if (last < 0 || last > maxDoc()) last = maxDoc();
    final int lastDocRequested = last;
    int nDocsReturned;
    int totalHits;
    float maxScore;
    int[] ids;
    float[] scores;
    DocSet set;

    boolean needScores = (cmd.getFlags() & GET_SCORES) != 0;
    int maxDoc = maxDoc();
    cmd.setMinExactCount(Integer.MAX_VALUE);// We need the full DocSet

    ProcessedFilter pf = getProcessedFilter(cmd.getFilter(), cmd.getFilterList());
    final Query query = QueryUtils.combineQueryAndFilter(QueryUtils.makeQueryable(cmd.getQuery()), pf.filter);

    // handle zero case...
    if (lastDocRequested <= 0) {
      final float[] topscore = new float[] {Float.NEGATIVE_INFINITY};

      Collector collector;
      final DocSetCollector setCollector = new DocSetCollector(maxDoc);

      if (!needScores) {
        collector = setCollector;
      } else {
        final Collector topScoreCollector = new SimpleCollector() {

          Scorable scorer;

          @Override
          public void setScorer(Scorable scorer) throws IOException {
            this.scorer = scorer;
          }

          @Override
          public void collect(int doc) throws IOException {
            float score = scorer.score();
            if (score > topscore[0]) topscore[0] = score;
          }

          @Override
          public ScoreMode scoreMode() {
            return ScoreMode.TOP_SCORES;
          }
        };

        collector = MultiCollector.wrap(setCollector, topScoreCollector);
      }

      buildAndRunCollectorChain(qr, query, collector, cmd, pf.postFilter);

      set = DocSetUtil.getDocSet(setCollector, this);

      nDocsReturned = 0;
      ids = new int[nDocsReturned];
      scores = new float[nDocsReturned];
      totalHits = set.size();
      maxScore = totalHits > 0 ? topscore[0] : 0.0f;
      // no docs on this page, so cursor doesn't change
      qr.setNextCursorMark(cmd.getCursorMark());
    } else {
      @SuppressWarnings({"rawtypes"})
      final TopDocsCollector topCollector = buildTopDocsCollector(len, cmd);
      DocSetCollector setCollector = new DocSetCollector(maxDoc);
      MaxScoreCollector maxScoreCollector = null;
      List<Collector> collectors = new ArrayList<>(Arrays.asList(topCollector, setCollector));

      if ((cmd.getFlags() & GET_SCORES) != 0) {
        maxScoreCollector = new MaxScoreCollector();
        collectors.add(maxScoreCollector);
      }

      Collector collector = MultiCollector.wrap(collectors);

      buildAndRunCollectorChain(qr, query, collector, cmd, pf.postFilter);

      set = DocSetUtil.getDocSet(setCollector, this);

      totalHits = topCollector.getTotalHits();
      assert (totalHits == set.size()) || qr.isPartialResults();

      TopDocs topDocs = topCollector.topDocs(0, len);
      if (cmd.getSort() != null && cmd.getQuery() instanceof RankQuery == false && (cmd.getFlags() & GET_SCORES) != 0) {
        TopFieldCollector.populateScores(topDocs.scoreDocs, this, query);
      }
      populateNextCursorMarkFromTopDocs(qr, cmd, topDocs);
      maxScore = totalHits > 0 ? (maxScoreCollector == null ? Float.NaN : maxScoreCollector.getMaxScore()) : 0.0f;
      nDocsReturned = topDocs.scoreDocs.length;

      ids = new int[nDocsReturned];
      scores = (cmd.getFlags() & GET_SCORES) != 0 ? new float[nDocsReturned] : null;
      for (int i = 0; i < nDocsReturned; i++) {
        ScoreDoc scoreDoc = topDocs.scoreDocs[i];
        ids[i] = scoreDoc.doc;
        if (scores != null) scores[i] = scoreDoc.score;
      }
    }

    int sliceLen = Math.min(lastDocRequested, nDocsReturned);
    if (sliceLen < 0) sliceLen = 0;

    qr.setDocList(new DocSlice(0, sliceLen, ids, scores, totalHits, maxScore, TotalHits.Relation.EQUAL_TO));
    // TODO: if we collect results before the filter, we just need to intersect with
    // that filter to generate the DocSet for qr.setDocSet()
    qr.setDocSet(set);

