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 }