如何配置CRC表的计算

Question

null

编辑：我应该如何单独控制每一个参数？我想了解init函数实际上是如何工作的，以及所有参数是如何实现的。

typedef unsigned char uint8_t;
typedef unsigned short crc;

crc  crcTable[256];
#define WIDTH  (8 * sizeof(crc))
#define TOPBIT (1 << (WIDTH - 1))
#define POLYNOMIAL 0x1021

template<typename t>
t reflect(t v)
{
    t r = 0;

    for (int i = 0; i < (8 * sizeof v); ++i)
    {
        r <<= 1;
        r |= v&1;
        v >>= 1;
    }

    return r;
}

void Init()
{
    crc  remainder;

    for (int dividend = 0; dividend < 256; ++dividend)
    {
        remainder = dividend << (WIDTH - 8);

        for (uint8_t bit = 8; bit > 0; --bit)
        {
            if (remainder & TOPBIT)
                remainder = (remainder << 1) ^ POLYNOMIAL;
            else
                remainder = (remainder << 1);
        }

        crcTable[dividend] = remainder;
    }
}

crc Calculate(const uint8_t *message, int nBytes, crc wOldCRC)
{
    uint8_t data;
    crc remainder = wOldCRC;

    for (int byte = 0; byte < nBytes; ++byte)
    {
        data = reflect(message[byte]) ^ (remainder >> (WIDTH - 8));
        remainder = crcTable[data] ^ (remainder << 8);
    }

    return reflect(remainder);
}

int main()
{
    crc expected = 0x6f91;
    uint8_t pattern[] = "123456789";

    Init();
    crc result = Calculate(pattern, 9, 0xFFFF);

    if (result != expected)
    {
        // this output is not relevant to the question, despite C++ tag
        printf("CRC 0x%04x wrong, expected 0x%04x\n", result, expected);
    }
}

Answer 1

而不是反映数据的进来，CRC的进来，CRC的出去，你只是反映多项式和运算。您只需要在编写代码时这样做一次。反射多项式为0x8408。

typedef unsigned char uint8_t;
typedef unsigned short crc;

crc  crcTable[256];
#define POLYNOMIAL 0x8408

void Init()
{
    crc  remainder;

    for (int dividend = 0; dividend < 256; ++dividend)
    {
        remainder = dividend;

        for (uint8_t bit = 8; bit > 0; --bit)
        {
            if (remainder & 1)
                remainder = (remainder >> 1) ^ POLYNOMIAL;
            else
                remainder = (remainder >> 1);
        }

        crcTable[dividend] = remainder;
    }
}

crc Calculate(const uint8_t *message, int nBytes, crc wOldCRC)
{
    uint8_t data;
    crc remainder = wOldCRC;

    for (int byte = 0; byte < nBytes; ++byte)
    {
        data = message[byte] ^ remainder;
        remainder = crcTable[data] ^ (remainder >> 8);
    }

    return remainder;
}

int main()
{
    crc expected = 0x6f91;
    uint8_t pattern[] = "123456789";

    Init();
    crc result = Calculate(pattern, 9, 0xFFFF);

    if (result != expected)
    {
        // this output is not relevant to the question, despite C++ tag
        printf("CRC 0x%04x wrong, expected 0x%04x\n", result, expected);
    }
}

在一般情况下，如果输入数据被反射，则反射如下面答案所示的多项式，在底部输入字节，检查低位以排除多项式，并向上移位。如果输入数据没有反映出来，那么就像问题中的代码一样，保持多项式不变，在顶部输入字节，检查高位，然后向下移动。

在几乎所有情况下，输出的反射与输入的反射是相同的。对于所有这些，不需要一点反向函数。如果输入和输出都没有反映，或者输入和输出都反映，则保持移位寄存器的结果不变。在RevEng站点编目的72个CRC中，只有一个CRC-12/3GPP(CRC-12/3GPP)的反射out不同于反射In(CRC-12/3GPP)。在这种情况下，您需要对输出进行位反转，因为输入没有被反映，而输出却被反映。