src/zlib/adler32.c

   1 /* adler32.c -- compute the Adler-32 checksum of a data stream
   2  * Copyright (C) 1995-2011 Mark Adler
   3  * For conditions of distribution and use, see copyright notice in zlib.h
   4  */
   5
   6 /* @(#) $Id$ */
   7
   8 #include "zutil.h"
   9
  10 #define local static
  11
  12 local uLong adler32_combine_ OF((uLong adler1, uLong adler2, z_off64_t len2));
  13
  14 #define BASE 65521      /* 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    try it both ways to see which is faster */
  26 #ifdef NO_DIVIDE
  27 /* note that this assumes BASE is 65521, where 65536 % 65521 == 15
  28    (thank you to John Reiser for pointing this out) */
  29 #  define CHOP(a) \
  30     do { \
  31         unsigned long tmp = a >> 16; \
  32         a &= 0xffffUL; \
  33         a += (tmp << 4) - tmp; \
  34     } while (0)
  35 #  define MOD28(a) \
  36     do { \
  37         CHOP(a); \
  38         if (a >= BASE) a -= BASE; \
  39     } while (0)
  40 #  define MOD(a) \
  41     do { \
  42         CHOP(a); \
  43         MOD28(a); \
  44     } while (0)
  45 #  define MOD63(a) \
  46     do { /* this assumes a is not negative */ \
  47         z_off64_t tmp = a >> 32; \
  48         a &= 0xffffffffL; \
  49         a += (tmp << 8) - (tmp << 5) + tmp; \
  50         tmp = a >> 16; \
  51         a &= 0xffffL; \
  52         a += (tmp << 4) - tmp; \
  53         tmp = a >> 16; \
  54         a &= 0xffffL; \
  55         a += (tmp << 4) - tmp; \
  56         if (a >= BASE) a -= BASE; \
  57     } while (0)
  58 #else
  59 #  define MOD(a) a %= BASE
  60 #  define MOD28(a) a %= BASE
  61 #  define MOD63(a) a %= BASE
  62 #endif
  63
  64 /* ========================================================================= */
  65 uLong ZEXPORT adler32(adler, buf, len)
  66     uLong adler;
  67     const Bytef *buf;
  68     uInt len;
  69 {
  70     unsigned long sum2;
  71     unsigned n;
  72
  73     /* split Adler-32 into component sums */
  74     sum2 = (adler >> 16) & 0xffff;
  75     adler &= 0xffff;
  76
  77     /* in case user likes doing a byte at a time, keep it fast */
  78     if (len == 1) {
  79         adler += buf[0];
  80         if (adler >= BASE)
  81             adler -= BASE;
  82         sum2 += adler;
  83         if (sum2 >= BASE)
  84             sum2 -= BASE;
  85         return adler | (sum2 << 16);
  86     }
  87
  88     /* initial Adler-32 value (deferred check for len == 1 speed) */
  89     if (buf == Z_NULL)
  90         return 1L;
  91
  92     /* in case short lengths are provided, keep it somewhat fast */
  93     if (len < 16) {
  94         while (len--) {
  95             adler += *buf++;
  96             sum2 += adler;
  97         }
  98         if (adler >= BASE)
  99             adler -= BASE;
 100         MOD28(sum2);            /* only added so many BASE's */
 101         return adler | (sum2 << 16);
 102     }
 103
 104     /* do length NMAX blocks -- requires just one modulo operation */
 105     while (len >= NMAX) {
 106         len -= NMAX;
 107         n = NMAX / 16;          /* NMAX is divisible by 16 */
 108         do {
 109             DO16(buf);          /* 16 sums unrolled */
 110             buf += 16;
 111         } while (--n);
 112         MOD(adler);
 113         MOD(sum2);
 114     }
 115
 116     /* do remaining bytes (less than NMAX, still just one modulo) */
 117     if (len) {                  /* avoid modulos if none remaining */
 118         while (len >= 16) {
 119             len -= 16;
 120             DO16(buf);
 121             buf += 16;
 122         }
 123         while (len--) {
 124             adler += *buf++;
 125             sum2 += adler;
 126         }
 127         MOD(adler);
 128         MOD(sum2);
 129     }
 130
 131     /* return recombined sums */
 132     return adler | (sum2 << 16);
 133 }
 134
 135 /* ========================================================================= */
 136 local uLong adler32_combine_(adler1, adler2, len2)
 137     uLong adler1;
 138     uLong adler2;
 139     z_off64_t len2;
 140 {
 141     unsigned long sum1;
 142     unsigned long sum2;
 143     unsigned rem;
 144
 145     /* for negative len, return invalid adler32 as a clue for debugging */
 146     if (len2 < 0)
 147         return 0xffffffffUL;
 148
 149     /* the derivation of this formula is left as an exercise for the reader */
 150     MOD63(len2);                /* assumes len2 >= 0 */
 151     rem = (unsigned)len2;
 152     sum1 = adler1 & 0xffff;
 153     sum2 = rem * sum1;
 154     MOD(sum2);
 155     sum1 += (adler2 & 0xffff) + BASE - 1;
 156     sum2 += ((adler1 >> 16) & 0xffff) + ((adler2 >> 16) & 0xffff) + BASE - rem;
 157     if (sum1 >= BASE) sum1 -= BASE;
 158     if (sum1 >= BASE) sum1 -= BASE;
 159     if (sum2 >= (BASE << 1)) sum2 -= (BASE << 1);
 160     if (sum2 >= BASE) sum2 -= BASE;
 161     return sum1 | (sum2 << 16);
 162 }
 163
 164 /* ========================================================================= */
 165 uLong ZEXPORT adler32_combine(adler1, adler2, len2)
 166     uLong adler1;
 167     uLong adler2;
 168     z_off_t len2;
 169 {
 170     return adler32_combine_(adler1, adler2, len2);
 171 }
 172
 173 #if 0 /* XCSoar doesn't need large file support */
 174 uLong ZEXPORT adler32_combine64(adler1, adler2, len2)
 175     uLong adler1;
 176     uLong adler2;
 177     z_off64_t len2;
 178 {
 179     return adler32_combine_(adler1, adler2, len2);
 180 }
 181 #endif