/*
 * 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 io.netty.handler.codec.compression;

import io.netty.buffer.ByteBuf;
import io.netty.channel.ChannelHandlerContext;
import io.netty.handler.codec.ByteToMessageDecoder;

import java.util.List;

import static io.netty.handler.codec.compression.Snappy.validateChecksum;

Uncompresses a ByteBuf encoded with the Snappy framing format. See Snappy framing format. Note that by default, validation of the checksum header in each chunk is DISABLED for performance improvements. If performance is less of an issue, or if you would prefer the safety that checksum validation brings, please use the SnappyFrameDecoder(boolean) constructor with the argument set to true. /**
 * Uncompresses a {@link ByteBuf} encoded with the Snappy framing format.
 *
 * See <a href="https://github.com/google/snappy/blob/master/framing_format.txt">Snappy framing format</a>.
 *
 * Note that by default, validation of the checksum header in each chunk is
 * DISABLED for performance improvements. If performance is less of an issue,
 * or if you would prefer the safety that checksum validation brings, please
 * use the {@link #SnappyFrameDecoder(boolean)} constructor with the argument
 * set to {@code true}.
 */
public class SnappyFrameDecoder extends ByteToMessageDecoder {

    private enum ChunkType {
        STREAM_IDENTIFIER,
        COMPRESSED_DATA,
        UNCOMPRESSED_DATA,
        RESERVED_UNSKIPPABLE,
        RESERVED_SKIPPABLE
    }

    private static final int SNAPPY_IDENTIFIER_LEN = 6;
    private static final int MAX_UNCOMPRESSED_DATA_SIZE = 65536 + 4;

    private final Snappy snappy = new Snappy();
    private final boolean validateChecksums;

    private boolean started;
    private boolean corrupted;

    
Creates a new snappy-framed decoder with validation of checksums turned OFF. To turn checksum validation on, please use the alternate SnappyFrameDecoder(boolean) constructor. /**
     * Creates a new snappy-framed decoder with validation of checksums
     * turned OFF. To turn checksum validation on, please use the alternate
     * {@link #SnappyFrameDecoder(boolean)} constructor.
     */
    public SnappyFrameDecoder() {
        this(false);
    }

    
Creates a new snappy-framed decoder with validation of checksums
as specified.
Params:
validateChecksums –  If true, the checksum field will be validated against the actual uncompressed data, and if the checksums do not match, a suitable DecompressionException will be thrown/**
     * Creates a new snappy-framed decoder with validation of checksums
     * as specified.
     *
     * @param validateChecksums
     *        If true, the checksum field will be validated against the actual
     *        uncompressed data, and if the checksums do not match, a suitable
     *        {@link DecompressionException} will be thrown
     */
    public SnappyFrameDecoder(boolean validateChecksums) {
        this.validateChecksums = validateChecksums;
    }

    @Override
    protected void decode(ChannelHandlerContext ctx, ByteBuf in, List<Object> out) throws Exception {
        if (corrupted) {
            in.skipBytes(in.readableBytes());
            return;
        }

        try {
            int idx = in.readerIndex();
            final int inSize = in.readableBytes();
            if (inSize < 4) {
                // We need to be at least able to read the chunk type identifier (one byte),
                // and the length of the chunk (3 bytes) in order to proceed
                return;
            }

            final int chunkTypeVal = in.getUnsignedByte(idx);
            final ChunkType chunkType = mapChunkType((byte) chunkTypeVal);
            final int chunkLength = in.getUnsignedMediumLE(idx + 1);

            switch (chunkType) {
                case STREAM_IDENTIFIER:
                    if (chunkLength != SNAPPY_IDENTIFIER_LEN) {
                        throw new DecompressionException("Unexpected length of stream identifier: " + chunkLength);
                    }

                    if (inSize < 4 + SNAPPY_IDENTIFIER_LEN) {
                        break;
                    }

                    in.skipBytes(4);
                    int offset = in.readerIndex();
                    in.skipBytes(SNAPPY_IDENTIFIER_LEN);

