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adler32.c revision 1.1 
      1 /*	$NetBSD: adler32.c,v 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 }
    152