LUCENE-6919: Make Scorer expose an iterator instead of extending DocIdSetIterator.

git-svn-id: https://svn.apache.org/repos/asf/lucene/dev/trunk@1719081 13f79535-47bb-0310-9956-ffa450edef68

lucene/CHANGES.txt

@@ -123,6 +123,10 @@ API Changes
 * LUCENE-6900: Grouping sortWithinGroup variables used to allow null to mean
   Sort.RELEVANCE.  Null is no longer permitted.  (David Smiley)
 
+* LUCENE-6919: The Scorer class has been refactored to expose an iterator
+  instead of extending DocIdSetIterator. asTwoPhaseIterator() has been renamed
+  to twoPhaseIterator() for consistency. (Adrien Grand)
+
 Optimizations
 
 * LUCENE-6889: BooleanQuery.rewrite now performs some query optimization, in

lucene/core/src/java/org/apache/lucene/index/BufferedUpdatesStream.java

@@ -30,6 +30,7 @@ import java.util.concurrent.atomic.AtomicLong;
 import org.apache.lucene.search.DocIdSetIterator;
 import org.apache.lucene.search.IndexSearcher;
 import org.apache.lucene.search.Query;
+import org.apache.lucene.search.Scorer;
 import org.apache.lucene.search.Weight;
 import org.apache.lucene.store.IOContext;
 import org.apache.lucene.util.Accountable;
@@ -690,8 +691,9 @@ class BufferedUpdatesStream implements Accountable {
       final IndexSearcher searcher = new IndexSearcher(readerContext.reader());
       searcher.setQueryCache(null);
       final Weight weight = searcher.createNormalizedWeight(query, false);
-      final DocIdSetIterator it = weight.scorer(readerContext);
+      final Scorer scorer = weight.scorer(readerContext);
-      if (it != null) {
+      if (scorer != null) {
+        final DocIdSetIterator it = scorer.iterator();
         final Bits liveDocs = readerContext.reader().getLiveDocs();
         while (true)  {
           int doc = it.nextDoc();

lucene/core/src/java/org/apache/lucene/search/BooleanTopLevelScorers.java

@@ -126,19 +126,22 @@ class BooleanTopLevelScorers {
 
     @Override
     public float score() throws IOException {
+      // TODO: sum into a double and cast to float if we ever send required clauses to BS1
       int curDoc = reqScorer.docID();
-      float reqScore = reqScorer.score();
+      float score = reqScorer.score();
-      if (optScorer == null) {
+
-        return reqScore * coordReq;
+      int optScorerDoc = optIterator.docID();
+      if (optScorerDoc < curDoc) {
+        optScorerDoc = optIterator.advance(curDoc);
       }
       
-      int optScorerDoc = optScorer.docID();
+      if (optScorerDoc == curDoc) {
-      if (optScorerDoc < curDoc && (optScorerDoc = optScorer.advance(curDoc)) == NO_MORE_DOCS) {
+        score = (score + optScorer.score()) * coordBoth;
-        optScorer = null;
+      } else {
-        return reqScore * coordReq;
+        score = score * coordReq;
       }
       
-      return optScorerDoc == curDoc ? (reqScore + optScorer.score()) * coordBoth : reqScore * coordReq;
+      return score;
     }
   }
 
@@ -158,19 +161,22 @@ class BooleanTopLevelScorers {
 
     @Override
     public float score() throws IOException {
+      // TODO: sum into a double and cast to float if we ever send required clauses to BS1
       int curDoc = reqScorer.docID();
-      float reqScore = reqScorer.score();
+      float score = reqScorer.score();
-      if (optScorer == null) {
+
-        return reqScore * coords[requiredCount];
+      int optScorerDoc = optIterator.docID();
+      if (optScorerDoc < curDoc) {
+        optScorerDoc = optIterator.advance(curDoc);
       }
       
-      int optScorerDoc = optScorer.docID();
+      if (optScorerDoc == curDoc) {
-      if (optScorerDoc < curDoc && (optScorerDoc = optScorer.advance(curDoc)) == NO_MORE_DOCS) {
+        score = (score + optScorer.score()) * coords[requiredCount + optScorer.freq()];
-        optScorer = null;
+      } else {
-        return reqScore * coords[requiredCount];
+        score = score * coords[requiredCount];
       }
       
-      return optScorerDoc == curDoc ? (reqScore + optScorer.score()) * coords[requiredCount + optScorer.freq()] : reqScore * coords[requiredCount];
+      return score;
     }
   }
 }

lucene/core/src/java/org/apache/lucene/search/BulkScorer.java

@@ -84,7 +84,7 @@ public abstract class BulkScorer {
   public abstract int score(LeafCollector collector, Bits acceptDocs, int min, int max) throws IOException;
 
   /**
-   * Same as {@link Scorer#cost()} for bulk scorers.
+   * Same as {@link DocIdSetIterator#cost()} for bulk scorers.
    */
   public abstract long cost();
 }

lucene/core/src/java/org/apache/lucene/search/CachingCollector.java

@@ -61,24 +61,22 @@ public abstract class CachingCollector extends FilterCollector {
 
     private CachedScorer() { super(null); }
 
+    @Override
+    public DocIdSetIterator iterator() {
+      throw new UnsupportedOperationException();
+    }
+
     @Override
     public final float score() { return score; }
 
     @Override
-    public final int advance(int target) { throw new UnsupportedOperationException(); }
+    public int docID() {
-
+      return doc;
-    @Override
+    }
-    public final int docID() { return doc; }
 
     @Override
     public final int freq() { throw new UnsupportedOperationException(); }
 
-    @Override
-    public final int nextDoc() { throw new UnsupportedOperationException(); }
-
-    @Override
-    public long cost() { return 1; }
-
   }
 
   private static class NoScoreCachingCollector extends CachingCollector {

lucene/core/src/java/org/apache/lucene/search/ConjunctionDISI.java

@@ -23,6 +23,7 @@ import java.util.Collections;
 import java.util.Comparator;
 import java.util.List;
 
+import org.apache.lucene.search.spans.Spans;
 import org.apache.lucene.util.CollectionUtil;
 
 /** A conjunction of DocIdSetIterators.
@@ -32,16 +33,16 @@ import org.apache.lucene.util.CollectionUtil;
  */
 public class ConjunctionDISI extends DocIdSetIterator {
 
-  /** Create a conjunction over the provided iterators, taking advantage of
+  /** Create a conjunction over the provided {@link Scorer}s, taking advantage
-   *  {@link TwoPhaseIterator}. */
+   *  of {@link TwoPhaseIterator}. */
-  public static ConjunctionDISI intersect(List<? extends DocIdSetIterator> iterators) {
+  public static ConjunctionDISI intersectScorers(List<Scorer> scorers) {
-    if (iterators.size() < 2) {
+    if (scorers.size() < 2) {
       throw new IllegalArgumentException("Cannot make a ConjunctionDISI of less than 2 iterators");
     }
     final List<DocIdSetIterator> allIterators = new ArrayList<>();
     final List<TwoPhaseIterator> twoPhaseIterators = new ArrayList<>();
-    for (DocIdSetIterator iter : iterators) {
+    for (Scorer scorer : scorers) {
-      addIterator(iter, allIterators, twoPhaseIterators);
+      addScorer(scorer, allIterators, twoPhaseIterators);
     }
 
     if (twoPhaseIterators.isEmpty()) {
@@ -51,12 +52,66 @@ public class ConjunctionDISI extends DocIdSetIterator {
     }
   }
 
-  /** Adds the iterator, possibly splitting up into two phases or collapsing if it is another conjunction */
+  /** Create a conjunction over the provided DocIdSetIterators. */
+  public static ConjunctionDISI intersectIterators(List<DocIdSetIterator> iterators) {
+    if (iterators.size() < 2) {
+      throw new IllegalArgumentException("Cannot make a ConjunctionDISI of less than 2 iterators");
+    }
+    final List<DocIdSetIterator> allIterators = new ArrayList<>();
+    final List<TwoPhaseIterator> twoPhaseIterators = new ArrayList<>();
+    for (DocIdSetIterator iterator : iterators) {
+      addIterator(iterator, allIterators, twoPhaseIterators);
+    }
+
+    if (twoPhaseIterators.isEmpty()) {
+      return new ConjunctionDISI(allIterators);
+    } else {
+      return new TwoPhase(allIterators, twoPhaseIterators);
+    }
+  }
+
+  /** Create a conjunction over the provided {@link Scorer}s, taking advantage
+   *  of {@link TwoPhaseIterator}. */
+  public static ConjunctionDISI intersectSpans(List<Spans> spanList) {
+    if (spanList.size() < 2) {
+      throw new IllegalArgumentException("Cannot make a ConjunctionDISI of less than 2 iterators");
+    }
+    final List<DocIdSetIterator> allIterators = new ArrayList<>();
+    final List<TwoPhaseIterator> twoPhaseIterators = new ArrayList<>();
+    for (Spans spans : spanList) {
+      addSpans(spans, allIterators, twoPhaseIterators);
+    }
+
+    if (twoPhaseIterators.isEmpty()) {
+      return new ConjunctionDISI(allIterators);
+    } else {
+      return new TwoPhase(allIterators, twoPhaseIterators);
+    }
+  }
+
+  /** Adds the scorer, possibly splitting up into two phases or collapsing if it is another conjunction */
+  private static void addScorer(Scorer scorer, List<DocIdSetIterator> allIterators, List<TwoPhaseIterator> twoPhaseIterators) {
+    TwoPhaseIterator twoPhaseIter = scorer.twoPhaseIterator();
+    if (twoPhaseIter != null) {
+      addTwoPhaseIterator(twoPhaseIter, allIterators, twoPhaseIterators);
+    } else { // no approximation support, use the iterator as-is
+      addIterator(scorer.iterator(), allIterators, twoPhaseIterators);
+    }
+  }
+
+  /** Adds the Spans. */
+  private static void addSpans(Spans spans, List<DocIdSetIterator> allIterators, List<TwoPhaseIterator> twoPhaseIterators) {
+    TwoPhaseIterator twoPhaseIter = spans.asTwoPhaseIterator();
+    if (twoPhaseIter != null) {
+      addTwoPhaseIterator(twoPhaseIter, allIterators, twoPhaseIterators);
+    } else { // no approximation support, use the iterator as-is
+      addIterator(spans, allIterators, twoPhaseIterators);
+    }
+  }
+
   private static void addIterator(DocIdSetIterator disi, List<DocIdSetIterator> allIterators, List<TwoPhaseIterator> twoPhaseIterators) {
     // Check for exactly this class for collapsing. Subclasses can do their own optimizations.
-    if (disi.getClass() == ConjunctionScorer.class) {
+    if (disi.getClass() == ConjunctionDISI.class || disi.getClass() == TwoPhase.class) {
-      addIterator(((ConjunctionScorer) disi).disi, allIterators, twoPhaseIterators);
-    } else if (disi.getClass() == ConjunctionDISI.class || disi.getClass() == TwoPhase.class) {
       ConjunctionDISI conjunction = (ConjunctionDISI) disi;
       // subconjuctions have already split themselves into two phase iterators and others, so we can take those
       // iterators as they are and move them up to this conjunction
@@ -67,14 +122,13 @@ public class ConjunctionDISI extends DocIdSetIterator {
         Collections.addAll(twoPhaseIterators, twoPhase.twoPhaseView.twoPhaseIterators);
       }
     } else {
-      TwoPhaseIterator twoPhaseIter = TwoPhaseIterator.asTwoPhaseIterator(disi);
-      if (twoPhaseIter != null) {
-        allIterators.add(twoPhaseIter.approximation());
-        twoPhaseIterators.add(twoPhaseIter);
-      } else { // no approximation support, use the iterator as-is
       allIterators.add(disi);
     }
   }
+
+  private static void addTwoPhaseIterator(TwoPhaseIterator twoPhaseIter, List<DocIdSetIterator> allIterators, List<TwoPhaseIterator> twoPhaseIterators) {
+    addIterator(twoPhaseIter.approximation(), allIterators, twoPhaseIterators);
+    twoPhaseIterators.add(twoPhaseIter);
   }
 
   final DocIdSetIterator lead;

lucene/core/src/java/org/apache/lucene/search/ConjunctionScorer.java

@@ -29,27 +29,27 @@ class ConjunctionScorer extends Scorer {
   final Scorer[] scorers;
   final float coord;
 
-  ConjunctionScorer(Weight weight, List<? extends DocIdSetIterator> required, List<Scorer> scorers) {
+  ConjunctionScorer(Weight weight, List<Scorer> required, List<Scorer> scorers) {
     this(weight, required, scorers, 1f);
   }
 
   /** Create a new {@link ConjunctionScorer}, note that {@code scorers} must be a subset of {@code required}. */
-  ConjunctionScorer(Weight weight, List<? extends DocIdSetIterator> required, List<Scorer> scorers, float coord) {
+  ConjunctionScorer(Weight weight, List<Scorer> required, List<Scorer> scorers, float coord) {
     super(weight);
     assert required.containsAll(scorers);
     this.coord = coord;
-    this.disi = ConjunctionDISI.intersect(required);
+    this.disi = ConjunctionDISI.intersectScorers(required);
     this.scorers = scorers.toArray(new Scorer[scorers.size()]);
   }
 
   @Override
-  public TwoPhaseIterator asTwoPhaseIterator() {
+  public TwoPhaseIterator twoPhaseIterator() {
     return disi.asTwoPhaseIterator();
   }
 
   @Override
-  public int advance(int target) throws IOException {
+  public DocIdSetIterator iterator() {
-    return disi.advance(target);
+    return disi;
   }
 
   @Override
@@ -57,11 +57,6 @@ class ConjunctionScorer extends Scorer {
     return disi.docID();
   }
 
-  @Override
-  public int nextDoc() throws IOException {
-    return disi.nextDoc();
-  }
-
   @Override
   public float score() throws IOException {
     double sum = 0.0d;
@@ -76,11 +71,6 @@ class ConjunctionScorer extends Scorer {
     return scorers.length;
   }
 
-  @Override
-  public long cost() {
-    return disi.cost();
-  }
-
   @Override
   public Collection<ChildScorer> getChildren() {
     ArrayList<ChildScorer> children = new ArrayList<>();

lucene/core/src/java/org/apache/lucene/search/ConstantScoreScorer.java

@@ -54,10 +54,20 @@ public final class ConstantScoreScorer extends Scorer {
   }
 
   @Override
-  public TwoPhaseIterator asTwoPhaseIterator() {
+  public DocIdSetIterator iterator() {
+    return disi;
+  }
+
+  @Override
+  public TwoPhaseIterator twoPhaseIterator() {
     return twoPhaseIterator;
   }
 
+  @Override
+  public int docID() {
+    return disi.docID();
+  }
+
   @Override
   public float score() throws IOException {
     return score;
@@ -68,24 +78,5 @@ public final class ConstantScoreScorer extends Scorer {
     return 1;
   }
 
-  @Override
-  public int docID() {
-    return disi.docID();
-  }
-
-  @Override
-  public int nextDoc() throws IOException {
-    return disi.nextDoc();
-  }
-
-  @Override
-  public int advance(int target) throws IOException {
-    return disi.advance(target);
-  }
-
-  @Override
-  public long cost() {
-    return disi.cost();
-  }
 }
 

lucene/core/src/java/org/apache/lucene/search/ConstantScoreWeight.java

@@ -82,9 +82,9 @@ public abstract class ConstantScoreWeight extends Weight {
     if (s == null) {
       exists = false;
     } else {
-      final TwoPhaseIterator twoPhase = s.asTwoPhaseIterator();
+      final TwoPhaseIterator twoPhase = s.twoPhaseIterator();
       if (twoPhase == null) {
-        exists = s.advance(doc) == doc;
+        exists = s.iterator().advance(doc) == doc;
       } else {
         exists = twoPhase.approximation().advance(doc) == doc && twoPhase.matches();
       }

lucene/core/src/java/org/apache/lucene/search/DisiPriorityQueue.java

@@ -28,8 +28,7 @@ import org.apache.lucene.util.PriorityQueue;
  * pluggable comparison function makes the rebalancing quite slow.
  * @lucene.internal
  */
-public final class DisiPriorityQueue<Iter extends DocIdSetIterator>
+public final class DisiPriorityQueue implements Iterable<DisiWrapper> {
-implements Iterable<DisiWrapper<Iter>> {
 
   static int leftNode(int node) {
     return ((node + 1) << 1) - 1;
@@ -43,10 +42,9 @@ implements Iterable<DisiWrapper<Iter>> {
     return ((node + 1) >>> 1) - 1;
   }
 
-  private final DisiWrapper<Iter>[] heap;
+  private final DisiWrapper[] heap;
   private int size;
 
-  @SuppressWarnings({"unchecked","rawtypes"})
   public DisiPriorityQueue(int maxSize) {
     heap = new DisiWrapper[maxSize];
     size = 0;
@@ -56,15 +54,15 @@ implements Iterable<DisiWrapper<Iter>> {
     return size;
   }
 
-  public DisiWrapper<Iter> top() {
+  public DisiWrapper top() {
     return heap[0];
   }
 
   /** Get the list of scorers which are on the current doc. */
-  public DisiWrapper<Iter> topList() {
+  public DisiWrapper topList() {
-    final DisiWrapper<Iter>[] heap = this.heap;
+    final DisiWrapper[] heap = this.heap;
     final int size = this.size;
-    DisiWrapper<Iter> list = heap[0];
+    DisiWrapper list = heap[0];
     list.next = null;
     if (size >= 3) {
       list = topList(list, heap, size, 1);
@@ -76,14 +74,14 @@ implements Iterable<DisiWrapper<Iter>> {
   }
 
   // prepend w1 (iterator) to w2 (list)
-  private DisiWrapper<Iter> prepend(DisiWrapper<Iter> w1, DisiWrapper<Iter> w2) {
+  private DisiWrapper prepend(DisiWrapper w1, DisiWrapper w2) {
     w1.next = w2;
     return w1;
   }
 
-  private DisiWrapper<Iter> topList(DisiWrapper<Iter> list, DisiWrapper<Iter>[] heap,
+  private DisiWrapper topList(DisiWrapper list, DisiWrapper[] heap,
                                     int size, int i) {
-    final DisiWrapper<Iter> w = heap[i];
+    final DisiWrapper w = heap[i];
     if (w.doc == list.doc) {
       list = prepend(w, list);
       final int left = leftNode(i);
@@ -98,8 +96,8 @@ implements Iterable<DisiWrapper<Iter>> {
     return list;
   }
 
-  public DisiWrapper<Iter> add(DisiWrapper<Iter> entry) {
+  public DisiWrapper add(DisiWrapper entry) {
-    final DisiWrapper<Iter>[] heap = this.heap;
+    final DisiWrapper[] heap = this.heap;
     final int size = this.size;
     heap[size] = entry;
     upHeap(size);
@@ -107,9 +105,9 @@ implements Iterable<DisiWrapper<Iter>> {
     return heap[0];
   }
 
-  public DisiWrapper<Iter> pop() {
+  public DisiWrapper pop() {
-    final DisiWrapper<Iter>[] heap = this.heap;
+    final DisiWrapper[] heap = this.heap;
-    final DisiWrapper<Iter> result = heap[0];
+    final DisiWrapper result = heap[0];
     final int i = --size;
     heap[0] = heap[i];
     heap[i] = null;
@@ -117,18 +115,18 @@ implements Iterable<DisiWrapper<Iter>> {
     return result;
   }
 
-  public DisiWrapper<Iter> updateTop() {
+  public DisiWrapper updateTop() {
     downHeap(size);
     return heap[0];
   }
 
-  DisiWrapper<Iter> updateTop(DisiWrapper<Iter> topReplacement) {
+  DisiWrapper updateTop(DisiWrapper topReplacement) {
     heap[0] = topReplacement;
     return updateTop();
   }
 
   void upHeap(int i) {
-    final DisiWrapper<Iter> node = heap[i];
+    final DisiWrapper node = heap[i];
     final int nodeDoc = node.doc;
     int j = parentNode(i);
     while (j >= 0 && nodeDoc < heap[j].doc) {
@@ -141,7 +139,7 @@ implements Iterable<DisiWrapper<Iter>> {
 
   void downHeap(int size) {
     int i = 0;
-    final DisiWrapper<Iter> node = heap[0];
+    final DisiWrapper node = heap[0];
     int j = leftNode(i);
     if (j < size) {
       int k = rightNode(j);
@@ -164,7 +162,7 @@ implements Iterable<DisiWrapper<Iter>> {
   }
 
   @Override
-  public Iterator<DisiWrapper<Iter>> iterator() {
+  public Iterator<DisiWrapper> iterator() {
     return Arrays.asList(heap).subList(0, size).iterator();
   }
 

lucene/core/src/java/org/apache/lucene/search/DisiWrapper.java

@@ -17,15 +17,18 @@ package org.apache.lucene.search;
  * limitations under the License.
  */
 
+import org.apache.lucene.search.spans.Spans;
+
 /**
  * Wrapper used in {@link DisiPriorityQueue}.
  * @lucene.internal
  */
-public class DisiWrapper<Iter extends DocIdSetIterator> {
+public class DisiWrapper {
-  public final Iter iterator;
+  public final DocIdSetIterator iterator;
+  public final Scorer scorer;
   public final long cost;
   public int doc; // the current doc, used for comparison
-  public DisiWrapper<Iter> next; // reference to a next element, see #topList
+  public DisiWrapper next; // reference to a next element, see #topList
 
   // An approximation of the iterator, or the iterator itself if it does not
   // support two-phase iteration
@@ -34,14 +37,33 @@ public class DisiWrapper<Iter extends DocIdSetIterator> {
   // two-phase iteration
   public final TwoPhaseIterator twoPhaseView;
 
+  // FOR SPANS
+  public final Spans spans;
   public int lastApproxMatchDoc; // last doc of approximation that did match
   public int lastApproxNonMatchDoc; // last doc of approximation that did not match
 
-  public DisiWrapper(Iter iterator) {
+  public DisiWrapper(Scorer scorer) {
-    this.iterator = iterator;
+    this.scorer = scorer;
+    this.spans = null;
+    this.iterator = scorer.iterator();
     this.cost = iterator.cost();
     this.doc = -1;
-    this.twoPhaseView = TwoPhaseIterator.asTwoPhaseIterator(iterator);
+    this.twoPhaseView = scorer.twoPhaseIterator();
+      
+    if (twoPhaseView != null) {
+      approximation = twoPhaseView.approximation();
+    } else {
+      approximation = iterator;
+    }
+  }
+
+  public DisiWrapper(Spans spans) {
+    this.scorer = null;
+    this.spans = spans;
+    this.iterator = spans;
+    this.cost = iterator.cost();
+    this.doc = -1;
+    this.twoPhaseView = spans.asTwoPhaseIterator();
       
     if (twoPhaseView != null) {
       approximation = twoPhaseView.approximation();

lucene/core/src/java/org/apache/lucene/search/DisjunctionDISIApproximation.java

@@ -23,16 +23,15 @@ import java.io.IOException;
  * the provided iterators.
  * @lucene.internal
  */
-public class DisjunctionDISIApproximation<Iter extends DocIdSetIterator>
+public class DisjunctionDISIApproximation extends DocIdSetIterator {
-extends DocIdSetIterator {
 
-  final DisiPriorityQueue<Iter> subIterators;
+  final DisiPriorityQueue subIterators;
   final long cost;
 
