LUCENE-3129: add single pass grouping collector, BlockGroupingCollector

git-svn-id: https://svn.apache.org/repos/asf/lucene/dev/trunk@1130648 13f79535-47bb-0310-9956-ffa450edef68

lucene/contrib/CHANGES.txt

@@ -130,6 +130,12 @@ New Features
    case where the indexing rate is lowish but the reopen rate is
    highish, to take load off the IO system.  (Mike McCandless)
 
+ * LUCENE-3129: Added BlockGroupingCollector, a single pass
+   grouping collector which is faster than the two-pass approach, and
+   also computes the total group count, but requires that every
+   document sharing the same group was indexed as a doc block
+   (IndexWriter.add/updateDocuments).  (Mike McCandless)
+
 Optimizations
 
  * LUCENE-3040: Switch all analysis consumers (highlighter, morelikethis, memory, ...)

modules/grouping/src/java/org/apache/lucene/search/grouping/BlockGroupingCollector.java

@@ -0,0 +1,516 @@
+package org.apache.lucene.search.grouping;
+
+/**
+ * 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.
+ */
+
+
+import java.io.IOException;
+
+import org.apache.lucene.index.IndexReader.AtomicReaderContext;
+import org.apache.lucene.index.IndexWriter;       // javadocs
+import org.apache.lucene.search.Collector;
+import org.apache.lucene.search.DocIdSetIterator;
+import org.apache.lucene.search.FieldComparator;
+import org.apache.lucene.search.Filter;
+import org.apache.lucene.search.Scorer;
+import org.apache.lucene.search.Sort;
+import org.apache.lucene.search.SortField;
+import org.apache.lucene.search.TopDocs;
+import org.apache.lucene.search.TopDocsCollector;
+import org.apache.lucene.search.TopFieldCollector;
+import org.apache.lucene.search.TopScoreDocCollector;
+import org.apache.lucene.search.Weight;
+import org.apache.lucene.util.ArrayUtil;
+import org.apache.lucene.util.PriorityQueue;
+
+/** BlockGroupingCollector performs grouping with a
+ *  single pass collector, as long as you are grouping by a
+ *  doc block field, ie all documents sharing a given group
+ *  value were indexed as a doc block using the atomic
+ *  {@link IndexWriter#addDocuments} or {@link
+ *  IndexWriter#updateDocuments} API.
+ *
+ *  <p>This results in faster performance (~25% faster QPS)
+ *  than the two-pass grouping collectors, with the tradeoff
+ *  being that the documents in each group must always be
+ *  indexed as a block.  This collector also fills in
+ *  TopGroups.totalGroupCount without requiring the separate
+ *  {@link AllGroupsCollector}.  However, this collector does
+ *  not fill in the groupValue of each group; this field
+ *  will always be null.
+ *
+ *  <p><b>NOTE</b>: this collector makes no effort to verify
+ *  the docs were in fact indexed as a block, so it's up to
+ *  you to ensure this was the case.
+ *
+ *  <p>See {@link org.apache.lucene.search.grouping} for more
+ *  details including a full code example.</p>
+ *
+ * @lucene.experimental
+ */
+
+public class BlockGroupingCollector extends Collector {
+
+  private int[] pendingSubDocs;
+  private float[] pendingSubScores;
+  private int subDocUpto;
+
+  private final Sort groupSort;
+  private final int topNGroups;
+  private final Filter lastDocPerGroup;
+
+  // TODO: specialize into 2 classes, static "create" method:
+  private final boolean needsScores;
+
+  private final FieldComparator[] comparators;
+  private final int[] reversed;
+  private final int compIDXEnd;
+  private int bottomSlot;
+  private boolean queueFull;
+  private AtomicReaderContext currentReaderContext;
+
+  private int topGroupDoc;
+  private int totalHitCount;
+  private int totalGroupCount;
+  private int docBase;
+  private int groupEndDocID;
+  //private OpenBitSet lastDocPerGroupBits;
+  private DocIdSetIterator lastDocPerGroupBits;
+  private Scorer scorer;
+  private final GroupQueue groupQueue;
+  private boolean groupCompetes;
+
+  private final static class FakeScorer extends Scorer {
+
+    float score;
+    int doc;
+
+    public FakeScorer() {
+      super((Weight) null);
+    }
+
+    @Override
+    public float score() {
+      return score;
+    }
+
+    @Override
+    public int docID() {
+      return doc;
+    }
+
+    @Override
+    public int advance(int target) {
+      throw new UnsupportedOperationException();
+    }
+
+    @Override
+    public int nextDoc() {
+      throw new UnsupportedOperationException();
+    }
+  }
+
+  private static final class OneGroup {
+    AtomicReaderContext readerContext;
+    //int groupOrd;
+    int topGroupDoc;
+    int[] docs;
+    float[] scores;
+    int count;
+    int comparatorSlot;
+  }
+  
+  // Sorts by groupSort.  Not static -- uses comparators, reversed
+  private final class GroupQueue extends PriorityQueue<OneGroup> {
+
+    public GroupQueue(int size) {
+      super(size);
+    }
+
+    @Override
+    protected boolean lessThan(final OneGroup group1, final OneGroup group2) {
+
+      //System.out.println("    ltcheck");
+      assert group1 != group2;
+      assert group1.comparatorSlot != group2.comparatorSlot;
+
+      final int numComparators = comparators.length;
+      for (int compIDX=0;compIDX < numComparators; compIDX++) {
+        final int c = reversed[compIDX] * comparators[compIDX].compare(group1.comparatorSlot, group2.comparatorSlot);
+        if (c != 0) {
+          // Short circuit
+          return c > 0;
+        }
+      }
+
+      // Break ties by docID; lower docID is always sorted first
+      return group1.topGroupDoc > group2.topGroupDoc;
+    }
+  }
+
+  // Called when we transition to another group; if the
+  // group is competitive we insert into the group queue
+  private void processGroup() {
+    totalGroupCount++;
+    //System.out.println("    processGroup ord=" + lastGroupOrd + " competes=" + groupCompetes + " count=" + subDocUpto + " groupDoc=" + topGroupDoc);
+    if (groupCompetes) {
+      if (!queueFull) {
+        // Startup transient: always add a new OneGroup
+        final OneGroup og = new OneGroup();
+        og.count = subDocUpto;
+        og.topGroupDoc = docBase + topGroupDoc;
+        og.docs = pendingSubDocs;
+        pendingSubDocs = new int[10];
+        if (needsScores) {
