crc32fast/
combine.rs

1const GF2_DIM: usize = 32;
2
3fn gf2_matrix_times(mat: &[u32; GF2_DIM], mut vec: u32) -> u32 {
4    let mut sum = 0;
5    let mut idx = 0;
6    while vec > 0 {
7        if vec & 1 == 1 {
8            sum ^= mat[idx];
9        }
10        vec >>= 1;
11        idx += 1;
12    }
13    return sum;
14}
15
16fn gf2_matrix_square(square: &mut [u32; GF2_DIM], mat: &[u32; GF2_DIM]) {
17    for n in 0..GF2_DIM {
18        square[n] = gf2_matrix_times(mat, mat[n]);
19    }
20}
21
22pub(crate) fn combine(mut crc1: u32, crc2: u32, mut len2: u64) -> u32 {
23    let mut row: u32;
24    let mut even = [0u32; GF2_DIM]; /* even-power-of-two zeros operator */
25    let mut odd = [0u32; GF2_DIM]; /* odd-power-of-two zeros operator */
26
27    /* degenerate case (also disallow negative lengths) */
28    if len2 <= 0 {
29        return crc1;
30    }
31
32    /* put operator for one zero bit in odd */
33    odd[0] = 0xedb88320; /* CRC-32 polynomial */
34    row = 1;
35    for n in 1..GF2_DIM {
36        odd[n] = row;
37        row <<= 1;
38    }
39
40    /* put operator for two zero bits in even */
41    gf2_matrix_square(&mut even, &odd);
42
43    /* put operator for four zero bits in odd */
44    gf2_matrix_square(&mut odd, &even);
45
46    /* apply len2 zeros to crc1 (first square will put the operator for one
47       zero byte, eight zero bits, in even) */
48    loop {
49        /* apply zeros operator for this bit of len2 */
50        gf2_matrix_square(&mut even, &odd);
51        if len2 & 1 == 1 {
52            crc1 = gf2_matrix_times(&even, crc1);
53        }
54        len2 >>= 1;
55
56        /* if no more bits set, then done */
57        if len2 == 0 {
58            break;
59        }
60
61        /* another iteration of the loop with odd and even swapped */
62        gf2_matrix_square(&mut odd, &even);
63        if len2 & 1 == 1 {
64            crc1 = gf2_matrix_times(&odd, crc1);
65        }
66        len2 >>= 1;
67
68        /* if no more bits set, then done */
69        if len2 == 0 {
70            break;
71        }
72    }
73
74    /* return combined crc */
75    crc1 ^= crc2;
76    return crc1;
77}