    // TODO: currently we don't generate the DocSet for the base query,
    // but the QueryDocSet == CompleteDocSet if filter==null.
    return pf.filter == null && pf.postFilter == null ? qr.getDocSet() : null;
  }

  /**
   * Returns documents matching both <code>query</code> and <code>filter</code> and sorted by <code>sort</code>. FUTURE:
   * The returned DocList may be retrieved from a cache.
   *
   * @param filter
   *          may be null
   * @param lsort
   *          criteria by which to sort (if null, query relevance is used)
   * @param offset
   *          offset into the list of documents to return
   * @param len
   *          maximum number of documents to return
   * @return DocList meeting the specified criteria, should <b>not</b> be modified by the caller.
   * @throws IOException
   *           If there is a low-level I/O error.
   */
  public DocList getDocList(Query query, DocSet filter, Sort lsort, int offset, int len) throws IOException {
    QueryCommand qc = new QueryCommand();
    qc.setQuery(query)
        .setFilter(filter)
        .setSort(lsort)
        .setOffset(offset)
        .setLen(len);
    QueryResult qr = new QueryResult();
    search(qr, qc);
    return qr.getDocList();
  }

  /**
   * Returns documents matching both <code>query</code> and <code>filter</code> and sorted by <code>sort</code>. Also
   * returns the complete set of documents matching <code>query</code> and <code>filter</code> (regardless of
   * <code>offset</code> and <code>len</code>).
   * <p>
   * This method is cache aware and may retrieve <code>filter</code> from the cache or make an insertion into the cache
   * as a result of this call.
   * <p>
   * FUTURE: The returned DocList may be retrieved from a cache.
   * <p>
   * The DocList and DocSet returned should <b>not</b> be modified.
   *
   * @param filter
   *          may be null
   * @param lsort
   *          criteria by which to sort (if null, query relevance is used)
   * @param offset
   *          offset into the list of documents to return
   * @param len
   *          maximum number of documents to return
   * @return DocListAndSet meeting the specified criteria, should <b>not</b> be modified by the caller.
   * @throws IOException
   *           If there is a low-level I/O error.
   */
  public DocListAndSet getDocListAndSet(Query query, Query filter, Sort lsort, int offset, int len) throws IOException {
    QueryCommand qc = new QueryCommand();
    qc.setQuery(query)
        .setFilterList(filter)
        .setSort(lsort)
        .setOffset(offset)
        .setLen(len)
        .setNeedDocSet(true);
    QueryResult qr = new QueryResult();
    search(qr, qc);
    return qr.getDocListAndSet();
  }

  /**
   * Returns documents matching both <code>query</code> and <code>filter</code> and sorted by <code>sort</code>. Also
   * returns the compete set of documents matching <code>query</code> and <code>filter</code> (regardless of
   * <code>offset</code> and <code>len</code>).
   * <p>
   * This method is cache aware and may retrieve <code>filter</code> from the cache or make an insertion into the cache
   * as a result of this call.
   * <p>
   * FUTURE: The returned DocList may be retrieved from a cache.
   * <p>
   * The DocList and DocSet returned should <b>not</b> be modified.
   *
   * @param filter
   *          may be null
   * @param lsort
   *          criteria by which to sort (if null, query relevance is used)
   * @param offset
   *          offset into the list of documents to return
   * @param len
   *          maximum number of documents to return
   * @param flags
   *          user supplied flags for the result set
   * @return DocListAndSet meeting the specified criteria, should <b>not</b> be modified by the caller.
   * @throws IOException
   *           If there is a low-level I/O error.
   */
  public DocListAndSet getDocListAndSet(Query query, Query filter, Sort lsort, int offset, int len, int flags)
      throws IOException {
    QueryCommand qc = new QueryCommand();
    qc.setQuery(query)
        .setFilterList(filter)
        .setSort(lsort)
        .setOffset(offset)
        .setLen(len)
        .setFlags(flags)
        .setNeedDocSet(true);
    QueryResult qr = new QueryResult();
    search(qr, qc);
    return qr.getDocListAndSet();
  }