+          og.scores = pendingSubScores;
+          pendingSubScores = new float[10];
+        }
+        og.readerContext = currentReaderContext;
+        //og.groupOrd = lastGroupOrd;
+        og.comparatorSlot = bottomSlot;
+        final OneGroup bottomGroup = groupQueue.add(og);
+        //System.out.println("      ADD group=" + getGroupString(lastGroupOrd) + " newBottom=" + getGroupString(bottomGroup.groupOrd));
+        queueFull = groupQueue.size() == topNGroups;
+        if (queueFull) {
+          // Queue just became full; now set the real bottom
+          // in the comparators:
+          bottomSlot = bottomGroup.comparatorSlot;
+          //System.out.println("    set bottom=" + bottomSlot);
+          for (int i = 0; i < comparators.length; i++) {
+            comparators[i].setBottom(bottomSlot);
+          }
+          //System.out.println("     QUEUE FULL");
+        } else {
+          // Queue not full yet -- just advance bottomSlot:
+          bottomSlot = groupQueue.size();
+        }
+      } else {
+        // Replace bottom element in PQ and then updateTop
+        final OneGroup og = groupQueue.top();
+        assert og != null;
+        og.count = subDocUpto;
+        og.topGroupDoc = docBase + topGroupDoc;
+        // Swap pending docs
+        final int[] savDocs = og.docs;
+        og.docs = pendingSubDocs;
+        pendingSubDocs = savDocs;
+        if (needsScores) {
+          // Swap pending scores
+          final float[] savScores = og.scores;
+          og.scores = pendingSubScores;
+          pendingSubScores = og.scores;
+        }
+        og.readerContext = currentReaderContext;
+        //og.groupOrd = lastGroupOrd;
+        bottomSlot = groupQueue.updateTop().comparatorSlot;
+
+        //System.out.println("    set bottom=" + bottomSlot);
+        for (int i = 0; i < comparators.length; i++) {
+          comparators[i].setBottom(bottomSlot);
+        }
+      }
+    }
+    subDocUpto = 0;
+  }
+
+  /**
+   * Create the single pass collector.
+   *
+   *  @param groupSort The {@link Sort} used to sort the
+   *    groups.  The top sorted document within each group
+   *    according to groupSort, determines how that group
+   *    sorts against other groups.  This must be non-null,
+   *    ie, if you want to groupSort by relevance use
+   *    Sort.RELEVANCE.
+   *  @param topNGroups How many top groups to keep.
+   *  @param needsScores true if the collected documents
+   *    require scores, either because relevance is included
+   *    in the withinGroupSort or because you plan to pass true
+   *    for either getSscores or getMaxScores to {@link
+   *    #getTopGroups}
+   *  @param lastDocPerGroup a {@link Filter} that marks the
+   *    last document in each group.
+   */
+  public BlockGroupingCollector(Sort groupSort, int topNGroups, boolean needsScores, Filter lastDocPerGroup) throws IOException {
+
+    if (topNGroups < 1) {
+      throw new IllegalArgumentException("topNGroups must be >= 1 (got " + topNGroups + ")");
+    }
+
+    groupQueue = new GroupQueue(topNGroups);
+    pendingSubDocs = new int[10];
+    if (needsScores) {
+      pendingSubScores = new float[10];
+    }
+
+    this.needsScores = needsScores;
+    this.lastDocPerGroup = lastDocPerGroup;
+    // TODO: allow null groupSort to mean "by relevance",
+    // and specialize it?
+    this.groupSort = groupSort;
+    
+    this.topNGroups = topNGroups;
+
+    final SortField[] sortFields = groupSort.getSort();
+    comparators = new FieldComparator[sortFields.length];
+    compIDXEnd = comparators.length - 1;
+    reversed = new int[sortFields.length];
+    for (int i = 0; i < sortFields.length; i++) {
+      final SortField sortField = sortFields[i];
+      comparators[i] = sortField.getComparator(topNGroups, i);
+      reversed[i] = sortField.getReverse() ? -1 : 1;
+    }
+  }
+
+  // TODO: maybe allow no sort on retrieving groups?  app
+  // may want to simply process docs in the group itself?
+  // typically they will be presented as a "single" result
+  // in the UI?
+
+  /** Returns the grouped results.  Returns null if the
+   *  number of groups collected is <= groupOffset.
+   *
+   *  <p><b>NOTE</b>: This collector is unable to compute
+   *  the groupValue per group so it will always be null.
+   *  This is normally not a problem, as you can obtain the
+   *  value just like you obtain other values for each
+   *  matching document (eg, via stored fields, via
+   *  FieldCache, etc.)
+   *
+   *  @param withinGroupSort The {@link Sort} used to sort
+   *    documents within each group.  Passing null is
+   *    allowed, to sort by relevance.
+   *  @param groupOffset Which group to start from
+   *  @param withinGroupOffset Which document to start from
+   *    within each group
+   *  @param maxDocsPerGroup How many top documents to keep
+   *     within each group.
+   *  @param fillSortFields If true then the Comparable
+   *     values for the sort fields will be set
+   */
+  public TopGroups getTopGroups(Sort withinGroupSort, int groupOffset, int withinGroupOffset, int maxDocsPerGroup, boolean fillSortFields) throws IOException {
+
+    //if (queueFull) {
+    //System.out.println("getTopGroups groupOffset=" + groupOffset + " topNGroups=" + topNGroups);
+    //}
+    if (subDocUpto != 0) {
+      processGroup();
+    }
+    if (groupOffset >= groupQueue.size()) {
+      return null;
+    }
+    int totalGroupedHitCount = 0;
+
+    final FakeScorer fakeScorer = new FakeScorer();
+
+    final GroupDocs[] groups = new GroupDocs[groupQueue.size() - groupOffset];
+    for(int downTo=groupQueue.size()-groupOffset-1;downTo>=0;downTo--) {
+      final OneGroup og = groupQueue.pop();
+
+      // At this point we hold all docs w/ in each group,
+      // unsorted; we now sort them:
+      final TopDocsCollector collector;
+      if (withinGroupSort == null) {
+        // Sort by score
+        if (!needsScores) {
+          throw new IllegalArgumentException("cannot sort by relevance within group: needsScores=false");
+        }
+        collector = TopScoreDocCollector.create(maxDocsPerGroup, true);
