crc algorithm

Peter Otten __peter__ at web.de
Tue Sep 2 20:24:54 CEST 2014


dream4soul at gmail.com wrote:

> I have trouble to implement crc algorithm in python 3.3
> 
> c version work  perfect. I try to use bytes, int and c_types without any
> success can some who help me:

ctypes is for interfacing with C; don't use it in regular code.

> c version:
> 
> unsigned short calc_crc(const void *p_dat, int l_dat){
>         unsigned char *dat_ptr;
>         int loopc;
>         unsigned short crc_dat;
>         unsigned char c_work;
> 
>         dat_ptr = (unsigned char*)p_dat;
>         crc_dat = 0x0000;
>         for (; l_dat > 0; l_dat--)
>         {
>                 c_work = *(dat_ptr++);
>                 for (loopc = 0; loopc < 8; loopc++)
>                 {
>                         if ((((unsigned char )(crc_dat & 0x0001)) ^
>                         (c_work & 0x01)) == 0x01)
>                         {
>                                 crc_dat >>=1 ;
>                                 crc_dat ^=0x8408;
>                         } else {
>                                 crc_dat >>=1;
> 
>                         }
>                         c_work >>=1;
>                 }
>         }
>         return(crc_dat);
> }

A near-literal translation would be:

def calc_crc(data):
    crc = 0
    for work in data:
        for i in range(8):
            if (crc & 1) ^ (work & 1):
                crc >>= 1
                crc ^= 0x8408
            else:
                crc >>= 1
            work >>= 1
    return crc

I don't see any operation where the "unboundedness" of Python's integer type 
could be a problem -- but no guarantees.




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