/*
    * Copyright 2025 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.channel.uring;
  
  import io.netty.buffer.ByteBuf;
  import io.netty.buffer.DuplicatedByteBuf;
  import io.netty.buffer.SlicedByteBuf;
  import io.netty.buffer.SwappedByteBuf;
  import io.netty.buffer.WrappedByteBuf;
  import io.netty.channel.unix.Buffer;
  
  import java.lang.invoke.MethodHandles;
  import java.lang.invoke.VarHandle;
  import java.nio.ByteBuffer;
  import java.nio.ByteOrder;
  import java.util.Arrays;
  import java.util.concurrent.atomic.AtomicInteger;
  
  final class IoUringBufferRing {
      private static final VarHandle SHORT_HANDLE =
              MethodHandles.byteBufferViewVarHandle(short[].class, ByteOrder.nativeOrder());
      private final ByteBuffer ioUringBufRing;
      private final int tailFieldPosition;
      private final short entries;
      private final int batchSize;
      private final int maxUnreleasedBuffers;
      private final short mask;
      private final short bufferGroupId;
      private final int ringFd;
      private final IoUringBufferRingByteBuf[] buffers;
      private final IoUringBufferRingAllocator allocator;
      private final IoUringBufferRingExhaustedEvent exhaustedEvent;
      private final boolean incremental;
      private final AtomicInteger unreleasedBuffers = new AtomicInteger();
      private volatile boolean usable;
      private boolean corrupted;
      private boolean closed;
      private int numBuffers;
      private boolean expanded;
  
      IoUringBufferRing(int ringFd, ByteBuffer ioUringBufRing,
                        short entries, int batchSize, int maxUnreleasedBuffers, short bufferGroupId, boolean incremental,
                        IoUringBufferRingAllocator allocator) {
          assert entries % 2 == 0;
          this.ioUringBufRing = ioUringBufRing;
          this.tailFieldPosition = Native.IO_URING_BUFFER_RING_TAIL;
          this.entries = entries;
          this.batchSize = batchSize;
          this.maxUnreleasedBuffers = maxUnreleasedBuffers;
          this.mask = (short) (entries - 1);
          this.bufferGroupId = bufferGroupId;
          this.ringFd = ringFd;
          this.buffers = new IoUringBufferRingByteBuf[entries];
          this.incremental = incremental;
          this.allocator = allocator;
          this.exhaustedEvent = new IoUringBufferRingExhaustedEvent(bufferGroupId);
      }
  
      boolean isUsable() {
          return !corrupted && usable;
      }
  
      void initialize() {
          fillBuffers();
          usable = true;
      }
  
      /**
       * Try to expand by adding more buffers to the ring if there is any space left.
       * This method might be called multiple times before we call {@link #fillBuffer()} again.
       */
      void expand() {
          // TODO: We could also shrink the number of elements again if we find out we not use all of it frequently.
          if (!expanded) {
              // Only expand once before we reset expanded in fillBuffer() which is called once a buffer was completely
              // used and moved out of the buffer ring.
              fillBuffers();
              expanded = true;
          }
      }
  
      private void fillBuffers() {
          int num = Math.min(batchSize, entries - numBuffers);
          for (short i = 0; i < num; i++) {
              fillBuffer();
          }
     }
 
     /**
      * @return the {@link IoUringBufferRingExhaustedEvent} that should be used to signal that there were no buffers
      * left for this buffer ring.
      */
     IoUringBufferRingExhaustedEvent getExhaustedEvent() {
         return exhaustedEvent;
     }
 
     void fillBuffer() {
         if (corrupted || closed) {
             return;
         }
         short oldTail = (short) SHORT_HANDLE.get(ioUringBufRing, tailFieldPosition);
         short ringIndex = (short) (oldTail & mask);
         assert buffers[ringIndex] == null;
         final ByteBuf byteBuf;
         try {
             byteBuf = allocator.allocate();
         } catch (OutOfMemoryError e) {
             // We did run out of memory, This buffer ring should be considered corrupted.
             // TODO: In the future we could try to recover it later by trying to refill it after some time and so
             //       bring it back to a non-corrupted state.
             corrupted = true;
             throw e;
         }
         byteBuf.writerIndex(byteBuf.capacity());
         buffers[ringIndex] = new IoUringBufferRingByteBuf(byteBuf);
 
         //  see:
         //  https://github.com/axboe/liburing/
         //      blob/19134a8fffd406b22595a5813a3e319c19630ac9/src/include/liburing.h#L1561
         int  position = Native.SIZEOF_IOURING_BUF * ringIndex;
         ioUringBufRing.putLong(position + Native.IOURING_BUFFER_OFFSETOF_ADDR,
                 IoUring.memoryAddress(byteBuf) + byteBuf.readerIndex());
         ioUringBufRing.putInt(position + Native.IOURING_BUFFER_OFFSETOF_LEN, byteBuf.capacity());
         ioUringBufRing.putShort(position + Native.IOURING_BUFFER_OFFSETOF_BID, ringIndex);
 
