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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:
    *
    *   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.buffer;
  
  import java.util.concurrent.locks.ReentrantLock;
  
  import static io.netty.buffer.PoolChunk.RUN_OFFSET_SHIFT;
  import static io.netty.buffer.PoolChunk.SIZE_SHIFT;
  import static io.netty.buffer.PoolChunk.IS_USED_SHIFT;
  import static io.netty.buffer.PoolChunk.IS_SUBPAGE_SHIFT;
  
  final class PoolSubpage<T> implements PoolSubpageMetric {
  
      final PoolChunk<T> chunk;
      final int elemSize;
      private final int pageShifts;
      private final int runOffset;
      private final int runSize;
      private final long[] bitmap;
      private final int bitmapLength;
      private final int maxNumElems;
      final int headIndex;
  
      PoolSubpage<T> prev;
      PoolSubpage<T> next;
  
      boolean doNotDestroy;
      private int nextAvail;
      private int numAvail;
  
      final ReentrantLock lock;
  
      // TODO: Test if adding padding helps under contention
      //private long pad0, pad1, pad2, pad3, pad4, pad5, pad6, pad7;
  
      /** Special constructor that creates a linked list head */
      PoolSubpage(int headIndex) {
          chunk = null;
          lock = new ReentrantLock();
          pageShifts = -1;
          runOffset = -1;
          elemSize = -1;
          runSize = -1;
          bitmap = null;
          bitmapLength = -1;
          maxNumElems = 0;
          this.headIndex = headIndex;
      }
  
      PoolSubpage(PoolSubpage<T> head, PoolChunk<T> chunk, int pageShifts, int runOffset, int runSize, int elemSize) {
          this.headIndex = head.headIndex;
          this.chunk = chunk;
          this.pageShifts = pageShifts;
          this.runOffset = runOffset;
          this.runSize = runSize;
          this.elemSize = elemSize;
  
          doNotDestroy = true;
  
          maxNumElems = numAvail = runSize / elemSize;
          int bitmapLength = maxNumElems >>> 6;
          if ((maxNumElems & 63) != 0) {
              bitmapLength ++;
          }
          this.bitmapLength = bitmapLength;
          bitmap = new long[bitmapLength];
          nextAvail = 0;
  
          lock = null;
          addToPool(head);
      }
  
      /**
       * Returns the bitmap index of the subpage allocation.
       */
      long allocate() {
          if (numAvail == 0 || !doNotDestroy) {
              return -1;
          }
  
          final int bitmapIdx = getNextAvail();
          if (bitmapIdx < 0) {
              removeFromPool(); // Subpage appear to be in an invalid state. Remove to prevent repeated errors.
              throw new AssertionError("No next available bitmap index found (bitmapIdx = " + bitmapIdx + "), " +
                      "even though there are supposed to be (numAvail = " + numAvail + ") " +
                     "out of (maxNumElems = " + maxNumElems + ") available indexes.");
         }
         int q = bitmapIdx >>> 6;
         int r = bitmapIdx & 63;
         assert (bitmap[q] >>> r & 1) == 0;
         bitmap[q] |= 1L << r;
 
         if (-- numAvail == 0) {
             removeFromPool();
         }
 
         return toHandle(bitmapIdx);
     }
 
     /**
      * @return {@code true} if this subpage is in use.
      *         {@code false} if this subpage is not used by its chunk and thus it's OK to be released.
      */
     boolean free(PoolSubpage<T> head, int bitmapIdx) {
         int q = bitmapIdx >>> 6;
         int r = bitmapIdx & 63;
         assert (bitmap[q] >>> r & 1) != 0;
         bitmap[q] ^= 1L << r;
 
         setNextAvail(bitmapIdx);
 
         if (numAvail ++ == 0) {
             addToPool(head);
             /* When maxNumElems == 1, the maximum numAvail is also 1.
              * Each of these PoolSubpages will go in here when they do free operation.
              * If they return true directly from here, then the rest of the code will be unreachable
              * and they will not actually be recycled. So return true only on maxNumElems > 1. */
             if (maxNumElems > 1) {
                 return true;
             }
         }
 
         if (numAvail != maxNumElems) {
             return true;
         } else {
             // Subpage not in use (numAvail == maxNumElems)
             if (prev == next) {
                 // Do not remove if this subpage is the only one left in the pool.
                 return true;
             }
 
