crc algorithm

Chris Kaynor ckaynor at zindagigames.com
Tue Sep 2 20:43:52 CEST 2014


Also, depending on the use-case, binascii.crc32 might also work fine:
https://docs.python.org/2/library/binascii.html#binascii.crc32

import binascii
def calc_crc(data):
    return binascii.crc32(data)

Much simpler.


Chris


On Tue, Sep 2, 2014 at 11:24 AM, Peter Otten <__peter__ at web.de> 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.
>
> --
> https://mail.python.org/mailman/listinfo/python-list
>
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