C＃中的CRC32失败

Question

I was trying to implement my own CRC32 function in C#. I saw an elegant solution in JS here JavaScript CRC32 So I came up with this:

internal static class Crc32
    {
        internal static long CalculateCrc32(string str)
        {
            long[] crcTable = Crc32.MakeCrcTable();
            long crc = 0 ^ (-1);

            for (int i = 0; i < str.Length; i++)
            {
                char c = str[i];
                crc = (crc >> 8) ^ crcTable[(crc ^ c) & 0xFF];
            }

            return ~crc; //(crc ^ (-1)) >> 0;
        }

        internal static long[] MakeCrcTable()
        {
            long c;
            long[] crcTable = new long[256];
            for (int n = 0; n < 256; n++)
            {
                c = n;
                for (int k = 0; k < 8; k++)
                {
                    var res = c & 1;
                    c = (res == 1) ? (0xEDB88320 ^ (c >> 1)) : (c >> 1);
                }
                crcTable[n] = c;
            }

            return crcTable;
        }
    }

The problem is that my solution does not return the same result. Console.WriteLine(Crc32.CalculateCrc32("l")); results in 1762050814 while the JS function produces 2517025534. The JS result is also the correct one. What am I doing wrong?

Answer 1

The problem here is that you are using the wrong datatype. I'm not familiar with the CRC32 algorithm so I googled to find http://sanity-free.org/12/crc32_implementation_in_csharp.html as a reference implementation.

the first thing I noticed is that they are using uint instead of long . This makes sense as I'd assume the 32 in CRC32 means that it would return a 32 bit number. long is a 64 bit signed int.

If you change all the long for uint then we nearly get a working program. The only line that won't work is c=n because it can't implicitly convert an int (n) to a uint (c). However since we know n is always going to be a positive integer we can just change n to be of type uint as well.

This leaves me with:

internal static uint CalculateCrc32(string str)
{
    uint[] crcTable = Crc32.MakeCrcTable();
    uint crc = 0xffffffff;
    for (int i = 0; i < str.Length; i++)
    {
        char c = str[i];
        crc = (crc >> 8) ^ crcTable[(crc ^ c) & 0xFF];
    }

    return ~crc; //(crc ^ (-1)) >> 0;
}

internal static uint[] MakeCrcTable()
{
    uint c;
    uint[] crcTable = new uint[256];
    for (uint n = 0; n < 256; n++)
    {
        c = n;
        for (int k = 0; k < 8; k++)
        {
            var res = c & 1;
            c = (res == 1) ? (0xEDB88320 ^ (c >> 1)) : (c >> 1);
        }
        crcTable[n] = c;
    }

    return crcTable;
}

With this code Console.WriteLine(Crc32.CalculateCrc32("l")); displays 2517025534 as expected.