-  public DisjunctionDISIApproximation(DisiPriorityQueue<Iter> subIterators) {
+  public DisjunctionDISIApproximation(DisiPriorityQueue subIterators) {
     this.subIterators = subIterators;
     long cost = 0;
-    for (DisiWrapper<Iter> w : subIterators) {
+    for (DisiWrapper w : subIterators) {
       cost += w.cost;
     }
     this.cost = cost;
@@ -50,7 +49,7 @@ extends DocIdSetIterator {
 
   @Override
   public int nextDoc() throws IOException {
-    DisiWrapper<Iter> top = subIterators.top();
+    DisiWrapper top = subIterators.top();
     final int doc = top.doc;
     do {
       top.doc = top.approximation.nextDoc();
@@ -62,7 +61,7 @@ extends DocIdSetIterator {
 
   @Override
   public int advance(int target) throws IOException {
-    DisiWrapper<Iter> top = subIterators.top();
+    DisiWrapper top = subIterators.top();
     do {
       top.doc = top.approximation.advance(target);
       top = subIterators.updateTop();

lucene/core/src/java/org/apache/lucene/search/DisjunctionMaxScorer.java

@@ -46,11 +46,11 @@ final class DisjunctionMaxScorer extends DisjunctionScorer {
   }
 
   @Override
-  protected float score(DisiWrapper<Scorer> topList) throws IOException {
+  protected float score(DisiWrapper topList) throws IOException {
     float scoreSum = 0;
     float scoreMax = 0;
-    for (DisiWrapper<Scorer> w = topList; w != null; w = w.next) {
+    for (DisiWrapper w = topList; w != null; w = w.next) {
-      final float subScore = w.iterator.score();
+      final float subScore = w.scorer.score();
       scoreSum += subScore;
       if (subScore > scoreMax) {
         scoreMax = subScore;

lucene/core/src/java/org/apache/lucene/search/DisjunctionScorer.java

@@ -28,21 +28,21 @@ import java.util.List;
 abstract class DisjunctionScorer extends Scorer {
 
   private final boolean needsScores;
-  final DisiPriorityQueue<Scorer> subScorers;
+  final DisiPriorityQueue subScorers;
   private final long cost;
 
   /** Linked list of scorers which are on the current doc */
-  private DisiWrapper<Scorer> topScorers;
+  private DisiWrapper topScorers;
 
   protected DisjunctionScorer(Weight weight, List<Scorer> subScorers, boolean needsScores) {
     super(weight);
     if (subScorers.size() <= 1) {
       throw new IllegalArgumentException("There must be at least 2 subScorers");
     }
-    this.subScorers = new DisiPriorityQueue<Scorer>(subScorers.size());
+    this.subScorers = new DisiPriorityQueue(subScorers.size());
     long cost = 0;
     for (Scorer scorer : subScorers) {
-      final DisiWrapper<Scorer> w = new DisiWrapper<>(scorer);
+      final DisiWrapper w = new DisiWrapper(scorer);
       cost += w.cost;
       this.subScorers.add(w);
     }
@@ -51,13 +51,55 @@ abstract class DisjunctionScorer extends Scorer {
   }
 
   @Override
-  public TwoPhaseIterator asTwoPhaseIterator() {
+  public DocIdSetIterator iterator() {
+    return new DocIdSetIterator() {
+
+      @Override
+      public int docID() {
+        return subScorers.top().doc;
+      }
+
+      @Override
+      public final int nextDoc() throws IOException {
+        topScorers = null;
+        DisiWrapper top = subScorers.top();
+        final int doc = top.doc;
+        do {
+          top.doc = top.iterator.nextDoc();
+          top = subScorers.updateTop();
+        } while (top.doc == doc);
+
+        return top.doc;
+      }
+
+      @Override
+      public final int advance(int target) throws IOException {
+        topScorers = null;
+        DisiWrapper top = subScorers.top();
+        do {
+          top.doc = top.iterator.advance(target);
+          top = subScorers.updateTop();
+        } while (top.doc < target);
+
+        return top.doc;
+      }
+
+      @Override
+      public final long cost() {
+        return cost;
+      }
+
+    };
+  }
+
+  @Override
+  public TwoPhaseIterator twoPhaseIterator() {
     float sumMatchCost = 0;
     long sumApproxCost = 0;
 
     // Compute matchCost as the avarage over the matchCost of the subScorers.
     // This is weighted by the cost, which is an expected number of matching documents.
-    for (DisiWrapper<Scorer> w : subScorers) {
+    for (DisiWrapper w : subScorers) {
       if (w.twoPhaseView != null) {
         long costWeight = (w.cost <= 1) ? 1 : w.cost;
         sumMatchCost += w.twoPhaseView.matchCost() * costWeight;
@@ -74,11 +116,11 @@ abstract class DisjunctionScorer extends Scorer {
     // note it is important to share the same pq as this scorer so that
     // rebalancing the pq through the approximation will also rebalance
     // the pq in this scorer.
-    return new TwoPhaseIterator(new DisjunctionDISIApproximation<Scorer>(subScorers)) {
+    return new TwoPhaseIterator(new DisjunctionDISIApproximation(subScorers)) {
 
       @Override
       public boolean matches() throws IOException {
-        DisiWrapper<Scorer> topScorers = subScorers.topList();
+        DisiWrapper topScorers = subScorers.topList();
         // remove the head of the list as long as it does not match
         while (topScorers.twoPhaseView != null && ! topScorers.twoPhaseView.matches()) {
           topScorers = topScorers.next;
@@ -90,8 +132,8 @@ abstract class DisjunctionScorer extends Scorer {
         if (needsScores) {
           // if scores or freqs are needed, we also need to remove scorers
           // from the top list that do not actually match
-          DisiWrapper<Scorer> previous = topScorers;
+          DisiWrapper previous = topScorers;
-          for (DisiWrapper<Scorer> w = topScorers.next; w != null; w = w.next) {
+          for (DisiWrapper w = topScorers.next; w != null; w = w.next) {
             if (w.twoPhaseView != null && ! w.twoPhaseView.matches()) {
               // w does not match, remove it
               previous.next = w.next;
@@ -119,48 +161,18 @@ abstract class DisjunctionScorer extends Scorer {
     };
   }
 
-  @Override
-  public final long cost() {
-    return cost;
-  }
-
   @Override
   public final int docID() {
    return subScorers.top().doc;
   }
 
-  @Override
-  public final int nextDoc() throws IOException {
-    topScorers = null;
-    DisiWrapper<Scorer> top = subScorers.top();
-    final int doc = top.doc;
-    do {
-      top.doc = top.iterator.nextDoc();
-      top = subScorers.updateTop();
-    } while (top.doc == doc);
-
-    return top.doc;
-  }
-
-  @Override
-  public final int advance(int target) throws IOException {
-    topScorers = null;
-    DisiWrapper<Scorer> top = subScorers.top();
-    do {
-      top.doc = top.iterator.advance(target);
-      top = subScorers.updateTop();
-    } while (top.doc < target);
-
-    return top.doc;
-  }
-
   @Override
   public final int freq() throws IOException {
     if (topScorers == null) {
       topScorers = subScorers.topList();
     }
     int freq = 1;
-    for (DisiWrapper<Scorer> w = topScorers.next; w != null; w = w.next) {
+    for (DisiWrapper w = topScorers.next; w != null; w = w.next) {
       freq += 1;
     }
     return freq;
@@ -175,13 +187,13 @@ abstract class DisjunctionScorer extends Scorer {
   }
 
   /** Compute the score for the given linked list of scorers. */
-  protected abstract float score(DisiWrapper<Scorer> topList) throws IOException;
+  protected abstract float score(DisiWrapper topList) throws IOException;
 
   @Override
   public final Collection<ChildScorer> getChildren() {
     ArrayList<ChildScorer> children = new ArrayList<>();
-    for (DisiWrapper<Scorer> scorer : subScorers) {
+    for (DisiWrapper scorer : subScorers) {
-      children.add(new ChildScorer(scorer.iterator, "SHOULD"));
+      children.add(new ChildScorer(scorer.scorer, "SHOULD"));
     }
     return children;
   }

lucene/core/src/java/org/apache/lucene/search/DisjunctionSumScorer.java

@@ -21,7 +21,6 @@ import java.io.IOException;
 import java.util.List;
 
 /** A Scorer for OR like queries, counterpart of <code>ConjunctionScorer</code>.
- * This Scorer implements {@link Scorer#advance(int)} and uses advance() on the given Scorers. 
  */
 final class DisjunctionSumScorer extends DisjunctionScorer { 
   private final float[] coord;
@@ -37,11 +36,11 @@ final class DisjunctionSumScorer extends DisjunctionScorer {
   }
 
   @Override
-  protected float score(DisiWrapper<Scorer> topList) throws IOException {
+  protected float score(DisiWrapper topList) throws IOException {
     double score = 0;
     int freq = 0;
-    for (DisiWrapper<Scorer> w = topList; w != null; w = w.next) {
+    for (DisiWrapper w = topList; w != null; w = w.next) {
-      score += w.iterator.score();
+      score += w.scorer.score();
       freq += 1;
     }
     return (float)score * coord[freq];

lucene/core/src/java/org/apache/lucene/search/ExactPhraseScorer.java

@@ -59,13 +59,13 @@ final class ExactPhraseScorer extends Scorer {
       iterators.add(posting.postings);
       postingsAndPositions.add(new PostingsAndPosition(posting.postings, posting.position));
     }
-    conjunction = ConjunctionDISI.intersect(iterators);
+    conjunction = ConjunctionDISI.intersectIterators(iterators);
     this.postings = postingsAndPositions.toArray(new PostingsAndPosition[postingsAndPositions.size()]);
     this.matchCost = matchCost;
   }
 
   @Override
-  public TwoPhaseIterator asTwoPhaseIterator() {
+  public TwoPhaseIterator twoPhaseIterator() {
     return new TwoPhaseIterator(conjunction) {
       @Override
       public boolean matches() throws IOException {
@@ -79,22 +79,9 @@ final class ExactPhraseScorer extends Scorer {
     };
   }
 
-  private int doNext(int doc) throws IOException {
-    for (;; doc = conjunction.nextDoc()) {
-      if (doc == NO_MORE_DOCS || phraseFreq() > 0) {
-        return doc;
-      }
-    }
-  }
-
   @Override
-  public int nextDoc() throws IOException {
+  public DocIdSetIterator iterator() {
-    return doNext(conjunction.nextDoc());
+    return TwoPhaseIterator.asDocIdSetIterator(twoPhaseIterator());
-  }
-
-  @Override
-  public int advance(int target) throws IOException {
-    return doNext(conjunction.advance(target));
   }
 
   @Override
@@ -180,8 +167,4 @@ final class ExactPhraseScorer extends Scorer {
     return this.freq = freq;
   }
 
-  @Override
-  public long cost() {
-    return conjunction.cost();
-  }
 }

lucene/core/src/java/org/apache/lucene/search/FakeScorer.java

@@ -30,11 +30,6 @@ final class FakeScorer extends Scorer {
     super(null);
   }
 
-  @Override
-  public int advance(int target) {
-    throw new UnsupportedOperationException("FakeScorer doesn't support advance(int)");
-  }
-
   @Override
   public int docID() {
     return doc;
@@ -45,19 +40,14 @@ final class FakeScorer extends Scorer {
     return freq;
   }
     
-  @Override
-  public int nextDoc() {
-    throw new UnsupportedOperationException("FakeScorer doesn't support nextDoc()");
-  }
-    
   @Override
   public float score() {
     return score;
   }
 
   @Override
-  public long cost() {
+  public DocIdSetIterator iterator() {
-    return 1;
+    throw new UnsupportedOperationException();
   }
 
   @Override

lucene/core/src/java/org/apache/lucene/search/FilterScorer.java

@@ -70,22 +70,12 @@ public abstract class FilterScorer extends Scorer {
   }
 
   @Override
-  public final int nextDoc() throws IOException {
+  public final DocIdSetIterator iterator() {
-    return in.nextDoc();
+    return in.iterator();
   }
   
   @Override
-  public final int advance(int target) throws IOException {
+  public final TwoPhaseIterator twoPhaseIterator() {
-    return in.advance(target);
+    return in.twoPhaseIterator();
-  }
-
-  @Override
-  public long cost() {
-    return in.cost();
-  }
-  
-  @Override
-  public final TwoPhaseIterator asTwoPhaseIterator() {
-    return in.asTwoPhaseIterator();
   }
 }

lucene/core/src/java/org/apache/lucene/search/MinShouldMatchSumScorer.java

@@ -64,7 +64,7 @@ final class MinShouldMatchSumScorer extends Scorer {
     final PriorityQueue<Scorer> pq = new PriorityQueue<Scorer>(scorers.size() - minShouldMatch + 1) {
       @Override
       protected boolean lessThan(Scorer a, Scorer b) {
-        return a.cost() > b.cost();
+        return a.iterator().cost() > b.iterator().cost();
       }
     };
     for (Scorer scorer : scorers) {
@@ -72,7 +72,7 @@ final class MinShouldMatchSumScorer extends Scorer {
     }
     long cost = 0;
     for (Scorer scorer = pq.pop(); scorer != null; scorer = pq.pop()) {
-      cost += scorer.cost();
+      cost += scorer.iterator().cost();
     }
     return cost;
   }
@@ -82,23 +82,22 @@ final class MinShouldMatchSumScorer extends Scorer {
 
   // list of scorers which 'lead' the iteration and are currently
   // positioned on 'doc'
-  DisiWrapper<Scorer> lead;
+  DisiWrapper lead;
   int doc;  // current doc ID of the leads
   int freq; // number of scorers on the desired doc ID
 
   // priority queue of scorers that are too advanced compared to the current
   // doc. Ordered by doc ID.
-  final DisiPriorityQueue<Scorer> head;
+  final DisiPriorityQueue head;
 
   // priority queue of scorers which are behind the current doc.
   // Ordered by cost.
-  final DisiWrapper<Scorer>[] tail;
+  final DisiWrapper[] tail;
   int tailSize;
 
   final Collection<ChildScorer> childScorers;
   final long cost;
 
-  @SuppressWarnings({"unchecked","rawtypes"})
   MinShouldMatchSumScorer(Weight weight, Collection<Scorer> scorers, int minShouldMatch, float[] coord) {
     super(weight);
 
@@ -113,13 +112,13 @@ final class MinShouldMatchSumScorer extends Scorer {
     this.coord = coord;
     this.doc = -1;
 
-    head = new DisiPriorityQueue<Scorer>(scorers.size() - minShouldMatch + 1);
+    head = new DisiPriorityQueue(scorers.size() - minShouldMatch + 1);
     // there can be at most minShouldMatch - 1 scorers beyond the current position
     // otherwise we might be skipping over matching documents
     tail = new DisiWrapper[minShouldMatch - 1];
 
     for (Scorer scorer : scorers) {
-      addLead(new DisiWrapper<Scorer>(scorer));
+      addLead(new DisiWrapper(scorer));
     }
 
     List<ChildScorer> children = new ArrayList<>();
@@ -131,13 +130,18 @@ final class MinShouldMatchSumScorer extends Scorer {
   }
 
   @Override
-  public long cost() {
+  public final Collection<ChildScorer> getChildren() {
-    return cost;
+    return childScorers;
   }
 
   @Override
-  public final Collection<ChildScorer> getChildren() {
+  public DocIdSetIterator iterator() {
-    return childScorers;
+    return new DocIdSetIterator() {
+
+      @Override
+      public int docID() {
+        assert doc == lead.doc;
+        return doc;
       }
 
       @Override
@@ -145,8 +149,8 @@ final class MinShouldMatchSumScorer extends Scorer {
         // We are moving to the next doc ID, so scorers in 'lead' need to go in
         // 'tail'. If there is not enough space in 'tail', then we take the least
         // costly scorers and advance them.
-    for (DisiWrapper<Scorer> s = lead; s != null; s = s.next) {
+        for (DisiWrapper s = lead; s != null; s = s.next) {
-      final DisiWrapper<Scorer> evicted = insertTailWithOverFlow(s);
+          final DisiWrapper evicted = insertTailWithOverFlow(s);
           if (evicted != null) {
             if (evicted.doc == doc) {
               evicted.doc = evicted.iterator.nextDoc();
@@ -164,8 +168,8 @@ final class MinShouldMatchSumScorer extends Scorer {
       @Override
       public int advance(int target) throws IOException {
         // Same logic as in nextDoc
-    for (DisiWrapper<Scorer> s = lead; s != null; s = s.next) {
+        for (DisiWrapper s = lead; s != null; s = s.next) {
-      final DisiWrapper<Scorer> evicted = insertTailWithOverFlow(s);
+          final DisiWrapper evicted = insertTailWithOverFlow(s);
           if (evicted != null) {
             evicted.doc = evicted.iterator.advance(target);
             head.add(evicted);
@@ -174,9 +178,9 @@ final class MinShouldMatchSumScorer extends Scorer {
 
         // But this time there might also be scorers in 'head' behind the desired
         // target so we need to do the same thing that we did on 'lead' on 'head'
-    DisiWrapper<Scorer> headTop = head.top();
+        DisiWrapper headTop = head.top();
         while (headTop.doc < target) {
-      final DisiWrapper<Scorer> evicted = insertTailWithOverFlow(headTop);
+          final DisiWrapper evicted = insertTailWithOverFlow(headTop);
           // We know that the tail is full since it contains at most
           // minShouldMatch - 1 entries and we just moved at least minShouldMatch
           // entries to it, so evicted is not null
@@ -188,19 +192,26 @@ final class MinShouldMatchSumScorer extends Scorer {
         return doNext();
       }
 
-  private void addLead(DisiWrapper<Scorer> lead) {
+      @Override
+      public long cost() {
+        return cost;
+      }
+    };
+  }
+
+  private void addLead(DisiWrapper lead) {
     lead.next = this.lead;
     this.lead = lead;
     freq += 1;
   }
 
   private void pushBackLeads() throws IOException {
-    for (DisiWrapper<Scorer> s = lead; s != null; s = s.next) {
+    for (DisiWrapper s = lead; s != null; s = s.next) {
       addTail(s);
     }
   }
 
-  private void advanceTail(DisiWrapper<Scorer> top) throws IOException {
+  private void advanceTail(DisiWrapper top) throws IOException {
     top.doc = top.iterator.advance(doc);
     if (top.doc == doc) {
       addLead(top);
@@ -210,7 +221,7 @@ final class MinShouldMatchSumScorer extends Scorer {
   }
 
   private void advanceTail() throws IOException {
-    final DisiWrapper<Scorer> top = popTail();
+    final DisiWrapper top = popTail();
     advanceTail(top);
   }
 
@@ -276,8 +287,8 @@ final class MinShouldMatchSumScorer extends Scorer {
     // we need to know about all matches
     updateFreq();
     double score = 0;
-    for (DisiWrapper<Scorer> s = lead; s != null; s = s.next) {
+    for (DisiWrapper s = lead; s != null; s = s.next) {
-      score += s.iterator.score();
+      score += s.scorer.score();
     }
     return coord[freq] * (float) score;
   }
@@ -289,12 +300,12 @@ final class MinShouldMatchSumScorer extends Scorer {
   }
 
   /** Insert an entry in 'tail' and evict the least-costly scorer if full. */
-  private DisiWrapper<Scorer> insertTailWithOverFlow(DisiWrapper<Scorer> s) {
+  private DisiWrapper insertTailWithOverFlow(DisiWrapper s) {
     if (tailSize < tail.length) {
       addTail(s);
       return null;
     } else if (tail.length >= 1) {
-      final DisiWrapper<Scorer> top = tail[0];
+      final DisiWrapper top = tail[0];
       if (top.cost < s.cost) {
         tail[0] = s;
         downHeapCost(tail, tailSize);
@@ -305,16 +316,16 @@ final class MinShouldMatchSumScorer extends Scorer {
   }
 
   /** Add an entry to 'tail'. Fails if over capacity. */
-  private void addTail(DisiWrapper<Scorer> s) {
+  private void addTail(DisiWrapper s) {
     tail[tailSize] = s;
     upHeapCost(tail, tailSize);
     tailSize += 1;
   }
 
   /** Pop the least-costly scorer from 'tail'. */
-  private DisiWrapper<Scorer> popTail() {
+  private DisiWrapper popTail() {
     assert tailSize > 0;
-    final DisiWrapper<Scorer> result = tail[0];
+    final DisiWrapper result = tail[0];
     tail[0] = tail[--tailSize];
     downHeapCost(tail, tailSize);
     return result;
@@ -322,8 +333,8 @@ final class MinShouldMatchSumScorer extends Scorer {
 
   /** Heap helpers */
 
-  private static void upHeapCost(DisiWrapper<Scorer>[] heap, int i) {
+  private static void upHeapCost(DisiWrapper[] heap, int i) {
-    final DisiWrapper<Scorer> node = heap[i];
+    final DisiWrapper node = heap[i];
     final long nodeCost = node.cost;
     int j = parentNode(i);
     while (j >= 0 && nodeCost < heap[j].cost) {
@@ -334,9 +345,9 @@ final class MinShouldMatchSumScorer extends Scorer {
     heap[i] = node;
   }
 
-  private static void downHeapCost(DisiWrapper<Scorer>[] heap, int size) {
+  private static void downHeapCost(DisiWrapper[] heap, int size) {
     int i = 0;
-    final DisiWrapper<Scorer> node = heap[0];
+    final DisiWrapper node = heap[0];
     int j = leftNode(i);
     if (j < size) {
       int k = rightNode(j);

lucene/core/src/java/org/apache/lucene/search/MultiPhraseQuery.java

@@ -241,7 +241,7 @@ public class MultiPhraseQuery extends Query {
     public Explanation explain(LeafReaderContext context, int doc) throws IOException {
       Scorer scorer = scorer(context);
       if (scorer != null) {
-        int newDoc = scorer.advance(doc);
+        int newDoc = scorer.iterator().advance(doc);
         if (newDoc == doc) {
           float freq = slop == 0 ? scorer.freq() : ((SloppyPhraseScorer)scorer).sloppyFreq();
           SimScorer docScorer = similarity.simScorer(stats, context);

lucene/core/src/java/org/apache/lucene/search/PhraseQuery.java

@@ -453,7 +453,7 @@ public class PhraseQuery extends Query {
     public Explanation explain(LeafReaderContext context, int doc) throws IOException {
       Scorer scorer = scorer(context);
       if (scorer != null) {
-        int newDoc = scorer.advance(doc);
+        int newDoc = scorer.iterator().advance(doc);
         if (newDoc == doc) {
           float freq = slop == 0 ? scorer.freq() : ((SloppyPhraseScorer)scorer).sloppyFreq();
           SimScorer docScorer = similarity.simScorer(stats, context);

lucene/core/src/java/org/apache/lucene/search/QueryRescorer.java

@@ -89,7 +89,7 @@ public abstract class QueryRescorer extends Rescorer {
         int targetDoc = docID - docBase;
         int actualDoc = scorer.docID();
         if (actualDoc < targetDoc) {
-          actualDoc = scorer.advance(targetDoc);
+          actualDoc = scorer.iterator().advance(targetDoc);
         }
 
         if (actualDoc == targetDoc) {

lucene/core/src/java/org/apache/lucene/search/ReqExclScorer.java

@@ -23,9 +23,6 @@ import java.util.Collections;
 
 /** A Scorer for queries with a required subscorer
  * and an excluding (prohibited) sub {@link Scorer}.
- * <br>
- * This <code>Scorer</code> implements {@link Scorer#advance(int)},
- * and it uses the advance() on the given scorers.
  */
 class ReqExclScorer extends Scorer {
 
