/*
    * Copyright 2013 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.lang.invoke.MethodHandles;
  import java.lang.invoke.VarHandle;
  import java.util.concurrent.atomic.AtomicIntegerFieldUpdater;
  
  import io.netty.util.internal.AtomicReferenceCountUpdater;
  import io.netty.util.internal.PlatformDependent;
  import io.netty.util.internal.ReferenceCountUpdater;
  import io.netty.util.internal.ReferenceCountUpdater.UpdaterType;
  import io.netty.util.internal.UnsafeReferenceCountUpdater;
  import io.netty.util.internal.VarHandleReferenceCountUpdater;
  
  import static io.netty.util.internal.ReferenceCountUpdater.getUnsafeOffset;
  import static java.util.concurrent.atomic.AtomicIntegerFieldUpdater.newUpdater;
  
  /**
   * Abstract base class for {@link ByteBuf} implementations that count references.
   */
  public abstract class AbstractReferenceCountedByteBuf extends AbstractByteBuf {
      private static final long REFCNT_FIELD_OFFSET;
      private static final AtomicIntegerFieldUpdater<AbstractReferenceCountedByteBuf> AIF_UPDATER;
      private static final Object REFCNT_FIELD_VH;
      private static final ReferenceCountUpdater<AbstractReferenceCountedByteBuf> updater;
  
      static {
          UpdaterType updaterType = ReferenceCountUpdater.updaterTypeOf(AbstractReferenceCountedByteBuf.class, "refCnt");
          switch (updaterType) {
              case Atomic:
                  AIF_UPDATER = newUpdater(AbstractReferenceCountedByteBuf.class, "refCnt");
                  REFCNT_FIELD_OFFSET = -1;
                  REFCNT_FIELD_VH = null;
                  updater = new AtomicReferenceCountUpdater<AbstractReferenceCountedByteBuf>() {
                      @Override
                      protected AtomicIntegerFieldUpdater<AbstractReferenceCountedByteBuf> updater() {
                          return AIF_UPDATER;
                      }
                  };
                  break;
              case Unsafe:
                  AIF_UPDATER = null;
                  REFCNT_FIELD_OFFSET = getUnsafeOffset(AbstractReferenceCountedByteBuf.class, "refCnt");
                  REFCNT_FIELD_VH = null;
                  updater = new UnsafeReferenceCountUpdater<AbstractReferenceCountedByteBuf>() {
                      @Override
                      protected long refCntFieldOffset() {
                          return REFCNT_FIELD_OFFSET;
                      }
                  };
                  break;
              case VarHandle:
                  AIF_UPDATER = null;
                  REFCNT_FIELD_OFFSET = -1;
                  REFCNT_FIELD_VH = PlatformDependent.findVarHandleOfIntField(MethodHandles.lookup(),
                          AbstractReferenceCountedByteBuf.class, "refCnt");
                  updater = new VarHandleReferenceCountUpdater<AbstractReferenceCountedByteBuf>() {
                      @Override
                      protected VarHandle varHandle() {
                          return (VarHandle) REFCNT_FIELD_VH;
                      }
                  };
                  break;
              default:
                  throw new Error("Unexpected updater type for AbstractReferenceCountedByteBuf: " + updaterType);
          }
      }
  
      // Value might not equal "real" reference count, all access should be via the updater
      @SuppressWarnings({"unused", "FieldMayBeFinal"})
      private volatile int refCnt;
  
      protected AbstractReferenceCountedByteBuf(int maxCapacity) {
          super(maxCapacity);
          updater.setInitialValue(this);
      }
  
      @Override
      boolean isAccessible() {
          // Try to do non-volatile read for performance as the ensureAccessible() is racy anyway and only provide
          // a best-effort guard.
          return updater.isLiveNonVolatile(this);
      }
  
     @Override
     public int refCnt() {
         return updater.refCnt(this);
     }
 
     /**
      * An unsafe operation intended for use by a subclass that sets the reference count of the buffer directly
      */
     protected final void setRefCnt(int refCnt) {
         updater.setRefCnt(this, refCnt);
     }
 
     /**
      * An unsafe operation intended for use by a subclass that resets the reference count of the buffer to 1
      */
     protected final void resetRefCnt() {
         updater.resetRefCnt(this);
     }
 
     @Override
     public ByteBuf retain() {
         return updater.retain(this);
     }
 
     @Override
     public ByteBuf retain(int increment) {
         return updater.retain(this, increment);
     }
 
     @Override
     public ByteBuf touch() {
         return this;
     }
 
     @Override
     public ByteBuf touch(Object hint) {
         return this;
     }
 
     @Override
     public boolean release() {
         return handleRelease(updater.release(this));
     }
 
     @Override
     public boolean release(int decrement) {
         return handleRelease(updater.release(this, decrement));
     }
 
     private boolean handleRelease(boolean result) {
         if (result) {
             deallocate();
         }
         return result;
     }
 
     /**
      * Called once {@link #refCnt()} is equals 0.
      */
     protected abstract void deallocate();
 }