Fix NDCG for empty search results

modules/rank-eval/src/main/java/org/elasticsearch/index/rankeval/DiscountedCumulativeGain.java

@@ -140,9 +140,12 @@ public EvalQueryQuality evaluate(String taskId, SearchHit[] hits,
 
         if (normalize) {
             Collections.sort(allRatings, Comparator.nullsLast(Collections.reverseOrder()));
-            double idcg = computeDCG(
-                    allRatings.subList(0, Math.min(ratingsInSearchHits.size(), allRatings.size())));
-            dcg = dcg / idcg;
+            double idcg = computeDCG(allRatings.subList(0, Math.min(ratingsInSearchHits.size(), allRatings.size())));
+            if (idcg > 0) {
+                dcg = dcg / idcg;
+            } else {
+                dcg = 0;
+            }
         }
         EvalQueryQuality evalQueryQuality = new EvalQueryQuality(taskId, dcg);
         evalQueryQuality.addHitsAndRatings(ratedHits);

modules/rank-eval/src/main/java/org/elasticsearch/index/rankeval/MeanReciprocalRank.java

@@ -228,6 +228,10 @@ public String getWriteableName() {
             return NAME;
         }
 
+        /**
+         * the ranking of the first relevant document, or -1 if no relevant document was
+         * found
+         */
         int getFirstRelevantRank() {
             return firstRelevantRank;
         }

modules/rank-eval/src/test/java/org/elasticsearch/index/rankeval/DiscountedCumulativeGainTests.java

@@ -205,6 +205,32 @@ public void testDCGAtFourMoreRatings() {
         assertEquals(12.392789260714371 / 13.347184833073591, dcg.evaluate("id", hits, ratedDocs).getQualityLevel(), DELTA);
     }
 
+    /**
+     * test that metric returns 0.0 when there are no search results
+     */
+    public void testNoResults() throws Exception {
+        Integer[] relevanceRatings = new Integer[] { 3, 2, 3, null, 1, null };
+        List<RatedDocument> ratedDocs = new ArrayList<>();
+        for (int i = 0; i < 6; i++) {
+            if (i < relevanceRatings.length) {
+                if (relevanceRatings[i] != null) {
+                    ratedDocs.add(new RatedDocument("index", Integer.toString(i), relevanceRatings[i]));
+                }
+            }
+        }
+        SearchHit[] hits = new SearchHit[0];
+        DiscountedCumulativeGain dcg = new DiscountedCumulativeGain();
+        EvalQueryQuality result = dcg.evaluate("id", hits, ratedDocs);
+        assertEquals(0.0d, result.getQualityLevel(), DELTA);
+        assertEquals(0, filterUnknownDocuments(result.getHitsAndRatings()).size());
+
+        // also check normalized
+        dcg = new DiscountedCumulativeGain(true, null, 10);
+        result = dcg.evaluate("id", hits, ratedDocs);
+        assertEquals(0.0d, result.getQualityLevel(), DELTA);
+        assertEquals(0, filterUnknownDocuments(result.getHitsAndRatings()).size());
+    }
+
     public void testParseFromXContent() throws IOException {
         assertParsedCorrect("{ \"unknown_doc_rating\": 2, \"normalize\": true, \"k\" : 15 }", 2, true, 15);
         assertParsedCorrect("{ \"normalize\": false, \"k\" : 15 }", null, false, 15);

modules/rank-eval/src/test/java/org/elasticsearch/index/rankeval/MeanReciprocalRankTests.java

@@ -158,6 +158,13 @@ public void testEvaluationNoRelevantInResults() {
         assertEquals(0.0, evaluation.getQualityLevel(), Double.MIN_VALUE);
     }
 
+    public void testNoResults() throws Exception {
+        SearchHit[] hits = new SearchHit[0];
+        EvalQueryQuality evaluated = (new MeanReciprocalRank()).evaluate("id", hits, Collections.emptyList());
+        assertEquals(0.0d, evaluated.getQualityLevel(), 0.00001);
+        assertEquals(-1, ((MeanReciprocalRank.Breakdown) evaluated.getMetricDetails()).getFirstRelevantRank());
+    }
+
     public void testXContentRoundtrip() throws IOException {
         MeanReciprocalRank testItem = createTestItem();
         XContentBuilder builder = XContentFactory.contentBuilder(randomFrom(XContentType.values()));

modules/rank-eval/src/test/java/org/elasticsearch/index/rankeval/PrecisionAtKTests.java

@@ -142,6 +142,14 @@ public void testNoRatedDocs() throws Exception {
         assertEquals(0, ((PrecisionAtK.Breakdown) evaluated.getMetricDetails()).getRetrieved());
     }
 
+    public void testNoResults() throws Exception {
+        SearchHit[] hits = new SearchHit[0];
+        EvalQueryQuality evaluated = (new PrecisionAtK()).evaluate("id", hits, Collections.emptyList());
+        assertEquals(0.0d, evaluated.getQualityLevel(), 0.00001);
+        assertEquals(0, ((PrecisionAtK.Breakdown) evaluated.getMetricDetails()).getRelevantRetrieved());
+        assertEquals(0, ((PrecisionAtK.Breakdown) evaluated.getMetricDetails()).getRetrieved());
+    }
+
     public void testParseFromXContent() throws IOException {
         String xContent = " {\n" + "   \"relevant_rating_threshold\" : 2" + "}";
         try (XContentParser parser = createParser(JsonXContent.jsonXContent, xContent)) {