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A module that is everything required to understands Druid Segments
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/*
 * 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.apache.druid.query.aggregation.mean;

import com.fasterxml.jackson.core.JsonGenerator;
import com.fasterxml.jackson.databind.JsonSerializer;
import com.fasterxml.jackson.databind.SerializerProvider;
import com.google.common.primitives.Doubles;

import java.io.IOException;
import java.nio.ByteBuffer;
import java.util.Comparator;
import java.util.Objects;

public class DoubleMeanHolder
{
  public static final int MAX_INTERMEDIATE_SIZE = Long.BYTES + Double.BYTES;
  public static final Comparator COMPARATOR = (o1, o2) -> Doubles.compare(o1.mean(), o2.mean());

  private double sum;
  private long count;

  public DoubleMeanHolder(double sum, long count)
  {
    this.sum = sum;
    this.count = count;
  }

  public void update(double sum)
  {
    this.sum += sum;
    count++;
  }

  public DoubleMeanHolder update(DoubleMeanHolder other)
  {
    sum += other.sum;
    count += other.count;
    return this;
  }

  public double mean()
  {
    return count == 0 ? 0 : sum / count;
  }

  public byte[] toBytes()
  {
    ByteBuffer buf = ByteBuffer.allocate(Double.BYTES + Long.BYTES);
    buf.putDouble(0, sum);
    buf.putLong(Double.BYTES, count);
    return buf.array();
  }

  @Override
  public boolean equals(Object o)
  {
    if (this == o) {
      return true;
    }
    if (o == null || getClass() != o.getClass()) {
      return false;
    }
    DoubleMeanHolder that = (DoubleMeanHolder) o;
    return Double.compare(that.sum, sum) == 0 &&
           count == that.count;
  }

  @Override
  public int hashCode()
  {
    return Objects.hash(sum, count);
  }

  public static DoubleMeanHolder fromBytes(byte[] data)
  {
    ByteBuffer buf = ByteBuffer.wrap(data);
    return new DoubleMeanHolder(buf.getDouble(0), buf.getLong(Double.BYTES));
  }

  public static void init(ByteBuffer buf, int position)
  {
    writeSum(buf, position, 0d);
    writeCount(buf, position, 0);
  }

  public static void update(ByteBuffer buf, int position, double sum)
  {
    writeSum(buf, position, getSum(buf, position) + sum);
    writeCount(buf, position, getCount(buf, position) + 1);
  }

  public static void update(ByteBuffer buf, int position, DoubleMeanHolder other)
  {
    writeSum(buf, position, getSum(buf, position) + other.sum);
    writeCount(buf, position, getCount(buf, position) + other.count);
  }

  public static DoubleMeanHolder get(ByteBuffer buf, int position)
  {
    return new DoubleMeanHolder(getSum(buf, position), getCount(buf, position));
  }

  private static void writeSum(ByteBuffer buf, int position, double sum)
  {
    buf.putDouble(position, sum);
  }

  private static double getSum(ByteBuffer buf, int position)
  {
    return buf.getDouble(position);
  }

  private static void writeCount(ByteBuffer buf, int position, long count)
  {
    buf.putLong(position + Double.BYTES, count);
  }

  private static long getCount(ByteBuffer buf, int position)
  {
    return buf.getLong(position + Double.BYTES);
  }

  public static class Serializer extends JsonSerializer
  {
    public static final Serializer INSTANCE = new Serializer();

    private Serializer()
    {

    }

    @Override
    public void serialize(DoubleMeanHolder obj, JsonGenerator jgen, SerializerProvider provider)
        throws IOException
    {
      jgen.writeBinary(obj.toBytes());
    }
  }
}