Is it possible to do CRC-32 calculation in splits?

Question

I use this trivial function to calculate the CRC checksum of a given file:

long i, j = 0;
int k = 0;
uint crc = 0xFFFFFFFF;
FileInfo file_info = new FileInfo(fil         


        

Answer 1

This equation is key:

CRC(a XOR b) == CRC(a) XOR CRC(b)

Suppose you want to compute the CRC of the following message:

"Always desire to learn something useful."

There exist functions to compute the CRC as:

crc_join(crc_part1("Always desire to lea"),
         crc_part2("rn something useful."))

If crc_part1 and crc_part2 zeros pad (\0) their arguments as shown, crc_join is just XOR.

crc_part1 = crc("Always desire to lea\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0")
crc_part2 = crc("\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0rn something useful.")

The trailing zeros can be accounted for in crc_part1 with lookup tables. The leading zeros can be ignored in crc_part2.

References:

1. High-speed Parallel Architecture for Software-based CRC
   Youngju. Do, Sung-Rok. Yoon, Taekyu. Kim, Kwang Eui. Pyun and Sin-Chong. Park
2. https://en.wikipedia.org/wiki/Cyclic_redundancy_check