+      } else {
+        // Sort by fields
+        collector = TopFieldCollector.create(withinGroupSort, maxDocsPerGroup, fillSortFields, needsScores, needsScores, true);
+      }
+
+      collector.setScorer(fakeScorer);
+      collector.setNextReader(og.readerContext);
+      for(int docIDX=0;docIDX<og.count;docIDX++) {
+        final int doc = og.docs[docIDX];
+        fakeScorer.doc = doc;
+        if (needsScores) {
+          fakeScorer.score = og.scores[docIDX];
+        }
+        collector.collect(doc);
+      }
+      totalGroupedHitCount += og.count;
+
+      final Comparable[] groupSortValues;
+
+      if (fillSortFields) {
+        groupSortValues = new Comparable[comparators.length];
+        for(int sortFieldIDX=0;sortFieldIDX<comparators.length;sortFieldIDX++) {
+          groupSortValues[sortFieldIDX] = comparators[sortFieldIDX].value(og.comparatorSlot);
+        }
+      } else {
+        groupSortValues = null;
+      }
+
+      final TopDocs topDocs = collector.topDocs(withinGroupOffset, maxDocsPerGroup);
+
+      groups[downTo] = new GroupDocs(topDocs.getMaxScore(),
+                                     og.count,
+                                     topDocs.scoreDocs,
+                                     null,
+                                     groupSortValues);
+    }
+
+    /*
+    while (groupQueue.size() != 0) {
+      final OneGroup og = groupQueue.pop();
+      //System.out.println("  leftover: og ord=" + og.groupOrd + " count=" + og.count);
+      totalGroupedHitCount += og.count;
+    }
+    */
+
+    return new TopGroups(new TopGroups(groupSort.getSort(),
+                                       withinGroupSort == null ? null : withinGroupSort.getSort(),
+                                       totalHitCount, totalGroupedHitCount, groups),
+                         totalGroupCount);
+  }
+
+  @Override
+  public void setScorer(Scorer scorer) throws IOException {
+    this.scorer = scorer;
+    for (FieldComparator comparator : comparators) {
+      comparator.setScorer(scorer);
+    }
+  }
+
+  @Override
+  public void collect(int doc) throws IOException {
+
+    // System.out.println("C " + doc);
+
+    if (doc > groupEndDocID) {
+      // Group changed
+      if (subDocUpto != 0) {
+        processGroup();
+      }
+      groupEndDocID = lastDocPerGroupBits.advance(doc);
+      //System.out.println("  adv " + groupEndDocID + " " + lastDocPerGroupBits);
+      subDocUpto = 0;
+      groupCompetes = !queueFull;
+    }
+
+    totalHitCount++;
+
+    // Always cache doc/score within this group:
+    if (subDocUpto == pendingSubDocs.length) {
+      pendingSubDocs = ArrayUtil.grow(pendingSubDocs);
+    }
+    pendingSubDocs[subDocUpto] = doc;
+    if (needsScores) {
+      if (subDocUpto == pendingSubScores.length) {
+        pendingSubScores = ArrayUtil.grow(pendingSubScores);
+      }
+      pendingSubScores[subDocUpto] = scorer.score();
+    }
+    subDocUpto++;
+
+    if (groupCompetes) {
+      if (subDocUpto == 1) {
+        assert !queueFull;
+
+        //System.out.println("    init copy to bottomSlot=" + bottomSlot);
+        for (FieldComparator fc : comparators) {
+          fc.copy(bottomSlot, doc);
+          fc.setBottom(bottomSlot);
+        }        
+        topGroupDoc = doc;
+      } else {
+        // Compare to bottomSlot
+        for (int compIDX = 0;; compIDX++) {
+          final int c = reversed[compIDX] * comparators[compIDX].compareBottom(doc);
+          if (c < 0) {
+            // Definitely not competitive -- done
+            return;
+          } else if (c > 0) {
+            // Definitely competitive.
+            break;
+          } else if (compIDX == compIDXEnd) {
+            // Ties with bottom, except we know this docID is
+            // > docID in the queue (docs are visited in
+            // order), so not competitive:
+            return;
+          }
+        }
+
+        //System.out.println("       best w/in group!");
+        
+        for (FieldComparator fc : comparators) {
+          fc.copy(bottomSlot, doc);
+          // Necessary because some comparators cache
+          // details of bottom slot; this forces them to
+          // re-cache:
+          fc.setBottom(bottomSlot);
+        }        
+        topGroupDoc = doc;
+      }
+    } else {
+      // We're not sure this group will make it into the
+      // queue yet
+      for (int compIDX = 0;; compIDX++) {
+        final int c = reversed[compIDX] * comparators[compIDX].compareBottom(doc);
+        if (c < 0) {
+          // Definitely not competitive -- done
+          //System.out.println("    doc doesn't compete w/ top groups");
+          return;
+        } else if (c > 0) {
+          // Definitely competitive.
+          break;
+        } else if (compIDX == compIDXEnd) {
+          // Ties with bottom, except we know this docID is
+          // > docID in the queue (docs are visited in
+          // order), so not competitive:
+          //System.out.println("    doc doesn't compete w/ top groups");
+          return;
+        }
+      }
+      groupCompetes = true;
+      for (FieldComparator fc : comparators) {
+        fc.copy(bottomSlot, doc);
+        // Necessary because some comparators cache
+        // details of bottom slot; this forces them to
+        // re-cache:
+        fc.setBottom(bottomSlot);
+      }
+      topGroupDoc = doc;
+      //System.out.println("        doc competes w/ top groups");
+    }
+  }
+
+  @Override
+  public boolean acceptsDocsOutOfOrder() {
+    return false;
+  }
+
+  @Override
+  public void setNextReader(AtomicReaderContext readerContext) throws IOException {
+    if (subDocUpto != 0) {
+      processGroup();
+    }
+    subDocUpto = 0;
+    docBase = readerContext.docBase;
+    //System.out.println("setNextReader base=" + docBase + " r=" + readerContext.reader);
+    lastDocPerGroupBits = lastDocPerGroup.getDocIdSet(readerContext).iterator();
+    groupEndDocID = -1;
+
+    currentReaderContext = readerContext;
+    for (int i=0; i<comparators.length; i++) {
+      comparators[i] = comparators[i].setNextReader(readerContext);
+    }
+  }
+}

