Make parent and child aggregator more obvious (#57490)

Pulls the way that the `ParentJoinAggregator` collects global ordinals
into a strategy object so it is a little simpler to reason about and
it'll be simpler to save memory by removing `asMultiBucketAggregator` in
the future.

Relates to #56487

modules/parent-join/src/main/java/org/elasticsearch/join/aggregations/ParentJoinAggregator.java

@@ -28,8 +28,10 @@ import org.apache.lucene.search.ScoreMode;
 import org.apache.lucene.search.Scorer;
 import org.apache.lucene.search.Weight;
 import org.apache.lucene.util.Bits;
+import org.elasticsearch.common.lease.Releasable;
 import org.elasticsearch.common.lease.Releasables;
 import org.elasticsearch.common.lucene.Lucene;
+import org.elasticsearch.common.util.BigArrays;
 import org.elasticsearch.common.util.BitArray;
 import org.elasticsearch.common.util.LongHash;
 import org.elasticsearch.search.aggregations.Aggregator;
@@ -52,16 +54,11 @@ public abstract class ParentJoinAggregator extends BucketsAggregator implements
     private final Weight inFilter;
     private final Weight outFilter;
     private final ValuesSource.Bytes.WithOrdinals valuesSource;
-    private final boolean singleAggregator;
 
     /**
-     * If this aggregator is nested under another aggregator we allocate a long hash per bucket.
+     * Strategy for collecting results.
      */
-    private final LongHash ordsHash;
-    /**
-     * Otherwise we use a dense bit array to record the global ordinals.
-     */
-    private final BitArray ordsBit;
+    private final CollectionStrategy collectionStrategy;
 
     public ParentJoinAggregator(String name,
                                     AggregatorFactories factories,
@@ -83,21 +80,9 @@ public abstract class ParentJoinAggregator extends BucketsAggregator implements
         this.inFilter = context.searcher().createWeight(context.searcher().rewrite(inFilter), ScoreMode.COMPLETE_NO_SCORES, 1f);
         this.outFilter = context.searcher().createWeight(context.searcher().rewrite(outFilter), ScoreMode.COMPLETE_NO_SCORES, 1f);
         this.valuesSource = valuesSource;
-        this.singleAggregator = parent == null;
-        this.ordsBit = singleAggregator ? new BitArray((int) maxOrd, context.bigArrays()) : null;
-        this.ordsHash = singleAggregator ? null : new LongHash(1, context.bigArrays());
-    }
-
-    private void addGlobalOrdinal(int globalOrdinal) {
-        if (singleAggregator) {
-            ordsBit.set(globalOrdinal);
-        } else {
-            ordsHash.add(globalOrdinal);
-        }
-    }
-
-    private boolean existsGlobalOrdinal(int globalOrdinal) {
-        return singleAggregator ? ordsBit.get(globalOrdinal): ordsHash.find(globalOrdinal) >= 0;
+        boolean singleAggregator = parent == null;
+        collectionStrategy = singleAggregator ?
+                new DenseCollectionStrategy(maxOrd, context.bigArrays()) : new SparseCollectionStrategy(context.bigArrays());
     }
 
     @Override
@@ -115,7 +100,7 @@ public abstract class ParentJoinAggregator extends BucketsAggregator implements
                 if (parentDocs.get(docId) && globalOrdinals.advanceExact(docId)) {
                     int globalOrdinal = (int) globalOrdinals.nextOrd();
                     assert globalOrdinal != -1 && globalOrdinals.nextOrd() == SortedSetDocValues.NO_MORE_ORDS;
-                    addGlobalOrdinal(globalOrdinal);
+                    collectionStrategy.addGlobalOrdinal(globalOrdinal);
                 }
             }
         };
@@ -155,7 +140,7 @@ public abstract class ParentJoinAggregator extends BucketsAggregator implements
                 if (globalOrdinals.advanceExact(docId)) {
                     int globalOrdinal = (int) globalOrdinals.nextOrd();
                     assert globalOrdinal != -1 && globalOrdinals.nextOrd() == SortedSetDocValues.NO_MORE_ORDS;
-                    if (existsGlobalOrdinal(globalOrdinal)) {
+                    if (collectionStrategy.existsGlobalOrdinal(globalOrdinal)) {
                         collectBucket(sub, docId, 0);
                     }
                 }
@@ -165,6 +150,75 @@ public abstract class ParentJoinAggregator extends BucketsAggregator implements
 
     @Override
     protected void doClose() {
-        Releasables.close(ordsBit, ordsHash);
+        Releasables.close(collectionStrategy);
+    }
+
+    /**
+     * Strategy for collecting the global ordinals of the join field in for all
+     * docs that match the {@code ParentJoinAggregator#inFilter} and then
+     * checking if which of the docs in the
+     * {@code ParentJoinAggregator#outFilter} also have the ordinal.
+     */
+    protected interface CollectionStrategy extends Releasable {
+        void addGlobalOrdinal(int globalOrdinal);
+        boolean existsGlobalOrdinal(int globalOrdinal);
+    }
+
+    /**
+     * Uses a dense, bit per ordinal representation of the join field in the
+     * docs that match {@code ParentJoinAggregator#inFilter}. Its memory usage
+     * is proportional to the maximum ordinal so it is only a good choice if
+     * most docs match.
+     */
+    protected class DenseCollectionStrategy implements CollectionStrategy {
+        private final BitArray ordsBits;
+
+        public DenseCollectionStrategy(long maxOrd, BigArrays bigArrays) {
+            ordsBits = new BitArray((int) maxOrd, context.bigArrays());
+        }
+
+        @Override
+        public void addGlobalOrdinal(int globalOrdinal) {
+            ordsBits.set(globalOrdinal);
+        }
+
+        @Override
+        public boolean existsGlobalOrdinal(int globalOrdinal) {
+            return ordsBits.get(globalOrdinal);
+        }
+
+        @Override
+        public void close() {
+            ordsBits.close();
+        }
+    }
+
+    /**
+     * Uses a hashed representation of whether of the join field in the docs
+     * that match {@code ParentJoinAggregator#inFilter}. Its memory usage is
+     * proportional to the number of matching global ordinals so it is used
+     * when only some docs might match.
+     */
+    protected class SparseCollectionStrategy implements CollectionStrategy {
+        private final LongHash ordsHash;
+
+        public SparseCollectionStrategy(BigArrays bigArrays) {
+            ordsHash = new LongHash(1, bigArrays);
+        }
+
+        @Override
+        public void addGlobalOrdinal(int globalOrdinal) {
+            ordsHash.add(globalOrdinal);
+        }
+
+        @Override
+        public boolean existsGlobalOrdinal(int globalOrdinal) {
+            return ordsHash.find(globalOrdinal) >= 0;
+        }
+
+        @Override
+        public void close() {
+            ordsHash.close();
+        }
     }
 }