LUCENE-8357: Fix function score explanations

lucene/CHANGES.txt

@@ -291,6 +291,9 @@ Bug Fixes
   position increment when the preservePositionIncrement setting is false.
   (David Smiley, Jim Ferenczi)
 
+* LUCENE-8357: FunctionScoreQuery.boostByQuery() and boostByValue() were
+  producing truncated Explanations (Markus Jelsma, Alan Woodward)
+
 Other
 
 * LUCENE-8301: Update randomizedtesting to 2.6.0. (Dawid Weiss)

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

@@ -270,6 +270,19 @@ public final class FunctionScoreQuery extends Query {
       return Objects.equals(boost, that.boost);
     }
 
+    @Override
+    public Explanation explain(LeafReaderContext ctx, int docId, Explanation scoreExplanation) throws IOException {
+      if (scoreExplanation.isMatch() == false) {
+        return scoreExplanation;
+      }
+      Explanation boostExpl = boost.explain(ctx, docId, scoreExplanation);
+      if (boostExpl.isMatch() == false) {
+        return scoreExplanation;
+      }
+      return Explanation.match(scoreExplanation.getValue().doubleValue() * boostExpl.getValue().doubleValue(),
+          "product of:", scoreExplanation, boostExpl);
+    }
+
     @Override
     public int hashCode() {
       return Objects.hash(boost);
@@ -345,5 +358,14 @@ public final class FunctionScoreQuery extends Query {
     public boolean isCacheable(LeafReaderContext ctx) {
       return query.isCacheable(ctx);
     }
+
+    @Override
+    public Explanation explain(LeafReaderContext ctx, int docId, Explanation scoreExplanation) throws IOException {
+      Explanation inner = query.explain(ctx, docId, scoreExplanation);
+      if (inner.isMatch() == false) {
+        return inner;
+      }
+      return Explanation.match(boost, "Matched boosting query " + query.toString());
+    }
   }
 }

lucene/queries/src/test/org/apache/lucene/queries/function/TestFunctionScoreQuery.java

@@ -134,9 +134,12 @@ public class TestFunctionScoreQuery extends FunctionTestSetup {
     assertEquals(plain.totalHits, docs.totalHits);
     for (int i = 0; i < expectedDocs.length; i++) {
       assertEquals(expectedDocs[i], docs.scoreDocs[i].doc);
-
     }
 
+    Explanation expl = searcher.explain(fq, 4);
+    assertTrue(expl.toString().contains("first"));
+    assertTrue(expl.toString().contains("iii"));
+
   }
 
   // BoostingQuery equivalent
@@ -155,8 +158,11 @@ public class TestFunctionScoreQuery extends FunctionTestSetup {
     assertEquals(plain.totalHits, docs.totalHits);
     for (int i = 0; i < expectedDocs.length; i++) {
       assertEquals(expectedDocs[i], docs.scoreDocs[i].doc);
-
     }
+
+    Explanation expl = searcher.explain(fq, 6);
+    assertTrue(expl.toString().contains("rechecking"));
+    assertTrue(expl.toString().contains("text"));
   }
 
   // check boosts with non-distributive score source