/*
* Licensed to the Apache Software Foundation (ASF) under one or more
* contributor license agreements. See the NOTICE file distributed with
* this work for additional information regarding copyright ownership.
* The ASF licenses this file to You under the Apache License, Version 2.0
* (the "License"); you may not use this file except in compliance with
* the License. You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/

package org.elasticsearch.search.aggregations.metrics.percentiles.tdigest;

import com.tdunning.math.stats.AVLTreeDigest;
import com.tdunning.math.stats.Centroid;
import org.elasticsearch.common.io.stream.StreamInput;
import org.elasticsearch.common.io.stream.StreamOutput;

import java.io.IOException;

/**
 * Extension of {@link TDigest} with custom serialization.
 */
public class TDigestState extends AVLTreeDigest {

    private final double compression;

    public TDigestState(double compression) {
        super(compression);
        this.compression = compression;
    }

    public static void write(TDigestState state, StreamOutput out) throws IOException {
        out.writeDouble(state.compression);
        out.writeVInt(state.centroidCount());
        for (Centroid centroid : state.centroids()) {
            out.writeDouble(centroid.mean());
            out.writeVLong(centroid.count());
        }
    }

    public static TDigestState read(StreamInput in) throws IOException {
        double compression = in.readDouble();
        TDigestState state = new TDigestState(compression);
        int n = in.readVInt();
        for (int i = 0; i < n; i++) {
            state.add(in.readDouble(), in.readVInt());
        }
        return state;
    }

}

/*
* Licensed to the Apache Software Foundation (ASF) under one or more
* contributor license agreements. See the NOTICE file distributed with
* this work for additional information regarding copyright ownership.
* The ASF licenses this file to You under the Apache License, Version 2.0
* (the "License"); you may not use this file except in compliance with
* the License. You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/

package org.elasticsearch.search.aggregations.metrics.percentiles.tdigest;

import com.tdunning.math.stats.AVLTreeDigest;
import com.tdunning.math.stats.Centroid;
import org.elasticsearch.common.io.stream.StreamInput;
import org.elasticsearch.common.io.stream.StreamOutput;

import java.io.IOException;

/**
 * Extension of {@link com.tdunning.math.stats.TDigest} with custom serialization.
 */
public class TDigestState extends AVLTreeDigest {

    private final double compression;

    public TDigestState(double compression) {
        super(compression);
        this.compression = compression;
    }

    public static void write(TDigestState state, StreamOutput out) throws IOException {
        out.writeDouble(state.compression);
        out.writeVInt(state.centroidCount());
        for (Centroid centroid : state.centroids()) {
            out.writeDouble(centroid.mean());
            out.writeVLong(centroid.count());
        }
    }

    public static TDigestState read(StreamInput in) throws IOException {
        double compression = in.readDouble();
        TDigestState state = new TDigestState(compression);
        int n = in.readVInt();
        for (int i = 0; i < n; i++) {
            state.add(in.readDouble(), in.readVInt());
        }
        return state;
    }

}

Add full link to TDigest class