                    checkByte(in.getByte(offset++), (byte) 's');
                    checkByte(in.getByte(offset++), (byte) 'N');
                    checkByte(in.getByte(offset++), (byte) 'a');
                    checkByte(in.getByte(offset++), (byte) 'P');
                    checkByte(in.getByte(offset++), (byte) 'p');
                    checkByte(in.getByte(offset), (byte) 'Y');

                    started = true;
                    break;
                case RESERVED_SKIPPABLE:
                    if (!started) {
                        throw new DecompressionException("Received RESERVED_SKIPPABLE tag before STREAM_IDENTIFIER");
                    }

                    if (inSize < 4 + chunkLength) {
                        // TODO: Don't keep skippable bytes
                        return;
                    }

                    in.skipBytes(4 + chunkLength);
                    break;
                case RESERVED_UNSKIPPABLE:
                    // The spec mandates that reserved unskippable chunks must immediately
                    // return an error, as we must assume that we cannot decode the stream
                    // correctly
                    throw new DecompressionException(
                            "Found reserved unskippable chunk type: 0x" + Integer.toHexString(chunkTypeVal));
                case UNCOMPRESSED_DATA:
                    if (!started) {
                        throw new DecompressionException("Received UNCOMPRESSED_DATA tag before STREAM_IDENTIFIER");
                    }
                    if (chunkLength > MAX_UNCOMPRESSED_DATA_SIZE) {
                        throw new DecompressionException("Received UNCOMPRESSED_DATA larger than 65540 bytes");
                    }

                    if (inSize < 4 + chunkLength) {
                        return;
                    }

                    in.skipBytes(4);
                    if (validateChecksums) {
                        int checksum = in.readIntLE();
                        validateChecksum(checksum, in, in.readerIndex(), chunkLength - 4);
                    } else {
                        in.skipBytes(4);
                    }
                    out.add(in.readRetainedSlice(chunkLength - 4));
                    break;
                case COMPRESSED_DATA:
                    if (!started) {
                        throw new DecompressionException("Received COMPRESSED_DATA tag before STREAM_IDENTIFIER");
                    }

                    if (inSize < 4 + chunkLength) {
                        return;
                    }

                    in.skipBytes(4);
                    int checksum = in.readIntLE();
                    ByteBuf uncompressed = ctx.alloc().buffer();
                    try {
                        if (validateChecksums) {
                            int oldWriterIndex = in.writerIndex();
                            try {
                                in.writerIndex(in.readerIndex() + chunkLength - 4);
                                snappy.decode(in, uncompressed);
                            } finally {
                                in.writerIndex(oldWriterIndex);
                            }
                            validateChecksum(checksum, uncompressed, 0, uncompressed.writerIndex());
                        } else {
                            snappy.decode(in.readSlice(chunkLength - 4), uncompressed);
                        }
                        out.add(uncompressed);
                        uncompressed = null;
                    } finally {
                        if (uncompressed != null) {
                            uncompressed.release();
                        }
                    }
                    snappy.reset();
                    break;
            }
        } catch (Exception e) {
            corrupted = true;
            throw e;
        }
    }

    private static void checkByte(byte actual, byte expect) {
        if (actual != expect) {
            throw new DecompressionException("Unexpected stream identifier contents. Mismatched snappy " +
                    "protocol version?");
        }
    }

    
Decodes the chunk type from the type tag byte.
Params:
type – The tag byte extracted from the stream
Returns:
The appropriate ChunkType, defaulting to ChunkType.RESERVED_UNSKIPPABLE/**
     * Decodes the chunk type from the type tag byte.
     *
     * @param type The tag byte extracted from the stream
     * @return The appropriate {@link ChunkType}, defaulting to {@link ChunkType#RESERVED_UNSKIPPABLE}
     */
    private static ChunkType mapChunkType(byte type) {
        if (type == 0) {
            return ChunkType.COMPRESSED_DATA;
        } else if (type == 1) {
            return ChunkType.UNCOMPRESSED_DATA;
        } else if (type == (byte) 0xff) {
            return ChunkType.STREAM_IDENTIFIER;
        } else if ((type & 0x80) == 0x80) {
            return ChunkType.RESERVED_SKIPPABLE;
        } else {
            return ChunkType.RESERVED_UNSKIPPABLE;
        }
    }
}