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    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: adler32.c,v 1.1.1.1 2012/02/17 15:09:30 misho Exp $ */
    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: }