Merge CombinedFieldQuery's WeightedDisiWrapper into DisiWrapper. (#14747)

This aims at simplifying things a bit to make it easier to implement
`CombinedFieldScorer#nextDocsAndScores` later.

Author: jpountz

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

@@ -378,8 +378,10 @@ public Scorer get(long leadCost) throws IOException {
           List<DisiWrapper> wrappers = new ArrayList<>(iterators.size());
           for (int i = 0; i < iterators.size(); i++) {
             float weight = fields.get(i).weight;
-            wrappers.add(
-                new WeightedDisiWrapper(new TermScorer(iterators.get(i), simWeight, null), weight));
+            Scorer scorer = new TermScorer(iterators.get(i), simWeight, null);
+            DisiWrapper w = new DisiWrapper(scorer, false, weight);
+            assert w.postingsEnum != null; // needed to access term frequencies
+            wrappers.add(w);
           }
           // Even though it is called approximation, it is accurate since none of
           // the sub iterators are two-phase iterators.
@@ -401,21 +403,6 @@ public boolean isCacheable(LeafReaderContext ctx) {
     }
   }
 
-  private static class WeightedDisiWrapper extends DisiWrapper {
-    final PostingsEnum postingsEnum;
-    final float weight;
-
-    WeightedDisiWrapper(Scorer scorer, float weight) {
-      super(scorer, false);
-      this.weight = weight;
-      this.postingsEnum = (PostingsEnum) scorer.iterator();
-    }
-
-    float freq() throws IOException {
-      return weight * postingsEnum.freq();
-    }
-  }
-
   private static class CombinedFieldScorer extends Scorer {
     private final DisjunctionDISIApproximation iterator;
     private final MultiNormsLeafSimScorer simScorer;
@@ -434,9 +421,9 @@ public int docID() {
 
     float freq() throws IOException {
       DisiWrapper w = iterator.topList();
-      float freq = ((WeightedDisiWrapper) w).freq();
+      float freq = w.postingsEnum.freq() * w.weight;
       for (w = w.next; w != null; w = w.next) {
-        freq += ((WeightedDisiWrapper) w).freq();
+        freq += w.postingsEnum.freq() * w.weight;
         if (freq < 0) { // overflow
           return Integer.MAX_VALUE;
         }

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

@@ -17,6 +17,7 @@
 package org.apache.lucene.search;
 
 import java.util.Objects;
+import org.apache.lucene.index.PostingsEnum;
 
 /**
  * Wrapper used in {@link DisiPriorityQueue}.
@@ -25,6 +26,8 @@
  */
 public class DisiWrapper {
   public final DocIdSetIterator iterator;
+  // Same as `iterator` if `iterator` is an instance of PostingsEnum, null otherwise
+  final PostingsEnum postingsEnum;
   public final Scorer scorer;
   public final Scorable scorable;
   public final long cost;
@@ -45,14 +48,22 @@ public class DisiWrapper {
   // for MaxScoreBulkScorer
   float maxWindowScore;
 
+  // for CombinedFieldQuery (BM25F)
+  final float weight;
+
   public DisiWrapper(Scorer scorer, boolean impacts) {
+    this(scorer, impacts, 1f);
+  }
+
+  DisiWrapper(Scorer scorer, boolean impacts, float weight) {
     this.scorer = Objects.requireNonNull(scorer);
     this.scorable = ScorerUtil.likelyTermScorer(scorer);
     if (impacts) {
       this.iterator = ScorerUtil.likelyImpactsEnum(scorer.iterator());
     } else {
       this.iterator = scorer.iterator();
     }
+    this.postingsEnum = iterator instanceof PostingsEnum pe ? pe : null;
     this.cost = iterator.cost();
     this.doc = -1;
     this.twoPhaseView = scorer.twoPhaseIterator();
@@ -64,5 +75,7 @@ public DisiWrapper(Scorer scorer, boolean impacts) {
       approximation = iterator;
       matchCost = 0f;
     }
+
+    this.weight = weight;
   }
 }