HLL field Mapper for Cardinality Rollups

CHANGELOG.md

@@ -15,7 +15,7 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 - Add circuit breaker support for gRPC transport to prevent out-of-memory errors ([#20203](https://github.com/opensearch-project/OpenSearch/pull/20203))
 - Add index-level-encryption support for snapshots and remote-store ([#20095](https://github.com/opensearch-project/OpenSearch/pull/20095))
 - Adding BackWardCompatibility test for remote publication enabled cluster ([#20221](https://github.com/opensearch-project/OpenSearch/pull/20221))
-
+- Support for hll field mapper to support cardinality rollups ([#20129](https://github.com/opensearch-project/OpenSearch/pull/20129))
 
 ### Changed
 - Handle custom metadata files in subdirectory-store ([#20157](https://github.com/opensearch-project/OpenSearch/pull/20157))

server/src/main/java/org/opensearch/index/fielddata/plain/HllFieldData.java

@@ -0,0 +1,177 @@
+/*
+ * 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.index.fielddata.plain;
+
+import org.apache.lucene.index.BinaryDocValues;
+import org.apache.lucene.index.LeafReader;
+import org.apache.lucene.index.LeafReaderContext;
+import org.apache.lucene.search.SortField;
+import org.apache.lucene.util.BytesRef;
+import org.opensearch.common.Nullable;
+import org.opensearch.common.util.BigArrays;
+import org.opensearch.core.common.bytes.BytesArray;
+import org.opensearch.core.indices.breaker.CircuitBreakerService;
+import org.opensearch.index.fielddata.IndexFieldData;
+import org.opensearch.index.fielddata.IndexFieldData.XFieldComparatorSource.Nested;
+import org.opensearch.index.fielddata.IndexFieldDataCache;
+import org.opensearch.index.fielddata.LeafFieldData;
+import org.opensearch.index.fielddata.ScriptDocValues;
+import org.opensearch.index.fielddata.SortedBinaryDocValues;
+import org.opensearch.search.DocValueFormat;
+import org.opensearch.search.MultiValueMode;
+import org.opensearch.search.aggregations.metrics.AbstractHyperLogLogPlusPlus;
+import org.opensearch.search.aggregations.support.ValuesSourceType;
+import org.opensearch.search.sort.BucketedSort;
+import org.opensearch.search.sort.SortOrder;
+
+import java.io.IOException;
+
+import static org.opensearch.search.aggregations.support.CoreValuesSourceType.BYTES;
+
+/**
+ * Field data implementation for HyperLogLog++ sketch fields.
+ * Provides access to HLL++ sketches stored as binary doc values.
+ *
+ * @opensearch.internal
+ */
+public class HllFieldData implements IndexFieldData<HllFieldData.HllLeafFieldData> {
+
+    /**
+     * Builder for HLL field data
+     *
+     * @opensearch.internal
+     */
+    public static class Builder implements IndexFieldData.Builder {
+        private final String name;
+        private final int precision;
+
+        public Builder(String name, int precision) {
+            this.name = name;
+            this.precision = precision;
+        }
+
+        @Override
+        public HllFieldData build(IndexFieldDataCache cache, CircuitBreakerService breakerService) {
+            return new HllFieldData(name, precision);
+        }
+    }
+
+    private final String fieldName;
+    private final int precision;
+
+    private HllFieldData(String fieldName, int precision) {
+        this.fieldName = fieldName;
+        this.precision = precision;
+    }
+
+    @Override
+    public final String getFieldName() {
+        return fieldName;
+    }
+
+    public int getPrecision() {
+        return precision;
+    }
+
+    @Override
+    public ValuesSourceType getValuesSourceType() {
+        // HLL fields use BYTES values source type since they store binary data
+        return BYTES;
+    }
+
+    @Override
+    public HllLeafFieldData load(LeafReaderContext context) {
+        return new HllLeafFieldData(context.reader(), fieldName);
+    }
+
+    @Override
+    public HllLeafFieldData loadDirect(LeafReaderContext context) throws Exception {
+        return load(context);
+    }
+
+    @Override
+    public SortField sortField(
+        @Nullable Object missingValue,
+        MultiValueMode sortMode,
+        XFieldComparatorSource.Nested nested,
+        boolean reverse
+    ) {
+        throw new IllegalArgumentException("Sorting is not supported on [hll] fields");
+    }
+
+    @Override
+    public BucketedSort newBucketedSort(
+        BigArrays bigArrays,
+        Object missingValue,
+        MultiValueMode sortMode,
+        Nested nested,
+        SortOrder sortOrder,
+        DocValueFormat format,
+        int bucketSize,
+        BucketedSort.ExtraData extra
+    ) {
+        throw new IllegalArgumentException("Bucketed sort is not supported on [hll] fields");
+    }
+
+    /**
+     * Leaf-level field data for HLL++ sketches.
+     *
+     * @opensearch.internal
+     */
+    public static class HllLeafFieldData implements LeafFieldData {
+
+        private final LeafReader reader;
+        private final String fieldName;
+
+        HllLeafFieldData(LeafReader reader, String fieldName) {
+            this.reader = reader;
+            this.fieldName = fieldName;
+        }
+
+        @Override
+        public long ramBytesUsed() {
+            return 0;
+        }
+
+        @Override
+        public void close() {
+            // Nothing to close
+        }
+
+        @Override
+        public ScriptDocValues<?> getScriptValues() {
+            throw new UnsupportedOperationException("HLL fields do not support getScriptValues");
+        }
+
+        @Override
+        public SortedBinaryDocValues getBytesValues() {
+            throw new UnsupportedOperationException("HLL fields do not support getBytesValues");
+        }
+
+        /**
+         * Get the HLL++ sketch for the given document ID.
+         *
+         * @param docId the document ID
+         * @return the HLL++ sketch, or null if the document doesn't have a value.
+         * The caller is responsible for closing the returned sketch to release resources.
+         * @throws IOException if an error occurs reading the sketch
+         */
+        public AbstractHyperLogLogPlusPlus getSketch(int docId) throws IOException {
+            BinaryDocValues docValues = reader.getBinaryDocValues(fieldName);
+            if (docValues != null && docValues.advanceExact(docId)) {
+                BytesRef sketchBytes = docValues.binaryValue();
+                return AbstractHyperLogLogPlusPlus.readFrom(
+                    new BytesArray(sketchBytes.bytes, sketchBytes.offset, sketchBytes.length).streamInput(),
+                    BigArrays.NON_RECYCLING_INSTANCE
+                );
+            }
+            return null;
+        }
+    }
+}