@@ -44,25 +41,20 @@ class ReqExclScorer extends Scorer {
   public ReqExclScorer(Scorer reqScorer, Scorer exclScorer) {
     super(reqScorer.weight);
     this.reqScorer = reqScorer;
-    reqTwoPhaseIterator = reqScorer.asTwoPhaseIterator();
+    reqTwoPhaseIterator = reqScorer.twoPhaseIterator();
     if (reqTwoPhaseIterator == null) {
-      reqApproximation = reqScorer;
+      reqApproximation = reqScorer.iterator();
     } else {
       reqApproximation = reqTwoPhaseIterator.approximation();
     }
-    exclTwoPhaseIterator = exclScorer.asTwoPhaseIterator();
+    exclTwoPhaseIterator = exclScorer.twoPhaseIterator();
     if (exclTwoPhaseIterator == null) {
-      exclApproximation = exclScorer;
+      exclApproximation = exclScorer.iterator();
     } else {
       exclApproximation = exclTwoPhaseIterator.approximation();
     }
   }
 
-  @Override
-  public int nextDoc() throws IOException {
-    return toNonExcluded(reqApproximation.nextDoc());
-  }
-
   /** Confirms whether or not the given {@link TwoPhaseIterator}
    *  matches on the current document. */
   private static boolean matches(TwoPhaseIterator it) throws IOException {
@@ -85,6 +77,10 @@ class ReqExclScorer extends Scorer {
     return matches(reqTwoPhaseIterator);
   }
 
+  @Override
+  public DocIdSetIterator iterator() {
+    return new DocIdSetIterator() {
+
       /** Advance to the next non-excluded doc. */
       private int toNonExcluded(int doc) throws IOException {
         int exclDoc = exclApproximation.docID();
@@ -101,9 +97,32 @@ class ReqExclScorer extends Scorer {
         }
       }
 
+      @Override
+      public int nextDoc() throws IOException {
+        return toNonExcluded(reqApproximation.nextDoc());
+      }
+
+      @Override
+      public int advance(int target) throws IOException {
+        return toNonExcluded(reqApproximation.advance(target));
+      }
+
       @Override
       public int docID() {
-    return reqScorer.docID();
+        return reqApproximation.docID();
+      }
+
+      @Override
+      public long cost() {
+        return reqApproximation.cost();
+      }
+
+    };
+  }
+
+  @Override
+  public int docID() {
+    return reqApproximation.docID();
   }
 
   @Override
@@ -111,11 +130,6 @@ class ReqExclScorer extends Scorer {
     return reqScorer.freq();
   }
 
-  @Override
-  public long cost() {
-    return reqScorer.cost();
-  }
-
   @Override
   public float score() throws IOException {
     return reqScorer.score(); // reqScorer may be null when next() or skipTo() already return false
@@ -127,12 +141,7 @@ class ReqExclScorer extends Scorer {
   }
 
   @Override
-  public int advance(int target) throws IOException {
+  public TwoPhaseIterator twoPhaseIterator() {
-    return toNonExcluded(reqApproximation.advance(target));
-  }
-
-  @Override
-  public TwoPhaseIterator asTwoPhaseIterator() {
     if (reqTwoPhaseIterator == null) {
       return null;
     }

lucene/core/src/java/org/apache/lucene/search/ReqOptSumScorer.java

@@ -22,15 +22,14 @@ import java.util.Collection;
 
 /** A Scorer for queries with a required part and an optional part.
  * Delays skipTo() on the optional part until a score() is needed.
- * <br>
- * This <code>Scorer</code> implements {@link Scorer#advance(int)}.
  */
 class ReqOptSumScorer extends Scorer {
   /** The scorers passed from the constructor.
    * These are set to null as soon as their next() or skipTo() returns false.
    */
   protected final Scorer reqScorer;
-  protected Scorer optScorer;
+  protected final Scorer optScorer;
+  protected final DocIdSetIterator optIterator;
 
   /** Construct a <code>ReqOptScorer</code>.
    * @param reqScorer The required scorer. This must match.
@@ -45,21 +44,17 @@ class ReqOptSumScorer extends Scorer {
     assert optScorer != null;
     this.reqScorer = reqScorer;
     this.optScorer = optScorer;
+    this.optIterator = optScorer.iterator();
   }
 
   @Override
-  public TwoPhaseIterator asTwoPhaseIterator() {
+  public TwoPhaseIterator twoPhaseIterator() {
-    return reqScorer.asTwoPhaseIterator();
+    return reqScorer.twoPhaseIterator();
   }
 
   @Override
-  public int nextDoc() throws IOException {
+  public DocIdSetIterator iterator() {
-    return reqScorer.nextDoc();
+    return reqScorer.iterator();
-  }
-  
-  @Override
-  public int advance(int target) throws IOException {
-    return reqScorer.advance(target);
   }
 
   @Override
@@ -68,7 +63,7 @@ class ReqOptSumScorer extends Scorer {
   }
 
   /** Returns the score of the current document matching the query.
-   * Initially invalid, until {@link #nextDoc()} is called the first time.
+   * Initially invalid, until the {@link #iterator()} is advanced the first time.
    * @return The score of the required scorer, eventually increased by the score
    * of the optional scorer when it also matches the current document.
    */
@@ -76,25 +71,25 @@ class ReqOptSumScorer extends Scorer {
   public float score() throws IOException {
     // TODO: sum into a double and cast to float if we ever send required clauses to BS1
     int curDoc = reqScorer.docID();
-    float reqScore = reqScorer.score();
+    float score = reqScorer.score();
-    if (optScorer == null) {
+
-      return reqScore;
+    int optScorerDoc = optIterator.docID();
+    if (optScorerDoc < curDoc) {
+      optScorerDoc = optIterator.advance(curDoc);
     }
     
-    int optScorerDoc = optScorer.docID();
+    if (optScorerDoc == curDoc) {
-    if (optScorerDoc < curDoc && (optScorerDoc = optScorer.advance(curDoc)) == NO_MORE_DOCS) {
+      score += optScorer.score();
-      optScorer = null;
-      return reqScore;
     }
     
-    return optScorerDoc == curDoc ? reqScore + optScorer.score() : reqScore;
+    return score;
   }
 
   @Override
   public int freq() throws IOException {
     // we might have deferred advance()
     score();
-    return (optScorer != null && optScorer.docID() == reqScorer.docID()) ? 2 : 1;
+    return optIterator.docID() == reqScorer.docID() ? 2 : 1;
   }
 
   @Override
@@ -105,9 +100,5 @@ class ReqOptSumScorer extends Scorer {
     return children;
   }
 
-  @Override
-  public long cost() {
-    return reqScorer.cost();
-  }
 }
 

lucene/core/src/java/org/apache/lucene/search/Scorer.java

@@ -25,8 +25,8 @@ import java.util.Collections;
  * Expert: Common scoring functionality for different types of queries.
  *
  * <p>
- * A <code>Scorer</code> iterates over documents matching a
+ * A <code>Scorer</code> exposes an {@link #iterator()} over documents
- * query in increasing order of doc Id.
+ * matching a query in increasing order of doc Id.
  * </p>
  * <p>
  * Document scores are computed using a given <code>Similarity</code>
@@ -39,7 +39,7 @@ import java.util.Collections;
  * TopScoreDocCollector}) will not properly collect hits
  * with these scores.
  */
-public abstract class Scorer extends DocIdSetIterator {
+public abstract class Scorer {
   /** the Scorer's parent Weight. in some cases this may be null */
   // TODO can we clean this up?
   protected final Weight weight;
@@ -52,10 +52,18 @@ public abstract class Scorer extends DocIdSetIterator {
     this.weight = weight;
   }
 
+  /**
+   * Returns the doc ID that is currently being scored.
+   * This will return {@code -1} if the {@link #iterator()} is not positioned
+   * or {@link DocIdSetIterator#NO_MORE_DOCS} if it has been entirely consumed.
+   * @see DocIdSetIterator#docID()
+   */
+  public abstract int docID();
+
   /** Returns the score of the current document matching the query.
-   * Initially invalid, until {@link #nextDoc()} or {@link #advance(int)}
+   * Initially invalid, until {@link DocIdSetIterator#nextDoc()} or
-   * is called the first time, or when called from within
+   * {@link DocIdSetIterator#advance(int)} is called on the {@link #iterator()}
-   * {@link LeafCollector#collect}.
+   * the first time, or when called from within {@link LeafCollector#collect}.
    */
   public abstract float score() throws IOException;
 
@@ -101,6 +109,19 @@ public abstract class Scorer extends DocIdSetIterator {
     }
   }
 
+  /**
+   * Return a {@link DocIdSetIterator} over matching documents.
+   *
+   * The returned iterator will either be positioned on {@code -1} if no
+   * documents have been scored yet, {@link DocIdSetIterator#NO_MORE_DOCS}
+   * if all documents have been scored already, or the last document id that
+   * has been scored otherwise.
+   *
+   * The returned iterator is a view: calling this method several times will
+   * return iterators that have the same state.
+   */
+  public abstract DocIdSetIterator iterator();
+
   /**
    * Optional method: Return a {@link TwoPhaseIterator} view of this
    * {@link Scorer}. A return value of {@code null} indicates that
@@ -108,15 +129,15 @@ public abstract class Scorer extends DocIdSetIterator {
    *
    * Note that the returned {@link TwoPhaseIterator}'s
    * {@link TwoPhaseIterator#approximation() approximation} must
-   * advance synchronously with this iterator: advancing the approximation must
+   * advance synchronously with the {@link #iterator()}: advancing the
-   * advance this iterator and vice-versa.
+   * approximation must advance the iterator and vice-versa.
    *
    * Implementing this method is typically useful on {@link Scorer}s
    * that have a high per-document overhead in order to confirm matches.
    *
    * The default implementation returns {@code null}.
    */
-  public TwoPhaseIterator asTwoPhaseIterator() {
+  public TwoPhaseIterator twoPhaseIterator() {
     return null;
   }
 }

lucene/core/src/java/org/apache/lucene/search/SloppyPhraseScorer.java

@@ -69,7 +69,7 @@ final class SloppyPhraseScorer extends Scorer {
       iterators[i] = postings[i].postings;
       phrasePositions[i] = new PhrasePositions(postings[i].postings, postings[i].position, i, postings[i].terms);
     }
-    conjunction = ConjunctionDISI.intersect(Arrays.asList(iterators));
+    conjunction = ConjunctionDISI.intersectIterators(Arrays.asList(iterators));
     this.matchCost = matchCost;
   }
 
@@ -551,48 +551,16 @@ final class SloppyPhraseScorer extends Scorer {
     return conjunction.docID(); 
   }
   
-  @Override
-  public int nextDoc() throws IOException {
-    int doc;
-    for (doc = conjunction.nextDoc(); doc != NO_MORE_DOCS; doc = conjunction.nextDoc()) {
-      sloppyFreq = phraseFreq(); // check for phrase
-      if (sloppyFreq != 0f) {
-        break;
-      }
-    }
-
-    return doc;
-  }
-  
   @Override
   public float score() {
     return docScorer.score(docID(), sloppyFreq);
   }
 
-  @Override
-  public int advance(int target) throws IOException {
-    assert target > docID();
-    int doc;
-    for (doc = conjunction.advance(target); doc != NO_MORE_DOCS; doc = conjunction.nextDoc()) {
-      sloppyFreq = phraseFreq(); // check for phrase
-      if (sloppyFreq != 0f) {
-        break;
-      }
-    }
-
-    return doc;
-  }
-
-  @Override
-  public long cost() {
-    return conjunction.cost();
-  }
-
   @Override
   public String toString() { return "scorer(" + weight + ")"; }
 
   @Override
-  public TwoPhaseIterator asTwoPhaseIterator() {
+  public TwoPhaseIterator twoPhaseIterator() {
     return new TwoPhaseIterator(conjunction) {
       @Override
       public boolean matches() throws IOException {
@@ -611,4 +579,9 @@ final class SloppyPhraseScorer extends Scorer {
       }
     };
   }
+
+  @Override
+  public DocIdSetIterator iterator() {
+    return TwoPhaseIterator.asDocIdSetIterator(twoPhaseIterator());
+  }
 }

lucene/core/src/java/org/apache/lucene/search/SynonymQuery.java

@@ -162,7 +162,7 @@ public final class SynonymQuery extends Query {
     public Explanation explain(LeafReaderContext context, int doc) throws IOException {
       Scorer scorer = scorer(context);
       if (scorer != null) {
-        int newDoc = scorer.advance(doc);
+        int newDoc = scorer.iterator().advance(doc);
         if (newDoc == doc) {
           final float freq;
           if (scorer instanceof SynonymScorer) {
@@ -229,15 +229,15 @@ public final class SynonymQuery extends Query {
     }
 
     @Override
-    protected float score(DisiWrapper<Scorer> topList) throws IOException {
+    protected float score(DisiWrapper topList) throws IOException {
       return similarity.score(topList.doc, tf(topList));
     }
     
     /** combines TF of all subs. */
-    final int tf(DisiWrapper<Scorer> topList) throws IOException {
+    final int tf(DisiWrapper topList) throws IOException {
       int tf = 0;
-      for (DisiWrapper<Scorer> w = topList; w != null; w = w.next) {
+      for (DisiWrapper w = topList; w != null; w = w.next) {
-        tf += w.iterator.freq();
+        tf += w.scorer.freq();
       }
       return tf;
     }

lucene/core/src/java/org/apache/lucene/search/TermQuery.java

@@ -136,7 +136,7 @@ public class TermQuery extends Query {
     public Explanation explain(LeafReaderContext context, int doc) throws IOException {
       Scorer scorer = scorer(context);
       if (scorer != null) {
-        int newDoc = scorer.advance(doc);
+        int newDoc = scorer.iterator().advance(doc);
         if (newDoc == doc) {
           float freq = scorer.freq();
           SimScorer docScorer = similarity.simScorer(stats, context);

lucene/core/src/java/org/apache/lucene/search/TermScorer.java

@@ -55,41 +55,17 @@ final class TermScorer extends Scorer {
     return postingsEnum.freq();
   }
 
-  /**
-   * Advances to the next document matching the query. <br>
-   *
-   * @return the document matching the query or NO_MORE_DOCS if there are no more documents.
-   */
   @Override
-  public int nextDoc() throws IOException {
+  public DocIdSetIterator iterator() {
-    return postingsEnum.nextDoc();
+    return postingsEnum;
   }
 
   @Override
   public float score() throws IOException {
-    assert docID() != NO_MORE_DOCS;
+    assert docID() != DocIdSetIterator.NO_MORE_DOCS;
     return docScorer.score(postingsEnum.docID(), postingsEnum.freq());
   }
 
-  /**
-   * Advances to the first match beyond the current whose document number is
-   * greater than or equal to a given target. <br>
-   * The implementation uses {@link org.apache.lucene.index.PostingsEnum#advance(int)}.
-   *
-   * @param target
-   *          The target document number.
-   * @return the matching document or NO_MORE_DOCS if none exist.
-   */
-  @Override
-  public int advance(int target) throws IOException {
-    return postingsEnum.advance(target);
-  }
-
-  @Override
-  public long cost() {
-    return postingsEnum.cost();
-  }
-
   /** Returns a string representation of this <code>TermScorer</code>. */
   @Override
   public String toString() { return "scorer(" + weight + ")[" + super.toString() + "]"; }

lucene/core/src/java/org/apache/lucene/search/TwoPhaseIterator.java

@@ -21,7 +21,7 @@ import java.io.IOException;
 import java.util.Objects;
 
 /**
- * Returned by {@link Scorer#asTwoPhaseIterator()}
+ * Returned by {@link Scorer#twoPhaseIterator()}
  * to expose an approximation of a {@link DocIdSetIterator}.
  * When the {@link #approximation()}'s
  * {@link DocIdSetIterator#nextDoc()} or {@link DocIdSetIterator#advance(int)}
@@ -98,14 +98,4 @@ public abstract class TwoPhaseIterator {
    */
   public abstract float matchCost();
 
-  /**
-   * Returns a {@link TwoPhaseIterator} for this {@link DocIdSetIterator}
-   * when available, otherwise returns null.
-   */
-  public static TwoPhaseIterator asTwoPhaseIterator(DocIdSetIterator iter) {
-    return (iter instanceof Scorer)
-            ? ((Scorer) iter).asTwoPhaseIterator()
-            : null;
-  }
-
 }

lucene/core/src/java/org/apache/lucene/search/Weight.java

@@ -147,6 +147,8 @@ public abstract class Weight {
    *  @lucene.internal */
   protected static class DefaultBulkScorer extends BulkScorer {
     private final Scorer scorer;
+    private final DocIdSetIterator iterator;
+    private final TwoPhaseIterator twoPhase;
 
     /** Sole constructor. */
     public DefaultBulkScorer(Scorer scorer) {
@@ -154,30 +156,31 @@ public abstract class Weight {
         throw new NullPointerException();
       }
       this.scorer = scorer;
+      this.iterator = scorer.iterator();
+      this.twoPhase = scorer.twoPhaseIterator();
     }
 
     @Override
     public long cost() {
-      return scorer.cost();
+      return iterator.cost();
     }
 
     @Override
     public int score(LeafCollector collector, Bits acceptDocs, int min, int max) throws IOException {
       collector.setScorer(scorer);
-      final TwoPhaseIterator twoPhase = scorer.asTwoPhaseIterator();
       if (scorer.docID() == -1 && min == 0 && max == DocIdSetIterator.NO_MORE_DOCS) {
-        scoreAll(collector, scorer, twoPhase, acceptDocs);
+        scoreAll(collector, iterator, twoPhase, acceptDocs);
         return DocIdSetIterator.NO_MORE_DOCS;
       } else {
         int doc = scorer.docID();
         if (doc < min) {
           if (twoPhase == null) {
-            doc = scorer.advance(min);
+            doc = iterator.advance(min);
           } else {
             doc = twoPhase.approximation().advance(min);
           }
         }
-        return scoreRange(collector, scorer, twoPhase, acceptDocs, doc, max);
+        return scoreRange(collector, iterator, twoPhase, acceptDocs, doc, max);
       }
     }
 
@@ -185,14 +188,14 @@ public abstract class Weight {
      *  separate this from {@link #scoreAll} to help out
      *  hotspot.
      *  See <a href="https://issues.apache.org/jira/browse/LUCENE-5487">LUCENE-5487</a> */
-    static int scoreRange(LeafCollector collector, Scorer scorer, TwoPhaseIterator twoPhase,
+    static int scoreRange(LeafCollector collector, DocIdSetIterator iterator, TwoPhaseIterator twoPhase,
         Bits acceptDocs, int currentDoc, int end) throws IOException {
       if (twoPhase == null) {
         while (currentDoc < end) {
           if (acceptDocs == null || acceptDocs.get(currentDoc)) {
             collector.collect(currentDoc);
           }
-          currentDoc = scorer.nextDoc();
+          currentDoc = iterator.nextDoc();
         }
         return currentDoc;
       } else {
@@ -211,9 +214,9 @@ public abstract class Weight {
      *  separate this from {@link #scoreRange} to help out
      *  hotspot.
      *  See <a href="https://issues.apache.org/jira/browse/LUCENE-5487">LUCENE-5487</a> */
-    static void scoreAll(LeafCollector collector, Scorer scorer, TwoPhaseIterator twoPhase, Bits acceptDocs) throws IOException {
+    static void scoreAll(LeafCollector collector, DocIdSetIterator iterator, TwoPhaseIterator twoPhase, Bits acceptDocs) throws IOException {
       if (twoPhase == null) {
-        for (int doc = scorer.nextDoc(); doc != DocIdSetIterator.NO_MORE_DOCS; doc = scorer.nextDoc()) {
+        for (int doc = iterator.nextDoc(); doc != DocIdSetIterator.NO_MORE_DOCS; doc = iterator.nextDoc()) {
           if (acceptDocs == null || acceptDocs.get(doc)) {
             collector.collect(doc);
           }

lucene/core/src/java/org/apache/lucene/search/package-info.java

@@ -427,13 +427,14 @@
  *     <p>The
  *         {@link org.apache.lucene.search.Scorer Scorer}
  *         abstract class provides common scoring functionality for all Scorer implementations and
- *         is the heart of the Lucene scoring process. The Scorer defines the following abstract (some of them are not
+ *         is the heart of the Lucene scoring process. The Scorer defines the following methods which
- *         yet abstract, but will be in future versions and should be considered as such now) methods which
+ *         must be implemented:
- *         must be implemented (some of them inherited from {@link org.apache.lucene.search.DocIdSetIterator DocIdSetIterator}):
  *         <ol>
  *             <li>
- *                 {@link org.apache.lucene.search.Scorer#nextDoc nextDoc()} &mdash; Advances to the next
+ *                 {@link org.apache.lucene.search.Scorer#iterator iterator()} &mdash; Return a
- *                 document that matches this Query, returning true if and only if there is another document that matches.</li>
+ *                 {@link org.apache.lucene.search.DocIdSetIterator DocIdSetIterator} that can iterate over all
+ *                 document that matches this Query.
+ *             </li>
  *             <li>
  *                 {@link org.apache.lucene.search.Scorer#docID docID()} &mdash; Returns the id of the
  *                 {@link org.apache.lucene.document.Document Document} that contains the match.
@@ -451,13 +452,6 @@
  *                 {@link org.apache.lucene.index.PostingsEnum#freq PostingsEnum.freq()}.
  *             </li>
  *             <li>
- *                 {@link org.apache.lucene.search.Scorer#advance advance()} &mdash; Skip ahead in
- *                 the document matches to the document whose id is greater than
- *                 or equal to the passed in value. In many instances, advance can be
- *                 implemented more efficiently than simply looping through all the matching documents until
- *                 the target document is identified.
- *             </li>
- *             <li>
  *                 {@link org.apache.lucene.search.Scorer#getChildren getChildren()} &mdash; Returns any child subscorers
  *                 underneath this scorer. This allows for users to navigate the scorer hierarchy and receive more fine-grained
  *                 details on the scoring process.
@@ -531,7 +525,7 @@
  * <p>Assuming a BooleanScorer2, we first initialize the Coordinator, which is used to apply the coord() 
  *   factor. We then get a internal Scorer based on the required, optional and prohibited parts of the query.
  *   Using this internal Scorer, the BooleanScorer2 then proceeds into a while loop based on the 
- *   {@link org.apache.lucene.search.Scorer#nextDoc Scorer.nextDoc()} method. The nextDoc() method advances 
+ *   {@link org.apache.lucene.search.DocIdSetIterator#nextDoc DocIdSetIterator.nextDoc()} method. The nextDoc() method advances 
  *   to the next document matching the query. This is an abstract method in the Scorer class and is thus 
  *   overridden by all derived  implementations. If you have a simple OR query your internal Scorer is most 
  *   likely a DisjunctionSumScorer, which essentially combines the scorers from the sub scorers of the OR'd terms.

lucene/core/src/java/org/apache/lucene/search/spans/ConjunctionSpans.java

@@ -23,7 +23,6 @@ import java.util.List;
 import org.apache.lucene.search.DocIdSetIterator;
 import org.apache.lucene.search.ConjunctionDISI;
 import org.apache.lucene.search.TwoPhaseIterator;
-import org.apache.lucene.search.similarities.Similarity;
 
