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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(file);
byte[] file_buffer = new byte[32768];
FileStream file_stream = new FileStream(@file, FileMode.Open);
while ((i = file_stream.Read(file_buffer, 0, file_buffer.Count())) > 0)
{
for (j = 0; j < i; j++)
{
uint before = crc;
k = (int)((crc ^ file_buffer[j]) & 0x000000FFL);
uint after = (uint)((crc >> 8) & 0x00FFFFFFL) ^ crc32_table[k];
crc = after;
uint test = (uint)((crc << 8) & 0x00FFFFFFL) ^ crc32_table[k];
MessageBox.Show((~crc).ToString("X"));
}
}
file_stream.Close();
return ~crc;
My question is this: Say I have a large file, of say 100MB. Is there any link between a CRC-32 calculation of the first 50MB and the last 50MB and the CRC-32 calculation of the 100MB file?
The reason I'm asking, is I have some very large files (~10GB give or take) which take some time to generate, but while they're being generated, most parts remain static, however, parts in the middle (known point) and right at the start (header, also known part/length). Calculating a CRC-32 checksum of a 10GB file takes quite some time, so I was wondering if there was any way to do it in chunks?
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The most common variant of the CRC32 checksum, sometimes called CRC-32b, is based on the following generator polynomial: g(x) = x32 + x26 + x23 + x22 + x16 + x12 + x11 + x10 + x8 + x7 + x5 + x4 + x2 + x + 1. This code processes one bit at a time.
CRC32 is an error-detecting function that uses a CRC32 algorithm to detect changes between source and target data. The CRC32 function converts a variable-length string into an 8-character string that is a text representation of the hexadecimal value of a 32 bit-binary sequence.
The CRC32 check value is 32 bits long, so the most-significant bit in the divisor doesn't factor into the computation at all. It's actually just there in the specification to clarify that the divisor is 32 bits long, rather than 26.
A CRC is pretty simple; you take a polynomial represented as bits and the data, and divide the polynomial into the data (or you represent the data as a polynomial and do the same thing). The remainder, which is between 0 and the polynomial is the CRC.
Yes. See crc32_combine_() in zlib for an example.
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