Groups | Search | Server Info | Keyboard shortcuts | Login | Register [http] [https] [nntp] [nntps]

Groups > comp.lang.python > #77447

Re: crc algorithm

Show all headers | View raw

dream4soul@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.

Back to comp.lang.python | Previous | Next — Previous in thread | Next in thread | Find similar | Unroll thread

Thread

crc algorithm dream4soul@gmail.com - 2014-09-02 10:50 -0700
  Re: crc algorithm Peter Otten <__peter__@web.de> - 2014-09-02 20:24 +0200
    Re: crc algorithm dream4soul@gmail.com - 2014-09-02 23:19 -0700
      Re: crc algorithm Peter Otten <__peter__@web.de> - 2014-09-03 09:19 +0200
        Re: crc algorithm dream4soul@gmail.com - 2014-09-03 01:43 -0700
        Re: crc algorithm dream4soul@gmail.com - 2014-09-03 01:46 -0700
          Re: crc algorithm Peter Otten <__peter__@web.de> - 2014-09-03 11:00 +0200
            Re: crc algorithm dream4soul@gmail.com - 2014-09-03 04:00 -0700
            Re: crc algorithm dream4soul@gmail.com - 2014-09-03 07:31 -0700
      Re: crc algorithm Mark Lawrence <breamoreboy@yahoo.co.uk> - 2014-09-03 08:22 +0100
  Re: crc algorithm Chris Kaynor <ckaynor@zindagigames.com> - 2014-09-02 11:43 -0700
    Re: crc algorithm dream4soul@gmail.com - 2014-09-02 23:25 -0700

csiph-web