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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);
    }
}