fix explain in function_score if no function filter matches (#19185)

* fix explain in function_score if no function filter matches

When each function in function_score has a filter but none of them matches
we always assume 1 for the combined functions and then combine that with the
sub query score.
But the explanation did not reflect that because in case no function matched
we did not even use the actual score that was computed in the explanation.

Author: brwe

core/src/main/java/org/elasticsearch/common/lucene/search/function/FiltersFunctionScoreQuery.java

@@ -37,6 +37,8 @@
 import java.io.IOException;
 import java.util.ArrayList;
 import java.util.Arrays;
+import java.util.Collection;
+import java.util.Collections;
 import java.util.List;
 import java.util.Locale;
 import java.util.Objects;
@@ -224,17 +226,23 @@ public Explanation explain(LeafReaderContext context, int doc) throws IOExceptio
                     filterExplanations.add(filterExplanation);
                 }
             }
+            FiltersFunctionFactorScorer scorer = functionScorer(context);
+            int actualDoc = scorer.iterator().advance(doc);
+            assert (actualDoc == doc);
+            double score = scorer.computeScore(doc, expl.getValue());
+            Explanation factorExplanation;
             if (filterExplanations.size() > 0) {
-                FiltersFunctionFactorScorer scorer = functionScorer(context);
-                int actualDoc = scorer.iterator().advance(doc);
-                assert (actualDoc == doc);
-                double score = scorer.computeScore(doc, expl.getValue());
-                Explanation factorExplanation = Explanation.match(
+                factorExplanation = Explanation.match(
                         CombineFunction.toFloat(score),
                         "function score, score mode [" + scoreMode.toString().toLowerCase(Locale.ROOT) + "]",
                         filterExplanations);
-                expl = combineFunction.explain(expl, factorExplanation, maxBoost);
+
+            } else {
+                // it is a little weird to add a match although no function matches but that is the way function_score behaves right now
+                factorExplanation = Explanation.match(1.0f,
+                    "No function matched", Collections.emptyList());
             }
+            expl = combineFunction.explain(expl, factorExplanation, maxBoost);
             if (minScore != null && minScore > expl.getValue()) {
                 expl = Explanation.noMatch("Score value is too low, expected at least " + minScore + " but got " + expl.getValue(), expl);
             }

core/src/test/java/org/elasticsearch/index/query/functionscore/FunctionScoreTests.java

@@ -599,7 +599,7 @@ public void testMinScoreExplain() throws IOException {
         Explanation ffsqExpl = searcher.explain(ffsq, 0);
         assertTrue(ffsqExpl.isMatch());
         assertEquals(queryExpl.getValue(), ffsqExpl.getValue(), 0f);
-        assertEquals(queryExpl.getDescription(), ffsqExpl.getDescription());
+        assertEquals(queryExpl.getDescription(), ffsqExpl.getDetails()[0].getDescription());
 
         ffsq = new FiltersFunctionScoreQuery(query, ScoreMode.SUM, new FilterFunction[0], Float.POSITIVE_INFINITY, 10f,
                 CombineFunction.MULTIPLY);
@@ -726,6 +726,31 @@ public void testFilterFunctionScoreHashCodeAndEquals() {
         }
     }
 
+    public void testExplanationAndScoreEqualsEvenIfNoFunctionMatches() throws IOException {
+        IndexSearcher localSearcher = newSearcher(reader);
+        ScoreMode scoreMode = randomFrom(new
+            ScoreMode[]{ScoreMode.SUM, ScoreMode.AVG, ScoreMode.FIRST, ScoreMode.MIN, ScoreMode.MAX, ScoreMode.MULTIPLY});
+        CombineFunction combineFunction = randomFrom(new
+            CombineFunction[]{CombineFunction.SUM, CombineFunction.AVG, CombineFunction.MIN, CombineFunction.MAX,
+            CombineFunction.MULTIPLY, CombineFunction.REPLACE});
+
+        // check for document that has no macthing function
+        FiltersFunctionScoreQuery query = new FiltersFunctionScoreQuery(new TermQuery(new Term(FIELD, "out")), scoreMode,
+            new FilterFunction[]{new FilterFunction(new TermQuery(new Term("_uid", "2")), new WeightFactorFunction(10))},
+            Float.MAX_VALUE, Float.NEGATIVE_INFINITY, combineFunction);
+        TopDocs searchResult = localSearcher.search(query, 1);
+        Explanation explanation = localSearcher.explain(query, searchResult.scoreDocs[0].doc);
+        assertThat(searchResult.scoreDocs[0].score, equalTo(explanation.getValue()));
+
+        // check for document that has a matching function
+        query = new FiltersFunctionScoreQuery(new TermQuery(new Term(FIELD, "out")), scoreMode,
+            new FilterFunction[]{new FilterFunction(new TermQuery(new Term("_uid", "1")), new WeightFactorFunction(10))},
+            Float.MAX_VALUE, Float.NEGATIVE_INFINITY, combineFunction);
+        searchResult = localSearcher.search(query, 1);
+        explanation = localSearcher.explain(query, searchResult.scoreDocs[0].doc);
+        assertThat(searchResult.scoreDocs[0].score, equalTo(explanation.getValue()));
+    }
+
     private static class DummyScoreFunction extends ScoreFunction {
         protected DummyScoreFunction(CombineFunction scoreCombiner) {
             super(scoreCombiner);