 /**
  * Common super class for multiple sub spans required in a document.
@@ -34,13 +33,12 @@ abstract class ConjunctionSpans extends Spans {
   boolean atFirstInCurrentDoc; // a first start position is available in current doc for nextStartPosition
   boolean oneExhaustedInCurrentDoc; // one subspans exhausted in current doc
 
-  ConjunctionSpans(List<Spans> subSpans, SpanWeight weight, Similarity.SimScorer docScorer) {
+  ConjunctionSpans(List<Spans> subSpans) {
-    super(weight, docScorer);
     if (subSpans.size() < 2) {
       throw new IllegalArgumentException("Less than 2 subSpans.size():" + subSpans.size());
     }
     this.subSpans = subSpans.toArray(new Spans[subSpans.size()]);
-    this.conjunction = ConjunctionDISI.intersect(subSpans);
+    this.conjunction = ConjunctionDISI.intersectSpans(subSpans);
     this.atFirstInCurrentDoc = true; // ensure for doc -1 that start/end positions are -1
   }
 

lucene/core/src/java/org/apache/lucene/search/spans/ContainSpans.java

@@ -21,15 +21,13 @@ import java.io.IOException;
 import java.util.Arrays;
 import java.util.Objects;
 
-import org.apache.lucene.search.similarities.Similarity;
-
 abstract class ContainSpans extends ConjunctionSpans {
   Spans sourceSpans;
   Spans bigSpans;
   Spans littleSpans;
 
-  ContainSpans(SpanWeight weight, Similarity.SimScorer simScorer, Spans bigSpans, Spans littleSpans, Spans sourceSpans) {
+  ContainSpans(Spans bigSpans, Spans littleSpans, Spans sourceSpans) {
-    super(Arrays.asList(bigSpans, littleSpans), weight, simScorer);
+    super(Arrays.asList(bigSpans, littleSpans));
     this.bigSpans = Objects.requireNonNull(bigSpans);
     this.littleSpans = Objects.requireNonNull(littleSpans);
     this.sourceSpans = Objects.requireNonNull(sourceSpans);

lucene/core/src/java/org/apache/lucene/search/spans/FilterSpans.java

@@ -21,7 +21,6 @@ import java.io.IOException;
 import java.util.Objects;
 
 import org.apache.lucene.search.TwoPhaseIterator;
-import org.apache.lucene.search.similarities.Similarity;
 
 /**
  * A {@link Spans} implementation wrapping another spans instance,
@@ -36,8 +35,7 @@ public abstract class FilterSpans extends Spans {
   private int startPos = -1;
   
   /** Wrap the given {@link Spans}. */
-  protected FilterSpans(Spans in, Similarity.SimScorer docScorer) {
+  protected FilterSpans(Spans in) {
-    super((SpanWeight)in.getWeight(), docScorer);
     this.in = Objects.requireNonNull(in);
   }
   

lucene/core/src/java/org/apache/lucene/search/spans/NearSpansOrdered.java

@@ -20,8 +20,6 @@ package org.apache.lucene.search.spans;
 import java.io.IOException;
 import java.util.List;
 
-import org.apache.lucene.search.similarities.Similarity;
-
 /**
  * A Spans that is formed from the ordered subspans of a SpanNearQuery
  * where the subspans do not overlap and have a maximum slop between them.
@@ -52,8 +50,8 @@ public class NearSpansOrdered extends ConjunctionSpans {
 
   private final int allowedSlop;
 
-  public NearSpansOrdered(SpanWeight weight, int allowedSlop, List<Spans> subSpans, Similarity.SimScorer simScorer) throws IOException {
+  public NearSpansOrdered(int allowedSlop, List<Spans> subSpans) throws IOException {
-    super(subSpans, weight, simScorer);
+    super(subSpans);
     this.atFirstInCurrentDoc = true; // -1 startPosition/endPosition also at doc -1
     this.allowedSlop = allowedSlop;
   }
@@ -152,10 +150,5 @@ public class NearSpansOrdered extends ConjunctionSpans {
     }
   }
 
-  @Override
-  public String toString() {
-    return "NearSpansOrdered("+weight.getQuery().toString()+")@"+docID()+": "+startPosition()+" - "+endPosition();
-  }
-
 }
 

lucene/core/src/java/org/apache/lucene/search/spans/NearSpansUnordered.java

@@ -22,7 +22,6 @@ import java.util.ArrayList;
 import java.util.List;
 
 import org.apache.lucene.search.TwoPhaseIterator;
-import org.apache.lucene.search.similarities.Similarity;
 import org.apache.lucene.util.PriorityQueue;
 
 /**
@@ -38,9 +37,9 @@ public class NearSpansUnordered extends ConjunctionSpans {
 
   private SpanPositionQueue spanPositionQueue;
 
-  public NearSpansUnordered(SpanWeight weight, int allowedSlop, List<Spans> subSpans, Similarity.SimScorer simScorer)
+  public NearSpansUnordered(int allowedSlop, List<Spans> subSpans)
   throws IOException {
-    super(subSpans, weight, simScorer);
+    super(subSpans);
 
     this.subSpanCells = new ArrayList<>(subSpans.size());
     for (Spans subSpan : subSpans) { // sub spans in query order
@@ -77,7 +76,6 @@ public class NearSpansUnordered extends ConjunctionSpans {
     final Spans in;
 
     public SpansCell(Spans spans) {
-      super((SpanWeight) NearSpansUnordered.this.weight, NearSpansUnordered.this.docScorer);
       this.in = spans;
     }
 
@@ -267,13 +265,4 @@ public class NearSpansUnordered extends ConjunctionSpans {
     }
   }
 
-  @Override
-  public String toString() {
-    if (minPositionCell() != null) {
-      return getClass().getName() + "("+weight.getQuery().toString()+")@"+
-        (docID()+":"+startPosition()+"-"+endPosition());
-    } else {
-      return getClass().getName() + "("+weight.getQuery().toString()+")@ ?START?";
-    }
-  }
 }

lucene/core/src/java/org/apache/lucene/search/spans/ScoringWrapperSpans.java

@@ -35,7 +35,6 @@ public class ScoringWrapperSpans extends Spans {
    * @param simScorer  the SimScorer to use for scoring
    */
   public ScoringWrapperSpans(Spans spans, Similarity.SimScorer simScorer) {
-    super((SpanWeight) spans.getWeight(), simScorer);
     this.in = spans;
   }
 

lucene/core/src/java/org/apache/lucene/search/spans/SpanBoostQuery.java

@@ -151,7 +151,7 @@ public final class SpanBoostQuery extends SpanQuery {
       }
 
       @Override
-      public Scorer scorer(LeafReaderContext context) throws IOException {
+      public SpanScorer scorer(LeafReaderContext context) throws IOException {
         return weight.scorer(context);
       }
 

lucene/core/src/java/org/apache/lucene/search/spans/SpanContainingQuery.java

@@ -71,7 +71,7 @@ public final class SpanContainingQuery extends SpanContainQuery {
       Spans big = containerContained.get(0);
       Spans little = containerContained.get(1);
 
-      return new ContainSpans(this, getSimScorer(context), big, little, big) {
+      return new ContainSpans(big, little, big) {
 
         @Override
         boolean twoPhaseCurrentDocMatches() throws IOException {

lucene/core/src/java/org/apache/lucene/search/spans/SpanNearQuery.java

@@ -219,8 +219,8 @@ public class SpanNearQuery extends SpanQuery implements Cloneable {
       }
 
       // all NearSpans require at least two subSpans
-      return (!inOrder) ? new NearSpansUnordered(this, slop, subSpans, getSimScorer(context))
+      return (!inOrder) ? new NearSpansUnordered(slop, subSpans)
-          : new NearSpansOrdered(this, slop, subSpans, getSimScorer(context));
+          : new NearSpansOrdered(slop, subSpans);
     }
 
     @Override
@@ -330,7 +330,6 @@ public class SpanNearQuery extends SpanQuery implements Cloneable {
     final int width;
 
     GapSpans(int width) {
-      super(null, null);
       this.width = width;
     }
 

lucene/core/src/java/org/apache/lucene/search/spans/SpanNotQuery.java

@@ -132,7 +132,7 @@ public final class SpanNotQuery extends SpanQuery {
       TwoPhaseIterator excludeTwoPhase = excludeSpans.asTwoPhaseIterator();
       DocIdSetIterator excludeApproximation = excludeTwoPhase == null ? null : excludeTwoPhase.approximation();
 
-      return new FilterSpans(includeSpans, getSimScorer(context)) {
+      return new FilterSpans(includeSpans) {
         // last document we have checked matches() against for the exclusion, and failed
         // when using approximations, so we don't call it again, and pass thru all inclusions.
         int lastApproxDoc = -1;

lucene/core/src/java/org/apache/lucene/search/spans/SpanOrQuery.java

@@ -169,20 +169,20 @@ public final class SpanOrQuery extends SpanQuery {
         return new ScoringWrapperSpans(subSpans.get(0), getSimScorer(context));
       }
 
-      DisiPriorityQueue<Spans> byDocQueue = new DisiPriorityQueue<>(subSpans.size());
+      DisiPriorityQueue byDocQueue = new DisiPriorityQueue(subSpans.size());
       for (Spans spans : subSpans) {
-        byDocQueue.add(new DisiWrapper<>(spans));
+        byDocQueue.add(new DisiWrapper(spans));
       }
 
       SpanPositionQueue byPositionQueue = new SpanPositionQueue(subSpans.size()); // when empty use -1
 
-      return new Spans(this, getSimScorer(context)) {
+      return new Spans() {
         Spans topPositionSpans = null;
 
         @Override
         public int nextDoc() throws IOException {
           topPositionSpans = null;
-          DisiWrapper<Spans> topDocSpans = byDocQueue.top();
+          DisiWrapper topDocSpans = byDocQueue.top();
           int currentDoc = topDocSpans.doc;
           do {
             topDocSpans.doc = topDocSpans.iterator.nextDoc();
@@ -194,7 +194,7 @@ public final class SpanOrQuery extends SpanQuery {
         @Override
         public int advance(int target) throws IOException {
           topPositionSpans = null;
-          DisiWrapper<Spans> topDocSpans = byDocQueue.top();
+          DisiWrapper topDocSpans = byDocQueue.top();
           do {
             topDocSpans.doc = topDocSpans.iterator.advance(target);
             topDocSpans = byDocQueue.updateTop();
@@ -204,7 +204,7 @@ public final class SpanOrQuery extends SpanQuery {
 
         @Override
         public int docID() {
-          DisiWrapper<Spans> topDocSpans = byDocQueue.top();
+          DisiWrapper topDocSpans = byDocQueue.top();
           return topDocSpans.doc;
         }
 
@@ -213,7 +213,7 @@ public final class SpanOrQuery extends SpanQuery {
           float sumMatchCost = 0; // See also DisjunctionScorer.asTwoPhaseIterator()
           long sumApproxCost = 0;
 
-          for (DisiWrapper<Spans> w : byDocQueue) {
+          for (DisiWrapper w : byDocQueue) {
             if (w.twoPhaseView != null) {
               long costWeight = (w.cost <= 1) ? 1 : w.cost;
               sumMatchCost += w.twoPhaseView.matchCost() * costWeight;
@@ -228,7 +228,7 @@ public final class SpanOrQuery extends SpanQuery {
 
           final float matchCost = sumMatchCost / sumApproxCost;
 
-          return new TwoPhaseIterator(new DisjunctionDISIApproximation<Spans>(byDocQueue)) {
+          return new TwoPhaseIterator(new DisjunctionDISIApproximation(byDocQueue)) {
             @Override
             public boolean matches() throws IOException {
               return twoPhaseCurrentDocMatches();
@@ -246,9 +246,9 @@ public final class SpanOrQuery extends SpanQuery {
         void computePositionsCost() {
           float sumPositionsCost = 0;
           long sumCost = 0;
-          for (DisiWrapper<Spans> w : byDocQueue) {
+          for (DisiWrapper w : byDocQueue) {
             long costWeight = (w.cost <= 1) ? 1 : w.cost;
-            sumPositionsCost += w.iterator.positionsCost() * costWeight;
+            sumPositionsCost += w.spans.positionsCost() * costWeight;
             sumCost += costWeight;
           }
           positionsCost = sumPositionsCost / sumCost;
@@ -265,7 +265,7 @@ public final class SpanOrQuery extends SpanQuery {
         int lastDocTwoPhaseMatched = -1;
 
         boolean twoPhaseCurrentDocMatches() throws IOException {
-          DisiWrapper<Spans> listAtCurrentDoc = byDocQueue.topList();
+          DisiWrapper listAtCurrentDoc = byDocQueue.topList();
           // remove the head of the list as long as it does not match
           final int currentDoc = listAtCurrentDoc.doc;
           while (listAtCurrentDoc.twoPhaseView != null) {
@@ -289,9 +289,9 @@ public final class SpanOrQuery extends SpanQuery {
         void fillPositionQueue() throws IOException { // called at first nextStartPosition
           assert byPositionQueue.size() == 0;
           // add all matching Spans at current doc to byPositionQueue
-          DisiWrapper<Spans> listAtCurrentDoc = byDocQueue.topList();
+          DisiWrapper listAtCurrentDoc = byDocQueue.topList();
           while (listAtCurrentDoc != null) {
-            Spans spansAtDoc = listAtCurrentDoc.iterator;
+            Spans spansAtDoc = listAtCurrentDoc.spans;
             if (lastDocTwoPhaseMatched == listAtCurrentDoc.doc) { // matched by DisjunctionDisiApproximation
               if (listAtCurrentDoc.twoPhaseView != null) { // matched by approximation
                 if (listAtCurrentDoc.lastApproxNonMatchDoc == listAtCurrentDoc.doc) { // matches() returned false

lucene/core/src/java/org/apache/lucene/search/spans/SpanPositionCheckQuery.java

@@ -94,7 +94,7 @@ public abstract class SpanPositionCheckQuery extends SpanQuery implements Clonea
     @Override
     public Spans getSpans(final LeafReaderContext context, Postings requiredPostings) throws IOException {
       Spans matchSpans = matchWeight.getSpans(context, requiredPostings);
-      return (matchSpans == null) ? null : new FilterSpans(matchSpans, getSimScorer(context)) {
+      return (matchSpans == null) ? null : new FilterSpans(matchSpans) {
         @Override
         protected AcceptStatus accept(Spans candidate) throws IOException {
           return acceptPosition(candidate);

lucene/core/src/java/org/apache/lucene/search/spans/SpanScorer.java

@@ -0,0 +1,145 @@
+package org.apache.lucene.search.spans;
+
+/*
+ * 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 java.util.Objects;
+
+import org.apache.lucene.search.DocIdSetIterator;
+import org.apache.lucene.search.Scorer;
+import org.apache.lucene.search.TwoPhaseIterator;
+import org.apache.lucene.search.similarities.Similarity;
+
+/**
+ * A basic {@link Scorer} over {@link Spans}.
+ * @lucene.experimental
+ */
+public class SpanScorer extends Scorer {
+
+  protected final Spans spans;
+  protected final Similarity.SimScorer docScorer;
+
+  /** accumulated sloppy freq (computed in setFreqCurrentDoc) */
+  private float freq;
+  /** number of matches (computed in setFreqCurrentDoc) */
+  private int numMatches;
+  private int lastScoredDoc = -1; // last doc we called setFreqCurrentDoc() for
+
+  /** Sole constructor. */
+  public SpanScorer(SpanWeight weight, Spans spans, Similarity.SimScorer docScorer) {
+    super(weight);
+    this.spans = Objects.requireNonNull(spans);
+    this.docScorer = docScorer;
+  }
+
+  @Override
+  public int docID() {
+    return spans.docID();
+  }
+
+  @Override
+  public DocIdSetIterator iterator() {
+    return spans;
+  }
+
+  @Override
+  public TwoPhaseIterator twoPhaseIterator() {
+    return spans.asTwoPhaseIterator();
+  }
+
+  /**
+   * Score the current doc. The default implementation scores the doc
+   * with the similarity using the slop-adjusted {@link #freq}.
+   */
+  protected float scoreCurrentDoc() throws IOException {
+    assert docScorer != null : getClass() + " has a null docScorer!";
+    return docScorer.score(docID(), freq);
+  }
+
+  /**
+   * Sets {@link #freq} and {@link #numMatches} for the current document.
+   * <p>
+   * This will be called at most once per document.
+   */
+  protected final void setFreqCurrentDoc() throws IOException {
+    freq = 0.0f;
+    numMatches = 0;
+
+    spans.doStartCurrentDoc();
+
+    assert spans.startPosition() == -1 : "incorrect initial start position, " + this.toString();
+    assert spans.endPosition() == -1 : "incorrect initial end position, " + this.toString();
+    int prevStartPos = -1;
+    int prevEndPos = -1;
+
+    int startPos = spans.nextStartPosition();
+    assert startPos != Spans.NO_MORE_POSITIONS : "initial startPos NO_MORE_POSITIONS, " + this.toString();
+    do {
+      assert startPos >= prevStartPos;
+      int endPos = spans.endPosition();
+      assert endPos != Spans.NO_MORE_POSITIONS;
+      // This assertion can fail for Or spans on the same term:
+      // assert (startPos != prevStartPos) || (endPos > prevEndPos) : "non increased endPos="+endPos;
+      assert (startPos != prevStartPos) || (endPos >= prevEndPos) : "decreased endPos="+endPos;
+      numMatches++;
+      if (docScorer == null) {  // scores not required, break out here
+        freq = 1;
+        return;
+      }
+      freq += docScorer.computeSlopFactor(spans.width());
+      spans.doCurrentSpans();
+      prevStartPos = startPos;
+      prevEndPos = endPos;
+      startPos = spans.nextStartPosition();
+    } while (startPos != Spans.NO_MORE_POSITIONS);
+
+    assert spans.startPosition() == Spans.NO_MORE_POSITIONS : "incorrect final start position, " + this.toString();
+    assert spans.endPosition() == Spans.NO_MORE_POSITIONS : "incorrect final end position, " + this.toString();
+  }
+
+  /**
+   * Ensure setFreqCurrentDoc is called, if not already called for the current doc.
+   */
+  private void ensureFreq() throws IOException {
+    int currentDoc = docID();
+    if (lastScoredDoc != currentDoc) {
+      setFreqCurrentDoc();
+      lastScoredDoc = currentDoc;
+    }
+  }
+
+  @Override
+  public final float score() throws IOException {
+    ensureFreq();
+    return scoreCurrentDoc();
+  }
+
+  @Override
+  public final int freq() throws IOException {
+    ensureFreq();
+    return numMatches;
+  }
+
+  /** Returns the intermediate "sloppy freq" adjusted for edit distance
+   *  @lucene.internal */
+  final float sloppyFreq() throws IOException {
+    ensureFreq();
+    return freq;
+  }
+
+}

lucene/core/src/java/org/apache/lucene/search/spans/SpanTermQuery.java

@@ -118,7 +118,7 @@ public class SpanTermQuery extends SpanQuery {
 
       final PostingsEnum postings = termsEnum.postings(null, requiredPostings.getRequiredPostings());
       float positionsCost = termPositionsCost(termsEnum) * PHRASE_TO_SPAN_TERM_POSITIONS_COST;
-      return new TermSpans(this, getSimScorer(context), postings, term, positionsCost);
+      return new TermSpans(getSimScorer(context), postings, term, positionsCost);
     }
   }
 

lucene/core/src/java/org/apache/lucene/search/spans/SpanWeight.java

@@ -27,7 +27,6 @@ import org.apache.lucene.index.TermContext;
 import org.apache.lucene.search.CollectionStatistics;
 import org.apache.lucene.search.Explanation;
 import org.apache.lucene.search.IndexSearcher;
-import org.apache.lucene.search.Scorer;
 import org.apache.lucene.search.TermStatistics;
 import org.apache.lucene.search.Weight;
 import org.apache.lucene.search.similarities.Similarity;
@@ -130,8 +129,13 @@ public abstract class SpanWeight extends Weight {
   }
 
   @Override
-  public Scorer scorer(LeafReaderContext context) throws IOException {
+  public SpanScorer scorer(LeafReaderContext context) throws IOException {
-    return getSpans(context, Postings.POSITIONS);
+    final Spans spans = getSpans(context, Postings.POSITIONS);
+    if (spans == null) {
+      return null;
+    }
+    final Similarity.SimScorer docScorer = getSimScorer(context);
+    return new SpanScorer(this, spans, docScorer);
   }
 
   /**
@@ -146,9 +150,9 @@ public abstract class SpanWeight extends Weight {
 
   @Override
   public Explanation explain(LeafReaderContext context, int doc) throws IOException {
-    Spans scorer = (Spans) scorer(context);
+    SpanScorer scorer = scorer(context);
     if (scorer != null) {
-      int newDoc = scorer.advance(doc);
+      int newDoc = scorer.iterator().advance(doc);
       if (newDoc == doc) {
         float freq = scorer.sloppyFreq();
         SimScorer docScorer = similarity.simScorer(simWeight, context);

lucene/core/src/java/org/apache/lucene/search/spans/SpanWithinQuery.java

@@ -72,7 +72,7 @@ public final class SpanWithinQuery extends SpanContainQuery {
       Spans big = containerContained.get(0);
       Spans little = containerContained.get(1);
 
-      return new ContainSpans(this, getSimScorer(context), big, little, little) {
+      return new ContainSpans(big, little, little) {
 
         @Override
         boolean twoPhaseCurrentDocMatches() throws IOException {

lucene/core/src/java/org/apache/lucene/search/spans/Spans.java

@@ -19,8 +19,9 @@ package org.apache.lucene.search.spans;
 
 import java.io.IOException;
 
+import org.apache.lucene.search.DocIdSetIterator;
 import org.apache.lucene.search.Scorer;
-import org.apache.lucene.search.similarities.Similarity;
+import org.apache.lucene.search.TwoPhaseIterator;
 import org.apache.lucene.search.similarities.Similarity.SimScorer;
 
 /** Iterates through combinations of start/end positions per-doc.
@@ -28,24 +29,10 @@ import org.apache.lucene.search.similarities.Similarity.SimScorer;
  *  These are enumerated in order, by increasing document number, within that by
  *  increasing start position and finally by increasing end position.
  */
-public abstract class Spans extends Scorer {
+public abstract class Spans extends DocIdSetIterator {
 
   public static final int NO_MORE_POSITIONS = Integer.MAX_VALUE;
 
-  protected final Similarity.SimScorer docScorer;
-
-  protected Spans(SpanWeight weight, SimScorer docScorer) {
-    super(weight);
-    this.docScorer = docScorer;
-  }
-
-  /** accumulated sloppy freq (computed in setFreqCurrentDoc) */
-  protected float freq;
-  /** number of matches (computed in setFreqCurrentDoc) */
-  protected int numMatches;
-
-  private int lastScoredDoc = -1; // last doc we called setFreqCurrentDoc() for
-
   /**
    * Returns the next start position for the current doc.
    * There is always at least one start/end position per doc.
@@ -97,6 +84,16 @@ public abstract class Spans extends Scorer {
    */
   public abstract float positionsCost();
 
+  /**
+   * Optional method: Return a {@link TwoPhaseIterator} view of this
+   * {@link Scorer}. A return value of {@code null} indicates that
+   * two-phase iteration is not supported.
+   * @see Scorer#twoPhaseIterator()
+   */
+  public TwoPhaseIterator asTwoPhaseIterator() {
+    return null;
+  }
+
   @Override
   public String toString() {
     StringBuilder sb = new StringBuilder();
@@ -109,58 +106,6 @@ public abstract class Spans extends Scorer {
     return sb.toString();
   }
 
