I needed to duplicate the Android CRC32 exactly in a separate desktop
C application, but the code that I found on-line did not exactly agree
with the Android version. So I did some trial and error tests and
developed the following C code which does agree with the Android CRC32
(assuming a 32-bit unsigned int in the desktop application):
unsigned int accum=0; //..equivalent to a CRC32.reset()
CRC32.update(byteValue) in Java is equivalent to:
accum ^= byteValue;
for(int i=0; i<8; i++)
{
unsigned int b = 0;
if(accum & 1) b = 0xedb88320;
accum =((accum>>1) & 0x7fffffff) ^ b;
}
accum ^= 0xd202ef8d;
At any time, accum is the CRC.getValue(). (You may be able to avoid
the "& 0x7fffffff" if you are sure your compiler does a real logical
right shift and not an arithmetic one.)
As with any such bit-wise CRC implementation, it can be speeded up by
using a pre-calculated table of 256 32-bit ints. That table can be
derived using the above bit-wise algorithm at design-time.
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