crc - OWI_ComputeCRC16 returning value 0 -

i checking out function owi_computecrc16() , when test function owi_computecrc16(0 >> 8, 0), return value 0.

/** * @brief    compute crc16 value of data set. * @details  function compute crc16 of indata using seed inital value crc. * @note     setting seed 0 computes crc16 of indata. * @note     passing return value of function seed argument computes crc16 value of longer string of data. * @bug      n/a * @warning  n/a * @param    unsigned char indata: 1 byte of data compute crc from. * @param    unsigned short seed: starting value of crc. * @return   crc16 of indata seed initial value. */ unsigned short owi_computecrc16(unsigned char indata, unsigned short seed) {     unsigned char bitsleft;     unsigned char temp;      (bitsleft = 8; bitsleft > 0; bitsleft--)     {         temp = ((seed ^ indata) & 0x01);         if (temp == 0)         {             seed >>= 1;         }         else         {             seed ^= 0x4002;             seed >>= 1;             seed |= 0x8000;         }         indata >>= 1;     }     return seed; } 

this making problems in legacy code have solve, function used parameters 0, 0 , sum of calculated crc values used.

example:

a) have data on example position = 0 key = 0 ... => owi_computecrc16(0 >> 8, 0) returns 0.

b) have data on example position = 0 key = 0 ... crc = 0. , have data record on example position = 1 key = 1 ... owi_computecrc16(1 >> 8, 1) returns 49345.

if sum on returned crc values, not know, if data position 0 included or not.

ok, think logical error , can solve it.

the question is: ok function, should calculate , return crc value, return value 0 (for possible given input values)?

p.s.: first question on stackoverflow, if did wrong or not understandable, please let me know. thanks.

the question is: ok function, should calculate , return crc value, return value 0 (for possible given input values)?

not any, sequences of bytes 0 return value. in fact 16-bit crc, should expect 0 crc for, on average, 1 out of every 65536 random sequences try.

this particular crc, no pre or post conditioning, give 0 crc empty sequence , sequence of zeros.

by way, odd implementation lot more operations needed. should replaced this:

unsigned short owi_computecrc16(unsigned char indata, unsigned short seed) {     seed ^= indata;     (int k = 0; k < 8; k++)         seed = seed & 1 ? (seed >> 1) ^ 0xa001 : seed >> 1;     return seed; } 

that doesn't obfuscate crc polynomial, in version familiar (reflected) 0xa001 or (not reflected) polynomial 0x8005, x¹⁶+x¹⁵+x²+1. faster still byte-wise or word-wise table-driven version. see crcany.c examples.