sys/src/libflate/crc.c

   1 #include <u.h>
   2 #include <libc.h>
   3 #include <flate.h>
   4
   5 ulong*
   6 mkcrctab(ulong poly)
   7 {
   8         ulong *crctab;
   9         ulong crc;
  10         int i, j;
  11
  12         crctab = malloc(256 * sizeof(ulong));
  13         if(crctab == nil)
  14                 return nil;
  15
  16         for(i = 0; i < 256; i++){
  17                 crc = i;
  18                 for(j = 0; j < 8; j++){
  19                         if(crc & 1)
  20                                 crc = (crc >> 1) ^ poly;
  21                         else
  22                                 crc >>= 1;
  23                 }
  24                 crctab[i] = crc;
  25         }
  26         return crctab;
  27 }
  28
  29 ulong
  30 blockcrc(ulong *crctab, ulong crc, void *vbuf, int n)
  31 {
  32         uchar *buf, *ebuf;
  33
  34         crc ^= 0xffffffff;
  35         buf = vbuf;
  36         ebuf = buf + n;
  37         while(buf < ebuf)
  38                 crc = crctab[(crc & 0xff) ^ *buf++] ^ (crc >> 8);
  39         return crc ^ 0xffffffff;
  40 }