lib/zlib/adler32.c

   1 /* adler32.c -- compute the Adler-32 checksum of a data stream
   2  * Copyright (C) 1995-2004 Mark Adler
   3  * For conditions of distribution and use, see copyright notice in zlib.h
   4  */
   5
   6 /* @(#) $Id$ */
   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)  {adler += (buf)[i]; sum2 += adler;}
  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 /* use NO_DIVIDE if your processor does not do division in hardware */
  22 #ifdef NO_DIVIDE
  23 #  define MOD(a) \
  24     do { \
  25         if (a >= (BASE << 16)) a -= (BASE << 16); \
  26         if (a >= (BASE << 15)) a -= (BASE << 15); \
  27         if (a >= (BASE << 14)) a -= (BASE << 14); \
  28         if (a >= (BASE << 13)) a -= (BASE << 13); \
  29         if (a >= (BASE << 12)) a -= (BASE << 12); \
  30         if (a >= (BASE << 11)) a -= (BASE << 11); \
  31         if (a >= (BASE << 10)) a -= (BASE << 10); \
  32         if (a >= (BASE << 9)) a -= (BASE << 9); \
  33         if (a >= (BASE << 8)) a -= (BASE << 8); \
  34         if (a >= (BASE << 7)) a -= (BASE << 7); \
  35         if (a >= (BASE << 6)) a -= (BASE << 6); \
  36         if (a >= (BASE << 5)) a -= (BASE << 5); \
  37         if (a >= (BASE << 4)) a -= (BASE << 4); \
  38         if (a >= (BASE << 3)) a -= (BASE << 3); \
  39         if (a >= (BASE << 2)) a -= (BASE << 2); \
  40         if (a >= (BASE << 1)) a -= (BASE << 1); \
  41         if (a >= BASE) a -= BASE; \
  42     } while (0)
  43 #  define MOD4(a) \
  44     do { \
  45         if (a >= (BASE << 4)) a -= (BASE << 4); \
  46         if (a >= (BASE << 3)) a -= (BASE << 3); \
  47         if (a >= (BASE << 2)) a -= (BASE << 2); \
  48         if (a >= (BASE << 1)) a -= (BASE << 1); \
  49         if (a >= BASE) a -= BASE; \
  50     } while (0)
  51 #else
  52 #  define MOD(a) a %= BASE
  53 #  define MOD4(a) a %= BASE
  54 #endif
  55
  56 /* ========================================================================= */
  57 uLong ZEXPORT adler32(uLong adler, const Bytef *buf, uInt len)
  58 {
  59     unsigned long sum2;
  60     unsigned n;
  61
  62     /* split Adler-32 into component sums */
  63     sum2 = (adler >> 16) & 0xffff;
  64     adler &= 0xffff;
  65
  66     /* in case user likes doing a byte at a time, keep it fast */
  67     if (len == 1) {
  68         adler += buf[0];
  69         if (adler >= BASE)
  70             adler -= BASE;
  71         sum2 += adler;
  72         if (sum2 >= BASE)
  73             sum2 -= BASE;
  74         return adler | (sum2 << 16);
  75     }
  76
  77     /* initial Adler-32 value (deferred check for len == 1 speed) */
  78     if (buf == Z_NULL)
  79         return 1L;
  80
  81     /* in case short lengths are provided, keep it somewhat fast */
  82     if (len < 16) {
  83         while (len--) {
  84             adler += *buf++;
  85             sum2 += adler;
  86         }
  87         if (adler >= BASE)
  88             adler -= BASE;
  89         MOD4(sum2);             /* only added so many BASE's */
  90         return adler | (sum2 << 16);
  91     }
  92
  93     /* do length NMAX blocks -- requires just one modulo operation */
  94     while (len >= NMAX) {
  95         len -= NMAX;
  96         n = NMAX / 16;          /* NMAX is divisible by 16 */
  97         do {
  98             DO16(buf);          /* 16 sums unrolled */
  99             buf += 16;
 100         } while (--n);
 101         MOD(adler);
 102         MOD(sum2);
 103     }
 104
 105     /* do remaining bytes (less than NMAX, still just one modulo) */
 106     if (len) {                  /* avoid modulos if none remaining */
 107         while (len >= 16) {
 108             len -= 16;
 109             DO16(buf);
 110             buf += 16;
 111         }
 112         while (len--) {
 113             adler += *buf++;
 114             sum2 += adler;
 115         }
 116         MOD(adler);
 117         MOD(sum2);
 118     }
 119
 120     /* return recombined sums */
 121     return adler | (sum2 << 16);
 122 }