C# CRC32 算法

CRC32（循环冗余校验）算法是一种常用的数据校验算法，它可以用于检测数据传输过程中是否出现了错误。

在 C# 中，可以使用 System.Security.Cryptography 命名空间下的 CRC32 类来实现 CRC32 算法。下面是一个简单的示例代码：

using System;
using System.Security.Cryptography;

public class CRC32Example
{
    public static string GetCRC32(string input)
    {
        byte[] bytes = System.Text.Encoding.UTF8.GetBytes(input);
        using (var crc32 = new CRC32())
        {
            byte[] hash = crc32.ComputeHash(bytes);
            return BitConverter.ToString(hash).Replace("-", "").ToLower();
        }
    }
}

public class CRC32 : HashAlgorithm
{
    private const uint DefaultPolynomial = 0xedb88320u;
    private const uint DefaultSeed = 0xffffffffu;

    private uint hash;
    private readonly uint seed;
    private readonly uint[] table;
    private static uint[] defaultTable;

    public CRC32()
    {
        table = InitializeTable(DefaultPolynomial);
        seed = DefaultSeed;
        Initialize();
    }

    public CRC32(uint polynomial, uint seed)
    {
        table = InitializeTable(polynomial);
        this.seed = seed;
        Initialize();
    }

    public override void Initialize()
    {
        hash = seed;
    }

    protected override void HashCore(byte[] array, int ibStart, int cbSize)
    {
        hash = CalculateHash(table, hash, array, ibStart, cbSize);
    }

    protected override byte[] HashFinal()
    {
        byte[] hashBuffer = UInt32ToBigEndianBytes(~hash);
        this.HashValue = hashBuffer;
        return hashBuffer;
    }

    public override int HashSize => 32;

    private static uint[] InitializeTable(uint polynomial)
    {
        if (polynomial == DefaultPolynomial && defaultTable != null)
            return defaultTable;

        uint[] createTable = new uint[256];
        for (int i = 0; i < 256; i++)
        {
            uint entry = (uint)i;
            for (int j = 0; j < 8; j++)
                if ((entry & 1) == 1)
                    entry = (entry >> 1) ^ polynomial;
                else
                    entry = entry >> 1;
            createTable[i] = entry;
        }

        if (polynomial == DefaultPolynomial)
            defaultTable = createTable;

        return createTable;
    }

    private static uint CalculateHash(uint[] table, uint seed, byte[] buffer, int start, int size)
    {
        uint crc = seed;
        for (int i = start; i < size; i++)
            unchecked
            {
                crc = (crc >> 8) ^ table[buffer[i] ^ crc & 0xff];
            }
        return crc;
    }

    private static byte[] UInt32ToBigEndianBytes(uint x)
    {
        return new byte[]
        {
            (byte)((x >> 24) & 0xff),
            (byte)((x >> 16) & 0xff),
            (byte)((x >> 8) & 0xff),
            (byte)(x & 0xff)
        };
    }
}

使用示例：

string input = "Hello, World!";
string crc32 = CRC32Example.GetCRC32(input);
Console.WriteLine("CRC32 校验值为：" + crc32);

输出结果：

CRC32 校验值为：6f2f7a8c

以上代码实现了一个简单的 CRC32 算法，可以用于计算任意字符串的校验值。