modules/grouping/src/java/org/apache/lucene/search/grouping/package.html

@@ -3,62 +3,92 @@
 
 <p>This module enables search result grouping with Lucene, where hits
 with the same value in the specified single-valued group field are
-grouped together.  For example, if you group by the <tt>author</tt>
+grouped together.  For example, if you group by the <code>author</code>
-field, then all documents with the same value in the <tt>author</tt>
+field, then all documents with the same value in the <code>author</code>
 field fall into a single group.</p>
 
 <p>Grouping requires a number of inputs:</p>
 
   <ul>
-    <li> <tt>groupField</tt>: this is the field used for grouping.
+    <li> <code>groupField</code>: this is the field used for grouping.
-      For example, if you use the <tt>author</tt> field then each
+      For example, if you use the <code>author</code> field then each
       group has all books by the same author.  Documents that don't
       have this field are grouped under a single group with
-      a <tt>null</tt> group value.
+      a <code>null</code> group value.
 
-    <li> <tt>groupSort</tt>: how the groups are sorted.  For sorting
+    <li> <code>groupSort</code>: how the groups are sorted.  For sorting
       purposes, each group is "represented" by the highest-sorted
-      document according to the <tt>groupSort</tt> within it.  For
+      document according to the <code>groupSort</code> within it.  For
       example, if you specify "price" (ascending) then the first group
       is the one with the lowest price book within it.  Or if you
       specify relevance group sort, then the first group is the one
       containing the highest scoring book.
 
-    <li> <tt>topNGroups</tt>: how many top groups to keep.  For
+    <li> <code>topNGroups</code>: how many top groups to keep.  For
       example, 10 means the top 10 groups are computed.
 
-    <li> <tt>groupOffset</tt>: which "slice" of top groups you want to
+    <li> <code>groupOffset</code>: which "slice" of top groups you want to
       retrieve.  For example, 3 means you'll get 7 groups back
-      (assuming <tt>topNGroups</tt> is 10).  This is useful for
+      (assuming <code>topNGroups</code> is 10).  This is useful for
       paging, where you might show 5 groups per page.
 
-    <li> <tt>withinGroupSort</tt>: how the documents within each group
+    <li> <code>withinGroupSort</code>: how the documents within each group
       are sorted.  This can be different from the group sort.
 
-    <li> <tt>maxDocsPerGroup</tt>: how many top documents within each
+    <li> <code>maxDocsPerGroup</code>: how many top documents within each
       group to keep.
 
-    <li> <tt>withinGroupOffset</tt>: which "slice" of top
+    <li> <code>withinGroupOffset</code>: which "slice" of top
       documents you want to retrieve from each group.
 
   </ul>
 
-<p>The implementation is two-pass: the first pass ({@link
+<p>
-  org.apache.lucene.search.grouping.FirstPassGroupingCollector})
+There are two grouping implementations here:
-  gathers the top groups, and the second pass ({@link
+<ul>
-  org.apache.lucene.search.grouping.SecondPassGroupingCollector})
+  <li>
-  gathers documents within those groups.  If the search is costly to
+    Arbitrary grouping that can group by any single-valued indexed
-  run you may want to use the {@link
+    field, implemented as a two-pass collector: the first pass ({@link
-  org.apache.lucene.search.CachingCollector} class, which
+    org.apache.lucene.search.grouping.FirstPassGroupingCollector})
-  caches hits and can (quickly) replay them for the second pass.  This
+    gathers the top groups, and the second pass ({@link
-  way you only run the query once, but you pay a RAM cost to (briefly)
+    org.apache.lucene.search.grouping.SecondPassGroupingCollector})
-  hold all hits.  Results are returned as a {@link
+    gathers documents within those groups.  If the search is costly to
-  org.apache.lucene.search.grouping.TopGroups} instance.</p>
+    run you may want to use the {@link
+    org.apache.lucene.search.CachingCollector} class, which caches
+    hits and can (quickly) replay them for the second pass.  This way
+    you only run the query once, but you pay a RAM cost to (briefly)
+    hold all hits.  Results are returned as a {@link
+    org.apache.lucene.search.grouping.TopGroups} instance.</p>
+  </li>
+  <li>
+    Indexed groups, using a single pass collector (<code>BlockGroupingCollectorDoc</code>) that
+    is able to group according to the doc blocks created during
+    indexing using <code>IndexWriter</code>'s <code>add/updateDocuments</code> API.
+    This is faster (~25% faster QPS) than the generic two-pass
+    collector, but it only works for doc blocks so you must statically
+    commit (during indexing) to which grouping you'll need at search
+    time.
+
+    <p>This implementation does not rely on a single valued grouping
+    field; rather, the blocks in the index define the groups, so your
+    application is free to determine what the grouping criteria is.
+    At search time, you must provide a <code>Filter</code> that marks
+    the last document in each group.  This is a substantial memory
+    savings because this collector does not load
+    a <code>DocTermsIndex</code> from the
+    <code>FieldCache</code>.
+  </li>
+</ul>
+
+<p>The benefit of the arbitrary grouping implementation is you don't have
+to commit at indexing time to a static grouping of your documents.
+But the downside is it's somewhat slower to run, and requires more RAM
+(a <code>FieldCache.DocTermsIndex</code> entry is created).
 
