/*
    * Copyright 2024 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.handler.codec.compression;
  
  import com.github.luben.zstd.ZstdInputStreamNoFinalizer;
  import io.netty.buffer.ByteBuf;
  import io.netty.channel.ChannelHandlerContext;
  import io.netty.handler.codec.ByteToMessageDecoder;
  import io.netty.util.internal.ObjectUtil;
  
  import java.io.Closeable;
  import java.io.IOException;
  import java.io.InputStream;
  import java.util.List;
  
  /**
   * Decompresses a compressed block {@link ByteBuf} using the Zstandard algorithm.
   * See <a href="https://facebook.github.io/zstd">Zstandard</a>.
   */
  public final class ZstdDecoder extends ByteToMessageDecoder {
      // Don't use static here as we want to still allow to load the classes.
      {
          try {
              Zstd.ensureAvailability();
          } catch (Throwable throwable) {
              throw new ExceptionInInitializerError(throwable);
          }
      }
  
      private static final int DEFAULT_MAX_FORWARD_BYTES = CompressionUtil.DEFAULT_MAX_FORWARD_BYTES;
      /**
       * Default maximum size of a single output buffer, in bytes (4 MiB).
       */
      public static final int DEFAULT_MAXIMUM_ALLOCATION_SIZE = 4 * 1024 * 1024;
      /**
       * Default upper bound on the {@code Window_Log} accepted by the decoder.
       * {@code 27} corresponds to a 128 MiB decompression window.
       */
      public static final int DEFAULT_MAX_WINDOW_LOG = 27;
      private static final int MIN_WINDOW_LOG = 10;
      private static final int MAX_WINDOW_LOG = 31;
      private final int maximumAllocationSize;
      private final int maxForwardBytes;
      private final int maxWindowLog;
      private final MutableByteBufInputStream inputStream = new MutableByteBufInputStream();
      private ZstdInputStreamNoFinalizer zstdIs;
  
      private boolean needsRead;
      private State currentState = State.DECOMPRESS_DATA;
  
      /**
       * Current state of stream.
       */
      private enum State {
          DECOMPRESS_DATA,
          CORRUPTED
      }
  
      /**
       * Creates a new decoder with the {@link #DEFAULT_MAXIMUM_ALLOCATION_SIZE},
       * and the {@link #DEFAULT_MAX_WINDOW_LOG} window log size.
       * <p>
       * The window log size bounds the memory usage of the sliding window for ZSTD frame decompression.
       * Frames declaring a larger window will be rejected to bound the memory the decoder may allocate per stream.
       *
       */
      public ZstdDecoder() {
          this(DEFAULT_MAXIMUM_ALLOCATION_SIZE, DEFAULT_MAX_WINDOW_LOG);
      }
  
      /**
       * Creates a new decoder with the given maximum allocation size,
       * and the {@link #DEFAULT_MAX_WINDOW_LOG} window log size.
       * <p>
       * The window log size bounds the memory usage of the sliding window for ZSTD frame decompression.
       * Frames declaring a larger window will be rejected to bound the memory the decoder may allocate per stream.
       *
       * @param maximumAllocationSize maximum size of a single output buffer.
       */
      public ZstdDecoder(int maximumAllocationSize) {
          this(maximumAllocationSize, DEFAULT_MAX_WINDOW_LOG);
      }
  
      /**
       * Creates a new decoder with an explicit upper bound on the accepted {@code Window_Log}.
       *
      * @param maximumAllocationSize maximum size of a single output buffer.
      * @param maxWindowLog          upper bound on the {@code Window_Log} field of incoming
      *                              frames; must be in {@code [10, 31]}. Frames declaring a
      *                              larger window will be rejected to bound the memory the
      *                              decoder may allocate per stream.
      */
     public ZstdDecoder(int maximumAllocationSize, int maxWindowLog) {
         this.maximumAllocationSize = ObjectUtil.checkPositiveOrZero(maximumAllocationSize, "maximumAllocationSize");
         this.maxForwardBytes = maximumAllocationSize > 0 ? maximumAllocationSize : DEFAULT_MAX_FORWARD_BYTES;
         this.maxWindowLog = ObjectUtil.checkInRange(maxWindowLog, MIN_WINDOW_LOG, MAX_WINDOW_LOG, "maxWindowLog");
     }
 
