/*
 * Copyright 2012 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:
 *
 *   http://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 common.net.handler.codec;

import java.util.List;

import common.net.buffer.ByteBuf;
import common.net.buffer.Unpooled;
import common.net.channel.ChannelHandlerContext;
import common.net.channel.ChannelInboundHandlerAdapter;
import common.net.util.internal.RecyclableArrayList;
import common.net.util.internal.StringUtil;

/**
 * {@link ChannelInboundHandlerAdapter} which decodes bytes in a stream-like fashion from one {@link ByteBuf} to an
 * other Message type.
 *
 * For example here is an implementation which reads all readable bytes from
 * the input {@link ByteBuf} and create a new {@link ByteBuf}.
 *
 * <pre>
 *     public class SquareDecoder extends {@link ByteToMessageDecoder} {
 *         {@code @Override}
 *         public void decode({@link ChannelHandlerContext} ctx, {@link ByteBuf} in, List&lt;Object&gt; out)
 *                 throws {@link Exception} {
 *             out.add(in.readBytes(in.readableBytes()));
 *         }
 *     }
 * </pre>
 *
 * Be aware that sub-classes of {@link ByteToMessageDecoder} <strong>MUST NOT</strong>
 * annotated with {@link @Sharable}.
 */
public abstract class ByteToMessageDecoder extends ChannelInboundHandlerAdapter {

    ByteBuf cumulation;
    private boolean singleDecode;
    private boolean decodeWasNull;
    private boolean first;

    protected ByteToMessageDecoder() {
        if (isSharable()) {
            throw new IllegalStateException("@Sharable annotation is not allowed");
        }
    }

    /**
     * If set then only one message is decoded on each {@link #channelRead(ChannelHandlerContext, Object)}
     * call. This may be useful if you need to do some protocol upgrade and want to make sure nothing is mixed up.
     *
     * Default is {@code false} as this has performance impacts.
     */
    public void setSingleDecode(boolean singleDecode) {
        this.singleDecode = singleDecode;
    }

    /**
     * If {@code true} then only one message is decoded on each
     * {@link #channelRead(ChannelHandlerContext, Object)} call.
     *
     * Default is {@code false} as this has performance impacts.
     */
    public boolean isSingleDecode() {
        return singleDecode;
    }

    /**
     * Returns the actual number of readable bytes in the internal cumulative
     * buffer of this decoder. You usually do not need to rely on this value
     * to write a decoder. Use it only when you must use it at your own risk.
     * This method is a shortcut to {@link #internalBuffer() internalBuffer().readableBytes()}.
     */
    protected int actualReadableBytes() {
        return internalBuffer().readableBytes();
    }

    /**
     * Returns the internal cumulative buffer of this decoder. You usually
     * do not need to access the internal buffer directly to write a decoder.
     * Use it only when you must use it at your own risk.
     */
    protected ByteBuf internalBuffer() {
        if (cumulation != null) {
            return cumulation;
        } else {
            return Unpooled.EMPTY_BUFFER;
        }
    }

    @Override
    public final void handlerRemoved(ChannelHandlerContext ctx) throws Exception {
        ByteBuf buf = internalBuffer();
        int readable = buf.readableBytes();
        if (buf.isReadable()) {
            ByteBuf bytes = buf.readBytes(readable);
            buf.release();
            ctx.fireChannelRead(bytes);
        } else {
            buf.release();
        }
        cumulation = null;
        ctx.fireChannelReadComplete();
        handlerRemoved0(ctx);
    }

    /**
     * Gets called after the {@link ByteToMessageDecoder} was removed from the actual context and it doesn't handle
     * events anymore.
     */
    protected void handlerRemoved0(ChannelHandlerContext ctx) throws Exception { }

    @Override
    public void channelRead(ChannelHandlerContext ctx, Object msg) throws Exception {
        if (msg instanceof ByteBuf) {
            RecyclableArrayList out = RecyclableArrayList.newInstance();
            try {
                ByteBuf data = (ByteBuf) msg;
                first = cumulation == null;
                if (first) {
                    cumulation = data;
                } else {
                    if (cumulation.writerIndex() > cumulation.maxCapacity() - data.readableBytes()
                            || cumulation.refCnt() > 1) {
                        // Expand cumulation (by replace it) when either there is not more room in the buffer
                        // or if the refCnt is greater then 1 which may happen when the user use slice().retain() or
                        // duplicate().retain().
                        //
                        // See:
                        // - https://github.com/netty/netty/issues/2327
                        // - https://github.com/netty/netty/issues/1764
                        expandCumulation(ctx, data.readableBytes());
                    }
                    cumulation.writeBytes(data);
                    data.release();
                }
                callDecode(ctx, cumulation, out);
            } catch (DecoderException e) {
                throw e;
            } catch (Throwable t) {
                throw new DecoderException(t);
            } finally {
                if (cumulation != null && !cumulation.isReadable()) {
                    cumulation.release();
                    cumulation = null;
                }
                int size = out.size();
                decodeWasNull = size == 0;

                for (int i = 0; i < size; i ++) {
                    ctx.fireChannelRead(out.get(i));
                }
                out.recycle();
            }
        } else {
            ctx.fireChannelRead(msg);
        }
    }

    private void expandCumulation(ChannelHandlerContext ctx, int readable) {
        ByteBuf oldCumulation = cumulation;
        cumulation = ctx.alloc().buffer(oldCumulation.readableBytes() + readable);
        cumulation.writeBytes(oldCumulation);
        oldCumulation.release();
    }

