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Druid HyperLogLog implementation

There is a newer version: 25.0.0

/*
 * 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.hll;

import com.google.common.hash.HashFunction;
import com.google.common.hash.Hashing;
import org.apache.druid.java.util.common.StringUtils;

/**
 * Contains data hashing methods used before applying Hyper Log Log.
 *
 * {@link HyperLogLogCollector#add(byte[])} requires hashed value on input. This class makes it easier to achieve
 * consistent value hashing for both internal aggregation and external hashing for pre-computed aggregation.
 * 
 * By default 128-bit murmur3 algorithm, x64 variant is used. Implementation is thread safe.
 * 
 * Caution! changing of implementation may cause improper cardinality estimation between data hashed with different
 * versions.
 */
public class HyperLogLogHash
{
  private static final HyperLogLogHash DEFAULT = new HyperLogLogHash(Hashing.murmur3_128());

  public static HyperLogLogHash getDefault()
  {
    return DEFAULT;
  }

  private final HashFunction hashFunction;

  public HyperLogLogHash(HashFunction hashFunction)
  {
    this.hashFunction = hashFunction;
  }

  public byte[] hash(byte[] rawValue)
  {
    return hashFunction.hashBytes(rawValue).asBytes();
  }

  public byte[] hash(String rawValue)
  {
    return hash(StringUtils.toUtf8(rawValue));
  }
}