zlib/adler32.c

   1 /* adler32.c -- compute the Adler-32 checksum of a data stream
   2  * Copyright (C) 1995-2011, 2016 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 local uLong adler32_combine_ OF((uLong adler1, uLong adler2, z_off64_t len2));
  11
  12 #define BASE 65521U     /* largest prime smaller than 65536 */
  13 #define NMAX 5552
  14 /* NMAX is the largest n such that 255n(n+1)/2 + (n+1)(BASE-1) <= 2^32-1 */
  15
  16 #define DO1(buf,i)  {adler += (buf)[i]; sum2 += adler;}
  17 #define DO2(buf,i)  DO1(buf,i); DO1(buf,i+1);
  18 #define DO4(buf,i)  DO2(buf,i); DO2(buf,i+2);
  19 #define DO8(buf,i)  DO4(buf,i); DO4(buf,i+4);
  20 #define DO16(buf)   DO8(buf,0); DO8(buf,8);
  21
  22 /* use NO_DIVIDE if your processor does not do division in hardware --
  23    try it both ways to see which is faster */
  24 #ifdef NO_DIVIDE
  25 /* note that this assumes BASE is 65521, where 65536 % 65521 == 15
  26    (thank you to John Reiser for pointing this out) */
  27 #  define CHOP(a) \
  28     do { \
  29         unsigned long tmp = a >> 16; \
  30         a &= 0xffffUL; \
  31         a += (tmp << 4) - tmp; \
  32     } while (0)
  33 #  define MOD28(a) \
  34     do { \
  35         CHOP(a); \
  36         if (a >= BASE) a -= BASE; \
  37     } while (0)
  38 #  define MOD(a) \
  39     do { \
  40         CHOP(a); \
  41         MOD28(a); \
  42     } while (0)
  43 #  define MOD63(a) \
  44     do { /* this assumes a is not negative */ \
  45         z_off64_t tmp = a >> 32; \
  46         a &= 0xffffffffL; \
  47         a += (tmp << 8) - (tmp << 5) + tmp; \
  48         tmp = a >> 16; \
  49         a &= 0xffffL; \
  50         a += (tmp << 4) - tmp; \
  51         tmp = a >> 16; \
  52         a &= 0xffffL; \
  53         a += (tmp << 4) - tmp; \
  54         if (a >= BASE) a -= BASE; \
  55     } while (0)
  56 #else
  57 #  define MOD(a) a %= BASE
  58 #  define MOD28(a) a %= BASE
  59 #  define MOD63(a) a %= BASE
  60 #endif
  61
  62 /* ========================================================================= */
  63 uLong ZEXPORT adler32_z(adler, buf, len)
  64     uLong adler;
  65     const Bytef *buf;
  66     z_size_t len;
  67 {
  68     unsigned long sum2;
  69     unsigned n;
  70
  71     /* split Adler-32 into component sums */
  72     sum2 = (adler >> 16) & 0xffff;
  73     adler &= 0xffff;
  74
  75     /* in case user likes doing a byte at a time, keep it fast */
  76     if (len == 1) {
  77         adler += buf[0];
  78         if (adler >= BASE)
  79             adler -= BASE;
  80         sum2 += adler;
  81         if (sum2 >= BASE)
  82             sum2 -= BASE;
  83         return adler | (sum2 << 16);
  84     }
  85
  86     /* initial Adler-32 value (deferred check for len == 1 speed) */
  87     if (buf == Z_NULL)
  88         return 1L;
  89
  90     /* in case short lengths are provided, keep it somewhat fast */
  91     if (len < 16) {
  92         while (len--) {
  93             adler += *buf++;
  94             sum2 += adler;
  95         }
  96         if (adler >= BASE)
  97             adler -= BASE;
  98         MOD28(sum2);            /* only added so many BASE's */
  99         return adler | (sum2 << 16);
 100     }
 101
 102     /* do length NMAX blocks -- requires just one modulo operation */
 103     while (len >= NMAX) {
 104         len -= NMAX;
 105         n = NMAX / 16;          /* NMAX is divisible by 16 */
 106         do {
 107             DO16(buf);          /* 16 sums unrolled */
 108             buf += 16;
 109         } while (--n);
 110         MOD(adler);
 111         MOD(sum2);
 112     }
 113
 114     /* do remaining bytes (less than NMAX, still just one modulo) */
 115     if (len) {                  /* avoid modulos if none remaining */
 116         while (len >= 16) {
 117             len -= 16;
 118             DO16(buf);
 119             buf += 16;
 120         }
 121         while (len--) {
 122             adler += *buf++;
 123             sum2 += adler;
 124         }
 125         MOD(adler);
 126         MOD(sum2);
 127     }
 128
 129     /* return recombined sums */
 130     return adler | (sum2 << 16);
 131 }
 132
 133 /* ========================================================================= */
 134 uLong ZEXPORT adler32(adler, buf, len)
 135     uLong adler;
 136     const Bytef *buf;
 137     uInt len;
 138 {
 139     return adler32_z(adler, buf, len);
 140 }
 141
 142 /* ========================================================================= */
 143 local uLong adler32_combine_(adler1, adler2, len2)
 144     uLong adler1;
 145     uLong adler2;
 146     z_off64_t len2;
 147 {
 148     unsigned long sum1;
 149     unsigned long sum2;
 150     unsigned rem;
 151
 152     /* for negative len, return invalid adler32 as a clue for debugging */
 153     if (len2 < 0)
 154         return 0xffffffffUL;
 155
 156     /* the derivation of this formula is left as an exercise for the reader */
 157     MOD63(len2);                /* assumes len2 >= 0 */
 158     rem = (unsigned)len2;
 159     sum1 = adler1 & 0xffff;
 160     sum2 = rem * sum1;
 161     MOD(sum2);
 162     sum1 += (adler2 & 0xffff) + BASE - 1;
 163     sum2 += ((adler1 >> 16) & 0xffff) + ((adler2 >> 16) & 0xffff) + BASE - rem;
 164     if (sum1 >= BASE) sum1 -= BASE;
 165     if (sum1 >= BASE) sum1 -= BASE;
 166     if (sum2 >= ((unsigned long)BASE << 1)) sum2 -= ((unsigned long)BASE << 1);
 167     if (sum2 >= BASE) sum2 -= BASE;
 168     return sum1 | (sum2 << 16);
 169 }
 170
 171 /* ========================================================================= */
 172 uLong ZEXPORT adler32_combine(adler1, adler2, len2)
 173     uLong adler1;
 174     uLong adler2;
 175     z_off_t len2;
 176 {
 177     return adler32_combine_(adler1, adler2, len2);
 178 }
 179
 180 uLong ZEXPORT adler32_combine64(adler1, adler2, len2)
 181     uLong adler1;
 182     uLong adler2;
 183     z_off64_t len2;
 184 {
 185     return adler32_combine_(adler1, adler2, len2);
 186 }