libbz2_rs_sys/
crctable.rs

1/// The polynomial used for the crc32 lookup table.
2///
3/// See also https://en.wikipedia.org/wiki/Cyclic_redundancy_check#Polynomial_representations
4const POLYNOMIAL: u32 = 0x04C11DB7;
5
6/// Most implementations (ethernet, zlib) use the reflected version of this polynomial.
7const _: () = assert!(POLYNOMIAL.reverse_bits() == 0xEDB88320);
8
9/// Lookup table to speed up crc32 checksum calculation.
10///
11/// The original C implementation notes:
12///
13/// > I think this is an implementation of the AUTODIN-II,
14/// > Ethernet & FDDI 32-bit CRC standard.  Vaguely derived
15/// > from code by Rob Warnock, in Section 51 of the
16/// > comp.compression FAQ.
17pub(crate) static BZ2_CRC32TABLE: [u32; 256] = generate_crc32_table(POLYNOMIAL);
18
19/// Generate the crc32 lookup table.
20///
21/// Note that contrary to most material you'll find on the internet, we're using the non-reflected
22/// polynomial, which impacts some of the logic (e.g. we bitwise and with 0x80000000 instead of 0x1).
23///
24/// This [article] has some excellent additional detail on how crc works, and how to make it fast.
25///
26/// [article]: https://create.stephan-brumme.com/crc32/
27const fn generate_crc32_table(polynomial: u32) -> [u32; 256] {
28    let mut table = [0u32; 256];
29
30    let mut i = 0;
31    while i < 256 {
32        let mut crc = (i as u32) << 24;
33
34        let mut j = 0;
35        while j < 8 {
36            if (crc & 0x80000000) != 0 {
37                crc = (crc << 1) ^ polynomial;
38            } else {
39                crc <<= 1;
40            }
41
42            j += 1;
43        }
44
45        table[i] = crc;
46
47        i += 1;
48    }
49
50    table
51}
libbz2_rs_sys/crctable.rs

libbz2_rs_sys/
crctable.rs
