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   /*
    * Copyright 2012 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:
    *
    *   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 io.netty.util;
  
  import java.util.concurrent.atomic.AtomicReference;
  import java.util.concurrent.atomic.AtomicReferenceArray;
  import java.util.concurrent.atomic.AtomicReferenceFieldUpdater;
  
  /**
   * Default {@link AttributeMap} implementation which use simple synchronization per bucket to keep the memory overhead
   * as low as possible.
   */
  public class DefaultAttributeMap implements AttributeMap {
  
      @SuppressWarnings("rawtypes")
      private static final AtomicReferenceFieldUpdater<DefaultAttributeMap, AtomicReferenceArray> updater =
              AtomicReferenceFieldUpdater.newUpdater(DefaultAttributeMap.class, AtomicReferenceArray.class, "attributes");
  
      private static final int BUCKET_SIZE = 4;
      private static final int MASK = BUCKET_SIZE  - 1;
  
      // Initialize lazily to reduce memory consumption; updated by AtomicReferenceFieldUpdater above.
      @SuppressWarnings("UnusedDeclaration")
      private volatile AtomicReferenceArray<DefaultAttribute<?>> attributes;
  
      @SuppressWarnings("unchecked")
      @Override
      public <T> Attribute<T> attr(AttributeKey<T> key) {
          if (key == null) {
              throw new NullPointerException("key");
          }
          AtomicReferenceArray<DefaultAttribute<?>> attributes = this.attributes;
          if (attributes == null) {
              // Not using ConcurrentHashMap due to high memory consumption.
              attributes = new AtomicReferenceArray<DefaultAttribute<?>>(BUCKET_SIZE);
  
              if (!updater.compareAndSet(this, null, attributes)) {
                  attributes = this.attributes;
              }
          }
  
          int i = index(key);
          DefaultAttribute<?> head = attributes.get(i);
          if (head == null) {
              // No head exists yet which means we may be able to add the attribute without synchronization and just
              // use compare and set. At worst we need to fallback to synchronization and waste two allocations.
              head = new DefaultAttribute();
              DefaultAttribute<T> attr = new DefaultAttribute<T>(head, key);
              head.next = attr;
              attr.prev = head;
              if (attributes.compareAndSet(i, null, head)) {
                  // we were able to add it so return the attr right away
                  return attr;
              } else {
                  head = attributes.get(i);
              }
          }
  
          synchronized (head) {
              DefaultAttribute<?> curr = head;
              for (;;) {
                  DefaultAttribute<?> next = curr.next;
                  if (next == null) {
                      DefaultAttribute<T> attr = new DefaultAttribute<T>(head, key);
                      curr.next = attr;
                      attr.prev = curr;
                      return attr;
                  }
  
                  if (next.key == key && !next.removed) {
                      return (Attribute<T>) next;
                  }
                  curr = next;
              }
          }
      }
  
      private static int index(AttributeKey<?> key) {
          return key.id() & MASK;
      }
  
      @SuppressWarnings("serial")
      private static final class DefaultAttribute<T> extends AtomicReference<T> implements Attribute<T> {
  
          private static final long serialVersionUID = -2661411462200283011L;
  
         // The head of the linked-list this attribute belongs to
         private final DefaultAttribute<?> head;
         private final AttributeKey<T> key;
 
         // Double-linked list to prev and next node to allow fast removal
         private DefaultAttribute<?> prev;
         private DefaultAttribute<?> next;
 
         // Will be set to true one the attribute is removed via getAndRemove() or remove()
         private volatile boolean removed;
 
         DefaultAttribute(DefaultAttribute<?> head, AttributeKey<T> key) {
             this.head = head;
             this.key = key;
         }
 
         // Special constructor for the head of the linked-list.
         DefaultAttribute() {
             head = this;
             key = null;
         }
 
         @Override
         public AttributeKey<T> key() {
             return key;
         }
 
         @Override
         public T setIfAbsent(T value) {
             while (!compareAndSet(null, value)) {
                 T old = get();
                 if (old != null) {
                     return old;
                 }
             }
             return null;
         }
 
         @Override
         public T getAndRemove() {
             removed = true;
             T oldValue = getAndSet(null);
             remove0();
             return oldValue;
         }
 
         @Override
         public void remove() {
             removed = true;
             set(null);
             remove0();
         }
 
         private void remove0() {
             synchronized (head) {
                 if (prev == null) {
                     // Removed before.
                     return;
                 }
 
                 prev.next = next;
 
                 if (next != null) {
                     next.prev = prev;
                 }
 
                 // Null out prev and next - this will guard against multiple remove0() calls which may corrupt
                 // the linked list for the bucket.
                 prev = null;
                 next = null;
             }
         }
     }
 }