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   /*
    * Copyright 2016 The Netty Project
    *
    * The Netty Project 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:
    *
    *   https://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 io.netty.util.concurrent;
  
  import io.netty.util.internal.UnstableApi;
  
  import java.util.concurrent.atomic.AtomicInteger;
  import java.util.concurrent.atomic.AtomicLong;
  
  /**
   * Default implementation which uses simple round-robin to choose next {@link EventExecutor}.
   */
  @UnstableApi
  public final class DefaultEventExecutorChooserFactory implements EventExecutorChooserFactory {
  
      public static final DefaultEventExecutorChooserFactory INSTANCE = new DefaultEventExecutorChooserFactory();
  
      private DefaultEventExecutorChooserFactory() { }
  
      @Override
      public EventExecutorChooser newChooser(EventExecutor[] executors) {
          if (isPowerOfTwo(executors.length)) {
              return new PowerOfTwoEventExecutorChooser(executors);
          } else {
              return new GenericEventExecutorChooser(executors);
          }
      }
  
      private static boolean isPowerOfTwo(int val) {
          return (val & -val) == val;
      }
  
      private static final class PowerOfTwoEventExecutorChooser implements EventExecutorChooser {
          private final AtomicInteger idx = new AtomicInteger();
          private final EventExecutor[] executors;
  
          PowerOfTwoEventExecutorChooser(EventExecutor[] executors) {
              this.executors = executors;
          }
  
          @Override
          public EventExecutor next() {
              return executors[idx.getAndIncrement() & executors.length - 1];
          }
      }
  
      private static final class GenericEventExecutorChooser implements EventExecutorChooser {
          // Use a 'long' counter to avoid non-round-robin behaviour at the 32-bit overflow boundary.
          // The 64-bit long solves this by placing the overflow so far into the future, that no system
          // will encounter this in practice.
          private final AtomicLong idx = new AtomicLong();
          private final EventExecutor[] executors;
  
          GenericEventExecutorChooser(EventExecutor[] executors) {
              this.executors = executors;
          }
  
          @Override
          public EventExecutor next() {
              return executors[(int) Math.abs(idx.getAndIncrement() % executors.length)];
          }
      }
  }