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#include <u.h>
#include <libc.h>
#include <flate.h>
enum
{
ADLERITERS = 5552, /* max iters before can overflow 32 bits */
ADLERBASE = 65521 /* largest prime smaller than 65536 */
};
ulong
adler32(ulong adler, void *vbuf, int n)
{
ulong s1, s2;
uchar *buf, *ebuf;
int m;
buf = vbuf;
s1 = adler & 0xffff;
s2 = (adler >> 16) & 0xffff;
for(; n >= 16; n -= m){
m = n;
if(m > ADLERITERS)
m = ADLERITERS;
m &= ~15;
for(ebuf = buf + m; buf < ebuf; buf += 16){
s1 += buf[0];
s2 += s1;
s1 += buf[1];
s2 += s1;
s1 += buf[2];
s2 += s1;
s1 += buf[3];
s2 += s1;
s1 += buf[4];
s2 += s1;
s1 += buf[5];
s2 += s1;
s1 += buf[6];
s2 += s1;
s1 += buf[7];
s2 += s1;
s1 += buf[8];
s2 += s1;
s1 += buf[9];
s2 += s1;
s1 += buf[10];
s2 += s1;
s1 += buf[11];
s2 += s1;
s1 += buf[12];
s2 += s1;
s1 += buf[13];
s2 += s1;
s1 += buf[14];
s2 += s1;
s1 += buf[15];
s2 += s1;
}
s1 %= ADLERBASE;
s2 %= ADLERBASE;
}
if(n){
for(ebuf = buf + n; buf < ebuf; buf++){
s1 += buf[0];
s2 += s1;
}
s1 %= ADLERBASE;
s2 %= ADLERBASE;
}
return (s2 << 16) + s1;
}
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