Minor cleanups for MMR Diversify Retriever

rest-api-spec/src/yamlRestTest/resources/rest-api-spec/test/search.retrievers/result-diversification/10_mmr_result_diversification_retriever.yml

@@ -322,9 +322,9 @@ teardown:
 
   - match: { hits.total.value: 10 }
   - length: { hits.hits: 3 }
-  - match: { hits.hits.0._source.textbody: "second text" }
-  - match: { hits.hits.1._source.textbody: "sixth text" }
-  - match: { hits.hits.2._source.textbody: "ninth text" }
+  - match: { hits.hits.0._source.textbody: "sixth text" }
+  - match: { hits.hits.1._source.textbody: "ninth text" }
+  - match: { hits.hits.2._source.textbody: "fourth text" }
 
   - do:
       search:
@@ -346,9 +346,9 @@ teardown:
 
   - match: { hits.total.value: 10 }
   - length: { hits.hits: 3 }
-  - match: { hits.hits.0._source.textbody: "second text" }
-  - match: { hits.hits.1._source.textbody: "fourth text" }
-  - match: { hits.hits.2._source.textbody: "fifth text" }
+  - match: { hits.hits.0._source.textbody: "fourth text" }
+  - match: { hits.hits.1._source.textbody: "fifth text" }
+  - match: { hits.hits.2._source.textbody: "seventh text" }
 
 ---
 "Test MMR result diversification byte vector type":
@@ -584,6 +584,27 @@ teardown:
   - match: { status: 400 }
   - match: { error.type: illegal_argument_exception }
 
+  - do:
+      catch: /\[diversify\] MMR result diversification \[size\] of -3 must be greater than zero/
+      search:
+        index: test-result-diversification-index
+        body:
+          retriever:
+            diversify:
+              type: "mmr"
+              field: "textvector"
+              size: -3
+              lambda: 0.7
+              retriever:
+                knn:
+                  field: "textvector"
+                  query_vector: [ 0.5, 0.2, 0.4, 0.4 ]
+                  k: 6
+                  num_candidates: 6
+
+  - match: { status: 400 }
+  - match: { error.type: action_request_validation_exception }
+
   - do:
       catch: /\[diversify\] MMR result diversification must have a \[lambda\] between 0.0 and 1.0. The value provided was null/
       search:

server/src/main/java/org/elasticsearch/search/diversification/DiversifyRetrieverBuilder.java

@@ -189,6 +189,19 @@ public ActionRequestValidationException validate(
     }
 
     private ActionRequestValidationException validateMMRDiversification(ActionRequestValidationException validationException) {
+        if (this.size <= 0) {
+            validationException = addValidationError(
+                String.format(
+                    Locale.ROOT,
+                    "[%s] MMR result diversification [%s] of %d must be greater than zero",
+                    getName(),
+                    SIZE_FIELD.getPreferredName(),
+                    this.size
+                ),
+                validationException
+            );
+        }
+
         if (this.size > this.rankWindowSize) {
             validationException = addValidationError(
                 String.format(

server/src/main/java/org/elasticsearch/search/diversification/mmr/MMRResultDiversification.java

@@ -45,18 +45,16 @@ public RankDoc[] diversify(RankDoc[] docs) throws IOException {
         // our chosen DocIDs to keep
         List<Integer> selectedDocRanks = new ArrayList<>();
 
-        // always add the highest scoring doc to the list
-        RankDoc highestScoreDoc = Arrays.stream(docs).max(Comparator.comparingDouble(doc -> doc.score)).orElse(docs[0]);
-        int highestScoreDocRank = highestScoreDoc.rank;
-        selectedDocRanks.add(highestScoreDocRank);
-
         // test the vector to see if we are using floats or bytes
-        VectorData firstVec = context.getFieldVector(highestScoreDocRank);
+        VectorData firstVec = context.getFieldVector(docs[0].rank);
         boolean useFloat = firstVec.isFloat();
 
         // cache the similarity scores for the query vector vs. searchHits
         Map<Integer, Float> querySimilarity = getQuerySimilarityForDocs(docs, useFloat, context);
 
