CRC.cpp

#include <assert.h>
#include <time.h>
#include <iostream>
#include <ctime>

unsigned int CalcCRC(const unsigned char* aInput, size_t aLength, const unsigned int* apTable)
{
        unsigned int CRC = 0;   
        for(size_t i=0; i<aLength; i++) {
                CRC = apTable[(CRC ^ aInput[i]) & 0xFF]^(CRC >> 8);
        }
        return (~CRC) & 0xFFFF;
}

void PrecomputeCRC(unsigned int* apTable)
{
        unsigned int i, j, CRC;

        for(i = 0; i < 256; i++) {
                CRC = i;
                for (j = 0; j < 8; ++j) {
                        if(CRC & 0x0001) CRC = (CRC >> 1) ^ 0xA6BC;
                        else CRC >>= 1;
                }
                apTable[i] = CRC;
        }
}

int main(int argc, char** argv)
{
        //setup the table and prove the crc is working correctly
        unsigned int table[256];
        PrecomputeCRC(table);
        unsigned char header[8] = {0x05, 0x64, 0x05, 0xC0, 0x01, 0x00, 0x00, 0x04};
        assert(CalcCRC(header, 8, table) == 0x21E9);

        size_t iter = 100*(1<<20);

        clock_t start = std::clock();
        for(size_t i=0; i<iter; ++i) CalcCRC(header, 8, table);
        clock_t stop = std::clock();

        std::cout << (stop-start) << " ms " << std::endl;

        return 0;
}