Add getMatchingChildren() method to Scorer

lucene/CHANGES.txt

@@ -130,6 +130,10 @@ New features
   SortedSetDocValues to allow filtering their TermsEnums with a
   CompiledAutomaton (Alan Woodward, Mike McCandless)
 
+* LUCENE-7628: Scorer now has a getMatchingChildren() method that will 
+  return all child scorers positioned on the current document.  (Alan
+  Woodward)
+
 Bug Fixes
 
 * LUCENE-7547: JapaneseTokenizerFactory was failing to close the

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

@@ -202,4 +202,12 @@ abstract class DisjunctionScorer extends Scorer {
     return children;
   }
 
+  @Override
+  public Collection<ChildScorer> getMatchingChildren() throws IOException {
+    List<ChildScorer> children = new ArrayList<>();
+    for (DisiWrapper w = getSubMatches(); w != null; w = w.next) {
+      children.add(new ChildScorer(w.scorer, "SHOULD"));
+    }
+    return children;
+  }
 }

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

@@ -132,6 +132,15 @@ final class MinShouldMatchSumScorer extends Scorer {
     return childScorers;
   }
 
+  @Override
+  public Collection<ChildScorer> getMatchingChildren() throws IOException {
+    List<ChildScorer> children = new ArrayList<>();
+    for (DisiWrapper s = lead; s != null; s = s.next) {
+      children.add(new ChildScorer(s.scorer, "SHOULD"));
+    }
+    return children;
+  }
+
   @Override
   public DocIdSetIterator iterator() {
     return new DocIdSetIterator() {

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

@@ -83,6 +83,14 @@ public abstract class Scorer {
     return Collections.emptyList();
   }
 
+  /**
+   * Returns child sub-scorers that match the current document
+   * @lucene.experimental
+   */
+  public Collection<ChildScorer> getMatchingChildren() throws IOException {
+    return getChildren();
+  }
+  
   /** A child Scorer and its relationship to its parent.
    * the meaning of the relationship depends upon the parent query. 
    * @lucene.experimental */

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

@@ -109,6 +109,39 @@ public class TestBooleanQueryVisitSubscorers extends LuceneTestCase {
     assertEquals(2, tfs.get(2).intValue()); // f2:search + f2:lucene
   }
 
+  public void testDisjunctionMatches() throws IOException {
+    BooleanQuery.Builder bq1 = new BooleanQuery.Builder();
+    bq1.add(new TermQuery(new Term(F1, "lucene")), Occur.SHOULD);
+    bq1.add(new PhraseQuery(F2, "search", "engine"), Occur.SHOULD);
+
+    Weight w1 = scorerSearcher.createNormalizedWeight(bq1.build(), true);
+    Scorer s1 = w1.scorer(reader.leaves().get(0));
+    assertEquals(0, s1.iterator().nextDoc());
+    assertEquals(2, s1.getMatchingChildren().size());
+
+    BooleanQuery.Builder bq2 = new BooleanQuery.Builder();
+    bq2.add(new TermQuery(new Term(F1, "lucene")), Occur.SHOULD);
+    bq2.add(new PhraseQuery(F2, "search", "library"), Occur.SHOULD);
+
+    Weight w2 = scorerSearcher.createNormalizedWeight(bq2.build(), true);
+    Scorer s2 = w2.scorer(reader.leaves().get(0));
+    assertEquals(0, s2.iterator().nextDoc());
+    assertEquals(1, s2.getMatchingChildren().size());
+  }
+
+  public void testMinShouldMatchMatches() throws IOException {
+    BooleanQuery.Builder bq = new BooleanQuery.Builder();
+    bq.add(new TermQuery(new Term(F1, "lucene")), Occur.SHOULD);
+    bq.add(new TermQuery(new Term(F2, "lucene")), Occur.SHOULD);
+    bq.add(new PhraseQuery(F2, "search", "library"), Occur.SHOULD);
+    bq.setMinimumNumberShouldMatch(2);
+
+    Weight w = scorerSearcher.createNormalizedWeight(bq.build(), true);
+    Scorer s = w.scorer(reader.leaves().get(0));
+    assertEquals(0, s.iterator().nextDoc());
+    assertEquals(2, s.getMatchingChildren().size());
+  }
+  
   public void testConjunctions() throws IOException {
     BooleanQuery.Builder bq = new BooleanQuery.Builder();
     bq.add(new TermQuery(new Term(F2, "lucene")), BooleanClause.Occur.MUST);