/*
 * 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 org.jboss.netty.handler.codec.frame;

import org.jboss.netty.buffer.ChannelBuffer;
import org.jboss.netty.buffer.ChannelBufferFactory;
import org.jboss.netty.channel.Channel;
import org.jboss.netty.channel.ChannelHandlerContext;
import org.jboss.netty.channel.Channels;
import org.jboss.netty.handler.codec.serialization.ObjectDecoder;

/**
 * A decoder that splits the received {@link ChannelBuffer}s dynamically by the
 * value of the length field in the message.  It is particularly useful when you
 * decode a binary message which has an integer header field that represents the
 * length of the message body or the whole message.
 * <p>
 * {@link LengthFieldBasedFrameDecoder} has many configuration parameters so
 * that it can decode any message with a length field, which is often seen in
 * proprietary client-server protocols. Here are some example that will give
 * you the basic idea on which option does what.
 *
 * <h3>2 bytes length field at offset 0, do not strip header</h3>
 *
 * The value of the length field in this example is <tt>12 (0x0C)</tt> which
 * represents the length of "HELLO, WORLD".  By default, the decoder assumes
 * that the length field represents the number of the bytes that follows the
 * length field.  Therefore, it can be decoded with the simplistic parameter
 * combination.
 * <pre>
 * <b>lengthFieldOffset</b>   = <b>0</b>
 * <b>lengthFieldLength</b>   = <b>2</b>
 * lengthAdjustment    = 0
 * initialBytesToStrip = 0 (= do not strip header)
 *
 * BEFORE DECODE (14 bytes)         AFTER DECODE (14 bytes)
 * +--------+----------------+      +--------+----------------+
 * | Length | Actual Content |----->| Length | Actual Content |
 * | 0x000C | "HELLO, WORLD" |      | 0x000C | "HELLO, WORLD" |
 * +--------+----------------+      +--------+----------------+
 * </pre>
 *
 * <h3>2 bytes length field at offset 0, strip header</h3>
 *
 * Because we can get the length of the content by calling
 * {@link ChannelBuffer#readableBytes()}, you might want to strip the length
 * field by specifying <tt>initialBytesToStrip</tt>.  In this example, we
 * specified <tt>2</tt>, that is same with the length of the length field, to
 * strip the first two bytes.
 * <pre>
 * lengthFieldOffset   = 0
 * lengthFieldLength   = 2
 * lengthAdjustment    = 0
 * <b>initialBytesToStrip</b> = <b>2</b> (= the length of the Length field)
 *
 * BEFORE DECODE (14 bytes)         AFTER DECODE (12 bytes)
 * +--------+----------------+      +----------------+
 * | Length | Actual Content |----->| Actual Content |
 * | 0x000C | "HELLO, WORLD" |      | "HELLO, WORLD" |
 * +--------+----------------+      +----------------+
 * </pre>
 *
 * <h3>2 bytes length field at offset 0, do not strip header, the length field
 *     represents the length of the whole message</h3>
 *
 * In most cases, the length field represents the length of the message body
 * only, as shown in the previous examples.  However, in some protocols, the
 * length field represents the length of the whole message, including the
 * message header.  In such a case, we specify a non-zero
 * <tt>lengthAdjustment</tt>.  Because the length value in this example message
 * is always greater than the body length by <tt>2</tt>, we specify <tt>-2</tt>
 * as <tt>lengthAdjustment</tt> for compensation.
 * <pre>
 * lengthFieldOffset   =  0
 * lengthFieldLength   =  2
 * <b>lengthAdjustment</b>    = <b>-2</b> (= the length of the Length field)
 * initialBytesToStrip =  0
 *
 * BEFORE DECODE (14 bytes)         AFTER DECODE (14 bytes)
 * +--------+----------------+      +--------+----------------+
 * | Length | Actual Content |----->| Length | Actual Content |
 * | 0x000E | "HELLO, WORLD" |      | 0x000E | "HELLO, WORLD" |
 * +--------+----------------+      +--------+----------------+
 * </pre>
 *
 * <h3>3 bytes length field at the end of 5 bytes header, do not strip header</h3>
 *
 * The following message is a simple variation of the first example.  An extra
 * header value is prepended to the message.  <tt>lengthAdjustment</tt> is zero
 * again because the decoder always takes the length of the prepended data into
 * account during frame length calculation.
