zlib/adler32.c

   1 /* adler32.c -- compute the Adler-32 checksum of a data stream
   2  * Copyright (C) 1995-2007 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 #include "zutil.h"
   9
  10 #define local static
  11
  12 local uLong adler32_combine_(uLong adler1, uLong adler2, z_off64_t len2);
  13
  14 #define BASE 65521UL    /* largest prime smaller than 65536 */
  15 #define NMAX 5552
  16 /* NMAX is the largest n such that 255n(n+1)/2 + (n+1)(BASE-1) <= 2^32-1 */
  17
  18 #define DO1(buf,i)  {adler += (buf)[i]; sum2 += adler;}
  19 #define DO2(buf,i)  DO1(buf,i); DO1(buf,i+1);
  20 #define DO4(buf,i)  DO2(buf,i); DO2(buf,i+2);
  21 #define DO8(buf,i)  DO4(buf,i); DO4(buf,i+4);
  22 #define DO16(buf)   DO8(buf,0); DO8(buf,8);
  23
  24 /* use NO_DIVIDE if your processor does not do division in hardware */
  25 #ifdef NO_DIVIDE
  26 #  define MOD(a) \
  27     do { \
  28         if (a >= (BASE << 16)) a -= (BASE << 16); \
  29         if (a >= (BASE << 15)) a -= (BASE << 15); \
  30         if (a >= (BASE << 14)) a -= (BASE << 14); \
  31         if (a >= (BASE << 13)) a -= (BASE << 13); \
  32         if (a >= (BASE << 12)) a -= (BASE << 12); \
  33         if (a >= (BASE << 11)) a -= (BASE << 11); \
  34         if (a >= (BASE << 10)) a -= (BASE << 10); \
  35         if (a >= (BASE << 9)) a -= (BASE << 9); \
  36         if (a >= (BASE << 8)) a -= (BASE << 8); \
  37         if (a >= (BASE << 7)) a -= (BASE << 7); \
  38         if (a >= (BASE << 6)) a -= (BASE << 6); \
  39         if (a >= (BASE << 5)) a -= (BASE << 5); \
  40         if (a >= (BASE << 4)) a -= (BASE << 4); \
  41         if (a >= (BASE << 3)) a -= (BASE << 3); \
  42         if (a >= (BASE << 2)) a -= (BASE << 2); \
  43         if (a >= (BASE << 1)) a -= (BASE << 1); \
  44         if (a >= BASE) a -= BASE; \
  45     } while (0)
  46 #  define MOD4(a) \
  47     do { \
  48         if (a >= (BASE << 4)) a -= (BASE << 4); \
  49         if (a >= (BASE << 3)) a -= (BASE << 3); \
  50         if (a >= (BASE << 2)) a -= (BASE << 2); \
  51         if (a >= (BASE << 1)) a -= (BASE << 1); \
  52         if (a >= BASE) a -= BASE; \
  53     } while (0)
  54 #else
  55 #  define MOD(a) a %= BASE
  56 #  define MOD4(a) a %= BASE
  57 #endif
  58
  59 /* ========================================================================= */
  60 uLong ZEXPORT adler32(adler, buf, len)
  61     uLong adler;
  62     const Bytef *buf;
  63     uInt len;
  64 {
  65     unsigned long sum2;
  66     unsigned n;
  67
  68     /* split Adler-32 into component sums */
  69     sum2 = (adler >> 16) & 0xffff;
  70     adler &= 0xffff;
  71
  72     /* in case user likes doing a byte at a time, keep it fast */
  73     if (len == 1) {
  74         adler += buf[0];
  75         if (adler >= BASE)
  76             adler -= BASE;
  77         sum2 += adler;
  78         if (sum2 >= BASE)
  79             sum2 -= BASE;
  80         return adler | (sum2 << 16);
  81     }
  82
  83     /* initial Adler-32 value (deferred check for len == 1 speed) */
  84     if (buf == Z_NULL)
  85         return 1L;
  86
  87     /* in case short lengths are provided, keep it somewhat fast */
  88     if (len < 16) {
  89         while (len--) {
  90             adler += *buf++;
  91             sum2 += adler;
  92         }
  93         if (adler >= BASE)
  94             adler -= BASE;
  95         MOD4(sum2);             /* only added so many BASE's */
  96         return adler | (sum2 << 16);
  97     }
  98
  99     /* do length NMAX blocks -- requires just one modulo operation */
 100     while (len >= NMAX) {
 101         len -= NMAX;
 102         n = NMAX / 16;          /* NMAX is divisible by 16 */
 103         do {
 104             DO16(buf);          /* 16 sums unrolled */
 105             buf += 16;
 106         } while (--n);
 107         MOD(adler);
 108         MOD(sum2);
 109     }
 110
 111     /* do remaining bytes (less than NMAX, still just one modulo) */
 112     if (len) {                  /* avoid modulos if none remaining */
 113         while (len >= 16) {
 114             len -= 16;
 115             DO16(buf);
 116             buf += 16;
 117         }
 118         while (len--) {
 119             adler += *buf++;
 120             sum2 += adler;
 121         }
 122         MOD(adler);
 123         MOD(sum2);
 124     }
 125
 126     /* return recombined sums */
 127     return adler | (sum2 << 16);
 128 }
 129
 130 /* ========================================================================= */
 131 local uLong adler32_combine_(adler1, adler2, len2)
 132     uLong adler1;
 133     uLong adler2;
 134     z_off64_t len2;
 135 {
 136     unsigned long sum1;
 137     unsigned long sum2;
 138     unsigned rem;
 139
 140     /* the derivation of this formula is left as an exercise for the reader */
 141     rem = (unsigned)(len2 % BASE);
 142     sum1 = adler1 & 0xffff;
 143     sum2 = rem * sum1;
 144     MOD(sum2);
 145     sum1 += (adler2 & 0xffff) + BASE - 1;
 146     sum2 += ((adler1 >> 16) & 0xffff) + ((adler2 >> 16) & 0xffff) + BASE - rem;
 147     if (sum1 >= BASE) sum1 -= BASE;
 148     if (sum1 >= BASE) sum1 -= BASE;
 149     if (sum2 >= (BASE << 1)) sum2 -= (BASE << 1);
 150     if (sum2 >= BASE) sum2 -= BASE;
 151     return sum1 | (sum2 << 16);
 152 }
 153
 154 /* ========================================================================= */
 155 uLong ZEXPORT adler32_combine(adler1, adler2, len2)
 156     uLong adler1;
 157     uLong adler2;
 158     z_off_t len2;
 159 {
 160     return adler32_combine_(adler1, adler2, len2);
 161 }
 162
 163 uLong ZEXPORT adler32_combine64(adler1, adler2, len2)
 164     uLong adler1;
 165     uLong adler2;
 166     z_off64_t len2;
 167 {
 168     return adler32_combine_(adler1, adler2, len2);
 169 }