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.timeline;
  
  import java.util.IdentityHashMap;
  import java.util.Map;
  import java.util.Objects;
  
  /**
   * A snapshot of some timeline data structures.
   * <br>
   * The snapshot contains historical data for several timeline data structures.
   * We use an IdentityHashMap to store this data.  This way, we can easily drop all of
   * the snapshot data.
   */
  class Snapshot {
      private final long epoch;
      private IdentityHashMap<Revertable, Delta> map = new IdentityHashMap<>(4);
      private Snapshot prev = this;
      private Snapshot next = this;
  
      Snapshot(long epoch) {
          this.epoch = epoch;
      }
  
      long epoch() {
          return epoch;
      }
  
      @SuppressWarnings("unchecked")
      <T extends Delta> T getDelta(Revertable owner) {
          return (T) map.get(owner);
      }
  
      void setDelta(Revertable owner, Delta delta) {
          Objects.requireNonNull(map, "Snapshot cannot be accessed after erase is called.");
          map.put(owner, delta);
      }
  
      void handleRevert() {
          Objects.requireNonNull(map, "Snapshot cannot be accessed after erase is called.");
          for (Map.Entry<Revertable, Delta> entry : map.entrySet()) {
              entry.getKey().executeRevert(epoch, entry.getValue());
          }
      }
  
      void mergeFrom(Snapshot source) {
          Objects.requireNonNull(map, "Snapshot cannot be accessed after erase is called.");
          // Merge the deltas from the source snapshot into this snapshot.
          for (Map.Entry<Revertable, Delta> entry : source.map.entrySet()) {
              // We first try to just copy over the object reference.  That will work if
              //we have no entry at all for the given Revertable.
              Delta destinationDelta = map.putIfAbsent(entry.getKey(), entry.getValue());
              if (destinationDelta != null) {
                  // If we already have an entry for the Revertable, we need to merge the
                  // source delta into our delta.
                  destinationDelta.mergeFrom(epoch, entry.getValue());
              }
          }
          // Delete the source snapshot to make sure nobody tries to reuse it.  We might now
          // share some delta entries with it.
          source.erase();
      }
  
      Snapshot prev() {
          return prev;
      }
  
      Snapshot next() {
          return next;
      }
  
      void appendNext(Snapshot newNext) {
          newNext.prev = this;
          newNext.next = next;
          next.prev = newNext;
          next = newNext;
      }
  
      void erase() {
          map = null;
          next.prev = prev;
          prev.next = next;
          prev = this;
          next = this;
      }
  }
  