tadler.c - plan9port - [fork] Plan 9 from user space
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       ---
       tadler.c (1161B)
       ---
            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 }