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Re: crc algorithm

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From Peter Otten <__peter__@web.de>
Subject Re: crc algorithm
Date Wed, 03 Sep 2014 09:19:29 +0200
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Xref csiph.com comp.lang.python:77469

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dream4soul@gmail.com wrote:

> On Tuesday, September 2, 2014 9:24:54 PM UTC+3, Peter Otten wrote:
>> 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.
> 
> this doesn't work
> 
> calc_crc(b'\x00\x00\x34\x35\x38\x35')
> rsult 0x9f41 , but c function gives us 0x8c40

Are you sure? I get 0x9f41 with the C version you posted:

$ cat crc.c
#include <stdio.h>

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);
}

main()
{
  unsigned char data[] = "\x00\x00\x34\x35\x38\x35";
  unsigned short crc = calc_crc(data, 6);
  printf("%x\n", crc);
}
$ gcc crc.c
$ ./a.out 
9f41

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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

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