  /**
   * Returns documents matching both <code>query</code> and the intersection of <code>filterList</code>, sorted by
   * <code>sort</code>. Also returns the compete set of documents matching <code>query</code> and <code>filter</code>
   * (regardless of <code>offset</code> and <code>len</code>).
   * <p>
   * This method is cache aware and may retrieve <code>filter</code> from the cache or make an insertion into the cache
   * as a result of this call.
   * <p>
   * FUTURE: The returned DocList may be retrieved from a cache.
   * <p>
   * The DocList and DocSet returned should <b>not</b> be modified.
   *
   * @param filterList
   *          may be null
   * @param lsort
   *          criteria by which to sort (if null, query relevance is used)
   * @param offset
   *          offset into the list of documents to return
   * @param len
   *          maximum number of documents to return
   * @return DocListAndSet meeting the specified criteria, should <b>not</b> be modified by the caller.
   * @throws IOException
   *           If there is a low-level I/O error.
   */
  public DocListAndSet getDocListAndSet(Query query, List<Query> filterList, Sort lsort, int offset, int len)
      throws IOException {
    QueryCommand qc = new QueryCommand();
    qc.setQuery(query)
        .setFilterList(filterList)
        .setSort(lsort)
        .setOffset(offset)
        .setLen(len)
        .setNeedDocSet(true);
    QueryResult qr = new QueryResult();
    search(qr, qc);
    return qr.getDocListAndSet();
  }

  /**
   * Returns documents matching both <code>query</code> and the intersection of <code>filterList</code>, sorted by
   * <code>sort</code>. Also returns the compete set of documents matching <code>query</code> and <code>filter</code>
   * (regardless of <code>offset</code> and <code>len</code>).
   * <p>
   * This method is cache aware and may retrieve <code>filter</code> from the cache or make an insertion into the cache
   * as a result of this call.
   * <p>
   * FUTURE: The returned DocList may be retrieved from a cache.
   * <p>
   * The DocList and DocSet returned should <b>not</b> be modified.
   *
   * @param filterList
   *          may be null
   * @param lsort
   *          criteria by which to sort (if null, query relevance is used)
   * @param offset
   *          offset into the list of documents to return
   * @param len
   *          maximum number of documents to return
   * @param flags
   *          user supplied flags for the result set
   * @return DocListAndSet meeting the specified criteria, should <b>not</b> be modified by the caller.
   * @throws IOException
   *           If there is a low-level I/O error.
   */
  public DocListAndSet getDocListAndSet(Query query, List<Query> filterList, Sort lsort, int offset, int len, int flags)
      throws IOException {
    QueryCommand qc = new QueryCommand();
    qc.setQuery(query)
        .setFilterList(filterList)
        .setSort(lsort)
        .setOffset(offset)
        .setLen(len)
        .setFlags(flags)
        .setNeedDocSet(true);
    QueryResult qr = new QueryResult();
    search(qr, qc);
    return qr.getDocListAndSet();
  }

  /**
   * Returns documents matching both <code>query</code> and <code>filter</code> and sorted by <code>sort</code>. Also
   * returns the compete set of documents matching <code>query</code> and <code>filter</code> (regardless of
   * <code>offset</code> and <code>len</code>).
   * <p>
   * FUTURE: The returned DocList may be retrieved from a cache.
   *
   * @param filter
   *          may be null
   * @param lsort
   *          criteria by which to sort (if null, query relevance is used)
   * @param offset
   *          offset into the list of documents to return
   * @param len
   *          maximum number of documents to return
   * @return DocListAndSet meeting the specified criteria, should <b>not</b> be modified by the caller.
   * @throws IOException
   *           If there is a low-level I/O error.
   */
  public DocListAndSet getDocListAndSet(Query query, DocSet filter, Sort lsort, int offset, int len)
      throws IOException {
    QueryCommand qc = new QueryCommand();
    qc.setQuery(query)
        .setFilter(filter)
        .setSort(lsort)
        .setOffset(offset)
        .setLen(len)
        .setNeedDocSet(true);
    QueryResult qr = new QueryResult();
    search(qr, qc);
    return qr.getDocListAndSet();
  }

