CRC-16

CRC-16 IBM  x ¹⁶ + x ¹⁵ + x ² + 1

CRC编码的原理，网上有很多，简单的说就是将数据用生成式进行模2除法。

这里我主要写一下，查表法的原理。

一、原理：

数据项：

先查表获得Bn的两字节余式，将该余式的高8位与Bn-1进行模2运算，假定B'n-1,低8位与Bn-2进行模2运算，假定结构为B'n-2，得到< B'n-1B'n-2Bn-3,...,B3B2B1 >,以此类推，最终获得两字节的CRC码.(参考字节型CRC算法实现一文)

二、实现

以下代码中中的查表法来自于 Linux-2.6.17，定义法是根据定义编写的，我使用随机数对两种方法进行了比对，结果一致

#include
#include
#include
typedef unsigned int u16;
typedef unsigned char u8;
u16 const crc16_table[256] = {
        0x0000, 0xC0C1, 0xC181, 0x0140, 0xC301, 0x03C0, 0x0280, 0xC241,
        0xC601, 0x06C0, 0x0780, 0xC741, 0x0500, 0xC5C1, 0xC481, 0x0440,
        0xCC01, 0x0CC0, 0x0D80, 0xCD41, 0x0F00, 0xCFC1, 0xCE81, 0x0E40,
        0x0A00, 0xCAC1, 0xCB81, 0x0B40, 0xC901, 0x09C0, 0x0880, 0xC841,
        0xD801, 0x18C0, 0x1980, 0xD941, 0x1B00, 0xDBC1, 0xDA81, 0x1A40,
        0x1E00, 0xDEC1, 0xDF81, 0x1F40, 0xDD01, 0x1DC0, 0x1C80, 0xDC41,
        0x1400, 0xD4C1, 0xD581, 0x1540, 0xD701, 0x17C0, 0x1680, 0xD641,
        0xD201, 0x12C0, 0x1380, 0xD341, 0x1100, 0xD1C1, 0xD081, 0x1040,
        0xF001, 0x30C0, 0x3180, 0xF141, 0x3300, 0xF3C1, 0xF281, 0x3240,
        0x3600, 0xF6C1, 0xF781, 0x3740, 0xF501, 0x35C0, 0x3480, 0xF441,
        0x3C00, 0xFCC1, 0xFD81, 0x3D40, 0xFF01, 0x3FC0, 0x3E80, 0xFE41,
        0xFA01, 0x3AC0, 0x3B80, 0xFB41, 0x3900, 0xF9C1, 0xF881, 0x3840,
        0x2800, 0xE8C1, 0xE981, 0x2940, 0xEB01, 0x2BC0, 0x2A80, 0xEA41,
        0xEE01, 0x2EC0, 0x2F80, 0xEF41, 0x2D00, 0xEDC1, 0xEC81, 0x2C40,
        0xE401, 0x24C0, 0x2580, 0xE541, 0x2700, 0xE7C1, 0xE681, 0x2640,
        0x2200, 0xE2C1, 0xE381, 0x2340, 0xE101, 0x21C0, 0x2080, 0xE041,
        0xA001, 0x60C0, 0x6180, 0xA141, 0x6300, 0xA3C1, 0xA281, 0x6240,
        0x6600, 0xA6C1, 0xA781, 0x6740, 0xA501, 0x65C0, 0x6480, 0xA441,
        0x6C00, 0xACC1, 0xAD81, 0x6D40, 0xAF01, 0x6FC0, 0x6E80, 0xAE41,
        0xAA01, 0x6AC0, 0x6B80, 0xAB41, 0x6900, 0xA9C1, 0xA881, 0x6840,
        0x7800, 0xB8C1, 0xB981, 0x7940, 0xBB01, 0x7BC0, 0x7A80, 0xBA41,
        0xBE01, 0x7EC0, 0x7F80, 0xBF41, 0x7D00, 0xBDC1, 0xBC81, 0x7C40,
        0xB401, 0x74C0, 0x7580, 0xB541, 0x7700, 0xB7C1, 0xB681, 0x7640,
        0x7200, 0xB2C1, 0xB381, 0x7340, 0xB101, 0x71C0, 0x7080, 0xB041,
        0x5000, 0x90C1, 0x9181, 0x5140, 0x9301, 0x53C0, 0x5280, 0x9241,
        0x9601, 0x56C0, 0x5780, 0x9741, 0x5500, 0x95C1, 0x9481, 0x5440,
        0x9C01, 0x5CC0, 0x5D80, 0x9D41, 0x5F00, 0x9FC1, 0x9E81, 0x5E40,
        0x5A00, 0x9AC1, 0x9B81, 0x5B40, 0x9901, 0x59C0, 0x5880, 0x9841,
        0x8801, 0x48C0, 0x4980, 0x8941, 0x4B00, 0x8BC1, 0x8A81, 0x4A40,
        0x4E00, 0x8EC1, 0x8F81, 0x4F40, 0x8D01, 0x4DC0, 0x4C80, 0x8C41,
        0x4400, 0x84C1, 0x8581, 0x4540, 0x8701, 0x47C0, 0x4680, 0x8641,
        0x8201, 0x42C0, 0x4380, 0x8341, 0x4100, 0x81C1, 0x8081, 0x4040
};

static inline u16 crc16_byte(u16 crc, const u8 data)
{
        return (crc >> 8) ^ crc16_table[(crc ^ data) & 0xff];
}

/**
 * crc16 - compute the CRC-16 for the data buffer
 * @crc:        previous CRC value
 * @buffer:     data pointer
 * @len:        number of bytes in the buffer
 *
 * Returns the updated CRC value.
 */
u16 crc16(u16 crc, u8 const *buffer, u16 len)
{
        while (len--)
                crc = crc16_byte(crc, *buffer++);
        return crc;
}
/**
 * crc16 - compute the CRC-16 for the data buffer according to the definition
 * @crc:        previous CRC value
 * @buffer:     data pointer
 * @len:        number of bytes in the buffer
 *
 * Returns the updated CRC value.
 */
u16 crc16_calculate(u16 crc,u8 const *buffer,u16 len)
{
    u16 i,j;
    u8 data;
    for ( j = 0; j < len; j++)
    {
         data= buffer[j];
        for ( i = 0; i < 8; i++)
        {
            crc = ((data ^ (u8)crc) & 1) ? ((crc >> 1) ^ 0xA001) : (crc >> 1);
            data>>= 1;
        }
    }
    return crc;
}

int main()
{
      u16 crc1,crc2;
      u8 test[32] ;
      u8 i;
      while(1)
      {
      for(i=0;i<32;i++)
      test[i]=rand();

      crc1=0;
      crc2=0;
     // crc=crc16(0,test,6);
     crc1=crc16(crc1,test,6);
     printf("CRC=%d/n",crc1);
      crc2=crc16_calculate(crc2,test,6);
      printf("CRC=%d/n",crc2);
      if(crc1!=crc2)
      break;
      }
      return 0;
}