dream4s...@gmail.com wrote:
> On Tuesday, September 2, 2014 9:24:54 PM UTC+3, Peter Otten wrote:
>> dream4s...@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
--
https://mail.python.org/mailman/listinfo/python-list
