计算CRC的逆(当数据长度等于CRC长度时)

 
// Demo_CRC.cpp

#include "stdafx.h"

#include <locale.h>

#include "CRC.h"

// 将一个字节逆序

unsigned char inv_bits(unsigned char c);

int _tmain(int argc, _TCHAR* argv[])

{

 setlocale(LC_ALL,"chs");

 _tprintf(_T("\nCRC逆计算的演示程序\n\n"));

 // 原始数据

 unsigned char data[] = {1,2};

 _tprintf(_T("原始数据: 0x%02X 0x%02X\n"), data[0], data[1]);

 // 初始crc

 unsigned short initCrc = 0;

 // 计算高位在前CRC

 CRC16_H crc_ccitt(0x1021);

 unsigned short crcH = crc_ccitt.do_crcH(initCrc, data, sizeof(data));

 unsigned char crc16_H[2] = {0};

 crc16_H[0] = HIBYTE(crcH);

 crc16_H[1] = LOBYTE(crcH);

 _tprintf(_T("crc_ccitt数据: 0x%02X 0x%02X\n"), crc16_H[0], crc16_H[1]);

 // 验证高位在前CRC, 拉通CRC为0

 unsigned char dataL[] =

 {

  // 原始数据

  data[0], data[1],

  // CRC

  crc16_H[0], crc16_H[1]

 };

 unsigned short crcCheckL = crc_ccitt.do_crcH(initCrc, dataL, sizeof(dataL));

 // 逆运算

 unsigned char inv_data[2] = {inv_bits(crc16_H[1]), inv_bits(crc16_H[0])};

 CRC16_H inv_ccitt(0x0811); // crc_ccitt的逆表达式为0x0811

 unsigned short inv_crcH = inv_ccitt.do_crcH(initCrc, inv_data, sizeof(inv_data));

 unsigned char inv_crc16_H[2] = {0};

 inv_crc16_H[0] = HIBYTE(inv_crcH);

 inv_crc16_H[1] = LOBYTE(inv_crcH);

 // CRC逆计算出的数据

 unsigned char inv_crc_Data[2] = {inv_bits(inv_crc16_H[1]), inv_bits(inv_crc16_H[0])};

 _tprintf(_T("根据CRC计算出的数据: 0x%02X 0x%02X\n"), inv_crc_Data[0], inv_crc_Data[1]);

 

 return 0;

}

// 将一个字节的bits逆序

// http://blog.csdn.net/jakee304/article/details/2152655

unsigned char inv_bits(unsigned char c)

{

 // 原始位序: 7 6 5 4 3 2 1 0

 // 位序变为: 6 7 4 5 2 3 0 1

 c = (c & 0xaa) >> 1 | (c & 0x55) << 1;

 

 // 位序变为: 4 5 6 7 0 1 2 3

 c = (c & 0xcc) >> 2 | (c & 0x33) << 2;

 

 // 位序变为: 0 1 2 3 4 5 6 7

 c = (c & 0xf0) >> 4 | (c & 0x0f) << 4;

 return c;

}

 

// CRC.h

#pragma once

class CRC16_H

{

public:

 CRC16_H(unsigned short polynomial)

 {

  // polynomial 多项式系数, 如CCITT-CRC16: x16+x12+x5+1, 则为0x1021

  for(int i=0; i<256; ++i)

  {

   crc16_tableH[i] = do_crcH(polynomial, (unsigned char)i);

  }

 }

 unsigned short do_crcH(unsigned short crcInit, unsigned char *message, unsigned int len)

 {

  unsigned short int crc = crcInit;

  while(len--)

  {

   crc = (crc<<8) ^ crc16_tableH[((crc>>8) ^ *message++) & 0xff];

  }

  return crc;

 }

private:

 unsigned short do_crcH(unsigned short polynomial, unsigned char message)

 {

  unsigned crc = 0;

  unsigned short bitTable[2] = {0, polynomial};

  for(int i=7; i>=0; --i)

  {

   crc = (crc<<1) ^ bitTable[((crc>>15) ^ (message>>i)) & 0x01];

  }

  return crc;

 }

 unsigned short crc16_tableH[256];

};