crc algorithm

dream4soul at gmail.com dream4soul at gmail.com
Wed Sep 3 08:19:07 CEST 2014


On Tuesday, September 2, 2014 9:24:54 PM UTC+3, Peter Otten wrote:
> 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.

this doesn't work 

calc_crc(b'\x00\x00\x34\x35\x38\x35')
rsult 0x9f41 , but c function gives us 0x8c40




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