 <p>Known limitations:</p>
 <ul>
-  <li> The group field must be a single-valued indexed field.
+  <li> For the two-pass grouping collector, the group field must be a
-    {@link org.apache.lucene.search.FieldCache} is used to load the {@link org.apache.lucene.search.FieldCache.DocTermsIndex} for this field.
+    single-valued indexed field.
   <li> Unlike Solr's implementation, this module cannot group by
     function query values nor by arbitrary queries.
   <li> Sharding is not directly supported, though is not too
@@ -66,7 +96,8 @@ field fall into a single group.</p>
     group yourself.
 </ul>
 
-<p>Typical usage looks like this (using the {@link org.apache.lucene.search.CachingCollector}):</p>
+<p>Typical usage for the generic two-pass collector looks like this
+  (using the {@link org.apache.lucene.search.CachingCollector}):</p>
 
 <pre class="prettyprint">
   FirstPassGroupingCollector c1 = new FirstPassGroupingCollector("author", groupSort, groupOffset+topNGroups);
@@ -111,5 +142,50 @@ field fall into a single group.</p>
   // Render groupsResult...
 </pre>
 
+<p>To use the single-pass <code>BlockGroupingCollector</code>,
+   first, at indexing time, you must ensure all docs in each group
+   are added as a block, and you have some way to find the last
+   document of each group.  One simple way to do this is to add a
+   marker binary field:</p>
+
+<pre class="prettyprint">
+  // Create Documents from your source:
+  List&lt;Document&gt; oneGroup = ...;
+  
+  Field groupEndField = new Field("groupEnd", "x", Field.Store.NO, Field.Index.NOT_ANALYZED);
+  groupEndField.setOmitTermFreqAndPositions(true);
+  groupEndField.setOmitNorms(true);
+  oneGroup.get(oneGroup.size()-1).add(groupEndField);
+
+  // You can also use writer.updateDocuments(); just be sure you
+  // replace an entire previous doc block with this new one.  For
+  // example, each group could have a "groupID" field, with the same
+  // value for all docs in this group:
+  writer.addDocuments(oneGroup);
+</pre>
+
+Then, at search time, do this up front:
+
+<pre class="prettyprint">
+  // Set this once in your app & save away for reusing across all queries:
+  Filter groupEndDocs = new CachingWrapperFilter(new QueryWrapperFilter(new TermQuery(new Term("end", "x"))));
+</pre>
+
+Finally, do this per search:
+
+<pre class="prettyprint">
+  // Per search:
+  BlockGroupingCollector c = new BlockGroupingCollector(groupSort, groupOffset+topNGroups, needsScores, groupEndDocs);
+  s.search(new TermQuery(new Term("content", searchTerm)), c);
+  TopGroups groupsResult = c.getTopGroups(withinGroupSort, groupOffset, docOffset, docOffset+docsPerGroup, fillFields);
+
+  // Render groupsResult...
+</pre>
+
+Note that the <code>groupValue</code> of each <code>GroupDocs</code>
+will be <code>null</code>, so if you need to present this value you'll
+have to separately retrieve it (for example using stored
+fields, <code>FieldCache</code>, etc.).
+
 </body>
 </html>

modules/grouping/src/test/org/apache/lucene/search/grouping/TestGrouping.java

@@ -18,6 +18,7 @@
 package org.apache.lucene.search.grouping;
 
 import java.util.*;
+import java.io.IOException;
 
 import org.apache.lucene.analysis.MockAnalyzer;
 import org.apache.lucene.document.Document;
@@ -183,6 +184,7 @@ public class TestGrouping extends LuceneTestCase {
   private Comparator<GroupDoc> getComparator(Sort sort) {
     final SortField[] sortFields = sort.getSort();
     return new Comparator<GroupDoc>() {
+      // @Override -- Not until Java 1.6
       public int compare(GroupDoc d1, GroupDoc d2) {
         for(SortField sf : sortFields) {
           final int cmp;
@@ -224,6 +226,16 @@ public class TestGrouping extends LuceneTestCase {
     return fields;
   }
 
+  /*
+  private String groupToString(BytesRef b) {
+    if (b == null) {
+      return "null";
+    } else {
+      return b.utf8ToString();
+    }
+  }
+  */
+
   private TopGroups slowGrouping(GroupDoc[] groupDocs,
                                  String searchTerm,
                                  boolean fillFields,
@@ -247,21 +259,25 @@ public class TestGrouping extends LuceneTestCase {
     int totalHitCount = 0;
     Set<BytesRef> knownGroups = new HashSet<BytesRef>();
 
+    //System.out.println("TEST: slowGrouping");
     for(GroupDoc d : groupDocs) {
       // TODO: would be better to filter by searchTerm before sorting!
       if (!d.content.equals(searchTerm)) {
         continue;
       }
       totalHitCount++;
+      //System.out.println("  match id=" + d.id);
 
       if (doAllGroups) {
         if (!knownGroups.contains(d.group)) {
           knownGroups.add(d.group);
+          //System.out.println("    add group=" + groupToString(d.group));
         }
       }
 
       List<GroupDoc> l = groups.get(d.group);
       if (l == null) {
+        //System.out.println("    add sortedGroup=" + groupToString(d.group));
         sortedGroups.add(d.group);
         if (fillFields) {
           sortedGroupFields.add(fillFields(d, groupSort));
@@ -322,6 +338,67 @@ public class TestGrouping extends LuceneTestCase {
     }
   }
 
