zlib/adler32.c

   1 /* adler32.c -- compute the Adler-32 checksum of a data stream
   2  * Copyright (C) 1995-1998 Mark Adler
   3  * For conditions of distribution and use, see copyright notice in zlib.h
   4  */
   5
   6 /* @(#) $Id$ */
   7
   8 #include "zlib.h"
   9
  10 #define BASE 65521L /* largest prime smaller than 65536 */
  11 #define NMAX 5552
  12 /* NMAX is the largest n such that 255n(n+1)/2 + (n+1)(BASE-1) <= 2^32-1 */
  13
  14 #define DO1(buf,i)  {s1 += buf[i]; s2 += s1;}
  15 #define DO2(buf,i)  DO1(buf,i); DO1(buf,i+1);
  16 #define DO4(buf,i)  DO2(buf,i); DO2(buf,i+2);
  17 #define DO8(buf,i)  DO4(buf,i); DO4(buf,i+4);
  18 #define DO16(buf)   DO8(buf,0); DO8(buf,8);
  19
  20 /* ========================================================================= */
  21 uLong ZEXPORT adler32(adler, buf, len)
  22     uLong adler;
  23     const Bytef *buf;
  24     uInt len;
  25 {
  26     unsigned long s1 = adler & 0xffff;
  27     unsigned long s2 = (adler >> 16) & 0xffff;
  28     int k;
  29
  30     if (buf == Z_NULL) return 1L;
  31
  32     while (len > 0) {
  33         k = len < NMAX ? len : NMAX;
  34         len -= k;
  35         while (k >= 16) {
  36             DO16(buf);
  37             buf += 16;
  38             k -= 16;
  39         }
  40         if (k != 0) do {
  41             s1 += *buf++;
  42             s2 += s1;
  43         } while (--k);
  44         s1 %= BASE;
  45         s2 %= BASE;
  46     }
  47     return (s2 << 16) | s1;
  48 }