-  /**
-   * Ensure setFreqCurrentDoc is called, if not already called for the current doc.
-   */
-  private void ensureFreq() throws IOException {
-    int currentDoc = docID();
-    if (lastScoredDoc != currentDoc) {
-      setFreqCurrentDoc();
-      lastScoredDoc = currentDoc;
-    }
-  }
-
-  /**
-   * Sets {@link #freq} and {@link #numMatches} for the current document.
-   * <p>
-   * This will be called at most once per document.
-   */
-  protected final void setFreqCurrentDoc() throws IOException {
-    freq = 0.0f;
-    numMatches = 0;
-
-    doStartCurrentDoc();
-
-    assert startPosition() == -1 : "incorrect initial start position, " + this.toString();
-    assert endPosition() == -1 : "incorrect initial end position, " + this.toString();
-    int prevStartPos = -1;
-    int prevEndPos = -1;
-
-    int startPos = nextStartPosition();
-    assert startPos != Spans.NO_MORE_POSITIONS : "initial startPos NO_MORE_POSITIONS, " + this.toString();
-    do {
-      assert startPos >= prevStartPos;
-      int endPos = endPosition();
-      assert endPos != Spans.NO_MORE_POSITIONS;
-      // This assertion can fail for Or spans on the same term:
-      // assert (startPos != prevStartPos) || (endPos > prevEndPos) : "non increased endPos="+endPos;
-      assert (startPos != prevStartPos) || (endPos >= prevEndPos) : "decreased endPos="+endPos;
-      numMatches++;
-      if (docScorer == null) {  // scores not required, break out here
-        freq = 1;
-        return;
-      }
-      freq += docScorer.computeSlopFactor(width());
-      doCurrentSpans();
-      prevStartPos = startPos;
-      prevEndPos = endPos;
-      startPos = nextStartPosition();
-    } while (startPos != Spans.NO_MORE_POSITIONS);
-
-    assert startPosition() == Spans.NO_MORE_POSITIONS : "incorrect final start position, " + this.toString();
-    assert endPosition() == Spans.NO_MORE_POSITIONS : "incorrect final end position, " + this.toString();
-  }
-
   /**
    * Called before the current doc's frequency is calculated
    */
@@ -171,32 +116,4 @@ public abstract class Spans extends Scorer {
    */
   protected void doCurrentSpans() throws IOException {}
 
-  /**
-   * Score the current doc. The default implementation scores the doc
-   * with the similarity using the slop-adjusted {@link #freq}.
-   */
-  protected float scoreCurrentDoc() throws IOException {
-    assert docScorer != null : getClass() + " has a null docScorer!";
-    return docScorer.score(docID(), freq);
-  }
-
-  @Override
-  public final float score() throws IOException {
-    ensureFreq();
-    return scoreCurrentDoc();
-  }
-
-  @Override
-  public final int freq() throws IOException {
-    ensureFreq();
-    return numMatches;
-  }
-
-  /** Returns the intermediate "sloppy freq" adjusted for edit distance
-   *  @lucene.internal */
-  final float sloppyFreq() throws IOException {
-    ensureFreq();
-    return freq;
-  }
-
 }

lucene/core/src/java/org/apache/lucene/search/spans/TermSpans.java

@@ -39,9 +39,8 @@ public class TermSpans extends Spans {
   protected boolean readPayload;
   private final float positionsCost;
 
-  public TermSpans(SpanWeight weight, Similarity.SimScorer scorer,
+  public TermSpans(Similarity.SimScorer scorer,
                     PostingsEnum postings, Term term, float positionsCost) {
-    super(weight, scorer);
     this.postings = Objects.requireNonNull(postings);
     this.term = Objects.requireNonNull(term);
     this.doc = -1;

lucene/core/src/test/org/apache/lucene/search/JustCompileSearch.java

@@ -177,17 +177,7 @@ final class JustCompileSearch {
     }
 
     @Override
-    public int nextDoc() {
+    public DocIdSetIterator iterator() {
-      throw new UnsupportedOperationException(UNSUPPORTED_MSG);
-    }
-    
-    @Override
-    public int advance(int target) {
-      throw new UnsupportedOperationException(UNSUPPORTED_MSG);
-    }
-
-    @Override
-    public long cost() {
       throw new UnsupportedOperationException(UNSUPPORTED_MSG);
     }
   }

lucene/core/src/test/org/apache/lucene/search/TestBooleanCoord.java

@@ -824,7 +824,7 @@ public class TestBooleanCoord extends LuceneTestCase {
     Weight weight = searcher.createNormalizedWeight(query, true);
     Scorer scorer = weight.scorer(reader.leaves().get(0));
     assertTrue(scorer.docID() == -1 || scorer.docID() == DocIdSetIterator.NO_MORE_DOCS);
-    assertEquals(0, scorer.nextDoc());
+    assertEquals(0, scorer.iterator().nextDoc());
     assertEquals(expected, scorer.score(), 0.0001f);
 
     // test bulk scorer

lucene/core/src/test/org/apache/lucene/search/TestBooleanQuery.java

@@ -357,7 +357,7 @@ public class TestBooleanQuery extends LuceneTestCase {
 
       // First pass: just use .nextDoc() to gather all hits
       final List<ScoreDoc> hits = new ArrayList<>();
-      while(scorer.nextDoc() != DocIdSetIterator.NO_MORE_DOCS) {
+      while(scorer.iterator().nextDoc() != DocIdSetIterator.NO_MORE_DOCS) {
         hits.add(new ScoreDoc(scorer.docID(), scorer.score()));
       }
 
@@ -384,12 +384,12 @@ public class TestBooleanQuery extends LuceneTestCase {
           if (left == 1 || random().nextBoolean()) {
             // next
             nextUpto = 1+upto;
-            nextDoc = scorer.nextDoc();
+            nextDoc = scorer.iterator().nextDoc();
           } else {
             // advance
             int inc = TestUtil.nextInt(random(), 1, left - 1);
             nextUpto = inc + upto;
-            nextDoc = scorer.advance(hits.get(nextUpto).doc);
+            nextDoc = scorer.iterator().advance(hits.get(nextUpto).doc);
           }
 
           if (nextUpto == hits.size()) {
@@ -658,7 +658,7 @@ public class TestBooleanQuery extends LuceneTestCase {
     final Weight weight = searcher.createNormalizedWeight(q.build(), random().nextBoolean());
     final Scorer scorer = weight.scorer(searcher.getIndexReader().leaves().get(0));
     assertTrue(scorer instanceof ConjunctionScorer);
-    assertNotNull(scorer.asTwoPhaseIterator());
+    assertNotNull(scorer.twoPhaseIterator());
 
     reader.close();
     w.close();
@@ -687,7 +687,7 @@ public class TestBooleanQuery extends LuceneTestCase {
     final Weight weight = searcher.createNormalizedWeight(q.build(), random().nextBoolean());
     final Scorer scorer = weight.scorer(reader.leaves().get(0));
     assertTrue(scorer instanceof DisjunctionScorer);
-    assertNotNull(scorer.asTwoPhaseIterator());
+    assertNotNull(scorer.twoPhaseIterator());
 
     reader.close();
     w.close();
@@ -718,7 +718,7 @@ public class TestBooleanQuery extends LuceneTestCase {
     final Weight weight = searcher.createNormalizedWeight(q.build(), random().nextBoolean());
     final Scorer scorer = weight.scorer(searcher.getIndexReader().leaves().get(0));
     assertTrue(scorer instanceof BoostedScorer || scorer instanceof ExactPhraseScorer);
-    assertNotNull(scorer.asTwoPhaseIterator());
+    assertNotNull(scorer.twoPhaseIterator());
 
     reader.close();
     w.close();
@@ -747,7 +747,7 @@ public class TestBooleanQuery extends LuceneTestCase {
     final Weight weight = searcher.createNormalizedWeight(q.build(), random().nextBoolean());
     final Scorer scorer = weight.scorer(reader.leaves().get(0));
     assertTrue(scorer instanceof ReqExclScorer);
-    assertNotNull(scorer.asTwoPhaseIterator());
+    assertNotNull(scorer.twoPhaseIterator());
 
     reader.close();
     w.close();
@@ -776,7 +776,7 @@ public class TestBooleanQuery extends LuceneTestCase {
     final Weight weight = searcher.createNormalizedWeight(q.build(), true);
     final Scorer scorer = weight.scorer(reader.leaves().get(0));
     assertTrue(scorer instanceof ReqOptSumScorer);
-    assertNotNull(scorer.asTwoPhaseIterator());
+    assertNotNull(scorer.twoPhaseIterator());
 
     reader.close();
     w.close();

lucene/core/src/test/org/apache/lucene/search/TestBooleanRewrites.java

@@ -91,7 +91,7 @@ public class TestBooleanRewrites extends LuceneTestCase {
     query2.add(new TermQuery(new Term("field", "b")), Occur.SHOULD);
     final Weight weight = searcher.createNormalizedWeight(query2.build(), true);
     final Scorer scorer = weight.scorer(reader.leaves().get(0));
-    assertEquals(0, scorer.nextDoc());
+    assertEquals(0, scorer.iterator().nextDoc());
     assertTrue(scorer.getClass().getName(), scorer instanceof FilterScorer);
     assertEquals(0f, scorer.score(), 0f);
 

lucene/core/src/test/org/apache/lucene/search/TestCachingCollector.java

@@ -41,14 +41,8 @@ public class TestCachingCollector extends LuceneTestCase {
     public int docID() { return 0; }
 
     @Override
-    public int nextDoc() throws IOException { return 0; }
+    public DocIdSetIterator iterator() {
-
+      throw new UnsupportedOperationException();
-    @Override
-    public int advance(int target) throws IOException { return 0; }
-
-    @Override
-    public long cost() {
-      return 1;
     }
   }
   

lucene/core/src/test/org/apache/lucene/search/TestConjunctionDISI.java

@@ -52,8 +52,8 @@ public class TestConjunctionDISI extends LuceneTestCase {
   /**
    * Create a {@link Scorer} that wraps the given {@link DocIdSetIterator}. It
    * also accepts a {@link TwoPhaseIterator} view, which is exposed in
-   * {@link Scorer#asTwoPhaseIterator()}. When the two-phase view is not null,
+   * {@link Scorer#twoPhaseIterator()}. When the two-phase view is not null,
-   * then {@link Scorer#nextDoc()} and {@link Scorer#advance(int)} will raise
+   * then {@link DocIdSetIterator#nextDoc()} and {@link DocIdSetIterator#advance(int)} will raise
    * an exception in order to make sure that {@link ConjunctionDISI} takes
    * advantage of the {@link TwoPhaseIterator} view.
    */
@@ -61,15 +61,11 @@ public class TestConjunctionDISI extends LuceneTestCase {
     return new Scorer(null) {
 
       @Override
-      public TwoPhaseIterator asTwoPhaseIterator() {
+      public DocIdSetIterator iterator() {
-        return twoPhaseIterator;
+        return new DocIdSetIterator() {
-      }
 
           @Override
           public int docID() {
-        if (twoPhaseIterator != null) {
-          throw new UnsupportedOperationException("ConjunctionDISI should call the two-phase iterator");
-        }
             return it.docID();
           }
 
@@ -96,6 +92,21 @@ public class TestConjunctionDISI extends LuceneTestCase {
             }
             return it.cost();
           }
+        };
+      }
+
+      @Override
+      public TwoPhaseIterator twoPhaseIterator() {
+        return twoPhaseIterator;
+      }
+
+      @Override
+      public int docID() {
+        if (twoPhaseIterator != null) {
+          throw new UnsupportedOperationException("ConjunctionDISI should call the two-phase iterator");
+        }
+        return it.docID();
+      }
 
       @Override
       public float score() throws IOException {
@@ -154,13 +165,13 @@ public class TestConjunctionDISI extends LuceneTestCase {
       final int maxDoc = TestUtil.nextInt(random(), 100, 10000);
       final int numIterators = TestUtil.nextInt(random(), 2, 5);
       final FixedBitSet[] sets = new FixedBitSet[numIterators];
-      final DocIdSetIterator[] iterators = new DocIdSetIterator[numIterators];
+      final Scorer[] iterators = new Scorer[numIterators];
       for (int i = 0; i < iterators.length; ++i) {
         final FixedBitSet set = randomSet(maxDoc);
         if (random().nextBoolean()) {
           // simple iterator
           sets[i] = set;
-          iterators[i] = new BitDocIdSet(set).iterator();
+          iterators[i] = new ConstantScoreScorer(null, 0f, new BitDocIdSet(set).iterator());
         } else {
           // scorer with approximation
           final FixedBitSet confirmed = clearRandomBits(set);
@@ -170,7 +181,7 @@ public class TestConjunctionDISI extends LuceneTestCase {
         }
       }
 
-      final ConjunctionDISI conjunction = ConjunctionDISI.intersect(Arrays.asList(iterators));
+      final ConjunctionDISI conjunction = ConjunctionDISI.intersectScorers(Arrays.asList(iterators));
       assertEquals(intersect(sets), toBitSet(maxDoc, conjunction));
     }
   }
@@ -182,14 +193,14 @@ public class TestConjunctionDISI extends LuceneTestCase {
       final int maxDoc = TestUtil.nextInt(random(), 100, 10000);
       final int numIterators = TestUtil.nextInt(random(), 2, 5);
       final FixedBitSet[] sets = new FixedBitSet[numIterators];
-      final DocIdSetIterator[] iterators = new DocIdSetIterator[numIterators];
+      final Scorer[] iterators = new Scorer[numIterators];
       boolean hasApproximation = false;
       for (int i = 0; i < iterators.length; ++i) {
         final FixedBitSet set = randomSet(maxDoc);
         if (random().nextBoolean()) {
           // simple iterator
           sets[i] = set;
-          iterators[i] = new BitDocIdSet(set).iterator();
+          iterators[i] = new ConstantScoreScorer(null, 0f, new BitDocIdSet(set).iterator());
         } else {
           // scorer with approximation
           final FixedBitSet confirmed = clearRandomBits(set);
@@ -200,7 +211,7 @@ public class TestConjunctionDISI extends LuceneTestCase {
         }
       }
 
-      final ConjunctionDISI conjunction = ConjunctionDISI.intersect(Arrays.asList(iterators));
+      final ConjunctionDISI conjunction = ConjunctionDISI.intersectScorers(Arrays.asList(iterators));
       TwoPhaseIterator twoPhaseIterator = conjunction.asTwoPhaseIterator();
       assertEquals(hasApproximation, twoPhaseIterator != null);
       if (hasApproximation) {
@@ -216,15 +227,15 @@ public class TestConjunctionDISI extends LuceneTestCase {
       final int maxDoc = TestUtil.nextInt(random(), 100, 10000);
       final int numIterators = TestUtil.nextInt(random(), 2, 5);
       final FixedBitSet[] sets = new FixedBitSet[numIterators];
-      DocIdSetIterator conjunction = null;
+      Scorer conjunction = null;
       boolean hasApproximation = false;
       for (int i = 0; i < numIterators; ++i) {
         final FixedBitSet set = randomSet(maxDoc);
-        final DocIdSetIterator newIterator;
+        final Scorer newIterator;
         if (random().nextBoolean()) {
           // simple iterator
           sets[i] = set;
-          newIterator = new BitDocIdSet(set).iterator();
+          newIterator = new ConstantScoreScorer(null, 0f, new BitDocIdSet(set).iterator());
         } else {
           // scorer with approximation
           final FixedBitSet confirmed = clearRandomBits(set);
@@ -237,17 +248,17 @@ public class TestConjunctionDISI extends LuceneTestCase {
         if (conjunction == null) {
           conjunction = newIterator;
         } else {
-          final ConjunctionDISI conj = ConjunctionDISI.intersect(Arrays.asList(conjunction, newIterator));
+          final ConjunctionDISI conj = ConjunctionDISI.intersectScorers(Arrays.asList(conjunction, newIterator));
           conjunction = scorer(conj, conj.asTwoPhaseIterator());
         }
       }
 
-      TwoPhaseIterator twoPhaseIterator = ((Scorer) conjunction).asTwoPhaseIterator();
+      TwoPhaseIterator twoPhaseIterator = ((Scorer) conjunction).twoPhaseIterator();
       assertEquals(hasApproximation, twoPhaseIterator != null);
       if (hasApproximation) {
         assertEquals(intersect(sets), toBitSet(maxDoc, TwoPhaseIterator.asDocIdSetIterator(twoPhaseIterator)));
       } else {
-        assertEquals(intersect(sets), toBitSet(maxDoc, conjunction));
+        assertEquals(intersect(sets), toBitSet(maxDoc, conjunction.iterator()));
       }
     }
   }
@@ -258,47 +269,47 @@ public class TestConjunctionDISI extends LuceneTestCase {
       final int maxDoc = TestUtil.nextInt(random(), 100, 10000);
       final int numIterators = TestUtil.nextInt(random(), 5, 10);
       final FixedBitSet[] sets = new FixedBitSet[numIterators];
-      final List<DocIdSetIterator> iterators = new LinkedList<>();
+      final List<Scorer> scorers = new LinkedList<>();
       for (int i = 0; i < numIterators; ++i) {
         final FixedBitSet set = randomSet(maxDoc);
         if (random().nextBoolean()) {
           // simple iterator
           sets[i] = set;
-          iterators.add(new BitDocIdSet(set).iterator());
+          scorers.add(new ConstantScoreScorer(null, 0f, new BitDocIdSet(set).iterator()));
         } else {
           // scorer with approximation
           final FixedBitSet confirmed = clearRandomBits(set);
           sets[i] = confirmed;
           final TwoPhaseIterator approximation = approximation(new BitDocIdSet(set).iterator(), confirmed);
-          iterators.add(scorer(approximation));
+          scorers.add(scorer(approximation));
         }
       }
 
       // make some sub sequences into sub conjunctions
       final int subIters = atLeast(3);
-      for (int subIter = 0; subIter < subIters && iterators.size() > 3; ++subIter) {
+      for (int subIter = 0; subIter < subIters && scorers.size() > 3; ++subIter) {
-        final int subSeqStart = TestUtil.nextInt(random(), 0, iterators.size() - 2);
+        final int subSeqStart = TestUtil.nextInt(random(), 0, scorers.size() - 2);
-        final int subSeqEnd = TestUtil.nextInt(random(), subSeqStart + 2, iterators.size());
+        final int subSeqEnd = TestUtil.nextInt(random(), subSeqStart + 2, scorers.size());
-        List<DocIdSetIterator> subIterators = iterators.subList(subSeqStart, subSeqEnd);
+        List<Scorer> subIterators = scorers.subList(subSeqStart, subSeqEnd);
-        DocIdSetIterator subConjunction;
+        Scorer subConjunction;
         if (wrapWithScorer) {
           subConjunction = new ConjunctionScorer(null, subIterators, Collections.emptyList());
         } else {
-          subConjunction = ConjunctionDISI.intersect(subIterators);
+          subConjunction = new ConstantScoreScorer(null, 0f, ConjunctionDISI.intersectScorers(subIterators));
         }
-        iterators.set(subSeqStart, subConjunction);
+        scorers.set(subSeqStart, subConjunction);
         int toRemove = subSeqEnd - subSeqStart - 1;
         while (toRemove-- > 0) {
-          iterators.remove(subSeqStart + 1);
+          scorers.remove(subSeqStart + 1);
         }
       }
-      if (iterators.size() == 1) {
+      if (scorers.size() == 1) {
         // ConjunctionDISI needs two iterators
-        iterators.add(DocIdSetIterator.all(maxDoc));
+        scorers.add(new ConstantScoreScorer(null, 0f, DocIdSetIterator.all(maxDoc)));
       }
 
 
-      final ConjunctionDISI conjunction = ConjunctionDISI.intersect(iterators);
+      final ConjunctionDISI conjunction = ConjunctionDISI.intersectScorers(scorers);
       assertEquals(intersect(sets), toBitSet(maxDoc, conjunction));
     }
   }

lucene/core/src/test/org/apache/lucene/search/TestConstantScoreQuery.java

@@ -216,7 +216,7 @@ public class TestConstantScoreQuery extends LuceneTestCase {
 
     final Weight weight = searcher.createNormalizedWeight(q, true);
     final Scorer scorer = weight.scorer(searcher.getIndexReader().leaves().get(0));
-    assertNotNull(scorer.asTwoPhaseIterator());
+    assertNotNull(scorer.twoPhaseIterator());
 
     reader.close();
     w.close();

lucene/core/src/test/org/apache/lucene/search/TestDisjunctionMaxQuery.java

@@ -180,7 +180,7 @@ public class TestDisjunctionMaxQuery extends LuceneTestCase {
     final Weight dw = s.createNormalizedWeight(dq, true);
     LeafReaderContext context = (LeafReaderContext)s.getTopReaderContext();
     final Scorer ds = dw.scorer(context);
-    final boolean skipOk = ds.advance(3) != DocIdSetIterator.NO_MORE_DOCS;
+    final boolean skipOk = ds.iterator().advance(3) != DocIdSetIterator.NO_MORE_DOCS;
     if (skipOk) {
       fail("firsttime skipTo found a match? ... "
           + r.document(ds.docID()).get("id"));
@@ -197,7 +197,7 @@ public class TestDisjunctionMaxQuery extends LuceneTestCase {
     LeafReaderContext context = (LeafReaderContext)s.getTopReaderContext();
     final Scorer ds = dw.scorer(context);
     assertTrue("firsttime skipTo found no match",
-        ds.advance(3) != DocIdSetIterator.NO_MORE_DOCS);
+        ds.iterator().advance(3) != DocIdSetIterator.NO_MORE_DOCS);
     assertEquals("found wrong docid", "d4", r.document(ds.docID()).get("id"));
   }
   

lucene/core/src/test/org/apache/lucene/search/TestMinShouldMatch2.java

@@ -147,36 +147,40 @@ public class TestMinShouldMatch2 extends LuceneTestCase {
   
   private void assertNext(Scorer expected, Scorer actual) throws Exception {
     if (actual == null) {
-      assertEquals(DocIdSetIterator.NO_MORE_DOCS, expected.nextDoc());
+      assertEquals(DocIdSetIterator.NO_MORE_DOCS, expected.iterator().nextDoc());
       return;
     }
     int doc;
-    while ((doc = expected.nextDoc()) != DocIdSetIterator.NO_MORE_DOCS) {
+    DocIdSetIterator expectedIt = expected.iterator();
-      assertEquals(doc, actual.nextDoc());
+    DocIdSetIterator actualIt = actual.iterator();
+    while ((doc = expectedIt.nextDoc()) != DocIdSetIterator.NO_MORE_DOCS) {
+      assertEquals(doc, actualIt.nextDoc());
       assertEquals(expected.freq(), actual.freq());
       float expectedScore = expected.score();
       float actualScore = actual.score();
       assertEquals(expectedScore, actualScore, CheckHits.explainToleranceDelta(expectedScore, actualScore));
     }
-    assertEquals(DocIdSetIterator.NO_MORE_DOCS, actual.nextDoc());
+    assertEquals(DocIdSetIterator.NO_MORE_DOCS, actualIt.nextDoc());
   }
   
   private void assertAdvance(Scorer expected, Scorer actual, int amount) throws Exception {
     if (actual == null) {
-      assertEquals(DocIdSetIterator.NO_MORE_DOCS, expected.nextDoc());
+      assertEquals(DocIdSetIterator.NO_MORE_DOCS, expected.iterator().nextDoc());
       return;
     }
+    DocIdSetIterator expectedIt = expected.iterator();
+    DocIdSetIterator actualIt = actual.iterator();
     int prevDoc = 0;
     int doc;
-    while ((doc = expected.advance(prevDoc+amount)) != DocIdSetIterator.NO_MORE_DOCS) {
+    while ((doc = expectedIt.advance(prevDoc+amount)) != DocIdSetIterator.NO_MORE_DOCS) {
-      assertEquals(doc, actual.advance(prevDoc+amount));
+      assertEquals(doc, actualIt.advance(prevDoc+amount));
       assertEquals(expected.freq(), actual.freq());
       float expectedScore = expected.score();
       float actualScore = actual.score();
       assertEquals(expectedScore, actualScore, CheckHits.explainToleranceDelta(expectedScore, actualScore));
       prevDoc = doc;
     }
-    assertEquals(DocIdSetIterator.NO_MORE_DOCS, actual.advance(prevDoc+amount));
+    assertEquals(DocIdSetIterator.NO_MORE_DOCS, actualIt.advance(prevDoc+amount));
   }
   
