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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.channel;
  
  import io.netty.buffer.ByteBuf;
  import io.netty.buffer.ByteBufAllocator;
  
  import java.util.ArrayList;
  import java.util.List;
  
  /**
   * The {@link RecvByteBufAllocator} that automatically increases and
   * decreases the predicted buffer size on feed back.
   * <p>
   * It gradually increases the expected number of readable bytes if the previous
   * read fully filled the allocated buffer.  It gradually decreases the expected
   * number of readable bytes if the read operation was not able to fill a certain
   * amount of the allocated buffer two times consecutively.  Otherwise, it keeps
   * returning the same prediction.
   */
  public class AdaptiveRecvByteBufAllocator implements RecvByteBufAllocator {
  
      static final int DEFAULT_MINIMUM = 64;
      static final int DEFAULT_INITIAL = 1024;
      static final int DEFAULT_MAXIMUM = 65536;
  
      private static final int INDEX_INCREMENT = 4;
      private static final int INDEX_DECREMENT = 1;
  
      private static final int[] SIZE_TABLE;
  
      static {
          List<Integer> sizeTable = new ArrayList<Integer>();
          for (int i = 16; i < 512; i += 16) {
              sizeTable.add(i);
          }
  
          for (int i = 512; i > 0; i <<= 1) {
              sizeTable.add(i);
          }
  
          SIZE_TABLE = new int[sizeTable.size()];
          for (int i = 0; i < SIZE_TABLE.length; i ++) {
              SIZE_TABLE[i] = sizeTable.get(i);
          }
      }
  
      public static final AdaptiveRecvByteBufAllocator DEFAULT = new AdaptiveRecvByteBufAllocator();
  
      private static int getSizeTableIndex(final int size) {
          for (int low = 0, high = SIZE_TABLE.length - 1;;) {
              if (high < low) {
                  return low;
              }
              if (high == low) {
                  return high;
              }
  
              int mid = low + high >>> 1;
              int a = SIZE_TABLE[mid];
              int b = SIZE_TABLE[mid + 1];
              if (size > b) {
                  low = mid + 1;
              } else if (size < a) {
                  high = mid - 1;
              } else if (size == a) {
                  return mid;
              } else {
                  return mid + 1;
              }
          }
      }
  
      private static final class HandleImpl implements Handle {
          private final int minIndex;
          private final int maxIndex;
          private int index;
          private int nextReceiveBufferSize;
          private boolean decreaseNow;
  
          HandleImpl(int minIndex, int maxIndex, int initial) {
              this.minIndex = minIndex;
              this.maxIndex = maxIndex;
  
              index = getSizeTableIndex(initial);
              nextReceiveBufferSize = SIZE_TABLE[index];
         }
 
         @Override
         public ByteBuf allocate(ByteBufAllocator alloc) {
             return alloc.ioBuffer(nextReceiveBufferSize);
         }
 
         @Override
         public int guess() {
             return nextReceiveBufferSize;
         }
 
         @Override
         public void record(int actualReadBytes) {
             if (actualReadBytes <= SIZE_TABLE[Math.max(0, index - INDEX_DECREMENT - 1)]) {
                 if (decreaseNow) {
                     index = Math.max(index - INDEX_DECREMENT, minIndex);
                     nextReceiveBufferSize = SIZE_TABLE[index];
                     decreaseNow = false;
                 } else {
                     decreaseNow = true;
                 }
             } else if (actualReadBytes >= nextReceiveBufferSize) {
                 index = Math.min(index + INDEX_INCREMENT, maxIndex);
                 nextReceiveBufferSize = SIZE_TABLE[index];
                 decreaseNow = false;
             }
         }
     }
 
     private final int minIndex;
     private final int maxIndex;
     private final int initial;
 
     /**
      * Creates a new predictor with the default parameters.  With the default
      * parameters, the expected buffer size starts from {@code 1024}, does not
      * go down below {@code 64}, and does not go up above {@code 65536}.
      */
     private AdaptiveRecvByteBufAllocator() {
         this(DEFAULT_MINIMUM, DEFAULT_INITIAL, DEFAULT_MAXIMUM);
     }
 
     /**
      * Creates a new predictor with the specified parameters.
      *
      * @param minimum  the inclusive lower bound of the expected buffer size
      * @param initial  the initial buffer size when no feed back was received
      * @param maximum  the inclusive upper bound of the expected buffer size
      */
     public AdaptiveRecvByteBufAllocator(int minimum, int initial, int maximum) {
         if (minimum <= 0) {
             throw new IllegalArgumentException("minimum: " + minimum);
         }
         if (initial < minimum) {
             throw new IllegalArgumentException("initial: " + initial);
         }
         if (maximum < initial) {
             throw new IllegalArgumentException("maximum: " + maximum);
         }
 
         int minIndex = getSizeTableIndex(minimum);
         if (SIZE_TABLE[minIndex] < minimum) {
             this.minIndex = minIndex + 1;
         } else {
             this.minIndex = minIndex;
         }
 
         int maxIndex = getSizeTableIndex(maximum);
         if (SIZE_TABLE[maxIndex] > maximum) {
             this.maxIndex = maxIndex - 1;
         } else {
             this.maxIndex = maxIndex;
         }
 
         this.initial = initial;
     }
 
     @Override
     public Handle newHandle() {
         return new HandleImpl(minIndex, maxIndex, initial);
     }
 }