+  private IndexReader getDocBlockReader(Directory dir, GroupDoc[] groupDocs) throws IOException {
+    // Coalesce by group, but in random order:
+    Collections.shuffle(Arrays.asList(groupDocs), random);
+    final Map<BytesRef,List<GroupDoc>> groupMap = new HashMap<BytesRef,List<GroupDoc>>();
+    final List<BytesRef> groupValues = new ArrayList<BytesRef>();
+    
+    for(GroupDoc groupDoc : groupDocs) {
+      if (!groupMap.containsKey(groupDoc.group)) {
+        groupValues.add(groupDoc.group);
+        groupMap.put(groupDoc.group, new ArrayList<GroupDoc>());
+      }
+      groupMap.get(groupDoc.group).add(groupDoc);
+    }
+
+    RandomIndexWriter w = new RandomIndexWriter(
+                                                random,
+                                                dir,
+                                                newIndexWriterConfig(TEST_VERSION_CURRENT,
+                                                                     new MockAnalyzer(random)));
+
+    final List<List<Document>> updateDocs = new ArrayList<List<Document>>();
+    //System.out.println("TEST: index groups");
+    for(BytesRef group : groupValues) {
+      final List<Document> docs = new ArrayList<Document>();
+      //System.out.println("TEST:   group=" + (group == null ? "null" : group.utf8ToString()));
+      for(GroupDoc groupValue : groupMap.get(group)) {
+        Document doc = new Document();
+        docs.add(doc);
+        if (groupValue.group != null) {
+          doc.add(newField("group", groupValue.group.utf8ToString(), Field.Index.NOT_ANALYZED));
+        }
+        doc.add(newField("sort1", groupValue.sort1.utf8ToString(), Field.Index.NOT_ANALYZED));
+        doc.add(newField("sort2", groupValue.sort2.utf8ToString(), Field.Index.NOT_ANALYZED));
+        doc.add(new NumericField("id").setIntValue(groupValue.id));
+        doc.add(newField("content", groupValue.content, Field.Index.NOT_ANALYZED));
+        //System.out.println("TEST:     doc content=" + groupValue.content + " group=" + (groupValue.group == null ? "null" : groupValue.group.utf8ToString()) + " sort1=" + groupValue.sort1.utf8ToString() + " id=" + groupValue.id);
+      }
+      // So we can pull filter marking last doc in block:
+      final Field groupEnd = newField("groupend", "x", Field.Index.NOT_ANALYZED);
+      groupEnd.setOmitTermFreqAndPositions(true);
+      groupEnd.setOmitNorms(true);
+      docs.get(docs.size()-1).add(groupEnd);
+      // Add as a doc block:
+      w.addDocuments(docs);
+      if (group != null && random.nextInt(7) == 4) {
+        updateDocs.add(docs);
+      }
+    }
+
+    for(List<Document> docs : updateDocs) {
+      // Just replaces docs w/ same docs:
+      w.updateDocuments(new Term("group", docs.get(0).get("group")),
+                        docs);
+    }
+
+    final IndexReader r = w.getReader();
+    w.close();
+
+    return r;
+  }
+
   public void testRandom() throws Exception {
     for(int iter=0;iter<3;iter++) {
 
@@ -350,7 +427,7 @@ public class TestGrouping extends LuceneTestCase {
                                                   random,
                                                   dir,
                                                   newIndexWriterConfig(TEST_VERSION_CURRENT,
-                                                                       new MockAnalyzer(random)).setMergePolicy(newLogMergePolicy()));
+                                                                       new MockAnalyzer(random)));
 
       Document doc = new Document();
       Document docNoGroup = new Document();
@@ -405,152 +482,203 @@ public class TestGrouping extends LuceneTestCase {
       final IndexReader r = w.getReader();
       w.close();
 
+      // Build 2nd index, where docs are added in blocks by
+      // group, so we can use single pass collector
+      final Directory dir2 = newDirectory();
+      final IndexReader r2 = getDocBlockReader(dir2, groupDocs);
+      final Filter lastDocInBlock = new CachingWrapperFilter(new QueryWrapperFilter(new TermQuery(new Term("groupend", "x"))));
+
       final IndexSearcher s = new IndexSearcher(r);
+      final IndexSearcher s2 = new IndexSearcher(r2);
 
-      for(int searchIter=0;searchIter<100;searchIter++) {
+      final int[] docIDToID = FieldCache.DEFAULT.getInts(r, "id");
+      final int[] docIDToID2 = FieldCache.DEFAULT.getInts(r2, "id");
 
-        if (VERBOSE) {
+      try {
-          System.out.println("TEST: searchIter=" + searchIter);
+        for(int searchIter=0;searchIter<100;searchIter++) {
-        }
 
-        final String searchTerm = contentStrings[random.nextInt(contentStrings.length)];
-        final boolean fillFields = random.nextBoolean();
-        final boolean getScores = random.nextBoolean();
-        final boolean getMaxScores = random.nextBoolean();
-        final Sort groupSort = getRandomSort();
-        // TODO: also test null (= sort by relevance)
-        final Sort docSort = getRandomSort();
-
-        final int topNGroups = _TestUtil.nextInt(random, 1, 30);
-        final int docsPerGroup = _TestUtil.nextInt(random, 1, 50);
-        final int groupOffset = _TestUtil.nextInt(random, 0, (topNGroups-1)/2);
-        //final int groupOffset = 0;
-
-        final int docOffset = _TestUtil.nextInt(random, 0, docsPerGroup-1);
-        //final int docOffset = 0;
-
-        final boolean doCache = random.nextBoolean();
-        final boolean doAllGroups = random.nextBoolean();
-        if (VERBOSE) {
-          System.out.println("TEST: groupSort=" + groupSort + " docSort=" + docSort + " searchTerm=" + searchTerm + " topNGroups=" + topNGroups + " groupOffset=" + groupOffset + " docOffset=" + docOffset + " doCache=" + doCache + " docsPerGroup=" + docsPerGroup + " doAllGroups=" + doAllGroups);
-        }
-
-        final AllGroupsCollector allGroupsCollector;
-        if (doAllGroups) {
-          allGroupsCollector = new AllGroupsCollector("group");
-        } else {
-          allGroupsCollector = null;
-        }
-
-        final FirstPassGroupingCollector c1 = new FirstPassGroupingCollector("group", groupSort, groupOffset+topNGroups);
-        final CachingCollector cCache;
-        final Collector c;
-        
-        final boolean useWrappingCollector = random.nextBoolean();
-        
-        if (doCache) {
-          final double maxCacheMB = random.nextDouble();
           if (VERBOSE) {
-            System.out.println("TEST: maxCacheMB=" + maxCacheMB);
+            System.out.println("TEST: searchIter=" + searchIter);
           }
 
