processing/src/main/java/org/apache/druid/query/groupby/epinephelinae/DictionaryBuildingUtils.java

/*
 * 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.groupby.epinephelinae;

import it.unimi.dsi.fastutil.Hash;
import it.unimi.dsi.fastutil.objects.Object2IntMap;
import it.unimi.dsi.fastutil.objects.Object2IntOpenCustomHashMap;
import org.apache.druid.segment.DimensionDictionary;

import java.util.ArrayList;
import java.util.List;

/**
 * Utilities for parts of the groupBy engine that need to build dictionaries.
 */
public class DictionaryBuildingUtils
{
  // Entry in dictionary, node pointer in reverseDictionary, hash + k/v/next pointer in reverseDictionary nodes
  private static final int ROUGH_OVERHEAD_PER_DICTIONARY_ENTRY = Long.BYTES * 5 + Integer.BYTES;

  /**
   * Creates a forward dictionary (dictionary ID -> value).
   */
  public static <T> List<T> createDictionary()
  {
    return new ArrayList<>();
  }

  /**
   * Create reverse dictionary (value -> dictionary ID) that relies on the given {@link Hash.Strategy} for
   * hashing and comparing equality. It explicitly requires a hashing strategy, so that callers are aware of the
   * correct implementation of the .hashCode and the .equals method used to store and address the objects
   *
   * If a value is not present in the reverse dictionary, {@link Object2IntMap#getInt} will
   * return {@link DimensionDictionary#ABSENT_VALUE_ID}.
   *
   * The object's {@link org.apache.druid.segment.column.NullableTypeStrategy} is often enough to create a reverse
   * dictionary for those objects
   */
  public static <T> Object2IntMap<T> createReverseDictionary(final Hash.Strategy<T> hashStrategy)
  {
    final Object2IntOpenCustomHashMap<T> m = new Object2IntOpenCustomHashMap<>(hashStrategy);
    m.defaultReturnValue(DimensionDictionary.ABSENT_VALUE_ID);
    return m;
  }

  /**
   * Estimated footprint of a new entry.
   */
  public static int estimateEntryFootprint(final int valueFootprint)
  {
    return valueFootprint + ROUGH_OVERHEAD_PER_DICTIONARY_ENTRY;
  }
}