/*
* 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 java.util.zip.CRC32;
final class SnappyChecksumUtil {
/**
* Computes the CRC32 checksum of the supplied data, performs the "mask" operation
* on the computed checksum, and then compares the resulting masked checksum to the
* supplied checksum.
*
* @param slice The input data to calculate the CRC32 checksum of
* @param checksum The checksum decoded from the stream to compare against
* @throws CompressionException If the calculated and supplied checksums do not match
*/
static void validateChecksum(ByteBuf slice, int checksum) {
if (calculateChecksum(slice) != checksum) {
throw new CompressionException("Uncompressed data did not match checksum");
}
}
/**
* Computes the CRC32 checksum of the supplied data and performs the "mask" operation
* on the computed checksum
*
* @param slice The input data to calculate the CRC32 checksum of
*/
static int calculateChecksum(ByteBuf slice) {
CRC32 crc32 = new CRC32();
try {
byte[] array = new byte[slice.readableBytes()];
slice.markReaderIndex();
slice.readBytes(array);
slice.resetReaderIndex();
crc32.update(array);
return maskChecksum((int) crc32.getValue());
} finally {
crc32.reset();
}
}
/**
* From the spec:
*
* "Checksums are not stored directly, but masked, as checksumming data and
* then its own checksum can be problematic. The masking is the same as used
* in Apache Hadoop: Rotate the checksum by 15 bits, then add the constant
* 0xa282ead8 (using wraparound as normal for unsigned integers)."
*
* @param checksum The actual checksum of the data
* @return The masked checksum
*/
static int maskChecksum(int checksum) {
return (checksum >> 15 | checksum << 17) + 0xa282ead8;
}
// utility class
private SnappyChecksumUtil() { }
}