请教CRC的问题

只看该作者 · 2007-11-22 09:44

//=============================================
//  关于CRC计算，到网上一搜，**一大把。经整理，得到如下几段代码。
//  用这几个函数算出来的值是不同的.其中1和2只是反转的关系。
//  按照对crc16的定义，只应存在两种算法。
//  那么3和4怎么理解？
//  代码3生成的CRC和MODEBUS协议吻合。
//  本人愚蠢，请指教
//=============================================
//  参数说明: 指针、数据长度、初值、多项式
//=============================================
//  Description: Calc CRC16.
//  Para: pointer of data. length of data, Init value, Poly
//  return: result
//=============================================
unsigned int CRC16_1(   unsigned char * puchMsg,
                        unsigned int usDataLen,
                        unsigned int CrcInit,
                        unsigned int Ploy )
{
    unsigned int crc;
    unsigned char i;

    crc = CrcInit;

    while( usDataLen-- )
    {
        for( i = 0x80; i != 0; i /= 2 )
        {
            if( ( crc & 0x8000 ) != 0 )
            {
                crc <<= 1;
                crc ^= Ploy;
            }
            else
                crc <<= 1;
            if( * puchMsg & i )
                crc ^= Ploy;
        }
        puchMsg++;
    }
    return( crc );
}
//=============================================
//  Description: Calc CRC16.
//  Para: pointer of data. length of data, Init value, Poly
//  return: result
//=============================================
unsigned int CRC16_2(   unsigned char * puchMsg,
                        unsigned int usDataLen,
                        unsigned int CrcInit,
                        unsigned int Ploy )
{
    unsigned int crc;
    unsigned char i;

    crc = CrcInit;

    while( usDataLen-- )
    {
        for( i = 0x01; i != 0; i *= 2 )
        {
            if( ( crc & 0x0001 ) != 0 )
            {
                crc >>= 1;
                crc ^= Ploy;
            }
            else
                crc >>= 1;
            if( * puchMsg & i )
                crc ^= Ploy;
        }
        puchMsg++;
    }
    return( crc );
}
//=============================================
//  Description: Calc CRC16.
//  Para: pointer of data. length of data, Init value, Poly
//  return: result
//=============================================
unsigned int CRC16_3(   unsigned char * puchMsg,
                        unsigned int usDataLen,
                        unsigned int CrcInit,
                        unsigned int Ploy )
{
    unsigned int Crc;
    int i = 0;
    unsigned char j = 0;

    Crc = CrcInit;

    for( i = 0; i < usDataLen; i++ )
    {
        Crc ^= puchMsg;
        for( j = 0; j < 8; j++ )
        {
            if( Crc & 0x0001 )
            {
                Crc >>= 1;
                Crc ^= Ploy;
            }
            else
            {
                Crc >>= 1;
            }
        }
    }
    return ( Crc );
}
//=============================================
//  Description: Calc CRC16.
//  Para: pointer of data. length of data, Init value, Poly
//  return: result
//=============================================
unsigned int CRC16_4(   unsigned char * puchMsg,
                        unsigned int usDataLen,
                        unsigned int CrcInit,
                        unsigned int Ploy )
{
    unsigned int Crc;
    int i = 0;
    unsigned char j = 0;

    Crc = CrcInit;

    for( i = 0; i < usDataLen; i++ )
    {
        Crc ^= puchMsg;
        for( j = 0; j < 8; j++ )
        {
            if( Crc & 0x0001 )
            {
                Crc >>= 1;
                Crc ^= Ploy;
            }
            else
            {
                Crc >>= 1;
            }
        }
    }
    return ( Crc );
}
//=============================================