content: /*
   * 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.kafka.common.record;
  
  import java.util.concurrent.ConcurrentHashMap;
  import java.util.concurrent.ConcurrentMap;
  
  
  /**
   * This class help estimate the compression ratio for each topic and compression type combination.
   */
  public class CompressionRatioEstimator {
      // The constant speed to increase compression ratio when a batch compresses better than expected.
      public static final float COMPRESSION_RATIO_IMPROVING_STEP = 0.005f;
      // The minimum speed to decrease compression ratio when a batch compresses worse than expected.
      public static final float COMPRESSION_RATIO_DETERIORATE_STEP = 0.05f;
      private static final ConcurrentMap<String, float[]> COMPRESSION_RATIO = new ConcurrentHashMap<>();
  
      /**
       * Update the compression ratio estimation for a topic and compression type.
       *
       * @param topic         the topic to update compression ratio estimation.
       * @param type          the compression type.
       * @param observedRatio the observed compression ratio.
       * @return the compression ratio estimation after the update.
       */
      public static float updateEstimation(String topic, CompressionType type, float observedRatio) {
          float[] compressionRatioForTopic = getAndCreateEstimationIfAbsent(topic);
          float currentEstimation = compressionRatioForTopic[type.id];
          synchronized (compressionRatioForTopic) {
              if (observedRatio > currentEstimation)
                  compressionRatioForTopic[type.id] = Math.max(currentEstimation + COMPRESSION_RATIO_DETERIORATE_STEP, observedRatio);
              else if (observedRatio < currentEstimation) {
                  compressionRatioForTopic[type.id] = Math.max(currentEstimation - COMPRESSION_RATIO_IMPROVING_STEP, observedRatio);
              }
          }
          return compressionRatioForTopic[type.id];
      }
  
      /**
       * Get the compression ratio estimation for a topic and compression type.
       */
      public static float estimation(String topic, CompressionType type) {
          float[] compressionRatioForTopic = getAndCreateEstimationIfAbsent(topic);
          return compressionRatioForTopic[type.id];
      }
  
      /**
       * Reset the compression ratio estimation to the initial values for a topic.
       */
      public static void resetEstimation(String topic) {
          float[] compressionRatioForTopic = getAndCreateEstimationIfAbsent(topic);
          synchronized (compressionRatioForTopic) {
              for (CompressionType type : CompressionType.values()) {
                  compressionRatioForTopic[type.id] = type.rate;
              }
          }
      }
  
      /**
       * Set the compression estimation for a topic compression type combination. This method is for unit test purpose.
       */
      public static void setEstimation(String topic, CompressionType type, float ratio) {
          float[] compressionRatioForTopic = getAndCreateEstimationIfAbsent(topic);
          synchronized (compressionRatioForTopic) {
              compressionRatioForTopic[type.id] = ratio;
          }
      }
  
      private static float[] getAndCreateEstimationIfAbsent(String topic) {
          float[] compressionRatioForTopic = COMPRESSION_RATIO.get(topic);
          if (compressionRatioForTopic == null) {
              compressionRatioForTopic = initialCompressionRatio();
              float[] existingCompressionRatio = COMPRESSION_RATIO.putIfAbsent(topic, compressionRatioForTopic);
              // Someone created the compression ratio array before us, use it.
              if (existingCompressionRatio != null)
                  return existingCompressionRatio;
          }
          return compressionRatioForTopic;
      }
  
      private static float[] initialCompressionRatio() {
          float[] compressionRatio = new float[CompressionType.values().length];
          for (CompressionType type : CompressionType.values()) {
              compressionRatio[type.id] = type.rate;
          }
          return compressionRatio;
      }
  }
  