  /**
   * Returns documents matching both <code>query</code> and <code>filter</code> and sorted by <code>sort</code>. Also
   * returns the compete set of documents matching <code>query</code> and <code>filter</code> (regardless of
   * <code>offset</code> and <code>len</code>).
   * <p>
   * This method is cache aware and may make an insertion into the cache as a result of this call.
   * <p>
   * FUTURE: The returned DocList may be retrieved from a cache.
   * <p>
   * The DocList and DocSet returned should <b>not</b> be modified.
   *
   * @param filter
   *          may be null
   * @param lsort
   *          criteria by which to sort (if null, query relevance is used)
   * @param offset
   *          offset into the list of documents to return
   * @param len
   *          maximum number of documents to return
   * @param flags
   *          user supplied flags for the result set
   * @return DocListAndSet meeting the specified criteria, should <b>not</b> be modified by the caller.
   * @throws IOException
   *           If there is a low-level I/O error.
   */
  public DocListAndSet getDocListAndSet(Query query, DocSet filter, Sort lsort, int offset, int len, int flags)
      throws IOException {
    QueryCommand qc = new QueryCommand();
    qc.setQuery(query)
        .setFilter(filter)
        .setSort(lsort)
        .setOffset(offset)
        .setLen(len)
        .setFlags(flags)
        .setNeedDocSet(true);
    QueryResult qr = new QueryResult();
    search(qr, qc);
    return qr.getDocListAndSet();
  }

  protected void sortDocSet(QueryResult qr, QueryCommand cmd) throws IOException {
    DocSet set = qr.getDocListAndSet().docSet;
    int nDocs = cmd.getSupersetMaxDoc();
    if (nDocs == 0) {
      // SOLR-2923
      qr.getDocListAndSet().docList = new DocSlice(0, 0, new int[0], null, set.size(), 0f, TotalHits.Relation.EQUAL_TO);
      qr.setNextCursorMark(cmd.getCursorMark());
      return;
    }

    // bit of a hack to tell if a set is sorted - do it better in the future.
    boolean inOrder = set instanceof BitDocSet || set instanceof SortedIntDocSet;

    @SuppressWarnings({"rawtypes"})
    TopDocsCollector topCollector = buildTopDocsCollector(nDocs, cmd);

    DocIterator iter = set.iterator();
    int base = 0;
    int end = 0;
    int readerIndex = 0;

    LeafCollector leafCollector = null;
    while (iter.hasNext()) {
      int doc = iter.nextDoc();
      while (doc >= end) {
        LeafReaderContext leaf = leafContexts.get(readerIndex++);
        base = leaf.docBase;
        end = base + leaf.reader().maxDoc();
        leafCollector = topCollector.getLeafCollector(leaf);
        // we should never need to set the scorer given the settings for the collector
      }
      leafCollector.collect(doc - base);
    }

    TopDocs topDocs = topCollector.topDocs(0, nDocs);

    int nDocsReturned = topDocs.scoreDocs.length;
    int[] ids = new int[nDocsReturned];

    for (int i = 0; i < nDocsReturned; i++) {
      ScoreDoc scoreDoc = topDocs.scoreDocs[i];
      ids[i] = scoreDoc.doc;
    }

    assert topDocs.totalHits.relation == TotalHits.Relation.EQUAL_TO;
    qr.getDocListAndSet().docList = new DocSlice(0, nDocsReturned, ids, null, topDocs.totalHits.value, 0.0f, topDocs.totalHits.relation);
    populateNextCursorMarkFromTopDocs(qr, cmd, topDocs);
  }

  /**
   * Returns the number of documents that match both <code>a</code> and <code>b</code>.
   * <p>
   * This method is cache-aware and may check as well as modify the cache.
   *
   * @return the number of documents in the intersection between <code>a</code> and <code>b</code>.
   * @throws IOException
   *           If there is a low-level I/O error.
   */
  public int numDocs(Query a, DocSet b) throws IOException {
    if (b.size() == 0) {
      return 0;
    }
    if (filterCache != null) {
      // Negative query if absolute value different from original
      Query absQ = QueryUtils.getAbs(a);
      DocSet positiveA = getPositiveDocSet(absQ);
      return a == absQ ? b.intersectionSize(positiveA) : b.andNotSize(positiveA);
    } else {
      // If there isn't a cache, then do a single filtered query
      // NOTE: we cannot use FilteredQuery, because BitDocSet assumes it will never
      // have deleted documents, but UninvertedField's doNegative has sets with deleted docs
      TotalHitCountCollector collector = new TotalHitCountCollector();
      BooleanQuery.Builder bq = new BooleanQuery.Builder();
      bq.add(QueryUtils.makeQueryable(a), BooleanClause.Occur.MUST);
      bq.add(new ConstantScoreQuery(b.getTopFilter()), BooleanClause.Occur.MUST);
      super.search(bq.build(), collector);
      return collector.getTotalHits();
    }
  }

