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The Apache Cassandra Project develops a highly scalable second-generation distributed database, bringing together Dynamo's fully distributed design and Bigtable's ColumnFamily-based data model.
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/*
 * Licensed to the Apache Software Foundation (ASF) under one
 * or more contributor license agreements.  See the NOTICE file
 * distributed with this work for additional information
 * regarding copyright ownership.  The ASF 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.apache.cassandra.net;

import java.io.IOException;
import java.nio.ByteBuffer;
import java.util.zip.CRC32;

import io.netty.buffer.ByteBuf;
import io.netty.util.concurrent.FastThreadLocal;

public class Crc
{
    private static final FastThreadLocal crc32 = new FastThreadLocal()
    {
        @Override
        protected CRC32 initialValue()
        {
            return new CRC32();
        }
    };

    private static final byte[] initialBytes = new byte[] { (byte) 0xFA, (byte) 0x2D, (byte) 0x55, (byte) 0xCA };

    public static final class InvalidCrc extends IOException
    {
        public InvalidCrc(int read, int computed)
        {
            super(String.format("Read %d, Computed %d", read, computed));
        }
    }

    static CRC32 crc32()
    {
        CRC32 crc = crc32.get();
        crc.reset();
        crc.update(initialBytes);
        return crc;
    }

    static int computeCrc32(ByteBuf buffer, int startReaderIndex, int endReaderIndex)
    {
        CRC32 crc = crc32();
        crc.update(buffer.internalNioBuffer(startReaderIndex, endReaderIndex - startReaderIndex));
        return (int) crc.getValue();
    }

    static int computeCrc32(ByteBuffer buffer, int start, int end)
    {
        CRC32 crc = crc32();
        updateCrc32(crc, buffer, start, end);
        return (int) crc.getValue();
    }

    static void updateCrc32(CRC32 crc, ByteBuffer buffer, int start, int end)
    {
        int savePosition = buffer.position();
        int saveLimit = buffer.limit();
        buffer.limit(end);
        buffer.position(start);
        crc.update(buffer);
        buffer.limit(saveLimit);
        buffer.position(savePosition);
    }

    private static final int CRC24_INIT = 0x875060;
    /**
     * Polynomial chosen from https://users.ece.cmu.edu/~koopman/crc/index.html, by Philip Koopman
     *
     * This webpage claims a copyright to Philip Koopman, which he licenses under the
     * Creative Commons Attribution 4.0 International License (https://creativecommons.org/licenses/by/4.0)
     *
     * It is unclear if this copyright can extend to a 'fact' such as this specific number, particularly
     * as we do not use Koopman's notation to represent the polynomial, but we anyway attribute his work and
     * link the terms of his license since they are not incompatible with our usage and we greatly appreciate his work.
     *
     * This polynomial provides hamming distance of 8 for messages up to length 105 bits;
     * we only support 8-64 bits at present, with an expected range of 40-48.
     */
    private static final int CRC24_POLY = 0x1974F0B;

    /**
     * NOTE: the order of bytes must reach the wire in the same order the CRC is computed, with the CRC
     * immediately following in a trailer.  Since we read in least significant byte order, if you
     * write to a buffer using putInt or putLong, the byte order will be reversed and
     * you will lose the guarantee of protection from burst corruptions of 24 bits in length.
     *
     * Make sure either to write byte-by-byte to the wire, or to use Integer/Long.reverseBytes if you
     * write to a BIG_ENDIAN buffer.
     *
     * See http://users.ece.cmu.edu/~koopman/pubs/ray06_crcalgorithms.pdf
     *
     * Complain to the ethernet spec writers, for having inverse bit to byte significance order.
     *
     * Note we use the most naive algorithm here.  We support at most 8 bytes, and typically supply
     * 5 or fewer, so any efficiency of a table approach is swallowed by the time to hit L3, even
     * for a tiny (4bit) table.
     *
     * @param bytes an up to 8-byte register containing bytes to compute the CRC over
     *              the bytes AND bits will be read least-significant to most significant.
     * @param len   the number of bytes, greater than 0 and fewer than 9, to be read from bytes
     * @return      the least-significant bit AND byte order crc24 using the CRC24_POLY polynomial
     */
    static int crc24(long bytes, int len)
    {
        int crc = CRC24_INIT;
        while (len-- > 0)
        {
            crc ^= (bytes & 0xff) << 16;
            bytes >>= 8;

            for (int i = 0; i < 8; i++)
            {
                crc <<= 1;
                if ((crc & 0x1000000) != 0)
                    crc ^= CRC24_POLY;
            }
        }
        return crc;
    }
}