adler32.c

static unsigned update_adler32(unsigned adler, const unsigned char* data, unsigned len) {
    unsigned s1 = adler & 0xffff;
    unsigned s2 = (adler >> 16) & 0xffff;

    while(len > 0) {
        /*at least 5550 sums can be done before the sums overflow, saving a lot of module divisions*/
        unsigned amount = len > 5550 ? 5550 : len;
        len -= amount;
        while(amount > 0) {
            s1 += (*data++);
            s2 += s1;
            --amount;
        }
        s1 %= 65521;
        s2 %= 65521;
    }

    return (s2 << 16) | s1;
}

adler32.rs

fn update_adler32(adler: u32, data: &[u8]) -> u32 {
    let mut s1 = adler & 0xffff;
    let mut s2 = (adler >> 16) & 0xffff;
  
    /*at least 5550 sums can be done before the sums overflow, saving a lot of module divisions*/
    for chunk in data.chunks(5550) {
        for b in chunk {
            s1 += *b as u32;
            s2 += s1;
        }
        s1 %= 65521;
        s2 %= 65521;
    }
  
    return (s2 << 16) | s1;
}