/*
    * Copyright 2021 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.netty5.buffer.api;
  
  import io.netty5.buffer.api.internal.ResourceSupport;
  import io.netty5.buffer.api.internal.Statics;
  import io.netty5.util.Resource;
  import io.netty5.util.Send;
  
  import java.lang.invoke.VarHandle;
  import java.util.Objects;
  
  import static java.lang.invoke.MethodHandles.lookup;
  
  /**
   * The {@link BufferHolder} is an abstract class that simplifies the implementation of objects that themselves contain
   * a {@link Buffer} instance.
   * <p>
   * The {@link BufferHolder} can only hold on to a single buffer, so objects and classes that need to hold on to multiple
   * buffers will have to do their implementation from scratch, though they can use the code of the {@link BufferHolder}
   * as inspiration. Alternatively, multiple buffers can be
   * {@linkplain BufferAllocator#compose(Iterable) composed} into a single buffer, which can then be put
   * in a buffer holder.
   * <p>
   * If you just want an object that is a reference to a buffer, then the {@link BufferRef} can be used for that purpose.
   * If you have an advanced use case where you wish to implement {@link Resource}, and tightly control lifetimes, then
   * {@link ResourceSupport} can be of help.
   *
   * @param <T> The concrete {@link BufferHolder} type.
   */
  public abstract class BufferHolder<T extends Resource<T>> implements Resource<T> {
      private static final VarHandle BUF = Statics.findVarHandle(lookup(), BufferHolder.class, "buf", Buffer.class);
      private Buffer buf;
  
      /**
       * Create a new {@link BufferHolder} to hold the given {@linkplain Buffer buffer}.
       *
       * @param buf The {@linkplain Buffer buffer} to be held by this holder.
       */
      protected BufferHolder(Buffer buf) {
          this.buf = Objects.requireNonNull(buf, "The buffer cannot be null.");
      }
  
      /**
       * Create a new {@link BufferHolder} to hold the {@linkplain Buffer buffer} received from the given {@link Send}.
       * <p>
       * The {@link BufferHolder} will then be holding exclusive ownership of the buffer.
       *
       * @param send The {@linkplain Buffer buffer} to be held by this holder.
       */
      protected BufferHolder(Send<Buffer> send) {
          buf = Objects.requireNonNull(send, "The Send-object cannot be null.").receive();
      }
  
      @Override
      public void close() {
          buf.close();
      }
  
      @SuppressWarnings("unchecked")
      @Override
      public Send<T> send() {
          return buf.send().map((Class<T>) getClass(), this::receive);
      }
  
      /**
       * Called when a {@linkplain #send() sent} {@link BufferHolder} is received by the recipient.
       * The {@link BufferHolder} should return a new concrete instance, that wraps the given {@link Buffer} object.
       *
       * @param buf The {@link Buffer} that is {@linkplain Send#receive() received} by the recipient,
       *           and needs to be wrapped in a new {@link BufferHolder} instance.
       * @return A new {@linkplain T buffer holder} instance, containing the given {@linkplain Buffer buffer}.
       */
      protected abstract T receive(Buffer buf);
  
      /**
       * Replace the underlying referenced buffer with the given buffer.
       * <p>
       * This method is protected to permit advanced use cases of {@link BufferHolder} sub-class implementations.
       * <p>
       * <strong>Note:</strong> This method closes the current buffer,
       * and takes exclusive ownership of the received buffer.
       * <p>
       * The buffer assignment is performed using a plain store.
       *
       * @param send The new {@link Buffer} instance that is replacing the currently held buffer.
      */
     protected final void replaceBuffer(Send<Buffer> send) {
         Buffer received = send.receive();
         buf.close();
         buf = received;
     }
 
     /**
      * Replace the underlying referenced buffer with the given buffer.
      * <p>
      * This method is protected to permit advanced use cases of {@link BufferHolder} sub-class implementations.
      * <p>
      * <strong>Note:</strong> this method closes the current buffer,
      * and takes exclusive ownership of the received buffer.
      * <p>
      * The buffer assignment is performed using a volatile store.
      *
      * @param send The {@link Send} with the new {@link Buffer} instance that is replacing the currently held buffer.
      */
     protected final void replaceBufferVolatile(Send<Buffer> send) {
         Buffer received = send.receive();
         var prev = (Buffer) BUF.getAndSet(this, received);
         prev.close();
     }
 
     /**
      * Access the held {@link Buffer} instance.
      * <p>
      * The access is performed using a plain load.
      *
      * @return The {@link Buffer} instance being held by this {@linkplain T buffer holder}.
      */
     protected final Buffer getBuffer() {
         return buf;
     }
 
     /**
      * Access the held {@link Buffer} instance.
      * <p>
      * The access is performed using a volatile load.
      *
      * @return The {@link Buffer} instance being held by this {@linkplain T buffer holder}.
      */
     protected final Buffer getBufferVolatile() {
         return (Buffer) BUF.getVolatile(this);
     }
 
     @Override
     public boolean isAccessible() {
         return buf.isAccessible();
     }
 
     @SuppressWarnings("unchecked")
     @Override
     public T touch(Object hint) {
         buf.touch(hint);
         return (T) this;
     }
 
     /**
      * This implementation of the {@code equals} operation is restricted to work only with instances of the same class.
      * The reason for that is that Netty library already has a number of classes that extend {@link BufferHolder} and
      * override {@code equals} method with an additional comparison logic, and we need the symmetric property of the
      * {@code equals} operation to be preserved.
      *
      * @param other The reference object with which to compare.
      * @return {@code true} if this object is the same as the obj argument; {@code false} otherwise.
      */
     @Override
     public boolean equals(Object other) {
         if (this == other) {
             return true;
         }
         if (other != null && getClass() == other.getClass()) {
             return buf.equals(((BufferHolder<?>) other).buf);
         }
         return false;
     }
 
     @Override
     public int hashCode() {
         int result = getClass().hashCode();
         result *= 31 + buf.hashCode();
         return result;
     }
 }