   /** simple test for next(): minShouldMatch=2 on 3 terms (one common, one medium, one rare) */
@@ -360,6 +364,10 @@ public class TestMinShouldMatch2 extends LuceneTestCase {
       return currentDoc;
     }
 
+    @Override
+    public DocIdSetIterator iterator() {
+      return new DocIdSetIterator() {
+        
         @Override
         public int nextDoc() throws IOException {
           assert currentDoc != NO_MORE_DOCS;
@@ -393,5 +401,12 @@ public class TestMinShouldMatch2 extends LuceneTestCase {
         public long cost() {
           return maxDoc;
         }
+        
+        @Override
+        public int docID() {
+          return currentDoc;
+        }
+      };
+    }
   }
 }

lucene/core/src/test/org/apache/lucene/search/TestPositiveScoresOnlyCollector.java

@@ -43,6 +43,14 @@ public class TestPositiveScoresOnlyCollector extends LuceneTestCase {
 
     @Override public int docID() { return idx; }
 
+    @Override
+    public DocIdSetIterator iterator() {
+      return new DocIdSetIterator() {
+        @Override
+        public int docID() {
+          return idx;
+        }
+
         @Override public int nextDoc() {
           return ++idx != scores.length ? idx : NO_MORE_DOCS;
         }
@@ -56,6 +64,8 @@ public class TestPositiveScoresOnlyCollector extends LuceneTestCase {
         public long cost() {
           return scores.length;
         } 
+      };
+    }
   }
 
   // The scores must have positive as well as negative values
@@ -90,7 +100,7 @@ public class TestPositiveScoresOnlyCollector extends LuceneTestCase {
     Collector c = new PositiveScoresOnlyCollector(tdc);
     LeafCollector ac = c.getLeafCollector(ir.leaves().get(0));
     ac.setScorer(s);
-    while (s.nextDoc() != DocIdSetIterator.NO_MORE_DOCS) {
+    while (s.iterator().nextDoc() != DocIdSetIterator.NO_MORE_DOCS) {
       ac.collect(0);
     }
     TopDocs td = tdc.topDocs();

lucene/core/src/test/org/apache/lucene/search/TestQueryRescorer.java

@@ -445,6 +445,15 @@ public class TestQueryRescorer extends LuceneTestCase {
               return 1;
             }
 
+            @Override
+            public DocIdSetIterator iterator() {
+              return new DocIdSetIterator() {
+
+                @Override
+                public int docID() {
+                  return docID;
+                }
+
                 @Override
                 public long cost() {
                   return 1;
@@ -464,6 +473,8 @@ public class TestQueryRescorer extends LuceneTestCase {
                   docID = target;
                   return docID;
                 }
+              };
+            }
 
             @Override
             public float score() throws IOException {

lucene/core/src/test/org/apache/lucene/search/TestScoreCachingWrappingScorer.java

@@ -49,6 +49,11 @@ public class TestScoreCachingWrappingScorer extends LuceneTestCase {
 
     @Override public int docID() { return doc; }
 
+    @Override
+    public DocIdSetIterator iterator() {
+      return new DocIdSetIterator() {
+        @Override public int docID() { return doc; }
+
         @Override public int nextDoc() {
           return ++doc < scores.length ? doc : NO_MORE_DOCS;
         }
@@ -62,6 +67,8 @@ public class TestScoreCachingWrappingScorer extends LuceneTestCase {
         public long cost() {
           return scores.length;
         }
+      };
+    }
   }
   
   private static final class ScoreCachingCollector extends SimpleCollector {
@@ -116,7 +123,7 @@ public class TestScoreCachingWrappingScorer extends LuceneTestCase {
     
     // We need to iterate on the scorer so that its doc() advances.
     int doc;
-    while ((doc = s.nextDoc()) != DocIdSetIterator.NO_MORE_DOCS) {
+    while ((doc = s.iterator().nextDoc()) != DocIdSetIterator.NO_MORE_DOCS) {
       scc.collect(doc);
     }
     

lucene/core/src/test/org/apache/lucene/search/TestTermScorer.java

@@ -133,11 +133,11 @@ public class TestTermScorer extends LuceneTestCase {
     LeafReaderContext context = (LeafReaderContext) indexSearcher.getTopReaderContext();
     Scorer ts = weight.scorer(context);
     assertTrue("next did not return a doc",
-        ts.nextDoc() != DocIdSetIterator.NO_MORE_DOCS);
+        ts.iterator().nextDoc() != DocIdSetIterator.NO_MORE_DOCS);
     assertTrue("next did not return a doc",
-        ts.nextDoc() != DocIdSetIterator.NO_MORE_DOCS);
+        ts.iterator().nextDoc() != DocIdSetIterator.NO_MORE_DOCS);
     assertTrue("next returned a doc and it should not have",
-        ts.nextDoc() == DocIdSetIterator.NO_MORE_DOCS);
+        ts.iterator().nextDoc() == DocIdSetIterator.NO_MORE_DOCS);
   }
   
   public void testAdvance() throws Exception {
@@ -149,7 +149,7 @@ public class TestTermScorer extends LuceneTestCase {
     assertTrue(indexSearcher.getTopReaderContext() instanceof LeafReaderContext);
     LeafReaderContext context = (LeafReaderContext) indexSearcher.getTopReaderContext();
     Scorer ts = weight.scorer(context);
-    assertTrue("Didn't skip", ts.advance(3) != DocIdSetIterator.NO_MORE_DOCS);
+    assertTrue("Didn't skip", ts.iterator().advance(3) != DocIdSetIterator.NO_MORE_DOCS);
     // The next doc should be doc 5
     assertTrue("doc should be number 5", ts.docID() == 5);
   }
@@ -186,7 +186,7 @@ public class TestTermScorer extends LuceneTestCase {
     
     Weight weight = indexSearcher.createNormalizedWeight(termQuery, true);
     try {
-      weight.scorer(forbiddenNorms.getContext()).nextDoc();
+      weight.scorer(forbiddenNorms.getContext()).iterator().nextDoc();
       fail("Should load norms");
     } catch (AssertionError e) {
       // ok
@@ -194,6 +194,6 @@ public class TestTermScorer extends LuceneTestCase {
     
     weight = indexSearcher.createNormalizedWeight(termQuery, false);
     // should not fail this time since norms are not necessary
-    weight.scorer(forbiddenNorms.getContext()).nextDoc();
+    weight.scorer(forbiddenNorms.getContext()).iterator().nextDoc();
   }
 }

lucene/core/src/test/org/apache/lucene/search/TestTopFieldCollector.java

@@ -236,18 +236,8 @@ public class TestTopFieldCollector extends LuceneTestCase {
                     }
 
                     @Override
-                    public int nextDoc() throws IOException {
+                    public DocIdSetIterator iterator() {
-                      return scorer.nextDoc();
+                      return scorer.iterator();
-                    }
-
-                    @Override
-                    public int advance(int target) throws IOException {
-                      return scorer.advance(target);
-                    }
-
-                    @Override
-                    public long cost() {
-                      return scorer.cost();
                     }
                     
                   };

lucene/core/src/test/org/apache/lucene/search/spans/JustCompileSearchSpans.java

@@ -34,10 +34,6 @@ final class JustCompileSearchSpans {
 
   static final class JustCompileSpans extends Spans {
 
-    JustCompileSpans() {
-      super(null, null);
-    }
-
     @Override
     public int docID() {
       throw new UnsupportedOperationException(UNSUPPORTED_MSG);

lucene/core/src/test/org/apache/lucene/search/spans/TestFilterSpans.java

@@ -18,8 +18,6 @@ package org.apache.lucene.search.spans;
  */
 
 import java.lang.reflect.Method;
-import java.util.HashSet;
-import java.util.Set;
 
 import org.apache.lucene.util.LuceneTestCase;
 import org.junit.Test;
@@ -29,13 +27,9 @@ public class TestFilterSpans extends LuceneTestCase {
   @Test
   public void testOverrides() throws Exception {
     // verify that all methods of Spans are overridden by FilterSpans,
-    // except those under the 'exclude' list
-    Set<Method> exclude = new HashSet<>();
-    exclude.add(FilterSpans.class.getMethod("freq"));
-    exclude.add(FilterSpans.class.getMethod("score"));
     for (Method m : FilterSpans.class.getMethods()) {
       if (m.getDeclaringClass() == Spans.class) {
-        assertTrue("method " + m.getName() + " not overridden!", exclude.contains(m));
+        fail("method " + m.getName() + " not overridden!");
       }
     }
   }

lucene/core/src/test/org/apache/lucene/search/spans/TestNearSpansOrdered.java

@@ -194,7 +194,7 @@ public class TestNearSpansOrdered extends LuceneTestCase {
     IndexReaderContext topReaderContext = searcher.getTopReaderContext();
     LeafReaderContext leave = topReaderContext.leaves().get(0);
     Scorer s = w.scorer(leave);
-    assertEquals(1, s.advance(1));
+    assertEquals(1, s.iterator().advance(1));
   }
 
   public void testOverlappedOrderedSpan() throws Exception {

lucene/core/src/test/org/apache/lucene/search/spans/TestSpans.java

@@ -303,12 +303,12 @@ public class TestSpans extends LuceneTestCase {
         searcher.setSimilarity(oldSim);
       }
       if (i == subIndex) {
-        assertTrue("first doc", spanScorer.nextDoc() != DocIdSetIterator.NO_MORE_DOCS);
+        assertTrue("first doc", spanScorer.iterator().nextDoc() != DocIdSetIterator.NO_MORE_DOCS);
         assertEquals("first doc number", spanScorer.docID() + ctx.docBase, 11);
         float score = spanScorer.score();
         assertTrue("first doc score should be zero, " + score, score == 0.0f);
       } else {
-        assertTrue("no second doc", spanScorer == null || spanScorer.nextDoc() == DocIdSetIterator.NO_MORE_DOCS);
+        assertTrue("no second doc", spanScorer == null || spanScorer.iterator().nextDoc() == DocIdSetIterator.NO_MORE_DOCS);
       }
     }
   }

lucene/expressions/src/java/org/apache/lucene/expressions/FakeScorer.java

@@ -19,6 +19,7 @@ package org.apache.lucene.expressions;
 
 import java.io.IOException;
 
+import org.apache.lucene.search.DocIdSetIterator;
 import org.apache.lucene.search.Scorer;
 
 class FakeScorer extends Scorer {
@@ -37,17 +38,7 @@ class FakeScorer extends Scorer {
   }
 
   @Override
-  public int nextDoc() throws IOException {
+  public DocIdSetIterator iterator() {
-    throw new UnsupportedOperationException();
-  }
-
-  @Override
-  public int advance(int target) throws IOException {
-    throw new UnsupportedOperationException();
-  }
-
-  @Override
-  public long cost() {
     throw new UnsupportedOperationException();
   }
 

lucene/expressions/src/test/org/apache/lucene/expressions/TestExpressionValueSource.java

@@ -128,17 +128,19 @@ public class TestExpressionValueSource extends LuceneTestCase {
     
     // everything
     ValueSourceScorer scorer = values.getRangeScorer(leaf.reader(), "4", "40", true, true);
-    assertEquals(-1, scorer.docID());
+    DocIdSetIterator iter = scorer.iterator();
-    assertEquals(0, scorer.nextDoc());
+    assertEquals(-1, iter.docID());
-    assertEquals(1, scorer.nextDoc());
+    assertEquals(0, iter.nextDoc());
-    assertEquals(2, scorer.nextDoc());
+    assertEquals(1, iter.nextDoc());
-    assertEquals(DocIdSetIterator.NO_MORE_DOCS, scorer.nextDoc());
+    assertEquals(2, iter.nextDoc());
+    assertEquals(DocIdSetIterator.NO_MORE_DOCS, iter.nextDoc());
 
     // just the first doc
     scorer = values.getRangeScorer(leaf.reader(), "4", "40", false, false);
+    iter = scorer.iterator();
     assertEquals(-1, scorer.docID());
-    assertEquals(0, scorer.nextDoc());
+    assertEquals(0, iter.nextDoc());
-    assertEquals(DocIdSetIterator.NO_MORE_DOCS, scorer.nextDoc());
+    assertEquals(DocIdSetIterator.NO_MORE_DOCS, iter.nextDoc());
   }
   
   public void testEquals() throws Exception {

lucene/facet/src/java/org/apache/lucene/facet/DrillSidewaysScorer.java

@@ -44,6 +44,7 @@ class DrillSidewaysScorer extends BulkScorer {
 
   // DrillDown DocsEnums:
   private final Scorer baseScorer;
+  private final DocIdSetIterator baseIterator;
 
   private final LeafReaderContext context;
 
@@ -60,13 +61,14 @@ class DrillSidewaysScorer extends BulkScorer {
     this.dims = dims;
     this.context = context;
     this.baseScorer = baseScorer;
+    this.baseIterator = baseScorer.iterator();
     this.drillDownCollector = drillDownCollector;
     this.scoreSubDocsAtOnce = scoreSubDocsAtOnce;
   }
 
   @Override
   public long cost() {
-    return baseScorer.cost();
+    return baseIterator.cost();
   }
 
   @Override
@@ -94,13 +96,8 @@ class DrillSidewaysScorer extends BulkScorer {
       dim.sidewaysLeafCollector.setScorer(scorer);
     }
 
-    // TODO: if we ever allow null baseScorer ... it will
-    // mean we DO score docs out of order ... hmm, or if we
-    // change up the order of the conjuntions below
-    assert baseScorer != null;
-
     // some scorers, eg ReqExlScorer, can hit NPE if cost is called after nextDoc
-    long baseQueryCost = baseScorer.cost();
+    long baseQueryCost = baseIterator.cost();
 
     final int numDims = dims.length;
 
@@ -115,7 +112,7 @@ class DrillSidewaysScorer extends BulkScorer {
     }
 
     // Position all scorers to their first matching doc:
-    baseScorer.nextDoc();
+    baseIterator.nextDoc();
     for (DocsAndCost dim : dims) {
       dim.approximation.nextDoc();
     }
@@ -157,7 +154,7 @@ class DrillSidewaysScorer extends BulkScorer {
 
     nextDoc: while (docID != PostingsEnum.NO_MORE_DOCS) {
       if (acceptDocs != null && acceptDocs.get(docID) == false) {
-        docID = baseScorer.nextDoc();
+        docID = baseIterator.nextDoc();
         continue;
       }
       LeafCollector failedCollector = null;
@@ -182,7 +179,7 @@ class DrillSidewaysScorer extends BulkScorer {
             // More than one dim fails on this document, so
             // it's neither a hit nor a near-miss; move to
             // next doc:
-            docID = baseScorer.nextDoc();
+            docID = baseIterator.nextDoc();
             continue nextDoc;
           } else {
             failedCollector = dim.sidewaysLeafCollector;
@@ -204,7 +201,7 @@ class DrillSidewaysScorer extends BulkScorer {
         collectNearMiss(failedCollector);
       }
 
-      docID = baseScorer.nextDoc();
+      docID = baseIterator.nextDoc();
     }
   }
 
@@ -316,9 +313,9 @@ class DrillSidewaysScorer extends BulkScorer {
         int ddDocID = docIDs[slot0];
         assert ddDocID != -1;
 
-        int baseDocID = baseScorer.docID();
+        int baseDocID = baseIterator.docID();
         if (baseDocID < ddDocID) {
-          baseDocID = baseScorer.advance(ddDocID);
+          baseDocID = baseIterator.advance(ddDocID);
         }
         if (baseDocID == ddDocID) {
           //if (DEBUG) {
@@ -437,7 +434,7 @@ class DrillSidewaysScorer extends BulkScorer {
       //  System.out.println("\ncycle nextChunkStart=" + nextChunkStart + " docIds[0]=" + docIDs[0]);
       //}
       int filledCount = 0;
-      int docID = baseScorer.docID();
+      int docID = baseIterator.docID();
       //if (DEBUG) {
       //  System.out.println("  base docID=" + docID);
       //}
@@ -456,7 +453,7 @@ class DrillSidewaysScorer extends BulkScorer {
           missingDims[slot] = 0;
           counts[slot] = 1;
         }
-        docID = baseScorer.nextDoc();
+        docID = baseIterator.nextDoc();
       }
 
       if (filledCount == 0) {
@@ -590,11 +587,6 @@ class DrillSidewaysScorer extends BulkScorer {
       super(null);
     }
 
-    @Override
-    public int advance(int target) {
-      throw new UnsupportedOperationException("FakeScorer doesn't support advance(int)");
-    }
-
     @Override
     public int docID() {
       return collectDocID;
@@ -606,7 +598,7 @@ class DrillSidewaysScorer extends BulkScorer {
     }
 
     @Override
-    public int nextDoc() {
+    public DocIdSetIterator iterator() {
       throw new UnsupportedOperationException("FakeScorer doesn't support nextDoc()");
     }
     
@@ -615,11 +607,6 @@ class DrillSidewaysScorer extends BulkScorer {
       return collectScore;
     }
 
-    @Override
-    public long cost() {
-      return baseScorer.cost();
-    }
-
     @Override
     public Collection<ChildScorer> getChildren() {
       return Collections.singletonList(new ChildScorer(baseScorer, "MUST"));
@@ -640,9 +627,9 @@ class DrillSidewaysScorer extends BulkScorer {
     LeafCollector sidewaysLeafCollector;
 
     DocsAndCost(Scorer scorer, Collector sidewaysCollector) {
-      final TwoPhaseIterator twoPhase = scorer.asTwoPhaseIterator();
+      final TwoPhaseIterator twoPhase = scorer.twoPhaseIterator();
       if (twoPhase == null) {
-        this.approximation = scorer;
+        this.approximation = scorer.iterator();
         this.twoPhase = null;
       } else {
         this.approximation = twoPhase.approximation();

lucene/facet/src/java/org/apache/lucene/facet/range/DoubleRange.java

@@ -164,10 +164,11 @@ public final class DoubleRange extends Range {
           if (fastMatchWeight == null) {
             approximation = DocIdSetIterator.all(maxDoc);
           } else {
-            approximation = fastMatchWeight.scorer(context);
+            Scorer s = fastMatchWeight.scorer(context);
-            if (approximation == null) {
+            if (s == null) {
               return null;
             }
+            approximation = s.iterator();
           }
 
           final FunctionValues values = valueSource.getValues(Collections.emptyMap(), context);

lucene/facet/src/java/org/apache/lucene/facet/range/DoubleRangeFacetCounts.java

@@ -36,6 +36,7 @@ import org.apache.lucene.search.DocIdSet;
 import org.apache.lucene.search.DocIdSetIterator;
 import org.apache.lucene.search.IndexSearcher;
 import org.apache.lucene.search.Query;
+import org.apache.lucene.search.Scorer;
 import org.apache.lucene.search.Weight;
 import org.apache.lucene.util.NumericUtils;
 
@@ -105,10 +106,11 @@ public class DoubleRangeFacetCounts extends RangeFacetCounts {
         final IndexSearcher searcher = new IndexSearcher(topLevelContext);
         searcher.setQueryCache(null);
         final Weight fastMatchWeight = searcher.createNormalizedWeight(fastMatchQuery, false);
-        fastMatchDocs = fastMatchWeight.scorer(hits.context);
+        Scorer s = fastMatchWeight.scorer(hits.context);
-        if (fastMatchDocs == null) {
+        if (s == null) {
           continue;
         }
+        fastMatchDocs = s.iterator();
       } else {
         fastMatchDocs = null;
       }

lucene/facet/src/java/org/apache/lucene/facet/range/LongRange.java

@@ -156,10 +156,11 @@ public final class LongRange extends Range {
           if (fastMatchWeight == null) {
             approximation = DocIdSetIterator.all(maxDoc);
           } else {
-            approximation = fastMatchWeight.scorer(context);
+            Scorer s = fastMatchWeight.scorer(context);
-            if (approximation == null) {
+            if (s == null) {
               return null;
             }
+            approximation = s.iterator();
           }
 
           final FunctionValues values = valueSource.getValues(Collections.emptyMap(), context);

lucene/facet/src/java/org/apache/lucene/facet/range/LongRangeFacetCounts.java

@@ -33,6 +33,7 @@ import org.apache.lucene.search.DocIdSet;
 import org.apache.lucene.search.DocIdSetIterator;
 import org.apache.lucene.search.IndexSearcher;
 import org.apache.lucene.search.Query;
+import org.apache.lucene.search.Scorer;
 import org.apache.lucene.search.Weight;
 
 /** {@link Facets} implementation that computes counts for
@@ -86,10 +87,11 @@ public class LongRangeFacetCounts extends RangeFacetCounts {
         final IndexSearcher searcher = new IndexSearcher(topLevelContext);
         searcher.setQueryCache(null);
         final Weight fastMatchWeight = searcher.createNormalizedWeight(fastMatchQuery, false);
-        fastMatchDocs = fastMatchWeight.scorer(hits.context);
+        Scorer s = fastMatchWeight.scorer(hits.context);
-        if (fastMatchDocs == null) {
+        if (s == null) {
           continue;
         }
+        fastMatchDocs = s.iterator();
       } else {
         fastMatchDocs = null;
       }

lucene/facet/src/java/org/apache/lucene/facet/taxonomy/FakeScorer.java

@@ -19,6 +19,7 @@ package org.apache.lucene.facet.taxonomy;
 
 import java.io.IOException;
 
+import org.apache.lucene.search.DocIdSetIterator;
 import org.apache.lucene.search.Scorer;
 
 class FakeScorer extends Scorer {
@@ -37,17 +38,7 @@ class FakeScorer extends Scorer {
   }
 
   @Override
-  public int nextDoc() throws IOException {
+  public DocIdSetIterator iterator() {
-    throw new UnsupportedOperationException();
-  }
-
-  @Override
-  public int advance(int target) throws IOException {
-    throw new UnsupportedOperationException();
-  }
-
-  @Override
-  public long cost() {
     throw new UnsupportedOperationException();
   }
 

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

@@ -478,7 +478,12 @@ public class BlockGroupingCollector extends SimpleCollector {
     subDocUpto = 0;
     docBase = readerContext.docBase;
     //System.out.println("setNextReader base=" + docBase + " r=" + readerContext.reader);
-    lastDocPerGroupBits = lastDocPerGroup.scorer(readerContext);
+    Scorer s = lastDocPerGroup.scorer(readerContext);
+    if (s == null) {
+      lastDocPerGroupBits = null;
+    } else {
+      lastDocPerGroupBits = s.iterator();
+    }
     groupEndDocID = -1;
 
     currentReaderContext = readerContext;

lucene/grouping/src/java/org/apache/lucene/search/grouping/FakeScorer.java

@@ -19,6 +19,7 @@ package org.apache.lucene.search.grouping;
 
 import java.io.IOException;
 