  /** @lucene.internal */
  public int numDocs(DocSet a, DocsEnumState deState) throws IOException {
    // Negative query if absolute value different from original
    return a.intersectionSize(getDocSet(deState));
  }

  public static class DocsEnumState {
    public String fieldName; // currently interned for as long as lucene requires it
    public TermsEnum termsEnum;
    public Bits liveDocs;
    public PostingsEnum postingsEnum;

    public int minSetSizeCached;

    public int[] scratch;
  }

  /**
   * Returns the number of documents that match both <code>a</code> and <code>b</code>.
   * <p>
   * This method is cache-aware and may check as well as modify the cache.
   *
   * @return the number of documents in the intersection between <code>a</code> and <code>b</code>.
   * @throws IOException
   *           If there is a low-level I/O error.
   */
  public int numDocs(Query a, Query b) throws IOException {
    Query absA = QueryUtils.getAbs(a);
    Query absB = QueryUtils.getAbs(b);
    DocSet positiveA = getPositiveDocSet(absA);
    DocSet positiveB = getPositiveDocSet(absB);

    // Negative query if absolute value different from original
    if (a == absA) {
      if (b == absB) return positiveA.intersectionSize(positiveB);
      return positiveA.andNotSize(positiveB);
    }
    if (b == absB) return positiveB.andNotSize(positiveA);

    // if both negative, we need to create a temp DocSet since we
    // don't have a counting method that takes three.
    DocSet all = getLiveDocSet();

    // -a -b == *:*.andNot(a).andNotSize(b) == *.*.andNotSize(a.union(b))
    // we use the last form since the intermediate DocSet should normally be smaller.
    return all.andNotSize(positiveA.union(positiveB));
  }

  /** @lucene.internal */
  public boolean intersects(DocSet a, DocsEnumState deState) throws IOException {
    return a.intersects(getDocSet(deState));
  }

  /**
   * Warm this searcher based on an old one (primarily for auto-cache warming).
   */
  @SuppressWarnings({"unchecked"})
  public void warm(SolrIndexSearcher old) {
    // Make sure this is first! filters can help queryResults execute!
    long warmingStartTime = System.nanoTime();
    // warm the caches in order...
    ModifiableSolrParams params = new ModifiableSolrParams();
    params.add("warming", "true");
    for (int i = 0; i < cacheList.length; i++) {
      if (log.isDebugEnabled()) {
        log.debug("autowarming [{}] from [{}]\n\t{}", this, old, old.cacheList[i]);
      }

      final SolrQueryRequest req = new LocalSolrQueryRequest(core, params) {
        @Override
        public SolrIndexSearcher getSearcher() {
          return SolrIndexSearcher.this;
        }

        @Override
        public void close() {}
      };

      final SolrQueryResponse rsp = new SolrQueryResponse();
      SolrRequestInfo.setRequestInfo(new SolrRequestInfo(req, rsp));
      try {
        cacheList[i].warm(this, old.cacheList[i]);
      } finally {
        try {
          req.close();
        } finally {
          SolrRequestInfo.clearRequestInfo();
        }
      }

      if (log.isDebugEnabled()) {
        log.debug("autowarming result for [{}]\n\t{}", this, cacheList[i]);
      }
    }
    warmupTime = TimeUnit.MILLISECONDS.convert(System.nanoTime() - warmingStartTime, TimeUnit.NANOSECONDS);
  }

  /**
   * return the named generic cache
   */
  @SuppressWarnings({"rawtypes"})
  public SolrCache getCache(String cacheName) {
    return cacheMap.get(cacheName);
  }

  /**
   * lookup an entry in a generic cache
   */
  @SuppressWarnings({"unchecked"})
  public Object cacheLookup(String cacheName, Object key) {
    @SuppressWarnings({"rawtypes"})
    SolrCache cache = cacheMap.get(cacheName);
    return cache == null ? null : cache.get(key);
  }

  /**
   * insert an entry in a generic cache
   */
  @SuppressWarnings({"unchecked"})
  public Object cacheInsert(String cacheName, Object key, Object val) {
    @SuppressWarnings({"rawtypes"})
    SolrCache cache = cacheMap.get(cacheName);
    return cache == null ? null : cache.put(key, val);
  }

  public Date getOpenTimeStamp() {
    return openTime;
  }

  // public but primarily for test case usage
  public long getOpenNanoTime() {
    return openNanoTime;
  }

