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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 org.jboss.netty.channel;
  
  import java.util.ArrayList;
  import java.util.List;
  
  /**
   * The {@link ReceiveBufferSizePredictor} 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 AdaptiveReceiveBufferSizePredictor implements
          ReceiveBufferSizePredictor {
  
      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 = 1; i <= 8; i ++) {
              sizeTable.add(i);
          }
  
          for (int i = 4; i < 32; i ++) {
              long v = 1L << i;
              long inc = v >>> 4;
              v -= inc << 3;
  
              for (int j = 0; j < 8; j ++) {
                  v += inc;
                  if (v > Integer.MAX_VALUE) {
                      sizeTable.add(Integer.MAX_VALUE);
                  } else {
                      sizeTable.add((int) v);
                  }
              }
          }
  
          SIZE_TABLE = new int[sizeTable.size()];
          for (int i = 0; i < SIZE_TABLE.length; i ++) {
              SIZE_TABLE[i] = sizeTable.get(i);
          }
      }
  
      private static int getSizeTableIndex(final int size) {
          if (size <= 16) {
              return size - 1;
          }
  
          int bits = 0;
          int v = size;
          do {
              v >>>= 1;
              bits ++;
          } while (v != 0);
  
          final int baseIdx = bits << 3;
          final int startIdx = baseIdx - 18;
          final int endIdx = baseIdx - 25;
  
          for (int i = startIdx; i >= endIdx; i --) {
              if (size >= SIZE_TABLE[i]) {
                  return i;
              }
          }
  
          throw new Error("shouldn't reach here; please file a bug report.");
      }
  
      private final int minIndex;
      private final int maxIndex;
      private int index;
      private int nextReceiveBufferSize;
      private boolean decreaseNow;
 
     /**
      * 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}.
      */
     public AdaptiveReceiveBufferSizePredictor() {
         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 AdaptiveReceiveBufferSizePredictor(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;
         }
 
         index = getSizeTableIndex(initial);
         nextReceiveBufferSize = SIZE_TABLE[index];
     }
 
     public int nextReceiveBufferSize() {
         return nextReceiveBufferSize;
     }
 
     public void previousReceiveBufferSize(int previousReceiveBufferSize) {
         if (previousReceiveBufferSize <= 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 (previousReceiveBufferSize >= nextReceiveBufferSize) {
             index = Math.min(index + INDEX_INCREMENT, maxIndex);
             nextReceiveBufferSize = SIZE_TABLE[index];
             decreaseNow = false;
         }
     }
 }