 * <pre>
 * <b>lengthFieldOffset</b>   = <b>2</b> (= the length of Header 1)
 * <b>lengthFieldLength</b>   = <b>3</b>
 * lengthAdjustment    = 0
 * initialBytesToStrip = 0
 *
 * BEFORE DECODE (17 bytes)                      AFTER DECODE (17 bytes)
 * +----------+----------+----------------+      +----------+----------+----------------+
 * | Header 1 |  Length  | Actual Content |----->| Header 1 |  Length  | Actual Content |
 * |  0xCAFE  | 0x00000C | "HELLO, WORLD" |      |  0xCAFE  | 0x00000C | "HELLO, WORLD" |
 * +----------+----------+----------------+      +----------+----------+----------------+
 * </pre>
 *
 * <h3>3 bytes length field at the beginning of 5 bytes header, do not strip header</h3>
 *
 * This is an advanced example that shows the case where there is an extra
 * header between the length field and the message body.  You have to specify a
 * positive <tt>lengthAdjustment</tt> so that the decoder counts the extra
 * header into the frame length calculation.
 * <pre>
 * lengthFieldOffset   = 0
 * lengthFieldLength   = 3
 * <b>lengthAdjustment</b>    = <b>2</b> (= the length of Header 1)
 * initialBytesToStrip = 0
 *
 * BEFORE DECODE (17 bytes)                      AFTER DECODE (17 bytes)
 * +----------+----------+----------------+      +----------+----------+----------------+
 * |  Length  | Header 1 | Actual Content |----->|  Length  | Header 1 | Actual Content |
 * | 0x00000C |  0xCAFE  | "HELLO, WORLD" |      | 0x00000C |  0xCAFE  | "HELLO, WORLD" |
 * +----------+----------+----------------+      +----------+----------+----------------+
 * </pre>
 *
 * <h3>2 bytes length field at offset 1 in the middle of 4 bytes header,
 *     strip the first header field and the length field</h3>
 *
 * This is a combination of all the examples above.  There are the prepended
 * header before the length field and the extra header after the length field.
 * The prepended header affects the <tt>lengthFieldOffset</tt> and the extra
 * header affects the <tt>lengthAdjustment</tt>.  We also specified a non-zero
 * <tt>initialBytesToStrip</tt> to strip the length field and the prepended
 * header from the frame.  If you don't want to strip the prepended header, you
 * could specify <tt>0</tt> for <tt>initialBytesToSkip</tt>.
 * <pre>
 * lengthFieldOffset   = 1 (= the length of HDR1)
 * lengthFieldLength   = 2
 * <b>lengthAdjustment</b>    = <b>1</b> (= the length of HDR2)
 * <b>initialBytesToStrip</b> = <b>3</b> (= the length of HDR1 + LEN)
 *
 * BEFORE DECODE (16 bytes)                       AFTER DECODE (13 bytes)
 * +------+--------+------+----------------+      +------+----------------+
 * | HDR1 | Length | HDR2 | Actual Content |----->| HDR2 | Actual Content |
 * | 0xCA | 0x000C | 0xFE | "HELLO, WORLD" |      | 0xFE | "HELLO, WORLD" |
 * +------+--------+------+----------------+      +------+----------------+
 * </pre>
 *
 * <h3>2 bytes length field at offset 1 in the middle of 4 bytes header,
 *     strip the first header field and the length field, the length field
 *     represents the length of the whole message</h3>
 *
 * Let's give another twist to the previous example.  The only difference from
 * the previous example is that the length field represents the length of the
 * whole message instead of the message body, just like the third example.
 * We have to count the length of HDR1 and Length into <tt>lengthAdjustment</tt>.
 * Please note that we don't need to take the length of HDR2 into account
 * because the length field already includes the whole header length.
 * <pre>
 * lengthFieldOffset   =  1
 * lengthFieldLength   =  2
 * <b>lengthAdjustment</b>    = <b>-3</b> (= the length of HDR1 + LEN, negative)
 * <b>initialBytesToStrip</b> = <b> 3</b>
 *
 * BEFORE DECODE (16 bytes)                       AFTER DECODE (13 bytes)
 * +------+--------+------+----------------+      +------+----------------+
 * | HDR1 | Length | HDR2 | Actual Content |----->| HDR2 | Actual Content |
 * | 0xCA | 0x0010 | 0xFE | "HELLO, WORLD" |      | 0xFE | "HELLO, WORLD" |
 * +------+--------+------+----------------+      +------+----------------+
 * </pre>
 *
 * @see LengthFieldPrepender
 */
public class LengthFieldBasedFrameDecoder extends FrameDecoder {

