zlib/adler32.c

   1 /* adler32.c -- compute the Adler-32 checksum of a data stream
   2  * Copyright (C) 1995-2003 Mark Adler
   3  * For conditions of distribution and use, see copyright notice in zlib.h
   4  */
   5
   6 /* @(#) $Id: adler32.c,v 1.1.1.2 2002/03/11 21:53:23 tromey Exp $ */
   7
   8 #define ZLIB_INTERNAL
   9 #include "zlib.h"
  10
  11 #define BASE 65521UL    /* 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 #ifdef NO_DIVIDE
  22 #  define MOD(a) \
  23     do { \
  24         if (a >= (BASE << 16)) a -= (BASE << 16); \
  25         if (a >= (BASE << 15)) a -= (BASE << 15); \
  26         if (a >= (BASE << 14)) a -= (BASE << 14); \
  27         if (a >= (BASE << 13)) a -= (BASE << 13); \
  28         if (a >= (BASE << 12)) a -= (BASE << 12); \
  29         if (a >= (BASE << 11)) a -= (BASE << 11); \
  30         if (a >= (BASE << 10)) a -= (BASE << 10); \
  31         if (a >= (BASE << 9)) a -= (BASE << 9); \
  32         if (a >= (BASE << 8)) a -= (BASE << 8); \
  33         if (a >= (BASE << 7)) a -= (BASE << 7); \
  34         if (a >= (BASE << 6)) a -= (BASE << 6); \
  35         if (a >= (BASE << 5)) a -= (BASE << 5); \
  36         if (a >= (BASE << 4)) a -= (BASE << 4); \
  37         if (a >= (BASE << 3)) a -= (BASE << 3); \
  38         if (a >= (BASE << 2)) a -= (BASE << 2); \
  39         if (a >= (BASE << 1)) a -= (BASE << 1); \
  40         if (a >= BASE) a -= BASE; \
  41     } while (0)
  42 #else
  43 #  define MOD(a) a %= BASE
  44 #endif
  45
  46 /* ========================================================================= */
  47 uLong ZEXPORT adler32(adler, buf, len)
  48     uLong adler;
  49     const Bytef *buf;
  50     uInt len;
  51 {
  52     unsigned long s1 = adler & 0xffff;
  53     unsigned long s2 = (adler >> 16) & 0xffff;
  54     int k;
  55
  56     if (buf == Z_NULL) return 1L;
  57
  58     while (len > 0) {
  59         k = len < NMAX ? (int)len : NMAX;
  60         len -= k;
  61         while (k >= 16) {
  62             DO16(buf);
  63             buf += 16;
  64             k -= 16;
  65         }
  66         if (k != 0) do {
  67             s1 += *buf++;
  68             s2 += s1;
  69         } while (--k);
  70         MOD(s1);
  71         MOD(s2);
  72     }
  73     return (s2 << 16) | s1;
  74 }