Clean up and test CollapseResponseProcessor

After realizing that we just need to keep the first hit for each group
(since results are already sorted by the sort criteria), I think
CollapseResponseProcessor might be worth including.

Combining it with the oversample + truncate processors, it can provide a
workaround for the lack of support for collapse + rescore.

Signed-off-by: Michael Froh <froh@amazon.com>

modules/search-pipeline-common/src/main/java/org/opensearch/search/pipeline/common/CollapseResponseProcessor.java

@@ -20,10 +20,10 @@
 import org.opensearch.search.pipeline.common.helpers.SearchResponseUtil;
 
 import java.util.ArrayList;
-import java.util.Comparator;
-import java.util.HashMap;
+import java.util.LinkedHashMap;
 import java.util.List;
 import java.util.Map;
+import java.util.Objects;
 
 /**
  * A simple implementation of field collapsing on search responses. Note that this is not going to work as well as
@@ -35,12 +35,12 @@ public class CollapseResponseProcessor extends AbstractProcessor implements Sear
      * Key to reference this processor type from a search pipeline.
      */
     public static final String TYPE = "collapse";
-    private static final String COLLAPSE_FIELD = "field";
+    static final String COLLAPSE_FIELD = "field";
     private final String collapseField;
 
     private CollapseResponseProcessor(String tag, String description, boolean ignoreFailure, String collapseField) {
         super(tag, description, ignoreFailure);
-        this.collapseField = collapseField;
+        this.collapseField = Objects.requireNonNull(collapseField);
     }
 
     @Override
@@ -49,34 +49,43 @@ public String getType() {
     }
 
     @Override
-    public SearchResponse processResponse(SearchRequest request, SearchResponse response) throws Exception {
+    public SearchResponse processResponse(SearchRequest request, SearchResponse response) {
 
         if (response.getHits() != null) {
-            Map<String, SearchHit> collapsedHits = new HashMap<>();
+            if (response.getHits().getCollapseField() != null) {
+                throw new IllegalStateException("Cannot collapse on " + collapseField + ". Results already collapsed on " + response.getHits().getCollapseField());
+            }
+            Map<String, SearchHit> collapsedHits = new LinkedHashMap<>();
+            List<Object> collapseValues = new ArrayList<>();
             for (SearchHit hit : response.getHits()) {
-                String fieldValue = "<missing>";
+                Object fieldValue = null;
                 DocumentField docField = hit.getFields().get(collapseField);
                 if (docField != null) {
                     if (docField.getValues().size() > 1) {
-                        throw new IllegalStateException("Document " + hit.getId() + " has multiple values for field " + collapseField);
-                    }
-                    fieldValue = docField.getValues().get(0).toString();
-                } else if (hit.hasSource()) {
-                    Object val = hit.getSourceAsMap().get(collapseField);
-                    if (val != null) {
-                        fieldValue = val.toString();
+                        throw new IllegalStateException("Failed to collapse " + hit.getId() + ": doc has multiple values for field " + collapseField);
                     }
+                    fieldValue = docField.getValues().get(0);
+                } else if (hit.getSourceAsMap() != null) {
+                    fieldValue = hit.getSourceAsMap().get(collapseField);
                 }
-                SearchHit previousHit = collapsedHits.get(fieldValue);
-                // TODO - Support the sort used in the request, rather than just score
-                if (previousHit == null || hit.getScore() > previousHit.getScore()) {
-                    collapsedHits.put(fieldValue, hit);
+                String fieldValueString;
+                if (fieldValue == null) {
+                    fieldValueString = "__missing__";
+                } else {
+                    fieldValueString = fieldValue.toString();
                 }
+
+                // Results are already sorted by sort criterion. Only keep the first hit for each field.
+                if (collapsedHits.containsKey(fieldValueString) == false) {
+                    collapsedHits.put(fieldValueString, hit);
+                    collapseValues.add(fieldValue);
+                }
+            }
+            SearchHit[] newHits = new SearchHit[collapsedHits.size()];
+            int i = 0;
+            for (SearchHit collapsedHit : collapsedHits.values()) {
+                newHits[i++] = collapsedHit;
             }
-            List<SearchHit> hitsToReturn = new ArrayList<>(collapsedHits.values());
-            hitsToReturn.sort(Comparator.comparingDouble(SearchHit::getScore).reversed());
-            SearchHit[] newHits = hitsToReturn.toArray(new SearchHit[0]);
-            List<String> collapseValues = new ArrayList<>(collapsedHits.keySet());
             SearchHits searchHits = new SearchHits(
                 newHits,
                 response.getHits().getTotalHits(),

modules/search-pipeline-common/src/test/java/org/opensearch/search/pipeline/common/CollapseResponseProcessorTests.java