    @Override
    public void channelReadComplete(ChannelHandlerContext ctx) throws Exception {
        if (cumulation != null && !first && cumulation.refCnt() == 1) {
            // discard some bytes if possible to make more room in the
            // buffer but only if the refCnt == 1  as otherwise the user may have
            // used slice().retain() or duplicate().retain().
            //
            // See:
            // - https://github.com/netty/netty/issues/2327
            // - https://github.com/netty/netty/issues/1764
            cumulation.discardSomeReadBytes();
        }
        if (decodeWasNull) {
            decodeWasNull = false;
            if (!ctx.channel().config().isAutoRead()) {
                ctx.read();
            }
        }
        ctx.fireChannelReadComplete();
    }

    @Override
    public void channelInactive(ChannelHandlerContext ctx) throws Exception {
        RecyclableArrayList out = RecyclableArrayList.newInstance();
        try {
            if (cumulation != null) {
                callDecode(ctx, cumulation, out);
                decodeLast(ctx, cumulation, out);
            } else {
                decodeLast(ctx, Unpooled.EMPTY_BUFFER, out);
            }
        } catch (DecoderException e) {
            throw e;
        } catch (Exception e) {
            throw new DecoderException(e);
        } finally {
            try {
                if (cumulation != null) {
                    cumulation.release();
                    cumulation = null;
                }
                int size = out.size();
                for (int i = 0; i < size; i++) {
                    ctx.fireChannelRead(out.get(i));
                }
                if (size > 0) {
                    // Something was read, call fireChannelReadComplete()
                    ctx.fireChannelReadComplete();
                }
                ctx.fireChannelInactive();
            } finally {
                // recycle in all cases
                out.recycle();
            }
        }
    }

    /**
     * Called once data should be decoded from the given {@link ByteBuf}. This method will call
     * {@link #decode(ChannelHandlerContext, ByteBuf, List)} as long as decoding should take place.
     *
     * @param ctx           the {@link ChannelHandlerContext} which this {@link ByteToMessageDecoder} belongs to
     * @param in            the {@link ByteBuf} from which to read data
     * @param out           the {@link List} to which decoded messages should be added
     */
    protected void callDecode(ChannelHandlerContext ctx, ByteBuf in, List<Object> out) {
        try {
            while (in.isReadable()) {
                int outSize = out.size();
                int oldInputLength = in.readableBytes();
                decode(ctx, in, out);

                // Check if this handler was removed before continuing the loop.
                // If it was removed, it is not safe to continue to operate on the buffer.
                //
                // See https://github.com/netty/netty/issues/1664
                if (ctx.isRemoved()) {
                    break;
                }

                if (outSize == out.size()) {
                    if (oldInputLength == in.readableBytes()) {
                        break;
                    } else {
                        continue;
                    }
                }

                if (oldInputLength == in.readableBytes()) {
                    throw new DecoderException(
                            StringUtil.simpleClassName(getClass()) +
                            ".decode() did not read anything but decoded a message.");
                }

                if (isSingleDecode()) {
                    break;
                }
            }
        } catch (DecoderException e) {
            throw e;
        } catch (Throwable cause) {
            throw new DecoderException(cause);
        }
    }

    /**
     * Decode the from one {@link ByteBuf} to an other. This method will be called till either the input
     * {@link ByteBuf} has nothing to read when return from this method or till nothing was read from the input
     * {@link ByteBuf}.
     *
     * @param ctx           the {@link ChannelHandlerContext} which this {@link ByteToMessageDecoder} belongs to
     * @param in            the {@link ByteBuf} from which to read data
     * @param out           the {@link List} to which decoded messages should be added
     * @throws Exception    is thrown if an error accour
     */
    protected abstract void decode(ChannelHandlerContext ctx, ByteBuf in, List<Object> out) throws Exception;

    /**
     * Is called one last time when the {@link ChannelHandlerContext} goes in-active. Which means the
     * {@link #channelInactive(ChannelHandlerContext)} was triggered.
     *
     * By default this will just call {@link #decode(ChannelHandlerContext, ByteBuf, List)} but sub-classes may
     * override this for some special cleanup operation.
     */
    protected void decodeLast(ChannelHandlerContext ctx, ByteBuf in, List<Object> out) throws Exception {
        decode(ctx, in, out);
    }
}