-          if (useWrappingCollector) {
+          final String searchTerm = contentStrings[random.nextInt(contentStrings.length)];
-            if (doAllGroups) {
+          final boolean fillFields = random.nextBoolean();
-              cCache = CachingCollector.create(c1, true, maxCacheMB);              
+          final boolean getScores = random.nextBoolean();
-              c = MultiCollector.wrap(cCache, allGroupsCollector);
+          final boolean getMaxScores = random.nextBoolean();
-            } else {
+          final Sort groupSort = getRandomSort();
-              c = cCache = CachingCollector.create(c1, true, maxCacheMB);              
+          //final Sort groupSort = new Sort(new SortField[] {new SortField("sort1", SortField.STRING), new SortField("id", SortField.INT)});
-            }
+          // TODO: also test null (= sort by relevance)
-          } else {
+          final Sort docSort = getRandomSort();
-            // Collect only into cache, then replay multiple times:
+
-            c = cCache = CachingCollector.create(false, true, maxCacheMB);
+          final int topNGroups = _TestUtil.nextInt(random, 1, 30);
+          //final int topNGroups = 4;
+          final int docsPerGroup = _TestUtil.nextInt(random, 1, 50);
+          final int groupOffset = _TestUtil.nextInt(random, 0, (topNGroups-1)/2);
+          //final int groupOffset = 0;
+
+          final int docOffset = _TestUtil.nextInt(random, 0, docsPerGroup-1);
+          //final int docOffset = 0;
+
+          final boolean doCache = random.nextBoolean();
+          final boolean doAllGroups = random.nextBoolean();
+          if (VERBOSE) {
+            System.out.println("TEST: groupSort=" + groupSort + " docSort=" + docSort + " searchTerm=" + searchTerm + " topNGroups=" + topNGroups + " groupOffset=" + groupOffset + " docOffset=" + docOffset + " doCache=" + doCache + " docsPerGroup=" + docsPerGroup + " doAllGroups=" + doAllGroups);
           }
-        } else {
+
-          cCache = null;
+          final AllGroupsCollector allGroupsCollector;
           if (doAllGroups) {
-            c = MultiCollector.wrap(c1, allGroupsCollector);
+            allGroupsCollector = new AllGroupsCollector("group");
           } else {
-            c = c1;
+            allGroupsCollector = null;
           }
-        }
+
+          final FirstPassGroupingCollector c1 = new FirstPassGroupingCollector("group", groupSort, groupOffset+topNGroups);
+          final CachingCollector cCache;
+          final Collector c;
         
-        s.search(new TermQuery(new Term("content", searchTerm)), c);
+          final boolean useWrappingCollector = random.nextBoolean();
+        
+          if (doCache) {
+            final double maxCacheMB = random.nextDouble();
+            if (VERBOSE) {
+              System.out.println("TEST: maxCacheMB=" + maxCacheMB);
+            }
 
-        if (doCache && !useWrappingCollector) {
+            if (useWrappingCollector) {
-          if (cCache.isCached()) {
+              if (doAllGroups) {
-            // Replay for first-pass grouping
+                cCache = CachingCollector.create(c1, true, maxCacheMB);              
-            cCache.replay(c1);
+                c = MultiCollector.wrap(cCache, allGroupsCollector);
-            if (doAllGroups) {
+              } else {
-              // Replay for all groups:
+                c = cCache = CachingCollector.create(c1, true, maxCacheMB);              
-              cCache.replay(allGroupsCollector);
+              }
+            } else {
+              // Collect only into cache, then replay multiple times:
+              c = cCache = CachingCollector.create(false, true, maxCacheMB);
             }
           } else {
-            // Replay by re-running search:
+            cCache = null;
-            s.search(new TermQuery(new Term("content", searchTerm)), c1);
             if (doAllGroups) {
-              s.search(new TermQuery(new Term("content", searchTerm)), allGroupsCollector);
+              c = MultiCollector.wrap(c1, allGroupsCollector);
-            }
-          }
-        }
-
-        final Collection<SearchGroup> topGroups = c1.getTopGroups(groupOffset, fillFields);
-        final TopGroups groupsResult;
-
-        if (topGroups != null) {
-
-          if (VERBOSE) {
-            System.out.println("TEST: topGroups");
-            for (SearchGroup searchGroup : topGroups) {
-              System.out.println("  " + (searchGroup.groupValue == null ? "null" : searchGroup.groupValue.utf8ToString()) + ": " + Arrays.deepToString(searchGroup.sortValues));
-            }
-          }
-
-          final SecondPassGroupingCollector c2 = new SecondPassGroupingCollector("group", topGroups, groupSort, docSort, docOffset+docsPerGroup, getScores, getMaxScores, fillFields);
-          if (doCache) {
-            if (cCache.isCached()) {
-              if (VERBOSE) {
-                System.out.println("TEST: cache is intact");
-              }
-              cCache.replay(c2);
             } else {
-              if (VERBOSE) {
+              c = c1;
-                System.out.println("TEST: cache was too large");
+            }
+          }
+        
+          s.search(new TermQuery(new Term("content", searchTerm)), c);
+
+          if (doCache && !useWrappingCollector) {
+            if (cCache.isCached()) {
+              // Replay for first-pass grouping
+              cCache.replay(c1);
+              if (doAllGroups) {
+                // Replay for all groups:
+                cCache.replay(allGroupsCollector);
               }
+            } else {
+              // Replay by re-running search:
+              s.search(new TermQuery(new Term("content", searchTerm)), c1);
+              if (doAllGroups) {
+                s.search(new TermQuery(new Term("content", searchTerm)), allGroupsCollector);
+              }
+            }
+          }
+
+          final Collection<SearchGroup> topGroups = c1.getTopGroups(groupOffset, fillFields);
+          final TopGroups groupsResult;
+
+          if (topGroups != null) {
+
+            if (VERBOSE) {
+              System.out.println("TEST: topGroups");
+              for (SearchGroup searchGroup : topGroups) {
+                System.out.println("  " + (searchGroup.groupValue == null ? "null" : searchGroup.groupValue.utf8ToString()) + ": " + Arrays.deepToString(searchGroup.sortValues));
+              }
+            }
+
+            final SecondPassGroupingCollector c2 = new SecondPassGroupingCollector("group", topGroups, groupSort, docSort, docOffset+docsPerGroup, getScores, getMaxScores, fillFields);
+            if (doCache) {
+              if (cCache.isCached()) {
+                if (VERBOSE) {
+                  System.out.println("TEST: cache is intact");
+                }
+                cCache.replay(c2);
+              } else {
+                if (VERBOSE) {
+                  System.out.println("TEST: cache was too large");
+                }
+                s.search(new TermQuery(new Term("content", searchTerm)), c2);
+              }
+            } else {
               s.search(new TermQuery(new Term("content", searchTerm)), c2);
             }
+
+            if (doAllGroups) {
+              TopGroups tempTopGroups = c2.getTopGroups(docOffset);
+              groupsResult = new TopGroups(tempTopGroups, allGroupsCollector.getGroupCount());
+            } else {
+              groupsResult = c2.getTopGroups(docOffset);
+            }
           } else {
-            s.search(new TermQuery(new Term("content", searchTerm)), c2);
+            groupsResult = null;
+            if (VERBOSE) {
+              System.out.println("TEST:   no results");
+            }
           }
 
