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
   7 #include "zutil.h"
   8
   9 #define local static
  10
  11 local uLong adler32_combine_ OF((uLong adler1, uLong adler2, z_off64_t len2));
  12
  13 #define BASE 65521      /* largest prime smaller than 65536 */
  14 #define NMAX 5552
  15 /* NMAX is the largest n such that 255n(n+1)/2 + (n+1)(BASE-1) <= 2^32-1 */
  16
  17 #define DO1(buf,i)  {adler += (buf)[i]; sum2 += adler;}
  18 #define DO2(buf,i)  DO1(buf,i); DO1(buf,i+1);
  19 #define DO4(buf,i)  DO2(buf,i); DO2(buf,i+2);
  20 #define DO8(buf,i)  DO4(buf,i); DO4(buf,i+4);
  21 #define DO16(buf)   DO8(buf,0); DO8(buf,8);
  22
  23 /* use NO_DIVIDE if your processor does not do division in hardware --
  24    try it both ways to see which is faster */
  25 #ifdef NO_DIVIDE
  26 /* note that this assumes BASE is 65521, where 65536 % 65521 == 15
  27    (thank you to John Reiser for pointing this out) */
  28 #  define CHOP(a) \
  29     do { \
  30         unsigned long tmp = a >> 16; \
  31         a &= 0xffffUL; \
  32         a += (tmp << 4) - tmp; \
  33     } while (0)
  34 #  define MOD28(a) \
  35     do { \
  36         CHOP(a); \
  37         if (a >= BASE) a -= BASE; \
  38     } while (0)
  39 #  define MOD(a) \
  40     do { \
  41         CHOP(a); \
  42         MOD28(a); \
  43     } while (0)
  44 #  define MOD63(a) \
  45     do { /* this assumes a is not negative */ \
  46         z_off64_t tmp = a >> 32; \
  47         a &= 0xffffffffL; \
  48         a += (tmp << 8) - (tmp << 5) + tmp; \
  49         tmp = a >> 16; \
  50         a &= 0xffffL; \
  51         a += (tmp << 4) - tmp; \
  52         tmp = a >> 16; \
  53         a &= 0xffffL; \
  54         a += (tmp << 4) - tmp; \
  55         if (a >= BASE) a -= BASE; \
  56     } while (0)
  57 #else
  58 #  define MOD(a) a %= BASE
  59 #  define MOD28(a) a %= BASE
  60 #  define MOD63(a) a %= BASE
  61 #endif
  62
  63 /* ========================================================================= */
  64 uLong ZEXPORT adler32(adler, buf, len)
  65     uLong adler;
  66     const Bytef *buf;
  67     uInt len;
  68 {
  69     unsigned long sum2;
  70     unsigned n;
  71
  72     /* split Adler-32 into component sums */
  73     sum2 = (adler >> 16) & 0xffff;
  74     adler &= 0xffff;
  75
  76     /* in case user likes doing a byte at a time, keep it fast */
  77     if (len == 1) {
  78         adler += buf[0];
  79         if (adler >= BASE)
  80             adler -= BASE;
  81         sum2 += adler;
  82         if (sum2 >= BASE)
  83             sum2 -= BASE;
  84         return adler | (sum2 << 16);
  85     }
  86
  87     /* initial Adler-32 value (deferred check for len == 1 speed) */
  88     if (buf == Z_NULL)
  89         return 1L;
  90
  91     /* in case short lengths are provided, keep it somewhat fast */
  92     if (len < 16) {
  93         while (len--) {
  94             adler += *buf++;
  95             sum2 += adler;
  96         }
  97         if (adler >= BASE)
  98             adler -= BASE;
  99         MOD28(sum2);            /* only added so many BASE's */
 100         return adler | (sum2 << 16);
 101     }
 102
 103     /* do length NMAX blocks -- requires just one modulo operation */
 104     while (len >= NMAX) {
 105         len -= NMAX;
 106         n = NMAX / 16;          /* NMAX is divisible by 16 */
 107         do {
 108             DO16(buf);          /* 16 sums unrolled */
 109             buf += 16;
 110         } while (--n);
 111         MOD(adler);
 112         MOD(sum2);
 113     }
 114
 115     /* do remaining bytes (less than NMAX, still just one modulo) */
 116     if (len) {                  /* avoid modulos if none remaining */
 117         while (len >= 16) {
 118             len -= 16;
 119             DO16(buf);
 120             buf += 16;
 121         }
 122         while (len--) {
 123             adler += *buf++;
 124             sum2 += adler;
 125         }
 126         MOD(adler);
 127         MOD(sum2);
 128     }
 129
 130     /* return recombined sums */
 131     return adler | (sum2 << 16);
 132 }
 133
 134 /* ========================================================================= */
 135 local uLong adler32_combine_(adler1, adler2, len2)
 136     uLong adler1;
 137     uLong adler2;
 138     z_off64_t len2;
 139 {
 140     unsigned long sum1;
 141     unsigned long sum2;
 142     unsigned rem;
 143
 144     /* for negative len, return invalid adler32 as a clue for debugging */
 145     if (len2 < 0)
 146         return 0xffffffffUL;
 147
 148     /* the derivation of this formula is left as an exercise for the reader */
 149     MOD63(len2);                /* assumes len2 >= 0 */
 150     rem = (unsigned)len2;
 151     sum1 = adler1 & 0xffff;
 152     sum2 = rem * sum1;
 153     MOD(sum2);
 154     sum1 += (adler2 & 0xffff) + BASE - 1;
 155     sum2 += ((adler1 >> 16) & 0xffff) + ((adler2 >> 16) & 0xffff) + BASE - rem;
 156     if (sum1 >= BASE) sum1 -= BASE;
 157     if (sum1 >= BASE) sum1 -= BASE;
 158     if (sum2 >= (BASE << 1)) sum2 -= (BASE << 1);
 159     if (sum2 >= BASE) sum2 -= BASE;
 160     return sum1 | (sum2 << 16);
 161 }
 162
 163 /* ========================================================================= */
 164 uLong ZEXPORT adler32_combine(adler1, adler2, len2)
 165     uLong adler1;
 166     uLong adler2;
 167     z_off_t len2;
 168 {
 169     return adler32_combine_(adler1, adler2, len2);
 170 }
 171
 172 uLong ZEXPORT adler32_combine64(adler1, adler2, len2)
 173     uLong adler1;
 174     uLong adler2;
 175     z_off64_t len2;
 176 {
 177     return adler32_combine_(adler1, adler2, len2);
 178 }