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