@@ -0,0 +1,79 @@
+/*
+ * SPDX-License-Identifier: Apache-2.0
+ *
+ * The OpenSearch Contributors require contributions made to
+ * this file be licensed under the Apache-2.0 license or a
+ * compatible open source license.
+ */
+
+package org.opensearch.search.pipeline.common;
+
+import org.apache.lucene.search.TotalHits;
+import org.opensearch.action.search.SearchRequest;
+import org.opensearch.action.search.SearchResponse;
+import org.opensearch.common.document.DocumentField;
+import org.opensearch.core.common.bytes.BytesArray;
+import org.opensearch.search.SearchHit;
+import org.opensearch.search.SearchHits;
+import org.opensearch.search.internal.InternalSearchResponse;
+import org.opensearch.test.OpenSearchTestCase;
+
+import java.util.Collections;
+import java.util.HashMap;
+import java.util.List;
+import java.util.Map;
+
+public class CollapseResponseProcessorTests extends OpenSearchTestCase {
+    public void testWithDocumentFields() {
+        testProcessor(true);
+    }
+
+    public void testWithSourceField() {
+        testProcessor(false);
+    }
+
+    private void testProcessor(boolean includeDocField) {
+        Map<String, Object> config = new HashMap<>(Map.of(CollapseResponseProcessor.COLLAPSE_FIELD, "groupid"));
+        CollapseResponseProcessor processor = new CollapseResponseProcessor.Factory().create(Collections.emptyMap(), null, null, false, config, null);
+        int numHits = randomIntBetween(1, 100);
+        SearchResponse inputResponse = generateResponse(numHits, includeDocField);
+
+        SearchResponse processedResponse = processor.processResponse(new SearchRequest(), inputResponse);
+        if (numHits % 2 == 0) {
+            assertEquals(numHits / 2, processedResponse.getHits().getHits().length);
+        } else {
+            assertEquals(numHits / 2 + 1, processedResponse.getHits().getHits().length);
+        }
+        for (SearchHit collapsedHit : processedResponse.getHits()) {
+            assertEquals(0, collapsedHit.docId() % 2);
+        }
+        assertEquals("groupid", processedResponse.getHits().getCollapseField());
+        assertEquals(processedResponse.getHits().getHits().length, processedResponse.getHits().getCollapseValues().length);
+        for (int i = 0; i < processedResponse.getHits().getHits().length; i++) {
+            assertEquals(i, processedResponse.getHits().getCollapseValues()[i]);
+        }
+    }
+
+    private static SearchResponse generateResponse(int numHits, boolean includeDocField) {
+        SearchHit[] hitsArray = new SearchHit[numHits];
+        for (int i = 0; i < numHits; i++) {
+            Map<String, DocumentField> docFields;
+            int groupValue = i / 2;
+            if (includeDocField) {
+                docFields = Map.of("groupid", new DocumentField("groupid", List.of(groupValue)));
+            } else {
+                docFields = Collections.emptyMap();
+            }
+            SearchHit hit = new SearchHit(i, Integer.toString(i), docFields, Collections.emptyMap());
+            hit.sourceRef(new BytesArray("{\"groupid\": "+ groupValue + "}"));
+            hitsArray[i] = hit;
+        }
+        SearchHits searchHits = new SearchHits(
+            hitsArray,
+            new TotalHits(Math.max(numHits, 1000), TotalHits.Relation.GREATER_THAN_OR_EQUAL_TO),
+            1.0f
+        );
+        InternalSearchResponse internalSearchResponse = new InternalSearchResponse(searchHits, null, null, null, false, false, 0);
+        return new SearchResponse(internalSearchResponse, null, 1, 1, 0, 10, null, null);
+    }
+}