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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.lucene.index;

import java.util.concurrent.Executor;
import org.apache.lucene.index.MergePolicy.OneMerge;
import org.apache.lucene.store.Directory;

/**
 * A {@link MergeScheduler} which never executes any merges. It is also a singleton and can be
 * accessed through {@link NoMergeScheduler#INSTANCE}. Use it if you want to prevent an {@link
 * IndexWriter} from ever executing merges, regardless of the {@link MergePolicy} used. Note that
 * you can achieve the same thing by using {@link NoMergePolicy}, however with {@link
 * NoMergeScheduler} you also ensure that no unnecessary code of any {@link MergeScheduler}
 * implementation is ever executed. Hence it is recommended to use both if you want to disable
 * merges from ever happening.
 */
public final class NoMergeScheduler extends MergeScheduler {

  /** The single instance of {@link NoMergeScheduler} */
  public static final MergeScheduler INSTANCE = new NoMergeScheduler();

  private NoMergeScheduler() {
    // prevent instantiation
  }

  @Override
  public void close() {}

  @Override
  public void merge(MergeSource mergeSource, MergeTrigger trigger) {}

  @Override
  public Directory wrapForMerge(OneMerge merge, Directory in) {
    return in;
  }

  @Override
  public MergeScheduler clone() {
    return this;
  }

  @Override
  public Executor getIntraMergeExecutor(OneMerge merge) {
    throw new UnsupportedOperationException("NoMergeScheduler does not support merges");
  }
}