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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;

import org.apache.druid.guice.annotations.ExtensionPoint;

import javax.annotation.Nullable;
import java.io.Closeable;

/**
 * An Aggregator is an object that can aggregate metrics.  Its aggregation-related methods (namely, aggregate() and get())
 * do not take any arguments as the assumption is that the Aggregator was given something in its constructor that
 * it can use to get at the next bit of data.
 *
 * Thus, an Aggregator can be thought of as a closure over some other thing that is stateful and changes between calls
 * to aggregate(). This is currently (as of this documentation) implemented through the use of {@link
 * org.apache.druid.segment.ColumnValueSelector} objects.
 *
 * During ingestion, {@link org.apache.druid.segment.incremental.OnheapIncrementalIndex} uses instances of this class
 * from multiple threads. In particular, ingestion threads call {@link #aggregate()} simultaneously with query threads
 * calling the various "get" methods. Instances of this class must be thread-safe.
 */
@ExtensionPoint
public interface Aggregator extends Closeable
{
  /**
   * Performs aggregation.
   */
  void aggregate();

  /**
   * Performs aggregation and returns the increase in required on-heap memory
   * caused by this aggregation step.
   * 
   * The default implementation of this method calls {@link #aggregate()} and returns 0.
   * 
   * If overridden, this method must include the JVM object overheads in the size
   * estimation and must ensure not to underestimate required memory as that might
   * lead to OOM errors.
   *
   * @return Increase in required on-heap memory caused by this aggregation step.
   */
  default long aggregateWithSize()
  {
    aggregate();
    return 0;
  }

  @Nullable
  Object get();
  float getFloat();
  long getLong();

  /**
   * The default implementation casts {@link Aggregator#getFloat()} to double.
   * This default method is added to enable smooth backward compatibility, please re-implement it if your aggregators
   * work with numeric double columns.
   */
  default double getDouble()
  {
    return getFloat();
  }

  /**
   * returns true if aggregator's output type is primitive long/double/float and aggregated value is null,
   * but when aggregated output type is Object, this method always returns false,
   * and users are advised to check nullability for the object returned by {@link #get()}
   * method.
   * The default implementation always return false to enable smooth backward compatibility,
   * re-implement if your aggregator is nullable.
   */
  default boolean isNull()
  {
    return false;
  }

  @Override
  void close();
}