    private final int maxFrameLength;
    private final int lengthFieldOffset;
    private final int lengthFieldLength;
    private final int lengthFieldEndOffset;
    private final int lengthAdjustment;
    private final int initialBytesToStrip;
    private final boolean failFast;
    private boolean discardingTooLongFrame;
    private long tooLongFrameLength;
    private long bytesToDiscard;

    /**
     * Creates a new instance.
     *
     * @param maxFrameLength
     *        the maximum length of the frame.  If the length of the frame is
     *        greater than this value, {@link TooLongFrameException} will be
     *        thrown.
     * @param lengthFieldOffset
     *        the offset of the length field
     * @param lengthFieldLength
     *        the length of the length field
     */
    public LengthFieldBasedFrameDecoder(
            int maxFrameLength,
            int lengthFieldOffset, int lengthFieldLength) {
        this(maxFrameLength, lengthFieldOffset, lengthFieldLength, 0, 0);
    }

    /**
     * Creates a new instance.
     *
     * @param maxFrameLength
     *        the maximum length of the frame.  If the length of the frame is
     *        greater than this value, {@link TooLongFrameException} will be
     *        thrown.
     * @param lengthFieldOffset
     *        the offset of the length field
     * @param lengthFieldLength
     *        the length of the length field
     * @param lengthAdjustment
     *        the compensation value to add to the value of the length field
     * @param initialBytesToStrip
     *        the number of first bytes to strip out from the decoded frame
     */
    public LengthFieldBasedFrameDecoder(
            int maxFrameLength,
            int lengthFieldOffset, int lengthFieldLength,
            int lengthAdjustment, int initialBytesToStrip) {
        this(
                maxFrameLength,
                lengthFieldOffset, lengthFieldLength, lengthAdjustment,
                initialBytesToStrip, false);
    }

    /**
     * Creates a new instance.
     *
     * @param maxFrameLength
     *        the maximum length of the frame.  If the length of the frame is
     *        greater than this value, {@link TooLongFrameException} will be
     *        thrown.
     * @param lengthFieldOffset
     *        the offset of the length field
     * @param lengthFieldLength
     *        the length of the length field
     * @param lengthAdjustment
     *        the compensation value to add to the value of the length field
     * @param initialBytesToStrip
     *        the number of first bytes to strip out from the decoded frame
     * @param failFast
     *        If <tt>true</tt>, a {@link TooLongFrameException} is thrown as
     *        soon as the decoder notices the length of the frame will exceed
     *        <tt>maxFrameLength</tt> regardless of whether the entire frame
     *        has been read.  If <tt>false</tt>, a {@link TooLongFrameException}
     *        is thrown after the entire frame that exceeds <tt>maxFrameLength</tt>
     *        has been read.
     */
    public LengthFieldBasedFrameDecoder(
            int maxFrameLength,
            int lengthFieldOffset, int lengthFieldLength,
            int lengthAdjustment, int initialBytesToStrip, boolean failFast) {
        if (maxFrameLength <= 0) {
            throw new IllegalArgumentException(
                    "maxFrameLength must be a positive integer: " +
                    maxFrameLength);
        }

        if (lengthFieldOffset < 0) {
            throw new IllegalArgumentException(
                    "lengthFieldOffset must be a non-negative integer: " +
                    lengthFieldOffset);
        }

        if (initialBytesToStrip < 0) {
            throw new IllegalArgumentException(
                    "initialBytesToStrip must be a non-negative integer: " +
                    initialBytesToStrip);
        }

        if (lengthFieldLength != 1 && lengthFieldLength != 2 &&
            lengthFieldLength != 3 && lengthFieldLength != 4 &&
            lengthFieldLength != 8) {
            throw new IllegalArgumentException(
                    "lengthFieldLength must be either 1, 2, 3, 4, or 8: " +
                    lengthFieldLength);
        }

        if (lengthFieldOffset > maxFrameLength - lengthFieldLength) {
            throw new IllegalArgumentException(
                    "maxFrameLength (" + maxFrameLength + ") " +
                    "must be equal to or greater than " +
                    "lengthFieldOffset (" + lengthFieldOffset + ") + " +
                    "lengthFieldLength (" + lengthFieldLength + ").");
        }

        this.maxFrameLength = maxFrameLength;
        this.lengthFieldOffset = lengthFieldOffset;
        this.lengthFieldLength = lengthFieldLength;
        this.lengthAdjustment = lengthAdjustment;
        lengthFieldEndOffset = lengthFieldOffset + lengthFieldLength;
        this.initialBytesToStrip = initialBytesToStrip;
        this.failFast = failFast;
    }