  @Override
  public Explanation explain(Query query, int doc) throws IOException {
    return super.explain(QueryUtils.makeQueryable(query), doc);
  }

  /** @lucene.internal
   * gets a cached version of the IndexFingerprint for this searcher
   **/
  public IndexFingerprint getIndexFingerprint(long maxVersion) throws IOException {
    final SolrIndexSearcher searcher = this;
    final AtomicReference<IOException> exception = new AtomicReference<>();
    try {
      return searcher.getTopReaderContext().leaves().stream()
          .map(ctx -> {
            try {
              return searcher.getCore().getIndexFingerprint(searcher, ctx, maxVersion);
            } catch (IOException e) {
              exception.set(e);
              return null;
            }
          })
          .filter(java.util.Objects::nonNull)
          .reduce(new IndexFingerprint(maxVersion), IndexFingerprint::reduce);

    } finally {
      if (exception.get() != null) throw exception.get();
    }
  }


  /////////////////////////////////////////////////////////////////////
  // SolrInfoBean stuff: Statistics and Module Info
  /////////////////////////////////////////////////////////////////////

  @Override
  public String getName() {
    return SolrIndexSearcher.class.getName();
  }

  @Override
  public String getDescription() {
    return "index searcher";
  }

  @Override
  public Category getCategory() {
    return Category.CORE;
  }

  @Override
  public SolrMetricsContext getSolrMetricsContext() {
    return solrMetricsContext;
  }

  @Override
  public void initializeMetrics(SolrMetricsContext parentContext, String scope) {
    parentContext.gauge(() -> name, true, "searcherName", Category.SEARCHER.toString(), scope);
    parentContext.gauge(() -> cachingEnabled, true, "caching", Category.SEARCHER.toString(), scope);
    parentContext.gauge(() -> openTime, true, "openedAt", Category.SEARCHER.toString(), scope);
    parentContext.gauge(() -> warmupTime, true, "warmupTime", Category.SEARCHER.toString(), scope);
    parentContext.gauge(() -> registerTime, true, "registeredAt", Category.SEARCHER.toString(), scope);
    // reader stats
    parentContext.gauge(rgauge(parentContext.nullNumber(), () -> reader.numDocs()), true, "numDocs", Category.SEARCHER.toString(), scope);
    parentContext.gauge(rgauge(parentContext.nullNumber(), () -> reader.maxDoc()), true, "maxDoc", Category.SEARCHER.toString(), scope);
    parentContext.gauge(rgauge(parentContext.nullNumber(), () -> reader.maxDoc() - reader.numDocs()), true, "deletedDocs", Category.SEARCHER.toString(), scope);
    parentContext.gauge(rgauge(parentContext.nullString(), () -> reader.toString()), true, "reader", Category.SEARCHER.toString(), scope);
    parentContext.gauge(rgauge(parentContext.nullString(), () -> reader.directory().toString()), true, "readerDir", Category.SEARCHER.toString(), scope);
    parentContext.gauge(rgauge(parentContext.nullNumber(), () -> reader.getVersion()), true, "indexVersion", Category.SEARCHER.toString(), scope);
    // size of the currently opened commit
    parentContext.gauge(() -> {
      try {
        Collection<String> files = reader.getIndexCommit().getFileNames();
        long total = 0;
        for (String file : files) {
          total += DirectoryFactory.sizeOf(reader.directory(), file);
        }
        return total;
      } catch (Exception e) {
        return parentContext.nullNumber();
      }
    }, true, "indexCommitSize", Category.SEARCHER.toString(), scope);
    // statsCache metrics
    parentContext.gauge(
        new MetricsMap(map -> {
          statsCache.getCacheMetrics().getSnapshot(map::putNoEx);
          map.put("statsCacheImpl", statsCache.getClass().getSimpleName());
        }), true, "statsCache", Category.CACHE.toString(), scope);
  }

  /** 
   * wraps a gauge (related to an IndexReader) and swallows any {@link AlreadyClosedException} that 
   * might be thrown, returning the specified default in it's place. 
   */
  private <T> Gauge<T> rgauge(T closedDefault, Gauge<T> g) {
    return () -> {
      try {
        return g.getValue();
      } catch (AlreadyClosedException ignore) {
        return closedDefault;
      }
    };
  }
  
  private static class FilterImpl extends Filter {
    private final Filter topFilter;
    private final List<Weight> weights;

    public FilterImpl(DocSet filter, List<Weight> weights) {
      this.weights = weights;
      this.topFilter = filter == null ? null : filter.getTopFilter();
    }

