1 #include <u.h>
2 #include <libc.h>
3 #include <flate.h>
4 
5 enum
6 {
7 	ADLERITERS	= 5552,	/* max iters before can overflow 32 bits */
8 	ADLERBASE	= 65521 /* largest prime smaller than 65536 */
9 };
10 
11 uint32
12 adler32(uint32 adler, void *vbuf, int n)
13 {
14 	uint32 s1, s2;
15 	uchar *buf, *ebuf;
16 	int m;
17 
18 	buf = vbuf;
19 	s1 = adler & 0xffff;
20 	s2 = (adler >> 16) & 0xffff;
21 	for(; n >= 16; n -= m){
22 		m = n;
23 		if(m > ADLERITERS)
24 			m = ADLERITERS;
25 		m &= ~15;
26 		for(ebuf = buf + m; buf < ebuf; buf += 16){
27 			s1 += buf[0];
28 			s2 += s1;
29 			s1 += buf[1];
30 			s2 += s1;
31 			s1 += buf[2];
32 			s2 += s1;
33 			s1 += buf[3];
34 			s2 += s1;
35 			s1 += buf[4];
36 			s2 += s1;
37 			s1 += buf[5];
38 			s2 += s1;
39 			s1 += buf[6];
40 			s2 += s1;
41 			s1 += buf[7];
42 			s2 += s1;
43 			s1 += buf[8];
44 			s2 += s1;
45 			s1 += buf[9];
46 			s2 += s1;
47 			s1 += buf[10];
48 			s2 += s1;
49 			s1 += buf[11];
50 			s2 += s1;
51 			s1 += buf[12];
52 			s2 += s1;
53 			s1 += buf[13];
54 			s2 += s1;
55 			s1 += buf[14];
56 			s2 += s1;
57 			s1 += buf[15];
58 			s2 += s1;
59 		}
60 		s1 %= ADLERBASE;
61 		s2 %= ADLERBASE;
62 	}
63 	if(n){
64 		for(ebuf = buf + n; buf < ebuf; buf++){
65 			s1 += buf[0];
66 			s2 += s1;
67 		}
68 		s1 %= ADLERBASE;
69 		s2 %= ADLERBASE;
70 	}
71 	return (s2 << 16) + s1;
72 }