     @Override
     protected void decode(ChannelHandlerContext ctx, ByteBuf in, List<Object> out) throws Exception {
         needsRead = true;
         try {
             if (currentState == State.CORRUPTED) {
                 in.skipBytes(in.readableBytes());
 
                 return;
             }
             inputStream.current = in;
 
             ByteBuf outBuffer = null;
 
             final int compressedLength = in.readableBytes();
             try {
                 long uncompressedLength;
                 if (in.isDirect()) {
                     uncompressedLength = com.github.luben.zstd.Zstd.getFrameContentSize(
                             CompressionUtil.safeNioBuffer(in, in.readerIndex(), in.readableBytes()));
                 } else {
                     uncompressedLength = com.github.luben.zstd.Zstd.getFrameContentSize(
                             in.array(), in.readerIndex() + in.arrayOffset(), in.readableBytes());
                 }
                 if (uncompressedLength <= 0) {
                     // Let's start with the compressedLength * 2 as often we will not have everything
                     // we need in the in buffer and don't want to reserve too much memory.
                     uncompressedLength = compressedLength * 2L;
                 }
 
                 int w;
                 do {
                     if (outBuffer == null) {
                         outBuffer = ctx.alloc().heapBuffer((int) (maximumAllocationSize == 0 ?
                                 uncompressedLength : Math.min(maximumAllocationSize, uncompressedLength)));
                     }
                     do {
                         w = outBuffer.writeBytes(zstdIs, outBuffer.writableBytes());
                     } while (w > 0 && outBuffer.isWritable());
                     if (!outBuffer.isWritable() || outBuffer.readableBytes() >= maxForwardBytes) {
                         needsRead = false;
                         ctx.fireChannelRead(outBuffer);
                         outBuffer = null;
                     }
                 } while (w > 0);
                 if (outBuffer != null && outBuffer.isReadable()) {
                     needsRead = false;
                     ctx.fireChannelRead(outBuffer);
                     outBuffer = null;
                 }
             } finally {
                 if (outBuffer != null) {
                     outBuffer.release();
                 }
             }
         } catch (Exception e) {
             currentState = State.CORRUPTED;
             throw new DecompressionException(e);
         } finally {
             inputStream.current = null;
         }
     }
 
     @Override
     public void channelReadComplete(ChannelHandlerContext ctx) throws Exception {
         // Discard bytes of the cumulation buffer if needed.
         discardSomeReadBytes();
 
         if (needsRead && !ctx.channel().config().isAutoRead()) {
             ctx.read();
         }
         ctx.fireChannelReadComplete();
     }
 
     @Override
     public void handlerAdded(ChannelHandlerContext ctx) throws Exception {
         super.handlerAdded(ctx);
         zstdIs = new ZstdInputStreamNoFinalizer(inputStream);
         zstdIs.setContinuous(true);
         // Bound the decompression window to mitigate memory amplification from frames that
         // declare an oversized Window_Size.
         zstdIs.setLongMax(maxWindowLog);
     }
 
     @Override
     protected void handlerRemoved0(ChannelHandlerContext ctx) throws Exception {
         try {
             closeSilently(zstdIs);
         } finally {
             super.handlerRemoved0(ctx);
         }
     }
 
     private static void closeSilently(Closeable closeable) {
         if (closeable != null) {
             try {
                 closeable.close();
             } catch (IOException ignore) {
                 // ignore
             }
         }
     }
 
     private static final class MutableByteBufInputStream extends InputStream {
         ByteBuf current;
 
         @Override
         public int read() {
             if (current == null || !current.isReadable()) {
                 return -1;
             }
             return current.readByte() & 0xff;
         }
 
         @Override
         public int read(byte[] b, int off, int len) {
             int available = available();
             if (available == 0) {
                 return -1;
             }
 
             len = Math.min(available, len);
             current.readBytes(b, off, len);
             return len;
         }
 
         @Override
         public int available() {
             return current == null ? 0 : current.readableBytes();
         }
     }
 }