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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.handler.codec;
  
  import io.netty.util.concurrent.FastThreadLocal;
  import io.netty.util.internal.MathUtil;
  
  import java.util.AbstractList;
  import java.util.RandomAccess;
  
  import static io.netty.util.internal.ObjectUtil.checkNotNull;
  
  /**
   * Special {@link AbstractList} implementation which is used within our codec base classes.
   */
  final class CodecOutputList extends AbstractList<Object> implements RandomAccess {
  
      private static final CodecOutputListRecycler NOOP_RECYCLER = new CodecOutputListRecycler() {
          @Override
          public void recycle(CodecOutputList object) {
              // drop on the floor and let the GC handle it.
          }
      };
  
      private static final FastThreadLocal<CodecOutputLists> CODEC_OUTPUT_LISTS_POOL =
              new FastThreadLocal<CodecOutputLists>() {
                  @Override
                  protected CodecOutputLists initialValue() throws Exception {
                      // 16 CodecOutputList per Thread are cached.
                      return new CodecOutputLists(16);
                  }
              };
  
      private interface CodecOutputListRecycler {
          void recycle(CodecOutputList codecOutputList);
      }
  
      private static final class CodecOutputLists implements CodecOutputListRecycler {
          private final CodecOutputList[] elements;
          private final int mask;
  
          private int currentIdx;
          private int count;
  
          CodecOutputLists(int numElements) {
              elements = new CodecOutputList[MathUtil.safeFindNextPositivePowerOfTwo(numElements)];
              for (int i = 0; i < elements.length; ++i) {
                  // Size of 16 should be good enough for the majority of all users as an initial capacity.
                  elements[i] = new CodecOutputList(this, 16);
              }
              count = elements.length;
              currentIdx = elements.length;
              mask = elements.length - 1;
          }
  
          public CodecOutputList getOrCreate() {
              if (count == 0) {
                  // Return a new CodecOutputList which will not be cached. We use a size of 4 to keep the overhead
                  // low.
                  return new CodecOutputList(NOOP_RECYCLER, 4);
              }
              --count;
  
              int idx = (currentIdx - 1) & mask;
              CodecOutputList list = elements[idx];
              currentIdx = idx;
              return list;
          }
  
          @Override
          public void recycle(CodecOutputList codecOutputList) {
              int idx = currentIdx;
              elements[idx] = codecOutputList;
              currentIdx = (idx + 1) & mask;
              ++count;
              assert count <= elements.length;
          }
      }
  
      static CodecOutputList newInstance() {
          return CODEC_OUTPUT_LISTS_POOL.get().getOrCreate();
      }
  
      private final CodecOutputListRecycler recycler;
      private int size;
      private Object[] array;
     private boolean insertSinceRecycled;
 
     private CodecOutputList(CodecOutputListRecycler recycler, int size) {
         this.recycler = recycler;
         array = new Object[size];
     }
 
     @Override
     public Object get(int index) {
         checkIndex(index);
         return array[index];
     }
 
     @Override
     public int size() {
         return size;
     }
 
     @Override
     public boolean add(Object element) {
         checkNotNull(element, "element");
         try {
             insert(size, element);
         } catch (IndexOutOfBoundsException ignore) {
             // This should happen very infrequently so we just catch the exception and try again.
             expandArray();
             insert(size, element);
         }
         ++ size;
         return true;
     }
 
     @Override
     public Object set(int index, Object element) {
         checkNotNull(element, "element");
         checkIndex(index);
 
         Object old = array[index];
         insert(index, element);
         return old;
     }
 
     @Override
     public void add(int index, Object element) {
         checkNotNull(element, "element");
         checkIndex(index);
 
         if (size == array.length) {
             expandArray();
         }
 
         if (index != size) {
             System.arraycopy(array, index, array, index + 1, size - index);
         }
 
         insert(index, element);
         ++ size;
     }
 
     @Override
     public Object remove(int index) {
         checkIndex(index);
         Object old = array[index];
 
         int len = size - index - 1;
         if (len > 0) {
             System.arraycopy(array, index + 1, array, index, len);
         }
         array[-- size] = null;
 
         return old;
     }
 
     @Override
     public void clear() {
         // We only set the size to 0 and not null out the array. Null out the array will explicit requested by
         // calling recycle()
         size = 0;
     }
 
     /**
      * Returns {@code true} if any elements where added or set. This will be reset once {@link #recycle()} was called.
      */
     boolean insertSinceRecycled() {
         return insertSinceRecycled;
     }
 
     /**
      * Recycle the array which will clear it and null out all entries in the internal storage.
      */
     void recycle() {
         for (int i = 0 ; i < size; i ++) {
             array[i] = null;
         }
         size = 0;
         insertSinceRecycled = false;
 
         recycler.recycle(this);
     }
 
     /**
      * Returns the element on the given index. This operation will not do any range-checks and so is considered unsafe.
      */
     Object getUnsafe(int index) {
         return array[index];
     }
 
     private void checkIndex(int index) {
         if (index >= size) {
             throw new IndexOutOfBoundsException("expected: index < ("
                     + size + "),but actual is (" + size + ")");
         }
     }
 
     private void insert(int index, Object element) {
         array[index] = element;
         insertSinceRecycled = true;
     }
 
     private void expandArray() {
         // double capacity
         int newCapacity = array.length << 1;
 
         if (newCapacity < 0) {
             throw new OutOfMemoryError();
         }
 
         Object[] newArray = new Object[newCapacity];
         System.arraycopy(array, 0, newArray, 0, array.length);
 
         array = newArray;
     }
 }