+        // always add the highest relevant doc to the list
+        selectedDocRanks.add(getHighestRelevantDocRank(docs, querySimilarity));
+
         Map<Integer, Map<Integer, Float>> cachedSimilarities = new HashMap<>();
         int topDocsSize = context.getSize();
 
@@ -113,6 +111,20 @@ public RankDoc[] diversify(RankDoc[] docs) throws IOException {
         return ret;
     }
 
+    private Integer getHighestRelevantDocRank(RankDoc[] docs, Map<Integer, Float> querySimilarity) {
+        Map.Entry<Integer, Float> highestRelevantDoc = querySimilarity.entrySet()
+            .stream()
+            .max(Comparator.comparingDouble(Map.Entry::getValue))
+            .orElse(null);
+
+        if (highestRelevantDoc != null) {
+            return highestRelevantDoc.getKey();
+        }
+
+        RankDoc highestScoreDoc = Arrays.stream(docs).max(Comparator.comparingDouble(doc -> doc.score)).orElse(docs[0]);
+        return highestScoreDoc.rank;
+    }
+
     private float getHighestScoreForSelectedVectors(
         int docRank,
         MMRResultDiversificationContext context,

server/src/test/java/org/elasticsearch/search/diversification/DiversifyRetrieverBuilderTests.java

@@ -57,6 +57,38 @@ public class DiversifyRetrieverBuilderTests extends ESTestCase {
     public void testValidate() {
         SearchSourceBuilder source = new SearchSourceBuilder();
 
+        var retrieverWithZeroSize = new DiversifyRetrieverBuilder(
+            getInnerRetriever(),
+            ResultDiversificationType.MMR,
+            "test_field",
+            10,
+            0,
+            getRandomQueryVector(),
+            0.3f
+        );
+        var validationZeroSize = retrieverWithZeroSize.validate(source, null, false, false);
+        assertEquals(1, validationZeroSize.validationErrors().size());
+        assertEquals(
+            "[diversify] MMR result diversification [size] of 0 must be greater than zero",
+            validationZeroSize.validationErrors().getFirst()
+        );
+
+        var retrieverWithNegativeSize = new DiversifyRetrieverBuilder(
+            getInnerRetriever(),
+            ResultDiversificationType.MMR,
+            "test_field",
+            10,
+            -1,
+            getRandomQueryVector(),
+            0.3f
+        );
+        var validationNegativeSize = retrieverWithNegativeSize.validate(source, null, false, false);
+        assertEquals(1, validationNegativeSize.validationErrors().size());
+        assertEquals(
+            "[diversify] MMR result diversification [size] of -1 must be greater than zero",
+            validationNegativeSize.validationErrors().getFirst()
+        );
+
         var retrieverWithLargeSize = new DiversifyRetrieverBuilder(
             getInnerRetriever(),
             ResultDiversificationType.MMR,
@@ -184,8 +216,8 @@ public void testMmrResultDiversification() {
         var result = retriever.combineInnerRetrieverResults(docs, false);
 
         assertEquals(3, result.length);
-        assertEquals(1, result[0].rank);
-        assertEquals(3, result[1].rank);
+        assertEquals(3, result[0].rank);
+        assertEquals(4, result[1].rank);
         assertEquals(6, result[2].rank);
 
         var retrieverWithoutRewrite = new DiversifyRetrieverBuilder(

server/src/test/java/org/elasticsearch/search/diversification/mmr/MMRResultDiversificationTests.java

@@ -89,7 +89,7 @@ private MMRResultDiversificationContext getRandomFloatContext(List<Integer> expe
             )
         );
 
-        expectedDocIds.addAll(List.of(1, 3, 6));
+        expectedDocIds.addAll(List.of(3, 4, 6));
 
         return diversificationContext;
     }
@@ -124,7 +124,7 @@ private MMRResultDiversificationContext getRandomByteContext(List<Integer> expec
             )
         );
 
-        expectedDocIds.addAll(List.of(1, 3, 6));
+        expectedDocIds.addAll(List.of(2, 3, 6));
 
         return diversificationContext;
     }