sys/src/libflate/adler.c

   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 ulong
  12 adler32(ulong adler, void *vbuf, int n)
  13 {
  14         ulong 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 }