Support getMaxScore of ConjunctionScorer for non top level scoring clause (#13043)

lucene/CHANGES.txt

@@ -202,6 +202,8 @@ Improvements
 
 * GITHUB#12999: Use Automaton for SurroundQuery prefix/pattern matching (Michael Gibney)
 
+* GITHUB#13043: Support getMaxScore of ConjunctionScorer for non top level scoring clause. (Shintaro Murakami)
+
 Optimizations
 ---------------------
 

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

@@ -65,15 +65,13 @@ class ConjunctionScorer extends Scorer {
 
   @Override
   public float getMaxScore(int upTo) throws IOException {
-    // This scorer is only used for TOP_SCORES when there is at most one scoring clause
-    switch (scorers.length) {
-      case 0:
-        return 0;
-      case 1:
-        return scorers[0].getMaxScore(upTo);
-      default:
-        return Float.POSITIVE_INFINITY;
+    double maxScore = 0;
+    for (Scorer s : scorers) {
+      if (s.docID() <= upTo) {
+        maxScore += s.getMaxScore(upTo);
+      }
     }
+    return (float) maxScore;
   }
 
   @Override
@@ -81,6 +79,9 @@ class ConjunctionScorer extends Scorer {
     if (scorers.length == 1) {
       return scorers[0].advanceShallow(target);
     }
+    for (Scorer scorer : scorers) {
+      scorer.advanceShallow(target);
+    }
     return super.advanceShallow(target);
   }
 

lucene/core/src/test/org/apache/lucene/search/TestBoolean2ScorerSupplier.java

@@ -521,4 +521,20 @@ public class TestBoolean2ScorerSupplier extends LuceneTestCase {
     assertTrue(clause1.topLevelScoringClause);
     assertFalse(clause2.topLevelScoringClause);
   }
+
+  public void testMaxScoreNonTopLevelScoringClause() throws Exception {
+    Map<Occur, Collection<ScorerSupplier>> subs = new EnumMap<>(Occur.class);
+    for (Occur occur : Occur.values()) {
+      subs.put(occur, new ArrayList<>());
+    }
+
+    FakeScorerSupplier clause1 = new FakeScorerSupplier(10, 10);
+    subs.get(Occur.MUST).add(clause1);
+    FakeScorerSupplier clause2 = new FakeScorerSupplier(10, 10);
+    subs.get(Occur.MUST).add(clause2);
+
+    Scorer scorer =
+        new Boolean2ScorerSupplier(new FakeWeight(), subs, ScoreMode.TOP_SCORES, 0).get(10);
+    assertEquals(2.0, scorer.getMaxScore(DocIdSetIterator.NO_MORE_DOCS), 0.0);
+  }
 }