linux/lib/zlib/adler32.c

   1 /* adler32.c -- compute the Adler-32 checksum of a data stream
   2  * Copyright (C) 1995-2002 Mark Adler
   3  * For conditions of distribution and use, see copyright notice in zlib.h
   4  */
   5
   6 /* @(#) $Id: adler32.c,v 1.1 2004/03/15 20:35:26 as Exp $ */
   7
   8 #include <zlib/zlib.h>
   9 #include "zconf.h"
  10
  11 #define BASE 65521L /* largest prime smaller than 65536 */
  12 #define NMAX 5552
  13 /* NMAX is the largest n such that 255n(n+1)/2 + (n+1)(BASE-1) <= 2^32-1 */
  14
  15 #define DO1(buf,i)  {s1 += buf[i]; s2 += s1;}
  16 #define DO2(buf,i)  DO1(buf,i); DO1(buf,i+1);
  17 #define DO4(buf,i)  DO2(buf,i); DO2(buf,i+2);
  18 #define DO8(buf,i)  DO4(buf,i); DO4(buf,i+4);
  19 #define DO16(buf)   DO8(buf,0); DO8(buf,8);
  20
  21 /* ========================================================================= */
  22 uLong ZEXPORT adler32(adler, buf, len)
  23     uLong adler;
  24     const Bytef *buf;
  25     uInt len;
  26 {
  27     unsigned long s1 = adler & 0xffff;
  28     unsigned long s2 = (adler >> 16) & 0xffff;
  29     int k;
  30
  31     if (buf == Z_NULL) return 1L;
  32
  33     while (len > 0) {
  34         k = len < NMAX ? len : NMAX;
  35         len -= k;
  36         while (k >= 16) {
  37             DO16(buf);
  38             buf += 16;
  39             k -= 16;
  40         }
  41         if (k != 0) do {
  42             s1 += *buf++;
  43             s2 += s1;
  44         } while (--k);
  45         s1 %= BASE;
  46         s2 %= BASE;
  47     }
  48     return (s2 << 16) | s1;
  49 }