common/dist/zlib/adler32.c

   1 /*      $NetBSD: adler32.c,v 1.1.1.1 2006/01/14 20:10:24 christos Exp $ */
   2
   3 /* adler32.c -- compute the Adler-32 checksum of a data stream
   4  * Copyright (C) 1995-2004 Mark Adler
   5  * For conditions of distribution and use, see copyright notice in zlib.h
   6  */
   7
   8 /* @(#) Id */
   9
  10 #define ZLIB_INTERNAL
  11 #include "zlib.h"
  12
  13 #define BASE 65521UL    /* 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 #ifdef NO_DIVIDE
  25 #  define MOD(a) \
  26     do { \
  27         if (a >= (BASE << 16)) a -= (BASE << 16); \
  28         if (a >= (BASE << 15)) a -= (BASE << 15); \
  29         if (a >= (BASE << 14)) a -= (BASE << 14); \
  30         if (a >= (BASE << 13)) a -= (BASE << 13); \
  31         if (a >= (BASE << 12)) a -= (BASE << 12); \
  32         if (a >= (BASE << 11)) a -= (BASE << 11); \
  33         if (a >= (BASE << 10)) a -= (BASE << 10); \
  34         if (a >= (BASE << 9)) a -= (BASE << 9); \
  35         if (a >= (BASE << 8)) a -= (BASE << 8); \
  36         if (a >= (BASE << 7)) a -= (BASE << 7); \
  37         if (a >= (BASE << 6)) a -= (BASE << 6); \
  38         if (a >= (BASE << 5)) a -= (BASE << 5); \
  39         if (a >= (BASE << 4)) a -= (BASE << 4); \
  40         if (a >= (BASE << 3)) a -= (BASE << 3); \
  41         if (a >= (BASE << 2)) a -= (BASE << 2); \
  42         if (a >= (BASE << 1)) a -= (BASE << 1); \
  43         if (a >= BASE) a -= BASE; \
  44     } while (0)
  45 #  define MOD4(a) \
  46     do { \
  47         if (a >= (BASE << 4)) a -= (BASE << 4); \
  48         if (a >= (BASE << 3)) a -= (BASE << 3); \
  49         if (a >= (BASE << 2)) a -= (BASE << 2); \
  50         if (a >= (BASE << 1)) a -= (BASE << 1); \
  51         if (a >= BASE) a -= BASE; \
  52     } while (0)
  53 #else
  54 #  define MOD(a) a %= BASE
  55 #  define MOD4(a) a %= BASE
  56 #endif
  57
  58 /* ========================================================================= */
  59 uLong ZEXPORT adler32(adler, buf, len)
  60     uLong adler;
  61     const Bytef *buf;
  62     uInt len;
  63 {
  64     unsigned long sum2;
  65     unsigned n;
  66
  67     /* split Adler-32 into component sums */
  68     sum2 = (adler >> 16) & 0xffff;
  69     adler &= 0xffff;
  70
  71     /* in case user likes doing a byte at a time, keep it fast */
  72     if (len == 1) {
  73         adler += buf[0];
  74         if (adler >= BASE)
  75             adler -= BASE;
  76         sum2 += adler;
  77         if (sum2 >= BASE)
  78             sum2 -= BASE;
  79         return adler | (sum2 << 16);
  80     }
  81
  82     /* initial Adler-32 value (deferred check for len == 1 speed) */
  83     if (buf == Z_NULL)
  84         return 1L;
  85
  86     /* in case short lengths are provided, keep it somewhat fast */
  87     if (len < 16) {
  88         while (len--) {
  89             adler += *buf++;
  90             sum2 += adler;
  91         }
  92         if (adler >= BASE)
  93             adler -= BASE;
  94         MOD4(sum2);             /* only added so many BASE's */
  95         return adler | (sum2 << 16);
  96     }
  97
  98     /* do length NMAX blocks -- requires just one modulo operation */
  99     while (len >= NMAX) {
 100         len -= NMAX;
 101         n = NMAX / 16;          /* NMAX is divisible by 16 */
 102         do {
 103             DO16(buf);          /* 16 sums unrolled */
 104             buf += 16;
 105         } while (--n);
 106         MOD(adler);
 107         MOD(sum2);
 108     }
 109
 110     /* do remaining bytes (less than NMAX, still just one modulo) */
 111     if (len) {                  /* avoid modulos if none remaining */
 112         while (len >= 16) {
 113             len -= 16;
 114             DO16(buf);
 115             buf += 16;
 116         }
 117         while (len--) {
 118             adler += *buf++;
 119             sum2 += adler;
 120         }
 121         MOD(adler);
 122         MOD(sum2);
 123     }
 124
 125     /* return recombined sums */
 126     return adler | (sum2 << 16);
 127 }
 128
 129 /* ========================================================================= */
 130 uLong ZEXPORT adler32_combine(adler1, adler2, len2)
 131     uLong adler1;
 132     uLong adler2;
 133     z_off_t len2;
 134 {
 135     unsigned long sum1;
 136     unsigned long sum2;
 137     unsigned rem;
 138
 139     /* the derivation of this formula is left as an exercise for the reader */
 140     rem = (unsigned)(len2 % BASE);
 141     sum1 = adler1 & 0xffff;
 142     sum2 = rem * sum1;
 143     MOD(sum2);
 144     sum1 += (adler2 & 0xffff) + BASE - 1;
 145     sum2 += ((adler1 >> 16) & 0xffff) + ((adler2 >> 16) & 0xffff) + BASE - rem;
 146     if (sum1 > BASE) sum1 -= BASE;
 147     if (sum1 > BASE) sum1 -= BASE;
 148     if (sum2 > (BASE << 1)) sum2 -= (BASE << 1);
 149     if (sum2 > BASE) sum2 -= BASE;
 150     return sum1 | (sum2 << 16);
 151 }