         // Now advanced the tail by the number of buffers that we just added.
         SHORT_HANDLE.setRelease(ioUringBufRing, tailFieldPosition, (short) (oldTail + 1));
         numBuffers++;
         // We added a buffer to the ring, let's reset the expanded variable so we can expand it if we receive
         // ENOBUFS.
         expanded = false;
     }
 
     /**
      * Return the amount of bytes that we attempted to read for the given id.
      * This method must be called before {@link #useBuffer(short, int, boolean)}.
      *
      * @param bid   the id of the buffer.
      * @return      the attempted bytes.
      */
     int attemptedBytesRead(short bid) {
         return buffers[bid].readableBytes();
     }
 
     /**
      * Use the buffer for the given buffer id. The returned {@link ByteBuf} must be released once not used anymore.
      *
      * @param bid           the id of the buffer
      * @param readableBytes the number of bytes that could be read. This value might be larger then what a single
      *                      {@link ByteBuf} can hold. Because of this, the caller should call
      *                      @link #useBuffer(short, int, boolean)} in a loop (obtaining the next bid to use by calling
      *                      {@link #nextBid(short)}) until all buffers could be obtained.
      * @return              the buffer.
      */
     ByteBuf useBuffer(short bid, int readableBytes, boolean more) {
         assert readableBytes > 0;
         IoUringBufferRingByteBuf byteBuf = buffers[bid];
 
         allocator.lastBytesRead(byteBuf.readableBytes(), readableBytes);
         if (incremental && more && byteBuf.readableBytes() > readableBytes) {
             // The buffer will be used later again, just slice out what we did read so far.
             return byteBuf.readRetainedSlice(readableBytes);
         }
 
         // The buffer is considered to be used, null out the slot.
         buffers[bid] = null;
         numBuffers--;
         byteBuf.markUsed();
         return byteBuf.writerIndex(byteBuf.readerIndex() +
                 Math.min(readableBytes, byteBuf.readableBytes()));
     }
 
     short nextBid(short bid) {
         return (short) ((bid + 1) & mask);
     }
 
     /**
      * The group id that is assigned to this buffer ring.
      *
      * @return group id.
      */
     short bufferGroupId() {
         return bufferGroupId;
     }
 
     /**
      * Close this {@link IoUringBufferRing}, using it after this method is called will lead to undefined behaviour.
      */
     void close() {
         if (closed) {
             return;
         }
         closed = true;
         Native.ioUringUnRegisterBufRing(ringFd, Buffer.memoryAddress(ioUringBufRing), entries, bufferGroupId);
         for (ByteBuf byteBuf : buffers) {
             if (byteBuf != null) {
                 byteBuf.release();
             }
         }
         Arrays.fill(buffers, null);
     }
 
     // Package-private for testing
     final class IoUringBufferRingByteBuf extends WrappedByteBuf {
         IoUringBufferRingByteBuf(ByteBuf buf) {
             super(buf);
         }
 
         void markUsed() {
             if (unreleasedBuffers.incrementAndGet() == maxUnreleasedBuffers) {
                 usable = false;
             }
         }
 
         @SuppressWarnings("deprecation")
         @Override
         public ByteBuf order(ByteOrder endianness) {
             if (endianness == order()) {
                 return this;
             }
             return new SwappedByteBuf(this);
         }
 
         @Override
         public ByteBuf slice() {
             return slice(readerIndex(), readableBytes());
         }
 
         @Override
         public ByteBuf retainedSlice() {
             return slice().retain();
         }
 
         @SuppressWarnings("deprecation")
         @Override
         public ByteBuf slice(int index, int length) {
             return new SlicedByteBuf(this, index, length);
         }
 
         @Override
         public ByteBuf retainedSlice(int index, int length) {
             return slice(index, length).retain();
         }
 
         @Override
         public ByteBuf readSlice(int length) {
             ByteBuf slice = slice(readerIndex(), length);
             skipBytes(length);
             return slice;
         }
 
         @Override
         public ByteBuf readRetainedSlice(int length) {
             ByteBuf slice = retainedSlice(readerIndex(), length);
             try {
                 skipBytes(length);
             } catch (Throwable cause) {
                 slice.release();
                 throw cause;
             }
             return slice;
         }
 
         @SuppressWarnings("deprecation")
         @Override
         public ByteBuf duplicate() {
             return new DuplicatedByteBuf(this);
         }
 
         @Override
         public ByteBuf retainedDuplicate() {
             return duplicate().retain();
         }
 
         @Override
         public boolean release() {
             if (super.release()) {
                 released();
                 return true;
             }
             return false;
         }
 
         @Override
         public boolean release(int decrement) {
             if (super.release(decrement)) {
                 released();
                 return true;
             }
             return false;
         }
 
         private void released() {
             if (unreleasedBuffers.decrementAndGet() == maxUnreleasedBuffers / 2) {
                 usable = true;
             }
         }
     }
 }