[Tutor] help with translating a c function to a python function

[Alan Gauld]

"shawn bright" <nephish at gmail.com> wrote

> oh, here is what i have so far, but is not giving me the right 
> values
>
> def crc16(data):
>       crc_hi = 0xff
>       crc_lo = 0xff
>       for x in data:
>           crc_index = crc_hi ^ x

So far so good..

>           crc_hi = crc_lo ^ (crc_hi | crc_index)

But the C code here is:

>          uchCRCHi = uchCRCLo ^ auchCRCHi[uIndex} ;
>          crc_lo = crc_lo | crc_index

now I assume the a in front indicates an array - which
since its not defined locally must be global somewhere.
And they are indexing it So your code should be something
like:

crc_hi = crc_lo ^ CRC_hiList[crc_index]

>         uchCRCLo = auchCRCLo[uIndex] ;

and

crc_lo = CRC_loList[crc_index]

      return (crc_hi << 8 | crc_lo)

> whaddya think?

I think I'm guessing quite a lot and that your C function is
not truly standalone. :-)

But I think the corrupt operations are indexes into an array.
But where the array is defined is anyones guess.
And in C a pointer can also be an array so auchCRCHi
could be defined as

SOMETYPE *auchCRCHi

or

SOMETYPE auchCRCHi[n]

HTH,

Alan G.



On 7/5/07, shawn bright <nephish at gmail.com> wrote:
> hello all,
>
> i have a c function from some modbus documentation that i need to
> translate into python.
>
> it looks like this:
>
>
> unsigned short CRC16(puchMsg, usDataLen)
> unsigned char *puchMsg ;
> unsigned short usDataLen ;
> {
>     unsigned char uchCRCHi = 0xFF ;
>     unsigned char uchCRCLo = 0xFF ;
>     unsigned uIndex ;
>     while (usDataLen––)
>         {
>         uIndex = uchCRCHi ^ *puchMsgg++ ;
>         uchCRCHi = uchCRCLo ^ auchCRCHi[uIndex} ;
>         uchCRCLo = auchCRCLo[uIndex] ;
>         }
>     return (uchCRCHi << 8 | uchCRCLo) ;
> }
>
> some of it i can make out, but i can't seem to figgure out
> this part ' auchCRCHi[uIndex};
> it looks like a typo, because there is a closing } that does not 
> match
> the opening [.
>
> anyway, if someone could kinda give me a push, it would help me out 
> a lot.
> thanks
>
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