    @Override
    protected Object decode(
            ChannelHandlerContext ctx, Channel channel, ChannelBuffer buffer) throws Exception {

        if (discardingTooLongFrame) {
            long bytesToDiscard = this.bytesToDiscard;
            int localBytesToDiscard = (int) Math.min(bytesToDiscard, buffer.readableBytes());
            buffer.skipBytes(localBytesToDiscard);
            bytesToDiscard -= localBytesToDiscard;
            this.bytesToDiscard = bytesToDiscard;
            failIfNecessary(ctx, false);
            return null;
        }

        if (buffer.readableBytes() < lengthFieldEndOffset) {
            return null;
        }

        int actualLengthFieldOffset = buffer.readerIndex() + lengthFieldOffset;
        long frameLength;
        switch (lengthFieldLength) {
        case 1:
            frameLength = buffer.getUnsignedByte(actualLengthFieldOffset);
            break;
        case 2:
            frameLength = buffer.getUnsignedShort(actualLengthFieldOffset);
            break;
        case 3:
            frameLength = buffer.getUnsignedMedium(actualLengthFieldOffset);
            break;
        case 4:
            frameLength = buffer.getUnsignedInt(actualLengthFieldOffset);
            break;
        case 8:
            frameLength = buffer.getLong(actualLengthFieldOffset);
            break;
        default:
            throw new Error("should not reach here");
        }

        if (frameLength < 0) {
            buffer.skipBytes(lengthFieldEndOffset);
            throw new CorruptedFrameException(
                    "negative pre-adjustment length field: " + frameLength);
        }

        frameLength += lengthAdjustment + lengthFieldEndOffset;
        if (frameLength < lengthFieldEndOffset) {
            buffer.skipBytes(lengthFieldEndOffset);
            throw new CorruptedFrameException(
                    "Adjusted frame length (" + frameLength + ") is less " +
                    "than lengthFieldEndOffset: " + lengthFieldEndOffset);
        }

        if (frameLength > maxFrameLength) {
            // Enter the discard mode and discard everything received so far.
            discardingTooLongFrame = true;
            tooLongFrameLength = frameLength;
            bytesToDiscard = frameLength - buffer.readableBytes();
            buffer.skipBytes(buffer.readableBytes());
            failIfNecessary(ctx, true);
            return null;
        }

        // never overflows because it's less than maxFrameLength
        int frameLengthInt = (int) frameLength;
        if (buffer.readableBytes() < frameLengthInt) {
            return null;
        }

        if (initialBytesToStrip > frameLengthInt) {
            buffer.skipBytes(frameLengthInt);
            throw new CorruptedFrameException(
                    "Adjusted frame length (" + frameLength + ") is less " +
                    "than initialBytesToStrip: " + initialBytesToStrip);
        }
        buffer.skipBytes(initialBytesToStrip);

        // extract frame
        int readerIndex = buffer.readerIndex();
        int actualFrameLength = frameLengthInt - initialBytesToStrip;
        ChannelBuffer frame = extractFrame(buffer, readerIndex, actualFrameLength);
        buffer.readerIndex(readerIndex + actualFrameLength);
        return frame;
    }

    private void failIfNecessary(ChannelHandlerContext ctx, boolean firstDetectionOfTooLongFrame) {
        if (bytesToDiscard == 0) {
            // Reset to the initial state and tell the handlers that
            // the frame was too large.
            long tooLongFrameLength = this.tooLongFrameLength;
            this.tooLongFrameLength = 0;
            discardingTooLongFrame = false;
            if (!failFast ||
                failFast && firstDetectionOfTooLongFrame) {
                fail(ctx, tooLongFrameLength);
            }
        } else {
            // Keep discarding and notify handlers if necessary.
            if (failFast && firstDetectionOfTooLongFrame) {
                fail(ctx, tooLongFrameLength);
            }
        }

    }

    /**
     * Extract the sub-region of the specified buffer. This method is called by
     * {@link #decode(ChannelHandlerContext, Channel, ChannelBuffer)} for each
     * frame.  The default implementation returns a copy of the sub-region.
     * For example, you could override this method to use an alternative
     * {@link ChannelBufferFactory}.
     * <p>
     * If you are sure that the frame and its content are not accessed after
     * the current {@link #decode(ChannelHandlerContext, Channel, ChannelBuffer)}
     * call returns, you can even avoid memory copy by returning the sliced
     * sub-region (i.e. <tt>return buffer.slice(index, length)</tt>).
     * It's often useful when you convert the extracted frame into an object.
     * Refer to the source code of {@link ObjectDecoder} to see how this method
     * is overridden to avoid memory copy.
     */
    protected ChannelBuffer extractFrame(ChannelBuffer buffer, int index, int length) {
        ChannelBuffer frame = buffer.factory().getBuffer(length);
        frame.writeBytes(buffer, index, length);
        return frame;
    }

    private void fail(ChannelHandlerContext ctx, long frameLength) {
        if (frameLength > 0) {
            Channels.fireExceptionCaught(
                    ctx.getChannel(),
                    new TooLongFrameException(
                            "Adjusted frame length exceeds " + maxFrameLength +
                            ": " + frameLength + " - discarded"));
        } else {
            Channels.fireExceptionCaught(
                    ctx.getChannel(),
                    new TooLongFrameException(
                            "Adjusted frame length exceeds " + maxFrameLength +
                            " - discarding"));
        }
    }
}
