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      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 /* @(#) $Id$ */
      7 
      8 #include "zutil.h"
      9 
     10 #define local static
     11 
     12 local uLong adler32_combine_ OF((uLong adler1, uLong adler2, z_off64_t len2));
     13 
     14 #define BASE 65521      /* largest prime smaller than 65536 */
     15 #define NMAX 5552
     16 /* NMAX is the largest n such that 255n(n+1)/2 + (n+1)(BASE-1) <= 2^32-1 */
     17 
     18 #define DO1(buf,i)  {adler += (buf)[i]; sum2 += adler;}
     19 #define DO2(buf,i)  DO1(buf,i); DO1(buf,i+1);
     20 #define DO4(buf,i)  DO2(buf,i); DO2(buf,i+2);
     21 #define DO8(buf,i)  DO4(buf,i); DO4(buf,i+4);
     22 #define DO16(buf)   DO8(buf,0); DO8(buf,8);
     23 
     24 /* use NO_DIVIDE if your processor does not do division in hardware --
     25    try it both ways to see which is faster */
     26 #ifdef NO_DIVIDE
     27 /* note that this assumes BASE is 65521, where 65536 % 65521 == 15
     28    (thank you to John Reiser for pointing this out) */
     29 #  define CHOP(a) \
     30     do { \
     31         unsigned long tmp = a >> 16; \
     32         a &= 0xffffUL; \
     33         a += (tmp << 4) - tmp; \
     34     } while (0)
     35 #  define MOD28(a) \
     36     do { \
     37         CHOP(a); \
     38         if (a >= BASE) a -= BASE; \
     39     } while (0)
     40 #  define MOD(a) \
     41     do { \
     42         CHOP(a); \
     43         MOD28(a); \
     44     } while (0)
     45 #  define MOD63(a) \
     46     do { /* this assumes a is not negative */ \
     47         z_off64_t tmp = a >> 32; \
     48         a &= 0xffffffffL; \
     49         a += (tmp << 8) - (tmp << 5) + tmp; \
     50         tmp = a >> 16; \
     51         a &= 0xffffL; \
     52         a += (tmp << 4) - tmp; \
     53         tmp = a >> 16; \
     54         a &= 0xffffL; \
     55         a += (tmp << 4) - tmp; \
     56         if (a >= BASE) a -= BASE; \
     57     } while (0)
     58 #else
     59 #  define MOD(a) a %= BASE
     60 #  define MOD28(a) a %= BASE
     61 #  define MOD63(a) a %= BASE
     62 #endif
     63 
     64 /* ========================================================================= */
     65 uLong ZEXPORT adler32(adler, buf, len)
     66     uLong adler;
     67     const Bytef *buf;
     68     uInt len;
     69 {
     70     unsigned long sum2;
     71     unsigned n;
     72 
     73     /* split Adler-32 into component sums */
     74     sum2 = (adler >> 16) & 0xffff;
     75     adler &= 0xffff;
     76 
     77     /* in case user likes doing a byte at a time, keep it fast */
     78     if (len == 1) {
     79         adler += buf[0];
     80         if (adler >= BASE)
     81             adler -= BASE;
     82         sum2 += adler;
     83         if (sum2 >= BASE)
     84             sum2 -= BASE;
     85         return adler | (sum2 << 16);
     86     }
     87 
     88     /* initial Adler-32 value (deferred check for len == 1 speed) */
     89     if (buf == Z_NULL)
     90         return 1L;
     91 
     92     /* in case short lengths are provided, keep it somewhat fast */
     93     if (len < 16) {
     94         while (len--) {
     95             adler += *buf++;
     96             sum2 += adler;
     97         }
     98         if (adler >= BASE)
     99             adler -= BASE;
    100         MOD28(sum2);            /* only added so many BASE's */
    101         return adler | (sum2 << 16);
    102     }
    103 
    104     /* do length NMAX blocks -- requires just one modulo operation */
    105     while (len >= NMAX) {
    106         len -= NMAX;
    107         n = NMAX / 16;          /* NMAX is divisible by 16 */
    108         do {
    109             DO16(buf);          /* 16 sums unrolled */
    110             buf += 16;
    111         } while (--n);
    112         MOD(adler);
    113         MOD(sum2);
    114     }
    115 
    116     /* do remaining bytes (less than NMAX, still just one modulo) */
    117     if (len) {                  /* avoid modulos if none remaining */
    118         while (len >= 16) {
    119             len -= 16;
    120             DO16(buf);
    121             buf += 16;
    122         }
    123         while (len--) {
    124             adler += *buf++;
    125             sum2 += adler;
    126         }
    127         MOD(adler);
    128         MOD(sum2);
    129     }
    130 
    131     /* return recombined sums */
    132     return adler | (sum2 << 16);
    133 }
    134 
    135 /* ========================================================================= */
    136 local uLong adler32_combine_(adler1, adler2, len2)
    137     uLong adler1;
    138     uLong adler2;
    139     z_off64_t len2;
    140 {
    141     unsigned long sum1;
    142     unsigned long sum2;
    143     unsigned rem;
    144 
    145     /* for negative len, return invalid adler32 as a clue for debugging */
    146     if (len2 < 0)
    147         return 0xffffffffUL;
    148 
    149     /* the derivation of this formula is left as an exercise for the reader */
    150     MOD63(len2);                /* assumes len2 >= 0 */
    151     rem = (unsigned)len2;
    152     sum1 = adler1 & 0xffff;
    153     sum2 = rem * sum1;
    154     MOD(sum2);
    155     sum1 += (adler2 & 0xffff) + BASE - 1;
    156     sum2 += ((adler1 >> 16) & 0xffff) + ((adler2 >> 16) & 0xffff) + BASE - rem;
    157     if (sum1 >= BASE) sum1 -= BASE;
    158     if (sum1 >= BASE) sum1 -= BASE;
    159     if (sum2 >= (BASE << 1)) sum2 -= (BASE << 1);
    160     if (sum2 >= BASE) sum2 -= BASE;
    161     return sum1 | (sum2 << 16);
    162 }
    163 
    164 /* ========================================================================= */
    165 uLong ZEXPORT adler32_combine(adler1, adler2, len2)
    166     uLong adler1;
    167     uLong adler2;
    168     z_off_t len2;
    169 {
    170     return adler32_combine_(adler1, adler2, len2);
    171 }
    172 
    173 uLong ZEXPORT adler32_combine64(adler1, adler2, len2)
    174     uLong adler1;
    175     uLong adler2;
    176     z_off64_t len2;
    177 {
    178     return adler32_combine_(adler1, adler2, len2);
    179 }
    180