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   /*
    * Copyright 2019 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.util.internal;
  
  import static io.netty.util.internal.ObjectUtil.checkPositive;
  
  import java.util.concurrent.atomic.AtomicIntegerFieldUpdater;
  
  import io.netty.util.IllegalReferenceCountException;
  import io.netty.util.ReferenceCounted;
  
  /**
   * Common logic for {@link ReferenceCounted} implementations
   */
  public abstract class ReferenceCountUpdater<T extends ReferenceCounted> {
      /*
       * Implementation notes:
       *
       * For the updated int field:
       *   Even => "real" refcount is (refCnt >>> 1)
       *   Odd  => "real" refcount is 0
       *
       * (x & y) appears to be surprisingly expensive relative to (x == y). Thus this class uses
       * a fast-path in some places for most common low values when checking for live (even) refcounts,
       * for example: if (rawCnt == 2 || rawCnt == 4 || (rawCnt & 1) == 0) { ...
       */
  
      protected ReferenceCountUpdater() { }
  
      public static long getUnsafeOffset(Class<? extends ReferenceCounted> clz, String fieldName) {
          try {
              if (PlatformDependent.hasUnsafe()) {
                  return PlatformDependent.objectFieldOffset(clz.getDeclaredField(fieldName));
              }
          } catch (Throwable ignore) {
              // fall-back
          }
          return -1;
      }
  
      protected abstract AtomicIntegerFieldUpdater<T> updater();
  
      protected abstract long unsafeOffset();
  
      public final int initialValue() {
          return 2;
      }
  
      public void setInitialValue(T instance) {
          final long offset = unsafeOffset();
          if (offset == -1) {
              updater().set(instance, initialValue());
          } else {
              PlatformDependent.safeConstructPutInt(instance, offset, initialValue());
          }
      }
  
      private static int realRefCnt(int rawCnt) {
          return rawCnt != 2 && rawCnt != 4 && (rawCnt & 1) != 0 ? 0 : rawCnt >>> 1;
      }
  
      /**
       * Like {@link #realRefCnt(int)} but throws if refCnt == 0
       */
      private static int toLiveRealRefCnt(int rawCnt, int decrement) {
          if (rawCnt == 2 || rawCnt == 4 || (rawCnt & 1) == 0) {
              return rawCnt >>> 1;
          }
          // odd rawCnt => already deallocated
          throw new IllegalReferenceCountException(0, -decrement);
      }
  
      private int nonVolatileRawCnt(T instance) {
          // TODO: Once we compile against later versions of Java we can replace the Unsafe usage here by varhandles.
          final long offset = unsafeOffset();
          return offset != -1 ? PlatformDependent.getInt(instance, offset) : updater().get(instance);
      }
  
      public final int refCnt(T instance) {
          return realRefCnt(updater().get(instance));
      }
  
      public final boolean isLiveNonVolatile(T instance) {
          final long offset = unsafeOffset();
          final int rawCnt = offset != -1 ? PlatformDependent.getInt(instance, offset) : updater().get(instance);
  
         // The "real" ref count is > 0 if the rawCnt is even.
         return rawCnt == 2 || rawCnt == 4 || rawCnt == 6 || rawCnt == 8 || (rawCnt & 1) == 0;
     }
 
     /**
      * An unsafe operation that sets the reference count directly
      */
     public final void setRefCnt(T instance, int refCnt) {
         updater().set(instance, refCnt > 0 ? refCnt << 1 : 1); // overflow OK here
     }
 
     /**
      * Resets the reference count to 1
      */
     public final void resetRefCnt(T instance) {
         // no need of a volatile set, it should happen in a quiescent state
         updater().lazySet(instance, initialValue());
     }
 
     public final T retain(T instance) {
         return retain0(instance, 1, 2);
     }
 
     public final T retain(T instance, int increment) {
         // all changes to the raw count are 2x the "real" change - overflow is OK
         int rawIncrement = checkPositive(increment, "increment") << 1;
         return retain0(instance, increment, rawIncrement);
     }
 
     // rawIncrement == increment << 1
     private T retain0(T instance, final int increment, final int rawIncrement) {
         int oldRef = updater().getAndAdd(instance, rawIncrement);
         if (oldRef != 2 && oldRef != 4 && (oldRef & 1) != 0) {
             throw new IllegalReferenceCountException(0, increment);
         }
         // don't pass 0!
         if ((oldRef <= 0 && oldRef + rawIncrement >= 0)
                 || (oldRef >= 0 && oldRef + rawIncrement < oldRef)) {
             // overflow case
             updater().getAndAdd(instance, -rawIncrement);
             throw new IllegalReferenceCountException(realRefCnt(oldRef), increment);
         }
         return instance;
     }
 
     public final boolean release(T instance) {
         int rawCnt = nonVolatileRawCnt(instance);
         return rawCnt == 2 ? tryFinalRelease0(instance, 2) || retryRelease0(instance, 1)
                 : nonFinalRelease0(instance, 1, rawCnt, toLiveRealRefCnt(rawCnt, 1));
     }
 
     public final boolean release(T instance, int decrement) {
         int rawCnt = nonVolatileRawCnt(instance);
         int realCnt = toLiveRealRefCnt(rawCnt, checkPositive(decrement, "decrement"));
         return decrement == realCnt ? tryFinalRelease0(instance, rawCnt) || retryRelease0(instance, decrement)
                 : nonFinalRelease0(instance, decrement, rawCnt, realCnt);
     }
 
     private boolean tryFinalRelease0(T instance, int expectRawCnt) {
         return updater().compareAndSet(instance, expectRawCnt, 1); // any odd number will work
     }
 
     private boolean nonFinalRelease0(T instance, int decrement, int rawCnt, int realCnt) {
         if (decrement < realCnt
                 // all changes to the raw count are 2x the "real" change - overflow is OK
                 && updater().compareAndSet(instance, rawCnt, rawCnt - (decrement << 1))) {
             return false;
         }
         return retryRelease0(instance, decrement);
     }
 
     private boolean retryRelease0(T instance, int decrement) {
         for (;;) {
             int rawCnt = updater().get(instance), realCnt = toLiveRealRefCnt(rawCnt, decrement);
             if (decrement == realCnt) {
                 if (tryFinalRelease0(instance, rawCnt)) {
                     return true;
                 }
             } else if (decrement < realCnt) {
                 // all changes to the raw count are 2x the "real" change
                 if (updater().compareAndSet(instance, rawCnt, rawCnt - (decrement << 1))) {
                     return false;
                 }
             } else {
                 throw new IllegalReferenceCountException(realCnt, -decrement);
             }
             Thread.yield(); // this benefits throughput under high contention
         }
     }
 }