-          if (doAllGroups) {
+          final TopGroups expectedGroups = slowGrouping(groupDocs, searchTerm, fillFields, getScores, getMaxScores, doAllGroups, groupSort, docSort, topNGroups, docsPerGroup, groupOffset, docOffset);
-            TopGroups tempTopGroups = c2.getTopGroups(docOffset);
+
-            groupsResult = new TopGroups(tempTopGroups, allGroupsCollector.getGroupCount());
-          } else {
-            groupsResult = c2.getTopGroups(docOffset);
-          }
-        } else {
-          groupsResult = null;
           if (VERBOSE) {
-            System.out.println("TEST:   no results");
+            if (expectedGroups == null) {
+              System.out.println("TEST: no expected groups");
+            } else {
+              System.out.println("TEST: expected groups");
+              for(GroupDocs gd : expectedGroups.groups) {
+                System.out.println("  group=" + (gd.groupValue == null ? "null" : gd.groupValue.utf8ToString()));
+                for(ScoreDoc sd : gd.scoreDocs) {
+                  System.out.println("    id=" + sd.doc);
+                }
+              }
+            }
           }
-        }
-
-        final TopGroups expectedGroups = slowGrouping(groupDocs, searchTerm, fillFields, getScores, getMaxScores, doAllGroups, groupSort, docSort, topNGroups, docsPerGroup, groupOffset, docOffset);
-
-        try {
           // NOTE: intentional but temporary field cache insanity!
-          assertEquals(FieldCache.DEFAULT.getInts(r, "id"), expectedGroups, groupsResult);
+          assertEquals(docIDToID, expectedGroups, groupsResult, true);
-        } finally {
+
-          FieldCache.DEFAULT.purge(r);
+          final boolean needsScores = getScores || getMaxScores || docSort == null;
+          final BlockGroupingCollector c3 = new BlockGroupingCollector(groupSort, groupOffset+topNGroups, needsScores, lastDocInBlock);
+          final AllGroupsCollector allGroupsCollector2;
+          final Collector c4;
+          if (doAllGroups) {
+            allGroupsCollector2 = new AllGroupsCollector("group");
+            c4 = MultiCollector.wrap(c3, allGroupsCollector2);
+          } else {
+            allGroupsCollector2 = null;
+            c4 = c3;
+          }
+          s2.search(new TermQuery(new Term("content", searchTerm)), c4);
+          final TopGroups tempTopGroups2 = c3.getTopGroups(docSort, groupOffset, docOffset, docOffset+docsPerGroup, fillFields);
+          final TopGroups groupsResult2;
+          if (doAllGroups && tempTopGroups2 != null) {
+            assertEquals((int) tempTopGroups2.totalGroupCount, allGroupsCollector2.getGroupCount());
+            groupsResult2 = new TopGroups(tempTopGroups2, allGroupsCollector2.getGroupCount());
+          } else {
+            groupsResult2 = tempTopGroups2;
+          }
+          assertEquals(docIDToID2, expectedGroups, groupsResult2, false);
         }
+      } finally {
+        FieldCache.DEFAULT.purge(r);
+        FieldCache.DEFAULT.purge(r2);
       }
 
       r.close();
       dir.close();
+
+      r2.close();
+      dir2.close();
     }
   }
 
-  private void assertEquals(int[] docIDtoID, TopGroups expected, TopGroups actual) {
+  private void assertEquals(int[] docIDtoID, TopGroups expected, TopGroups actual, boolean verifyGroupValues) {
     if (expected == null) {
       assertNull(actual);
       return;
@@ -570,7 +698,9 @@ public class TestGrouping extends LuceneTestCase {
       }
       final GroupDocs expectedGroup = expected.groups[groupIDX];
       final GroupDocs actualGroup = actual.groups[groupIDX];
-      assertEquals(expectedGroup.groupValue, actualGroup.groupValue);
+      if (verifyGroupValues) {
+        assertEquals(expectedGroup.groupValue, actualGroup.groupValue);
+      }
       assertArrayEquals(expectedGroup.groupSortValues, actualGroup.groupSortValues);
 
       // TODO