    @Override
    public DocIdSet getDocIdSet(LeafReaderContext context, Bits acceptDocs) throws IOException {
      final DocIdSet sub = topFilter == null ? null : topFilter.getDocIdSet(context, acceptDocs);
      if (weights.size() == 0) return sub;
      return new FilterSet(sub, context);
    }

    @Override
    public String toString(String field) {
      return "SolrFilter";
    }

    @Override
    public void visit(QueryVisitor visitor) {
      visitor.visitLeaf(this);
    }

    private class FilterSet extends DocIdSet {
      private final DocIdSet docIdSet;
      private final LeafReaderContext context;

      public FilterSet(DocIdSet docIdSet, LeafReaderContext context) {
        this.docIdSet = docIdSet;
        this.context = context;
      }

      @Override
      public DocIdSetIterator iterator() throws IOException {
        List<DocIdSetIterator> iterators = new ArrayList<>(weights.size() + 1);
        if (docIdSet != null) {
          final DocIdSetIterator iter = docIdSet.iterator();
          if (iter == null) return null;
          iterators.add(iter);
        }
        for (Weight w : weights) {
          final Scorer scorer = w.scorer(context);
          if (scorer == null) return null;
          iterators.add(scorer.iterator());
        }
        if (iterators.isEmpty()) return null;
        if (iterators.size() == 1) return iterators.get(0);
        if (iterators.size() == 2) return new DualFilterIterator(iterators.get(0), iterators.get(1));
        return new FilterIterator(iterators.toArray(new DocIdSetIterator[iterators.size()]));
      }

      @Override
      public Bits bits() throws IOException {
        return null; // don't use random access
      }

      @Override
      public long ramBytesUsed() {
        return docIdSet != null ? docIdSet.ramBytesUsed() : 0L;
      }
    }

    private static class FilterIterator extends DocIdSetIterator {
      private final DocIdSetIterator[] iterators;
      private final DocIdSetIterator first;

      public FilterIterator(DocIdSetIterator[] iterators) {
        this.iterators = iterators;
        this.first = iterators[0];
      }

      @Override
      public int docID() {
        return first.docID();
      }

      private int advanceAllTo(int doc) throws IOException {
        int highestDocIter = 0; // index of the iterator with the highest id
        int i = 1; // We already advanced the first iterator before calling this method
        while (i < iterators.length) {
          if (i != highestDocIter) {
            final int next = iterators[i].advance(doc);
            if (next != doc) { // We need to advance all iterators to a new target
              doc = next;
              highestDocIter = i;
              i = 0;
              continue;
            }
          }
          ++i;
        }
        return doc;
      }

      @Override
      public int nextDoc() throws IOException {
        return advanceAllTo(first.nextDoc());
      }

      @Override
      public int advance(int target) throws IOException {
        return advanceAllTo(first.advance(target));
      }

      @Override
      public long cost() {
        return first.cost();
      }
    }

    private static class DualFilterIterator extends DocIdSetIterator {
      private final DocIdSetIterator a;
      private final DocIdSetIterator b;

      public DualFilterIterator(DocIdSetIterator a, DocIdSetIterator b) {
        this.a = a;
        this.b = b;
      }

      @Override
      public int docID() {
        return a.docID();
      }

      @Override
      public int nextDoc() throws IOException {
        return doNext(a.nextDoc());
      }

      @Override
      public int advance(int target) throws IOException {
        return doNext(a.advance(target));
      }

      @Override
      public long cost() {
        return Math.min(a.cost(), b.cost());
      }

      private int doNext(int doc) throws IOException {
        for (;;) {
          int other = b.advance(doc);
          if (other == doc) return doc;
          doc = a.advance(other);
          if (other == doc) return doc;
        }
      }

    }

    @Override
    public boolean equals(Object other) {
      return sameClassAs(other) &&
             equalsTo(getClass().cast(other));
    }

    private boolean equalsTo(FilterImpl other) {
      return Objects.equals(this.topFilter, other.topFilter) &&
             Objects.equals(this.weights, other.weights);
    }

    @Override
    public int hashCode() {
      return classHash() 
          + 31 * Objects.hashCode(topFilter)
          + 31 * Objects.hashCode(weights);
    }
  }
  public long getWarmupTime() {
    return warmupTime;
  }

}