On Wednesday, September 3, 2014 10:19:29 AM UTC+3, Peter Otten wrote:
> 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
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> >>
>
> >> > success can some who help me:
>
> >>
>
> >>
>
> >>
>
> >> ctypes is for interfacing with C; don't use it in regular code.
>
> >>
>
> >>
>
> >>
>
> >> > c version:
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> >>
>
> >> >
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> >>
>
> >> > unsigned short calc_crc(const void *p_dat, int l_dat){
>
> >>
>
> >> > unsigned char *dat_ptr;
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> >>
>
> >> > int loopc;
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> >>
>
> >> > unsigned short crc_dat;
>
> >>
>
> >> > unsigned char c_work;
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> >>
>
> >> >
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> >>
>
> >> > dat_ptr = (unsigned char*)p_dat;
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> >>
>
> >> > crc_dat = 0x0000;
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> >>
>
> >> > for (; l_dat > 0; l_dat--)
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> >>
>
> >> > {
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> >>
>
> >> > c_work = *(dat_ptr++);
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> >>
>
> >> > for (loopc = 0; loopc < 8; loopc++)
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> >>
>
> >> > {
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> >>
>
> >> > if ((((unsigned char )(crc_dat & 0x0001)) ^
>
> >>
>
> >> > (c_work & 0x01)) == 0x01)
>
> >>
>
> >> > {
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> >>
>
> >> > crc_dat >>=1 ;
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> >>
>
> >> > crc_dat ^=0x8408;
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> >>
>
> >> > } else {
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> >>
>
> >> > crc_dat >>=1;
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> >>
>
> >> >
>
> >>
>
> >> > }
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> >>
>
> >> > c_work >>=1;
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> >>
>
> >> > }
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> >>
>
> >> > }
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> >>
>
> >> > return(crc_dat);
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> >>
>
> >> > }
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> >>
>
> >>
>
> >>
>
> >> A near-literal translation would be:
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> >>
>
> >>
>
> >>
>
> >> def calc_crc(data):
>
> >>
>
> >> crc = 0
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> >>
>
> >> for work in data:
>
> >>
>
> >> for i in range(8):
>
> >>
>
> >> if (crc & 1) ^ (work & 1):
>
> >>
>
> >> crc >>= 1
>
> >>
>
> >> crc ^= 0x8408
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> >>
>
> >> else:
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> >>
>
> >> crc >>= 1
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> >>
>
> >> work >>= 1
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> >>
>
> >> return crc
>
> >>
>
> >>
>
> >>
>
> >> I don't see any operation where the "unboundedness" of Python's integer
>
> >> type
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> >>
>
> >> 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
int main(int argc, char const *argv[])
{
unsigned short rez;
unsigned char a[]={0x30,0x30,0x34,0x35,0x38,0x35};
unsigned short val;
rez=calc_crc(a,(int)sizeof(a));
printf("%#hx\n",rez );
return 0;
}
o$ gcc main.c
o$ ./a.out
0x8c40
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