/*
    * 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 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() {
      }
  
      protected abstract void safeInitializeRawRefCnt(T refCntObj, int value);
  
      protected abstract int getAndAddRawRefCnt(T refCntObj, int increment);
  
      protected abstract int getRawRefCnt(T refCnt);
  
      protected abstract int getAcquireRawRefCnt(T refCnt);
  
      protected abstract void setReleaseRawRefCnt(T refCnt, int value);
  
      protected abstract boolean casRawRefCnt(T refCnt, int expected, int value);
  
      public final int initialValue() {
          return 2;
      }
  
      public final void setInitialValue(T instance) {
          safeInitializeRawRefCnt(instance, 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);
      }
  
      public final int refCnt(T instance) {
          return realRefCnt(getAcquireRawRefCnt(instance));
      }
  
      public final boolean isLiveNonVolatile(T instance) {
          final int rawCnt = getRawRefCnt(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) {
          int rawRefCnt = refCnt > 0 ? refCnt << 1 : 1; // overflow OK here
          setReleaseRawRefCnt(instance, rawRefCnt);
      }
  
      /**
       * 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
         setReleaseRawRefCnt(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 = getAndAddRawRefCnt(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
             getAndAddRawRefCnt(instance, -rawIncrement);
             throw new IllegalReferenceCountException(realRefCnt(oldRef), increment);
         }
         return instance;
     }
 
     public final boolean release(T instance) {
         int rawCnt = getRawRefCnt(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 = getRawRefCnt(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 casRawRefCnt(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
                 && casRawRefCnt(instance, rawCnt, rawCnt - (decrement << 1))) {
             return false;
         }
         return retryRelease0(instance, decrement);
     }
 
     private boolean retryRelease0(T instance, int decrement) {
         for (;;) {
             int rawCnt = getRawRefCnt(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 (casRawRefCnt(instance, rawCnt, rawCnt - (decrement << 1))) {
                     return false;
                 }
             } else {
                 throw new IllegalReferenceCountException(realCnt, -decrement);
             }
             Thread.yield(); // this benefits throughput under high contention
         }
     }
 
     public enum UpdaterType {
         Unsafe,
         VarHandle,
         Atomic
     }
 
     public static <T extends ReferenceCounted> UpdaterType updaterTypeOf(Class<T> clz, String fieldName) {
         long fieldOffset = getUnsafeOffset(clz, fieldName);
         if (fieldOffset >= 0) {
             return UpdaterType.Unsafe;
         }
         if (PlatformDependent.hasVarHandle()) {
             return UpdaterType.VarHandle;
         }
         return UpdaterType.Atomic;
     }
 
     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;
     }
 }