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The Netty project is an effort to provide an asynchronous event-driven network application framework and tools for rapid development of maintainable high performance and high scalability protocol servers and clients. In other words, Netty is a NIO client server framework which enables quick and easy development of network applications such as protocol servers and clients. It greatly simplifies and streamlines network programming such as TCP and UDP socket server.

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/*
 * 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 static org.jboss.netty.buffer.ChannelBuffers.*;

import java.nio.ByteOrder;

import org.jboss.netty.buffer.ChannelBuffer;
import org.jboss.netty.buffer.ChannelBufferFactory;
import org.jboss.netty.channel.Channel;
import org.jboss.netty.channel.ChannelHandler.Sharable;
import org.jboss.netty.channel.ChannelHandlerContext;
import org.jboss.netty.handler.codec.oneone.OneToOneEncoder;

/**
 * An encoder that prepends the length of the message.  The length value is
 * prepended as a binary form.  It is encoded in either big endian or little
 * endian depending on the default {@link ByteOrder} of the current
 * {@link ChannelBufferFactory}.
 * 
 * For example, {@link LengthFieldPrepender}(2) will encode the
 * following 12-bytes string:
 * 
 * +----------------+
 * | "HELLO, WORLD" |
 * +----------------+
 * 
 * into the following:
 *  * +--------+----------------+
 * + 0x000C | "HELLO, WORLD" |
 * +--------+----------------+
 * 
 * If you turned on the {@code lengthIncludesLengthFieldLength} flag in the
 * constructor, the encoded data would look like the following
 * (12 (original data) + 2 (prepended data) = 14 (0xE)):
 *  * +--------+----------------+
 * + 0x000E | "HELLO, WORLD" |
 * +--------+----------------+
 * 
 */
@Sharable
public class LengthFieldPrepender extends OneToOneEncoder {

    private final int lengthFieldLength;
    private final boolean lengthIncludesLengthFieldLength;

    /**
     * Creates a new instance.
     *
     * @param lengthFieldLength the length of the prepended length field.
     *                          Only 1, 2, 3, 4, and 8 are allowed.
     *
     * @throws IllegalArgumentException
     *         if {@code lengthFieldLength} is not 1, 2, 3, 4, or 8
     */
    public LengthFieldPrepender(int lengthFieldLength) {
        this(lengthFieldLength, false);
    }

    /**
     * Creates a new instance.
     *
     * @param lengthFieldLength the length of the prepended length field.
     *                          Only 1, 2, 3, 4, and 8 are allowed.
     * @param lengthIncludesLengthFieldLength
     *                          if {@code true}, the length of the prepended
     *                          length field is added to the value of the
     *                          prepended length field.
     *
     * @throws IllegalArgumentException
     *         if {@code lengthFieldLength} is not 1, 2, 3, 4, or 8
     */
    public LengthFieldPrepender(
            int lengthFieldLength, boolean lengthIncludesLengthFieldLength) {
        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);
        }

        this.lengthFieldLength = lengthFieldLength;
        this.lengthIncludesLengthFieldLength = lengthIncludesLengthFieldLength;
    }

    @Override
    protected Object encode(
            ChannelHandlerContext ctx, Channel channel, Object msg) throws Exception {
        if (!(msg instanceof ChannelBuffer)) {
            return msg;
        }

        ChannelBuffer body = (ChannelBuffer) msg;
        ChannelBuffer header = channel.getConfig().getBufferFactory().getBuffer(body.order(), lengthFieldLength);

        int length = lengthIncludesLengthFieldLength?
                body.readableBytes() + lengthFieldLength : body.readableBytes();
        switch (lengthFieldLength) {
        case 1:
            if (length >= 256) {
                throw new IllegalArgumentException(
                        "length does not fit into a byte: " + length);
            }
            header.writeByte((byte) length);
            break;
        case 2:
            if (length >= 65536) {
                throw new IllegalArgumentException(
                        "length does not fit into a short integer: " + length);
            }
            header.writeShort((short) length);
            break;
        case 3:
            if (length >= 16777216) {
                throw new IllegalArgumentException(
                        "length does not fit into a medium integer: " + length);
            }
            header.writeMedium(length);
            break;
        case 4:
            header.writeInt(length);
            break;
        case 8:
            header.writeLong(length);
            break;
        default:
            throw new Error("should not reach here");
        }
        return wrappedBuffer(header, body);
    }
}