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

[Tiger12506]

>> i have a c function from some modbus documentation that i need to
>> translate into python.

>> 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) ;
>> }

I found this link which may provide some insight into what's going on here.
(google "modbus CRC16")

http://www.modbustools.com/modbus_crc16.htm

This proves to me that auchCRCHi is a lookup table that you do not have 
access to. Happily :-) that link provides the table.

Hmmm... let's see, the difficult C stuff... *puchMsgg++ means to return the 
current character in a string, and then increment the pointer, so that when 
the C code encounters *puchMsgg++ again it reads the next character, 
increments, etc. You can emulate this with an index and array notation in 
python.

 ^ ,  <<   , and | are all bitwise operators, and python uses all of these 
in the same way as C

'^' means XOR exclusive OR.
0101 ^ 0011 = 0110    i.e.  5 ^ 3 = 6

'<< ' means left - shift
0010 << 2 = 1000  i.e. a << b = a * (2**b)

'|' means OR.
0101 ^ 0011 = 0111   i.e. 5 ^ 3 = 7

puchMsgg   is basically a string
and all the unsigned stuff are (very roughly) integers.

HTH,
Jacob S.