Remove MultiCollectorWrapper and use MultiCollector in Lucene instead (#19595)

* Remove MultiCollectorWrapper and use MultiCollector in Lucene instead

Signed-off-by: Binlong Gao <gbinlong@amazon.com>

* Modify change log

Signed-off-by: Binlong Gao <gbinlong@amazon.com>

* Optimize some code

Signed-off-by: Binlong Gao <gbinlong@amazon.com>

* pattern matching for instanceof

Signed-off-by: Binlong Gao <gbinlong@amazon.com>

* Remove redundant bracket

Signed-off-by: Binlong Gao <gbinlong@amazon.com>

---------

Signed-off-by: Binlong Gao <gbinlong@amazon.com>

Author: gaobinlong

CHANGELOG.md

@@ -14,6 +14,7 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 - Faster `terms` query creation for `keyword` field with index and docValues enabled ([#19350](https://github.com/opensearch-project/OpenSearch/pull/19350))
 - Refactor to move prepareIndex and prepareDelete methods to Engine class ([#19551](https://github.com/opensearch-project/OpenSearch/pull/19551))
 - Omit maxScoreCollector in SimpleTopDocsCollectorContext when concurrent segment search enabled ([#19584](https://github.com/opensearch-project/OpenSearch/pull/19584))
+- Remove MultiCollectorWrapper and use MultiCollector in Lucene instead ([#19595](https://github.com/opensearch-project/OpenSearch/pull/19595))
 
 ### Fixed
 - Fix Allocation and Rebalance Constraints of WeightFunction are incorrectly reset ([#19012](https://github.com/opensearch-project/OpenSearch/pull/19012))

server/src/main/java/org/opensearch/search/query/MultiCollectorWrapper.java

@@ -85,6 +85,7 @@
 import java.util.ArrayList;
 import java.util.Arrays;
 import java.util.Collection;
+import java.util.List;
 import java.util.Objects;
 import java.util.function.Function;
 import java.util.function.Supplier;
@@ -322,15 +323,15 @@ public Collector newCollector() throws IOException {
                         maxScoreCollector = new MaxScoreCollector();
                     }
 
-                    return MultiCollectorWrapper.wrap(collapseContext.createTopDocs(sort, numHits, searchAfter), maxScoreCollector);
+                    return MultiCollector.wrap(collapseContext.createTopDocs(sort, numHits, searchAfter), maxScoreCollector);
                 }
 
                 @Override
                 public ReduceableSearchResult reduce(Collection<Collector> collectors) throws IOException {
                     final Collection<Collector> subs = new ArrayList<>();
                     for (final Collector collector : collectors) {
-                        if (collector instanceof MultiCollectorWrapper) {
-                            subs.addAll(((MultiCollectorWrapper) collector).getCollectors());
+                        if (collector instanceof MultiCollector m) {
+                            subs.addAll(List.of(m.getCollectors()));
                         } else {
                             subs.add(collector);
                         }
@@ -340,10 +341,10 @@ public ReduceableSearchResult reduce(Collection<Collector> collectors) throws IO
                     float maxScore = Float.NaN;
 
                     for (final Collector collector : subs) {
-                        if (collector instanceof CollapsingTopDocsCollector<?>) {
-                            topFieldDocs.add(((CollapsingTopDocsCollector<?>) collector).getTopDocs());
-                        } else if (collector instanceof MaxScoreCollector) {
-                            float score = ((MaxScoreCollector) collector).getMaxScore();
+                        if (collector instanceof CollapsingTopDocsCollector<?> c) {
+                            topFieldDocs.add(c.getTopDocs());
+                        } else if (collector instanceof MaxScoreCollector msc) {
+                            float score = msc.getMaxScore();
                             if (Float.isNaN(maxScore)) {
                                 maxScore = score;
                             } else {
@@ -549,7 +550,7 @@ public Collector newCollector() throws IOException {
                     maxScoreCollector = new MaxScoreCollector();
                 }
 
-                return MultiCollectorWrapper.wrap(manager.newCollector(), maxScoreCollector);
+                return MultiCollector.wrap(manager.newCollector(), maxScoreCollector);
             }
 
             @SuppressWarnings("unchecked")
@@ -559,18 +560,18 @@ public ReduceableSearchResult reduce(Collection<Collector> collectors) throws IO
                 final Collection<MaxScoreCollector> maxScoreCollectors = new ArrayList<>();
 
                 for (final Collector collector : collectors) {
-                    if (collector instanceof MultiCollectorWrapper) {
-                        for (final Collector sub : (((MultiCollectorWrapper) collector).getCollectors())) {
-                            if (sub instanceof TopDocsCollector<?>) {
-                                topDocsCollectors.add((TopDocsCollector<?>) sub);
-                            } else if (sub instanceof MaxScoreCollector) {
-                                maxScoreCollectors.add((MaxScoreCollector) sub);
+                    if (collector instanceof MultiCollector m) {
+                        for (final Collector sub : m.getCollectors()) {
+                            if (sub instanceof TopDocsCollector<?> tdc) {
+                                topDocsCollectors.add(tdc);
+                            } else if (sub instanceof MaxScoreCollector msc) {
+                                maxScoreCollectors.add(msc);
                             }
                         }
-                    } else if (collector instanceof TopDocsCollector<?>) {
-                        topDocsCollectors.add((TopDocsCollector<?>) collector);
-                    } else if (collector instanceof MaxScoreCollector) {
-                        maxScoreCollectors.add((MaxScoreCollector) collector);
+                    } else if (collector instanceof TopDocsCollector<?> c) {
+                        topDocsCollectors.add(c);
+                    } else if (collector instanceof MaxScoreCollector msc) {
+                        maxScoreCollectors.add(msc);
                     }
                 }