1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
//! Adler-32 checksums (RFC 1950, <https://www.rfc-editor.org/rfc/rfc1950.html>).
/// Largest prime smaller than 65536.
const BASE: u32 = 65521;
/// `NMAX` is the largest `n` such that `255n(n+1)/2 + (n+1)(BASE-1) <= 2^32-1`.
const NMAX: usize = 5552;
/// Adler-32 checksums.
#[derive(Debug)]
pub struct Adler32;
impl Adler32 {
/// The Adler-32 of `buffer`, continuing from `adler`: pass `1` to start a
/// new checksum, or a previous result to continue one. Uses wrapping
/// arithmetic.
pub fn calculate(adler: u32, buffer: &[u8]) -> u32 {
let mut s1 = adler & 0xFFFF;
let mut s2 = (adler >> 16) & 0xFFFF;
let mut buffer = buffer;
while !buffer.is_empty() {
// Fold in at most NMAX bytes before reducing modulo BASE, so the
// sums can't overflow.
let k = std::cmp::min(buffer.len(), NMAX);
let mut remaining = k;
while remaining >= 16 {
for &byte in &buffer[..16] {
s1 = s1.wrapping_add(u32::from(byte));
s2 = s2.wrapping_add(s1);
}
buffer = &buffer[16..];
remaining -= 16;
}
for &byte in &buffer[..remaining] {
s1 = s1.wrapping_add(u32::from(byte));
s2 = s2.wrapping_add(s1);
}
buffer = &buffer[remaining..];
s1 %= BASE;
s2 %= BASE;
}
(s2 << 16) | s1
}
}