+import org.apache.lucene.search.DocIdSetIterator;
 import org.apache.lucene.search.Scorer;
 
 class FakeScorer extends Scorer {
@@ -37,17 +38,7 @@ class FakeScorer extends Scorer {
   }
 
   @Override
-  public int nextDoc() throws IOException {
+  public DocIdSetIterator iterator() {
-    throw new UnsupportedOperationException();
-  }
-
-  @Override
-  public int advance(int target) throws IOException {
-    throw new UnsupportedOperationException();
-  }
-
-  @Override
-  public long cost() {
     throw new UnsupportedOperationException();
   }
 

lucene/join/src/java/org/apache/lucene/search/join/BaseGlobalOrdinalScorer.java

@@ -22,21 +22,20 @@ import org.apache.lucene.search.DocIdSetIterator;
 import org.apache.lucene.search.Scorer;
 import org.apache.lucene.search.TwoPhaseIterator;
 import org.apache.lucene.search.Weight;
-import org.apache.lucene.util.LongBitSet;
 
 import java.io.IOException;
 
 abstract class BaseGlobalOrdinalScorer extends Scorer {
 
   final SortedDocValues values;
-  final Scorer approximationScorer;
+  final DocIdSetIterator approximation;
 
   float score;
 
-  public BaseGlobalOrdinalScorer(Weight weight, SortedDocValues values, Scorer approximationScorer) {
+  public BaseGlobalOrdinalScorer(Weight weight, SortedDocValues values, DocIdSetIterator approximationScorer) {
     super(weight);
     this.values = values;
-    this.approximationScorer = approximationScorer;
+    this.approximation = approximationScorer;
   }
 
   @Override
@@ -46,45 +45,19 @@ abstract class BaseGlobalOrdinalScorer extends Scorer {
 
   @Override
   public int docID() {
-    return approximationScorer.docID();
+    return approximation.docID();
   }
 
   @Override
-  public int nextDoc() throws IOException {
+  public DocIdSetIterator iterator() {
-    return advance(approximationScorer.docID() + 1);
+    return TwoPhaseIterator.asDocIdSetIterator(twoPhaseIterator());
   }
 
   @Override
-  public TwoPhaseIterator asTwoPhaseIterator() {
+  public TwoPhaseIterator twoPhaseIterator() {
-    final DocIdSetIterator approximation = new DocIdSetIterator() {
-      @Override
-      public int docID() {
-        return approximationScorer.docID();
-      }
-
-      @Override
-      public int nextDoc() throws IOException {
-        return approximationScorer.nextDoc();
-      }
-
-      @Override
-      public int advance(int target) throws IOException {
-        return approximationScorer.advance(target);
-      }
-
-      @Override
-      public long cost() {
-        return approximationScorer.cost();
-      }
-    };
     return createTwoPhaseIterator(approximation);
   }
 
-  @Override
-  public long cost() {
-    return approximationScorer.cost();
-  }
-
   @Override
   public int freq() throws IOException {
     return 1;

lucene/join/src/java/org/apache/lucene/search/join/FakeScorer.java

@@ -19,6 +19,7 @@ package org.apache.lucene.search.join;
 
 import java.io.IOException;
 
+import org.apache.lucene.search.DocIdSetIterator;
 import org.apache.lucene.search.Scorer;
 
 class FakeScorer extends Scorer {
@@ -37,17 +38,7 @@ class FakeScorer extends Scorer {
   }
 
   @Override
-  public int nextDoc() throws IOException {
+  public DocIdSetIterator iterator() {
-    throw new UnsupportedOperationException();
-  }
-
-  @Override
-  public int advance(int target) throws IOException {
-    throw new UnsupportedOperationException();
-  }
-
-  @Override
-  public long cost() {
     throw new UnsupportedOperationException();
   }
 

lucene/join/src/java/org/apache/lucene/search/join/GlobalOrdinalsQuery.java

@@ -135,9 +135,9 @@ final class GlobalOrdinalsQuery extends Query {
         return null;
       }
       if (globalOrds != null) {
-        return new OrdinalMapScorer(this, score(), foundOrds, values, approximationScorer, globalOrds.getGlobalOrds(context.ord));
+        return new OrdinalMapScorer(this, score(), foundOrds, values, approximationScorer.iterator(), globalOrds.getGlobalOrds(context.ord));
       } {
-        return new SegmentOrdinalScorer(this, score(), foundOrds, values, approximationScorer);
+        return new SegmentOrdinalScorer(this, score(), foundOrds, values, approximationScorer.iterator());
       }
     }
 
@@ -148,34 +148,20 @@ final class GlobalOrdinalsQuery extends Query {
     final LongBitSet foundOrds;
     final LongValues segmentOrdToGlobalOrdLookup;
 
-    public OrdinalMapScorer(Weight weight, float score, LongBitSet foundOrds, SortedDocValues values, Scorer approximationScorer, LongValues segmentOrdToGlobalOrdLookup) {
+    public OrdinalMapScorer(Weight weight, float score, LongBitSet foundOrds, SortedDocValues values, DocIdSetIterator approximationScorer, LongValues segmentOrdToGlobalOrdLookup) {
       super(weight, values, approximationScorer);
       this.score = score;
       this.foundOrds = foundOrds;
       this.segmentOrdToGlobalOrdLookup = segmentOrdToGlobalOrdLookup;
     }
 
-    @Override
-    public int advance(int target) throws IOException {
-      for (int docID = approximationScorer.advance(target); docID < NO_MORE_DOCS; docID = approximationScorer.nextDoc()) {
-        final long segmentOrd = values.getOrd(docID);
-        if (segmentOrd != -1) {
-          final long globalOrd = segmentOrdToGlobalOrdLookup.get(segmentOrd);
-          if (foundOrds.get(globalOrd)) {
-            return docID;
-          }
-        }
-      }
-      return NO_MORE_DOCS;
-    }
-
     @Override
     protected TwoPhaseIterator createTwoPhaseIterator(DocIdSetIterator approximation) {
       return new TwoPhaseIterator(approximation) {
 
         @Override
         public boolean matches() throws IOException {
-          final long segmentOrd = values.getOrd(approximationScorer.docID());
+          final long segmentOrd = values.getOrd(approximation.docID());
           if (segmentOrd != -1) {
             final long globalOrd = segmentOrdToGlobalOrdLookup.get(segmentOrd);
             if (foundOrds.get(globalOrd)) {
@@ -197,32 +183,19 @@ final class GlobalOrdinalsQuery extends Query {
 
     final LongBitSet foundOrds;
 
-    public SegmentOrdinalScorer(Weight weight, float score, LongBitSet foundOrds, SortedDocValues values, Scorer approximationScorer) {
+    public SegmentOrdinalScorer(Weight weight, float score, LongBitSet foundOrds, SortedDocValues values, DocIdSetIterator approximationScorer) {
       super(weight, values, approximationScorer);
       this.score = score;
       this.foundOrds = foundOrds;
     }
 
-    @Override
-    public int advance(int target) throws IOException {
-      for (int docID = approximationScorer.advance(target); docID < NO_MORE_DOCS; docID = approximationScorer.nextDoc()) {
-        final long segmentOrd = values.getOrd(docID);
-        if (segmentOrd != -1) {
-          if (foundOrds.get(segmentOrd)) {
-            return docID;
-          }
-        }
-      }
-      return NO_MORE_DOCS;
-    }
-
     @Override
     protected TwoPhaseIterator createTwoPhaseIterator(DocIdSetIterator approximation) {
       return new TwoPhaseIterator(approximation) {
 
         @Override
         public boolean matches() throws IOException {
-          final long segmentOrd = values.getOrd(approximationScorer.docID());
+          final long segmentOrd = values.getOrd(approximation.docID());
           if (segmentOrd != -1) {
             if (foundOrds.get(segmentOrd)) {
               return true;

lucene/join/src/java/org/apache/lucene/search/join/GlobalOrdinalsWithScoreQuery.java

@@ -160,9 +160,9 @@ final class GlobalOrdinalsWithScoreQuery extends Query {
       if (approximationScorer == null) {
         return null;
       } else if (globalOrds != null) {
-        return new OrdinalMapScorer(this, collector, values, approximationScorer, globalOrds.getGlobalOrds(context.ord));
+        return new OrdinalMapScorer(this, collector, values, approximationScorer.iterator(), globalOrds.getGlobalOrds(context.ord));
       } else {
-        return new SegmentOrdinalScorer(this, collector, values, approximationScorer);
+        return new SegmentOrdinalScorer(this, collector, values, approximationScorer.iterator());
       }
     }
 
@@ -173,34 +173,19 @@ final class GlobalOrdinalsWithScoreQuery extends Query {
     final LongValues segmentOrdToGlobalOrdLookup;
     final GlobalOrdinalsWithScoreCollector collector;
 
-    public OrdinalMapScorer(Weight weight, GlobalOrdinalsWithScoreCollector collector, SortedDocValues values, Scorer approximationScorer, LongValues segmentOrdToGlobalOrdLookup) {
+    public OrdinalMapScorer(Weight weight, GlobalOrdinalsWithScoreCollector collector, SortedDocValues values, DocIdSetIterator approximation, LongValues segmentOrdToGlobalOrdLookup) {
-      super(weight, values, approximationScorer);
+      super(weight, values, approximation);
       this.segmentOrdToGlobalOrdLookup = segmentOrdToGlobalOrdLookup;
       this.collector = collector;
     }
 
-    @Override
-    public int advance(int target) throws IOException {
-      for (int docID = approximationScorer.advance(target); docID < NO_MORE_DOCS; docID = approximationScorer.nextDoc()) {
-        final long segmentOrd = values.getOrd(docID);
-        if (segmentOrd != -1) {
-          final int globalOrd = (int) segmentOrdToGlobalOrdLookup.get(segmentOrd);
-          if (collector.match(globalOrd)) {
-            score = collector.score(globalOrd);
-            return docID;
-          }
-        }
-      }
-      return NO_MORE_DOCS;
-    }
-
     @Override
     protected TwoPhaseIterator createTwoPhaseIterator(DocIdSetIterator approximation) {
       return new TwoPhaseIterator(approximation) {
 
         @Override
         public boolean matches() throws IOException {
-          final long segmentOrd = values.getOrd(approximationScorer.docID());
+          final long segmentOrd = values.getOrd(approximation.docID());
           if (segmentOrd != -1) {
             final int globalOrd = (int) segmentOrdToGlobalOrdLookup.get(segmentOrd);
             if (collector.match(globalOrd)) {
@@ -223,32 +208,18 @@ final class GlobalOrdinalsWithScoreQuery extends Query {
 
     final GlobalOrdinalsWithScoreCollector collector;
 
-    public SegmentOrdinalScorer(Weight weight, GlobalOrdinalsWithScoreCollector collector, SortedDocValues values, Scorer approximationScorer) {
+    public SegmentOrdinalScorer(Weight weight, GlobalOrdinalsWithScoreCollector collector, SortedDocValues values, DocIdSetIterator approximation) {
-      super(weight, values, approximationScorer);
+      super(weight, values, approximation);
       this.collector = collector;
     }
 
-    @Override
-    public int advance(int target) throws IOException {
-      for (int docID = approximationScorer.advance(target); docID < NO_MORE_DOCS; docID = approximationScorer.nextDoc()) {
-        final int segmentOrd = values.getOrd(docID);
-        if (segmentOrd != -1) {
-          if (collector.match(segmentOrd)) {
-            score = collector.score(segmentOrd);
-            return docID;
-          }
-        }
-      }
-      return NO_MORE_DOCS;
-    }
-
     @Override
     protected TwoPhaseIterator createTwoPhaseIterator(DocIdSetIterator approximation) {
       return new TwoPhaseIterator(approximation) {
 
         @Override
         public boolean matches() throws IOException {
-          final int segmentOrd = values.getOrd(approximationScorer.docID());
+          final int segmentOrd = values.getOrd(approximation.docID());
           if (segmentOrd != -1) {
             if (collector.match(segmentOrd)) {
               score = collector.score(segmentOrd);

lucene/join/src/java/org/apache/lucene/search/join/QueryBitSetProducer.java

@@ -27,9 +27,9 @@ import org.apache.lucene.index.LeafReader;
 import org.apache.lucene.index.LeafReaderContext;
 import org.apache.lucene.index.ReaderUtil;
 import org.apache.lucene.search.DocIdSet;
-import org.apache.lucene.search.DocIdSetIterator;
 import org.apache.lucene.search.IndexSearcher;
 import org.apache.lucene.search.Query;
+import org.apache.lucene.search.Scorer;
 import org.apache.lucene.search.Weight;
 import org.apache.lucene.util.BitDocIdSet;
 import org.apache.lucene.util.BitSet;
@@ -68,12 +68,12 @@ public class QueryBitSetProducer implements BitSetProducer {
       final IndexSearcher searcher = new IndexSearcher(topLevelContext);
       searcher.setQueryCache(null);
       final Weight weight = searcher.createNormalizedWeight(query, false);
-      final DocIdSetIterator it = weight.scorer(context);
+      final Scorer s = weight.scorer(context);
 
-      if (it == null) {
+      if (s == null) {
         docIdSet = DocIdSet.EMPTY;
       } else {
-        docIdSet = new BitDocIdSet(BitSet.of(it, context.reader().maxDoc()));
+        docIdSet = new BitDocIdSet(BitSet.of(s.iterator(), context.reader().maxDoc()));
       }
       cache.put(key, docIdSet);
     }

lucene/join/src/java/org/apache/lucene/search/join/TermsIncludingScoreQuery.java

@@ -180,8 +180,6 @@ class TermsIncludingScoreQuery extends Query {
     final float[] scores;
     final long cost;
 
-    int currentDoc = -1;
-
     SVInOrderScorer(Weight weight, TermsEnum termsEnum, int maxDoc, long cost) throws IOException {
       super(weight);
       FixedBitSet matchingDocs = new FixedBitSet(maxDoc);
@@ -210,7 +208,7 @@ class TermsIncludingScoreQuery extends Query {
 
     @Override
     public float score() throws IOException {
-      return scores[currentDoc];
+      return scores[docID()];
     }
 
     @Override
@@ -220,23 +218,14 @@ class TermsIncludingScoreQuery extends Query {
 
     @Override
     public int docID() {
-      return currentDoc;
+      return matchingDocsIterator.docID();
     }
 
     @Override
-    public int nextDoc() throws IOException {
+    public DocIdSetIterator iterator() {
-      return currentDoc = matchingDocsIterator.nextDoc();
+      return matchingDocsIterator;
     }
 
-    @Override
-    public int advance(int target) throws IOException {
-      return currentDoc = matchingDocsIterator.advance(target);
-    }
-
-    @Override
-    public long cost() {
-      return cost;
-    }
   }
 
   // This scorer deals with the fact that a document can have more than one score from multiple related documents.

lucene/join/src/java/org/apache/lucene/search/join/ToChildBlockJoinQuery.java

@@ -26,6 +26,7 @@ import java.util.Set;
 import org.apache.lucene.index.IndexReader;
 import org.apache.lucene.index.LeafReaderContext;
 import org.apache.lucene.index.Term;
+import org.apache.lucene.search.DocIdSetIterator;
 import org.apache.lucene.search.Explanation;
 import org.apache.lucene.search.IndexSearcher;
 import org.apache.lucene.search.Query;
@@ -141,7 +142,7 @@ public class ToChildBlockJoinQuery extends Query {
     @Override
     public Explanation explain(LeafReaderContext context, int doc) throws IOException {
       ToChildBlockJoinScorer scorer = (ToChildBlockJoinScorer) scorer(context);
-      if (scorer != null && scorer.advance(doc) == doc) {
+      if (scorer != null && scorer.iterator().advance(doc) == doc) {
         int parentDoc = scorer.getParentDoc();
         return Explanation.match(
           scorer.score(), 
@@ -155,6 +156,7 @@ public class ToChildBlockJoinQuery extends Query {
 
   static class ToChildBlockJoinScorer extends Scorer {
     private final Scorer parentScorer;
+    private final DocIdSetIterator parentIt;
     private final BitSet parentBits;
     private final boolean doScores;
 
@@ -169,6 +171,7 @@ public class ToChildBlockJoinQuery extends Query {
       this.doScores = doScores;
       this.parentBits = parentBits;
       this.parentScorer = parentScorer;
+      this.parentIt = parentScorer.iterator();
     }
 
     @Override
@@ -176,6 +179,15 @@ public class ToChildBlockJoinQuery extends Query {
       return Collections.singleton(new ChildScorer(parentScorer, "BLOCK_JOIN"));
     }
 
+    @Override
+    public DocIdSetIterator iterator() {
+      return new DocIdSetIterator() {
+
+        @Override
+        public int docID() {
+          return childDoc;
+        }
+
         @Override
         public int nextDoc() throws IOException {
           //System.out.println("Q.nextDoc() parentDoc=" + parentDoc + " childDoc=" + childDoc);
@@ -188,7 +200,7 @@ public class ToChildBlockJoinQuery extends Query {
               // to skip over some number of parents w/ no
               // children:
               while (true) {
-            parentDoc = parentScorer.nextDoc();
+                parentDoc = parentIt.nextDoc();
                 validateParentDoc();
 
                 if (parentDoc == 0) {
@@ -197,7 +209,7 @@ public class ToChildBlockJoinQuery extends Query {
                   // into ctor so we can skip this if... but it's
                   // tricky because scorer must return -1 for
                   // .doc() on init...
-              parentDoc = parentScorer.nextDoc();
+                  parentDoc = parentIt.nextDoc();
                   validateParentDoc();
                 }
 
@@ -236,36 +248,13 @@ public class ToChildBlockJoinQuery extends Query {
           }
         }
 
-    /** Detect mis-use, where provided parent query in fact
-     *  sometimes returns child documents.  */
-    private void validateParentDoc() {
-      if (parentDoc != NO_MORE_DOCS && !parentBits.get(parentDoc)) {
-        throw new IllegalStateException(INVALID_QUERY_MESSAGE + parentDoc);
-      }
-    }
-
-    @Override
-    public int docID() {
-      return childDoc;
-    }
-
-    @Override
-    public float score() throws IOException {
-      return parentScore;
-    }
-
-    @Override
-    public int freq() throws IOException {
-      return parentFreq;
-    }
-
         @Override
         public int advance(int childTarget) throws IOException {
           if (childTarget >= parentDoc) {
             if (childTarget == NO_MORE_DOCS) {
               return childDoc = parentDoc = NO_MORE_DOCS;
             }
-        parentDoc = parentScorer.advance(childTarget + 1);
+            parentDoc = parentIt.advance(childTarget + 1);
             validateParentDoc();
 
             if (parentDoc == NO_MORE_DOCS) {
@@ -281,7 +270,7 @@ public class ToChildBlockJoinQuery extends Query {
                 break;
               }
               // parent with no children, move to the next one
-          parentDoc = parentScorer.nextDoc();
+              parentDoc = parentIt.nextDoc();
               validateParentDoc();
               if (parentDoc == NO_MORE_DOCS) {
                 return childDoc = NO_MORE_DOCS;
@@ -303,7 +292,32 @@ public class ToChildBlockJoinQuery extends Query {
 
         @Override
         public long cost() {
-      return parentScorer.cost();
+          return parentIt.cost();
+        }
+      };
+    }
+
+    /** Detect mis-use, where provided parent query in fact
+     *  sometimes returns child documents.  */
+    private void validateParentDoc() {
+      if (parentDoc != DocIdSetIterator.NO_MORE_DOCS && !parentBits.get(parentDoc)) {
+        throw new IllegalStateException(INVALID_QUERY_MESSAGE + parentDoc);
+      }
+    }
+
+    @Override
+    public int docID() {
+      return childDoc;
+    }
+
+    @Override
+    public float score() throws IOException {
+      return parentScore;
+    }
+
+    @Override
+    public int freq() throws IOException {
+      return parentFreq;
     }
     
     int getParentDoc() {

lucene/join/src/java/org/apache/lucene/search/join/ToParentBlockJoinIndexSearcher.java

@@ -61,7 +61,8 @@ public class ToParentBlockJoinIndexSearcher extends IndexSearcher {
         final LeafCollector leafCollector = collector.getLeafCollector(ctx);
         leafCollector.setScorer(scorer);
         final Bits liveDocs = ctx.reader().getLiveDocs();
-        for (int doc = scorer.nextDoc(); doc != DocIdSetIterator.NO_MORE_DOCS; doc = scorer.nextDoc()) {
+        final DocIdSetIterator it = scorer.iterator();
+        for (int doc = it.nextDoc(); doc != DocIdSetIterator.NO_MORE_DOCS; doc = it.nextDoc()) {
           if (liveDocs == null || liveDocs.get(doc)) {
             leafCollector.collect(doc);
           }

lucene/join/src/java/org/apache/lucene/search/join/ToParentBlockJoinQuery.java

@@ -163,7 +163,7 @@ public class ToParentBlockJoinQuery extends Query {
         return null;
       }
 
-      final int firstChildDoc = childScorer.nextDoc();
+      final int firstChildDoc = childScorer.iterator().nextDoc();
       if (firstChildDoc == DocIdSetIterator.NO_MORE_DOCS) {
         // No matches
         return null;
@@ -184,7 +184,7 @@ public class ToParentBlockJoinQuery extends Query {
     @Override
     public Explanation explain(LeafReaderContext context, int doc) throws IOException {
       BlockJoinScorer scorer = (BlockJoinScorer) scorer(context);
-      if (scorer != null && scorer.advance(doc) == doc) {
+      if (scorer != null && scorer.iterator().advance(doc) == doc) {
         return scorer.explain(context.docBase);
       }
       return Explanation.noMatch("Not a match");
@@ -249,6 +249,11 @@ public class ToParentBlockJoinQuery extends Query {
       return ret;
     }
 
+    @Override
+    public DocIdSetIterator iterator() {
+      return new DocIdSetIterator() {
+        final DocIdSetIterator childIt = childScorer.iterator();
+
         @Override
         public int nextDoc() throws IOException {
           //System.out.println("Q.nextDoc() nextChildDoc=" + nextChildDoc);
@@ -302,7 +307,7 @@ public class ToParentBlockJoinQuery extends Query {
               parentFreq += childFreq;
             }
             childDocUpto++;
-        nextChildDoc = childScorer.nextDoc();
+            nextChildDoc = childIt.nextDoc();
           } while (nextChildDoc < parentDoc);
 
           // Parent & child docs are supposed to be
@@ -332,21 +337,6 @@ public class ToParentBlockJoinQuery extends Query {
           return parentDoc;
         }
 
-    @Override
-    public int docID() {
-      return parentDoc;
-    }
-
-    @Override
-    public float score() throws IOException {
-      return parentScore;
-    }
-    
-    @Override
-    public int freq() {
-      return parentFreq;
-    }
-
         @Override
         public int advance(int parentTarget) throws IOException {
 
