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