src/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(adler, buf, len)
  58     uLong adler;
  59     const Bytef *buf;
  60     uInt len;
  61 {
  62     unsigned long sum2;
  63     unsigned n;
  64
  65     /* split Adler-32 into component sums */
  66     sum2 = (adler >> 16) & 0xffff;
  67     adler &= 0xffff;
  68
  69     /* in case user likes doing a byte at a time, keep it fast */
  70     if (len == 1) {
  71         adler += buf[0];
  72         if (adler >= BASE)
  73             adler -= BASE;
  74         sum2 += adler;
  75         if (sum2 >= BASE)
  76             sum2 -= BASE;
  77         return adler | (sum2 << 16);
  78     }
  79
  80     /* initial Adler-32 value (deferred check for len == 1 speed) */
  81     if (buf == Z_NULL)
  82         return 1L;
  83
  84     /* in case short lengths are provided, keep it somewhat fast */
  85     if (len < 16) {
  86         while (len--) {
  87             adler += *buf++;
  88             sum2 += adler;
  89         }
  90         if (adler >= BASE)
  91             adler -= BASE;
  92         MOD4(sum2);             /* only added so many BASE's */
  93         return adler | (sum2 << 16);
  94     }
  95
  96     /* do length NMAX blocks -- requires just one modulo operation */
  97     while (len >= NMAX) {
  98         len -= NMAX;
  99         n = NMAX / 16;          /* NMAX is divisible by 16 */
 100         do {
 101             DO16(buf);          /* 16 sums unrolled */
 102             buf += 16;
 103         } while (--n);
 104         MOD(adler);
 105         MOD(sum2);
 106     }
 107
 108     /* do remaining bytes (less than NMAX, still just one modulo) */
 109     if (len) {                  /* avoid modulos if none remaining */
 110         while (len >= 16) {
 111             len -= 16;
 112             DO16(buf);
 113             buf += 16;
 114         }
 115         while (len--) {
 116             adler += *buf++;
 117             sum2 += adler;
 118         }
 119         MOD(adler);
 120         MOD(sum2);
 121     }
 122
 123     /* return recombined sums */
 124     return adler | (sum2 << 16);
 125 }
 126
 127 /* ========================================================================= */
 128 uLong ZEXPORT adler32_combine(adler1, adler2, len2)
 129     uLong adler1;
 130     uLong adler2;
 131     z_off_t len2;
 132 {
 133     unsigned long sum1;
 134     unsigned long sum2;
 135     unsigned rem;
 136
 137     /* the derivation of this formula is left as an exercise for the reader */
 138     rem = (unsigned)(len2 % BASE);
 139     sum1 = adler1 & 0xffff;
 140     sum2 = rem * sum1;
 141     MOD(sum2);
 142     sum1 += (adler2 & 0xffff) + BASE - 1;
 143     sum2 += ((adler1 >> 16) & 0xffff) + ((adler2 >> 16) & 0xffff) + BASE - rem;
 144     if (sum1 > BASE) sum1 -= BASE;
 145     if (sum1 > BASE) sum1 -= BASE;
 146     if (sum2 > (BASE << 1)) sum2 -= (BASE << 1);
 147     if (sum2 > BASE) sum2 -= BASE;
 148     return sum1 | (sum2 << 16);
 149 }