             // Remove this subpage from the pool if there are other subpages left in the pool.
             doNotDestroy = false;
             removeFromPool();
             return false;
         }
     }
 
     private void addToPool(PoolSubpage<T> head) {
         assert prev == null && next == null;
         prev = head;
         next = head.next;
         next.prev = this;
         head.next = this;
     }
 
     private void removeFromPool() {
         assert prev != null && next != null;
         prev.next = next;
         next.prev = prev;
         next = null;
         prev = null;
     }
 
     private void setNextAvail(int bitmapIdx) {
         nextAvail = bitmapIdx;
     }
 
     private int getNextAvail() {
         int nextAvail = this.nextAvail;
         if (nextAvail >= 0) {
             this.nextAvail = -1;
             return nextAvail;
         }
         return findNextAvail();
     }
 
     private int findNextAvail() {
         for (int i = 0; i < bitmapLength; i ++) {
             long bits = bitmap[i];
             if (~bits != 0) {
                 return findNextAvail0(i, bits);
             }
         }
         return -1;
     }
 
     private int findNextAvail0(int i, long bits) {
         final int baseVal = i << 6;
         for (int j = 0; j < 64; j ++) {
             if ((bits & 1) == 0) {
                 int val = baseVal | j;
                 if (val < maxNumElems) {
                     return val;
                 } else {
                     break;
                 }
             }
             bits >>>= 1;
         }
         return -1;
     }
 
     private long toHandle(int bitmapIdx) {
         int pages = runSize >> pageShifts;
         return (long) runOffset << RUN_OFFSET_SHIFT
                | (long) pages << SIZE_SHIFT
                | 1L << IS_USED_SHIFT
                | 1L << IS_SUBPAGE_SHIFT
                | bitmapIdx;
     }
 
     @Override
     public String toString() {
         final int numAvail;
         if (chunk == null) {
             // This is the head so there is no need to synchronize at all as these never change.
             numAvail = 0;
         } else {
             final boolean doNotDestroy;
             PoolSubpage<T> head = chunk.arena.smallSubpagePools[headIndex];
             head.lock();
             try {
                 doNotDestroy = this.doNotDestroy;
                 numAvail = this.numAvail;
             } finally {
                 head.unlock();
             }
             if (!doNotDestroy) {
                 // Not used for creating the String.
                 return "(" + runOffset + ": not in use)";
             }
         }
 
         return "(" + this.runOffset + ": " + (this.maxNumElems - numAvail) + '/' + this.maxNumElems +
                 ", offset: " + this.runOffset + ", length: " + this.runSize + ", elemSize: " + this.elemSize + ')';
     }
 
     @Override
     public int maxNumElements() {
         return maxNumElems;
     }
 
     @Override
     public int numAvailable() {
         if (chunk == null) {
             // It's the head.
             return 0;
         }
         PoolSubpage<T> head = chunk.arena.smallSubpagePools[headIndex];
         head.lock();
         try {
             return numAvail;
         } finally {
             head.unlock();
         }
     }
 
     @Override
     public int elementSize() {
         return elemSize;
     }
 
     @Override
     public int pageSize() {
         return 1 << pageShifts;
     }
 
     boolean isDoNotDestroy() {
         if (chunk == null) {
             // It's the head.
             return true;
         }
         PoolSubpage<T> head = chunk.arena.smallSubpagePools[headIndex];
         head.lock();
         try {
             return doNotDestroy;
         } finally {
             head.unlock();
         }
     }
 
     void destroy() {
         if (chunk != null) {
             chunk.destroy();
         }
     }
 
     void lock() {
         lock.lock();
     }
 
     void unlock() {
         lock.unlock();
     }
 }