@@ -370,7 +360,7 @@ public class ToParentBlockJoinQuery extends Query {
           //System.out.println("  rolled back to prevParentDoc=" + prevParentDoc + " vs parentDoc=" + parentDoc);
           assert prevParentDoc >= parentDoc;
           if (prevParentDoc > nextChildDoc) {
-        nextChildDoc = childScorer.advance(prevParentDoc);
+            nextChildDoc = childIt.advance(prevParentDoc);
             // System.out.println("  childScorer advanced to child docID=" + nextChildDoc);
           //} else {
             //System.out.println("  skip childScorer advance");
@@ -386,6 +376,33 @@ public class ToParentBlockJoinQuery extends Query {
           return nd;
         }
 
+        @Override
+        public int docID() {
+          return parentDoc;
+        }
+
+        @Override
+        public long cost() {
+          return childIt.cost();
+        }
+      };
+    }
+
+    @Override
+    public int docID() {
+      return parentDoc;
+    }
+
+    @Override
+    public float score() throws IOException {
+      return parentScore;
+    }
+    
+    @Override
+    public int freq() {
+      return parentFreq;
+    }
+
     public Explanation explain(int docBase) throws IOException {
       int start = docBase + prevParentDoc + 1; // +1 b/c prevParentDoc is previous parent doc
       int end = docBase + parentDoc - 1; // -1 b/c parentDoc is parent doc
@@ -393,11 +410,6 @@ public class ToParentBlockJoinQuery extends Query {
       );
     }
 
-    @Override
-    public long cost() {
-      return childScorer.cost();
-    }
-
     /**
      * Instructs this scorer to keep track of the child docIds and score ids for retrieval purposes.
      */

lucene/join/src/test/org/apache/lucene/search/join/TestBlockJoin.java

@@ -63,6 +63,7 @@ import org.apache.lucene.search.Query;
 import org.apache.lucene.search.QueryUtils;
 import org.apache.lucene.search.RandomApproximationQuery;
 import org.apache.lucene.search.ScoreDoc;
+import org.apache.lucene.search.Scorer;
 import org.apache.lucene.search.Sort;
 import org.apache.lucene.search.SortField;
 import org.apache.lucene.search.TermQuery;
@@ -1143,8 +1144,8 @@ public class TestBlockJoin extends LuceneTestCase {
 
     ToParentBlockJoinQuery q = new ToParentBlockJoinQuery(tq, parentFilter, ScoreMode.Avg);
     Weight weight = s.createNormalizedWeight(q, true);
-    DocIdSetIterator disi = weight.scorer(s.getIndexReader().leaves().get(0));
+    Scorer sc = weight.scorer(s.getIndexReader().leaves().get(0));
-    assertEquals(1, disi.advance(1));
+    assertEquals(1, sc.iterator().advance(1));
     r.close();
     dir.close();
   }
@@ -1177,8 +1178,8 @@ public class TestBlockJoin extends LuceneTestCase {
 
     ToParentBlockJoinQuery q = new ToParentBlockJoinQuery(tq, parentFilter, ScoreMode.Avg);
     Weight weight = s.createNormalizedWeight(q, true);
-    DocIdSetIterator disi = weight.scorer(s.getIndexReader().leaves().get(0));
+    Scorer sc = weight.scorer(s.getIndexReader().leaves().get(0));
-    assertEquals(2, disi.advance(0));
+    assertEquals(2, sc.iterator().advance(0));
     r.close();
     dir.close();
   }
@@ -1657,12 +1658,12 @@ public class TestBlockJoin extends LuceneTestCase {
     ToChildBlockJoinQuery parentJoinQuery = new ToChildBlockJoinQuery(parentQuery, parentFilter);
 
     Weight weight = s.createNormalizedWeight(parentJoinQuery, random().nextBoolean());
-    DocIdSetIterator advancingScorer = weight.scorer(s.getIndexReader().leaves().get(0));
+    Scorer advancingScorer = weight.scorer(s.getIndexReader().leaves().get(0));
-    DocIdSetIterator nextDocScorer = weight.scorer(s.getIndexReader().leaves().get(0));
+    Scorer nextDocScorer = weight.scorer(s.getIndexReader().leaves().get(0));
 
-    final int firstKid = nextDocScorer.nextDoc();
+    final int firstKid = nextDocScorer.iterator().nextDoc();
     assertTrue("firstKid not found", DocIdSetIterator.NO_MORE_DOCS != firstKid);
-    assertEquals(firstKid, advancingScorer.advance(0));
+    assertEquals(firstKid, advancingScorer.iterator().advance(0));
     
     r.close();
     dir.close();

lucene/join/src/test/org/apache/lucene/search/join/TestBlockJoinValidation.java

@@ -140,7 +140,7 @@ public class TestBlockJoinValidation extends LuceneTestCase {
     } while (parentDocs.get(target + 1));
 
     try {
-      scorer.advance(target);
+      scorer.iterator().advance(target);
       fail();
     } catch (IllegalStateException expected) {
       assertTrue(expected.getMessage() != null && expected.getMessage().contains(ToChildBlockJoinQuery.INVALID_QUERY_MESSAGE));

lucene/misc/src/java/org/apache/lucene/index/PKIndexSplitter.java

@@ -24,6 +24,7 @@ import org.apache.lucene.index.IndexWriterConfig.OpenMode;
 import org.apache.lucene.search.DocIdSetIterator;
 import org.apache.lucene.search.IndexSearcher;
 import org.apache.lucene.search.Query;
+import org.apache.lucene.search.Scorer;
 import org.apache.lucene.search.TermRangeQuery;
 import org.apache.lucene.search.Weight;
 import org.apache.lucene.store.Directory;
@@ -134,9 +135,9 @@ public class PKIndexSplitter {
       final int maxDoc = in.maxDoc();
       final FixedBitSet bits = new FixedBitSet(maxDoc);
       // ignore livedocs here, as we filter them later:
-      final DocIdSetIterator preserveIt = preserveWeight.scorer(context);
+      final Scorer preverveScorer = preserveWeight.scorer(context);
-      if (preserveIt != null) {
+      if (preverveScorer != null) {
-        bits.or(preserveIt);
+        bits.or(preverveScorer.iterator());
       }
       if (negateFilter) {
         bits.flip(0, maxDoc);

lucene/misc/src/java/org/apache/lucene/index/Sorter.java

@@ -20,6 +20,7 @@ package org.apache.lucene.index;
 import java.io.IOException;
 import java.util.Comparator;
 
+import org.apache.lucene.search.DocIdSetIterator;
 import org.apache.lucene.search.LeafFieldComparator;
 import org.apache.lucene.search.Scorer;
 import org.apache.lucene.search.Sort;
@@ -269,18 +270,7 @@ final class Sorter {
       return doc;
     }
 
-    @Override
+    public DocIdSetIterator iterator() {
-    public int nextDoc() throws IOException {
-      throw new UnsupportedOperationException();
-    }
-
-    @Override
-    public int advance(int target) throws IOException {
-      throw new UnsupportedOperationException();
-    }
-
-    @Override
-    public long cost() {
       throw new UnsupportedOperationException();
     }
 

lucene/misc/src/java/org/apache/lucene/search/BlockJoinComparatorSource.java

@@ -122,11 +122,11 @@ public class BlockJoinComparatorSource extends FieldComparatorSource {
         IndexSearcher searcher = new IndexSearcher(ReaderUtil.getTopLevelContext(context));
         searcher.setQueryCache(null);
         final Weight weight = searcher.createNormalizedWeight(parentsFilter, false);
-        final DocIdSetIterator parents = weight.scorer(context);
+        final Scorer parents = weight.scorer(context);
         if (parents == null) {
           throw new IllegalStateException("LeafReader " + context.reader() + " contains no parents!");
         }
-        parentBits = BitSet.of(parents, context.reader().maxDoc());
+        parentBits = BitSet.of(parents.iterator(), context.reader().maxDoc());
         parentLeafComparators = new LeafFieldComparator[parentComparators.length];
         for (int i = 0; i < parentComparators.length; i++) {
           parentLeafComparators[i] = parentComparators[i].getLeafComparator(context);

lucene/misc/src/test/org/apache/lucene/index/TestBlockJoinSorter.java

@@ -27,9 +27,9 @@ import org.apache.lucene.document.Field.Store;
 import org.apache.lucene.document.NumericDocValuesField;
 import org.apache.lucene.document.StringField;
 import org.apache.lucene.search.BlockJoinComparatorSource;
-import org.apache.lucene.search.DocIdSetIterator;
 import org.apache.lucene.search.IndexSearcher;
 import org.apache.lucene.search.Query;
+import org.apache.lucene.search.Scorer;
 import org.apache.lucene.search.Sort;
 import org.apache.lucene.search.SortField;
 import org.apache.lucene.search.TermQuery;
@@ -37,7 +37,6 @@ import org.apache.lucene.search.Weight;
 import org.apache.lucene.util.ArrayUtil;
 import org.apache.lucene.util.BitSet;
 import org.apache.lucene.util.LuceneTestCase;
-import org.junit.Ignore;
 
 public class TestBlockJoinSorter extends LuceneTestCase {
 
@@ -74,8 +73,8 @@ public class TestBlockJoinSorter extends LuceneTestCase {
     final Query parentsFilter = new TermQuery(new Term("parent", "true"));
 
     final Weight weight = searcher.createNormalizedWeight(parentsFilter, false);
-    final DocIdSetIterator parents = weight.scorer(indexReader.leaves().get(0));
+    final Scorer parents = weight.scorer(indexReader.leaves().get(0));
-    final BitSet parentBits = BitSet.of(parents, reader.maxDoc());
+    final BitSet parentBits = BitSet.of(parents.iterator(), reader.maxDoc());
     final NumericDocValues parentValues = reader.getNumericDocValues("parent_val");
     final NumericDocValues childValues = reader.getNumericDocValues("child_val");
 

lucene/queries/src/java/org/apache/lucene/queries/BoostingQuery.java

@@ -112,9 +112,9 @@ public class BoostingQuery extends Query {
           if (contextScorer == null) {
             return matchScorer;
           }
-          TwoPhaseIterator contextTwoPhase = contextScorer.asTwoPhaseIterator();
+          TwoPhaseIterator contextTwoPhase = contextScorer.twoPhaseIterator();
           DocIdSetIterator contextApproximation = contextTwoPhase == null
-              ? contextScorer
+              ? contextScorer.iterator()
               : contextTwoPhase.approximation();
           return new FilterScorer(matchScorer) {
             @Override

lucene/queries/src/java/org/apache/lucene/queries/CustomScoreQuery.java

@@ -294,7 +294,7 @@ public class CustomScoreQuery extends Query implements Cloneable {
       int doc = docID();
       if (doc > valSrcDocID) {
         for (Scorer valSrcScorer : valSrcScorers) {
-          valSrcScorer.advance(doc);
+          valSrcScorer.iterator().advance(doc);
         }
         valSrcDocID = doc;
       }

lucene/queries/src/java/org/apache/lucene/queries/function/FunctionQuery.java

@@ -24,6 +24,7 @@ import java.util.Set;
 import org.apache.lucene.index.IndexReader;
 import org.apache.lucene.index.LeafReaderContext;
 import org.apache.lucene.index.Term;
+import org.apache.lucene.search.DocIdSetIterator;
 import org.apache.lucene.search.Explanation;
 import org.apache.lucene.search.IndexSearcher;
 import org.apache.lucene.search.Query;
@@ -94,7 +95,7 @@ public class FunctionQuery extends Query {
     final FunctionWeight weight;
     final int maxDoc;
     final float qWeight;
-    int doc=-1;
+    final DocIdSetIterator iterator;
     final FunctionValues vals;
 
     public AllScorer(LeafReaderContext context, FunctionWeight w, float qWeight) throws IOException {
@@ -103,35 +104,23 @@ public class FunctionQuery extends Query {
       this.qWeight = qWeight;
       this.reader = context.reader();
       this.maxDoc = reader.maxDoc();
+      iterator = DocIdSetIterator.all(context.reader().maxDoc());
       vals = func.getValues(weight.context, context);
     }
 
+    @Override
+    public DocIdSetIterator iterator() {
+      return iterator;
+    }
+
     @Override
     public int docID() {
-      return doc;
+      return iterator.docID();
-    }
-
-    // instead of matching all docs, we could also embed a query.
-    // the score could either ignore the subscore, or boost it.
-    // Containment:  floatline(foo:myTerm, "myFloatField", 1.0, 0.0f)
-    // Boost:        foo:myTerm^floatline("myFloatField",1.0,0.0f)
-    @Override
-    public int nextDoc() throws IOException {
-      ++doc;
-      if (doc>=maxDoc) {
-        return doc=NO_MORE_DOCS;
-      }
-      return doc;
-    }
-
-    @Override
-    public int advance(int target) throws IOException {
-      return slowAdvance(target);
     }
 
     @Override
     public float score() throws IOException {
-      float score = qWeight * vals.floatVal(doc);
+      float score = qWeight * vals.floatVal(docID());
 
       // Current Lucene priority queues can't handle NaN and -Infinity, so
       // map to -Float.MAX_VALUE. This conditional handles both -infinity
@@ -139,11 +128,6 @@ public class FunctionQuery extends Query {
       return score>Float.NEGATIVE_INFINITY ? score : -Float.MAX_VALUE;
     }
 
-    @Override
-    public long cost() {
-      return maxDoc;
-    }
-
     @Override
     public int freq() throws IOException {
       return 1;

lucene/queries/src/java/org/apache/lucene/queries/function/FunctionRangeQuery.java

@@ -132,7 +132,7 @@ public class FunctionRangeQuery extends Query {
       //  which can be slow since if that doc doesn't match, it has to linearly find the next matching
       ValueSourceScorer scorer = scorer(context);
       if (scorer.matches(doc)) {
-        scorer.advance(doc);
+        scorer.iterator().advance(doc);
         return Explanation.match(scorer.score(), FunctionRangeQuery.this.toString(), functionValues.explain(doc));
       } else {
         return Explanation.noMatch(FunctionRangeQuery.this.toString(), functionValues.explain(doc));

lucene/queries/src/java/org/apache/lucene/queries/function/ValueSourceScorer.java

@@ -48,10 +48,11 @@ public abstract class ValueSourceScorer extends Scorer {
   protected ValueSourceScorer(IndexReader reader, FunctionValues values) {
     super(null);//no weight
     this.values = values;
-    this.twoPhaseIterator = new TwoPhaseIterator(DocIdSetIterator.all(reader.maxDoc())) { // no approximation!
+    final DocIdSetIterator approximation = DocIdSetIterator.all(reader.maxDoc()); // no approximation!
+    this.twoPhaseIterator = new TwoPhaseIterator(approximation) {
       @Override
       public boolean matches() throws IOException {
-        return ValueSourceScorer.this.matches(docID());
+        return ValueSourceScorer.this.matches(approximation.docID());
       }
 
       @Override
@@ -66,7 +67,12 @@ public abstract class ValueSourceScorer extends Scorer {
   public abstract boolean matches(int doc);
 
   @Override
-  public TwoPhaseIterator asTwoPhaseIterator() {
+  public DocIdSetIterator iterator() {
+    return disi;
+  }
+
+  @Override
+  public TwoPhaseIterator twoPhaseIterator() {
     return twoPhaseIterator;
   }
 
@@ -75,16 +81,6 @@ public abstract class ValueSourceScorer extends Scorer {
     return disi.docID();
   }
 
-  @Override
-  public int nextDoc() throws IOException {
-    return disi.nextDoc();
-  }
-
-  @Override
-  public int advance(int target) throws IOException {
-    return disi.advance(target);
-  }
-
   @Override
   public float score() throws IOException {
     // (same as FunctionQuery, but no qWeight)  TODO consider adding configurable qWeight
@@ -100,8 +96,4 @@ public abstract class ValueSourceScorer extends Scorer {
     return 1;
   }
 
-  @Override
-  public long cost() {
-    return disi.cost();
-  }
 }

lucene/queries/src/java/org/apache/lucene/queries/function/valuesource/QueryValueSource.java

@@ -25,11 +25,11 @@ import org.apache.lucene.index.ReaderUtil;
 import org.apache.lucene.queries.function.FunctionValues;
 import org.apache.lucene.queries.function.ValueSource;
 import org.apache.lucene.queries.function.docvalues.FloatDocValues;
+import org.apache.lucene.search.DocIdSetIterator;
 import org.apache.lucene.search.IndexSearcher;
 import org.apache.lucene.search.Query;
 import org.apache.lucene.search.Scorer;
 import org.apache.lucene.search.Weight;
-import org.apache.lucene.util.Bits;
 import org.apache.lucene.util.mutable.MutableValue;
 import org.apache.lucene.util.mutable.MutableValueFloat;
 
@@ -86,6 +86,7 @@ class QueryDocValues extends FloatDocValues {
   final Query q;
 
   Scorer scorer;
+  DocIdSetIterator it;
   int scorerDoc; // the document the scorer is on
   boolean noMatches=false;
 
@@ -129,12 +130,13 @@ class QueryDocValues extends FloatDocValues {
           noMatches = true;
           return defVal;
         }
+        it = scorer.iterator();
         scorerDoc = -1;
       }
       lastDocRequested = doc;
 
       if (scorerDoc < doc) {
-        scorerDoc = scorer.advance(doc);
+        scorerDoc = it.advance(doc);
       }
 
       if (scorerDoc > doc) {
@@ -161,11 +163,12 @@ class QueryDocValues extends FloatDocValues {
           noMatches = true;
           return false;
         }
+        it = scorer.iterator();
       }
       lastDocRequested = doc;
 
       if (scorerDoc < doc) {
-        scorerDoc = scorer.advance(doc);
+        scorerDoc = it.advance(doc);
       }
 
       if (scorerDoc > doc) {
@@ -221,10 +224,11 @@ class QueryDocValues extends FloatDocValues {
             mval.exists = false;
             return;
           }
+          it = scorer.iterator();
           lastDocRequested = doc;
 
           if (scorerDoc < doc) {
-            scorerDoc = scorer.advance(doc);
+            scorerDoc = it.advance(doc);
           }
 
           if (scorerDoc > doc) {

lucene/queries/src/java/org/apache/lucene/queries/payloads/PayloadScoreQuery.java

@@ -27,11 +27,13 @@ import org.apache.lucene.index.Term;
 import org.apache.lucene.index.TermContext;
 import org.apache.lucene.search.Explanation;
 import org.apache.lucene.search.IndexSearcher;
-import org.apache.lucene.search.Scorer;
 import org.apache.lucene.search.similarities.ClassicSimilarity;
 import org.apache.lucene.search.similarities.Similarity;
+import org.apache.lucene.search.similarities.Similarity.SimScorer;
+import org.apache.lucene.search.spans.FilterSpans;
 import org.apache.lucene.search.spans.SpanCollector;
 import org.apache.lucene.search.spans.SpanQuery;
+import org.apache.lucene.search.spans.SpanScorer;
 import org.apache.lucene.search.spans.SpanWeight;
 import org.apache.lucene.search.spans.Spans;
 import org.apache.lucene.util.BytesRef;
@@ -132,11 +134,13 @@ public class PayloadScoreQuery extends SpanQuery {
     }
 
     @Override
-    public Scorer scorer(LeafReaderContext context) throws IOException {
+    public PayloadSpanScorer scorer(LeafReaderContext context) throws IOException {
       Spans spans = getSpans(context, Postings.PAYLOADS);
       if (spans == null)
         return null;
-      return new PayloadSpans(spans, this, innerWeight.getSimScorer(context));
+      SimScorer docScorer = innerWeight.getSimScorer(context);
+      PayloadSpans payloadSpans = new PayloadSpans(spans, docScorer);
+      return new PayloadSpanScorer(this, payloadSpans, docScorer);
     }
 
     @Override
@@ -156,8 +160,8 @@ public class PayloadScoreQuery extends SpanQuery {
 
     @Override
     public Explanation explain(LeafReaderContext context, int doc) throws IOException {
-      PayloadSpans scorer = (PayloadSpans) scorer(context);
+      PayloadSpanScorer scorer = scorer(context);
-      if (scorer == null || scorer.advance(doc) != doc)
+      if (scorer == null || scorer.iterator().advance(doc) != doc)
         return Explanation.noMatch("No match");
 
       scorer.freq();  // force freq calculation
@@ -173,40 +177,20 @@ public class PayloadScoreQuery extends SpanQuery {
     }
   }
 
-  private class PayloadSpans extends Spans implements SpanCollector {
+  private class PayloadSpans extends FilterSpans implements SpanCollector {
 
-    private int payloadsSeen;
+    private final SimScorer docScorer;
-    private float payloadScore;
+    public int payloadsSeen;
-    private final Spans in;
+    public float payloadScore;
 
-    private PayloadSpans(Spans spans, SpanWeight weight, Similarity.SimScorer docScorer) throws IOException {
+    private PayloadSpans(Spans in, SimScorer docScorer) {
-      super(weight, docScorer);
+      super(in);
-      this.in = spans;
+      this.docScorer = docScorer;
     }
     
     @Override
-    public int nextStartPosition() throws IOException {
+    protected AcceptStatus accept(Spans candidate) throws IOException {
-      return in.nextStartPosition();
+      return AcceptStatus.YES;
-    }
-
-    @Override
-    public int startPosition() {
-      return in.startPosition();
-    }
-
-    @Override
-    public int endPosition() {
-      return in.endPosition();
-    }
-
-    @Override
-    public int width() {
-      return in.width();
-    }
-
-    @Override
-    public void collect(SpanCollector collector) throws IOException {
-      in.collect(collector);
     }
     
     @Override
@@ -215,11 +199,6 @@ public class PayloadScoreQuery extends SpanQuery {
       payloadsSeen = 0;
     }
 
-    @Override
-    protected void doCurrentSpans() throws IOException {
-      in.collect(this);
-    }
-
     @Override
     public void collectLeaf(PostingsEnum postings, int position, Term term) throws IOException {
       BytesRef payload = postings.getPayload();
@@ -231,12 +210,30 @@ public class PayloadScoreQuery extends SpanQuery {
       payloadsSeen++;
     }
 
+    @Override
+    public void reset() {}
+
+    @Override
+    protected void doCurrentSpans() throws IOException {
+      in.collect(this);
+    }
+  }
+
+  private class PayloadSpanScorer extends SpanScorer {
+
+    private final PayloadSpans spans;
+
+    private PayloadSpanScorer(SpanWeight weight, PayloadSpans spans, Similarity.SimScorer docScorer) throws IOException {
+      super(weight, spans, docScorer);
+      this.spans = spans;
+    }
+
     protected float getPayloadScore() {
-      return function.docScore(docID(), getField(), payloadsSeen, payloadScore);
+      return function.docScore(docID(), getField(), spans.payloadsSeen, spans.payloadScore);
     }
 
     protected Explanation getPayloadExplanation() {
-      return function.explain(docID(), getField(), payloadsSeen, payloadScore);
+      return function.explain(docID(), getField(), spans.payloadsSeen, spans.payloadScore);
     }
 
     protected float getSpanScore() throws IOException {
@@ -250,35 +247,6 @@ public class PayloadScoreQuery extends SpanQuery {
       return getPayloadScore();
     }
 
-    @Override
-    public void reset() {
-
-    }
-
-    @Override
-    public int docID() {
-      return in.docID();
-    }
-
-    @Override
-    public int nextDoc() throws IOException {
-      return in.nextDoc();
-    }
-
-    @Override
-    public int advance(int target) throws IOException {
-      return in.advance(target);
-    }
-
-    @Override
-    public long cost() {
-      return in.cost();
-    }
-
-    @Override
-    public float positionsCost() {
-      return in.positionsCost();
-    }
   }
 
 }