File indexing completed on 2024-04-21 03:48:28

 /* adler32.c -- compute the Adler-32 checksum of a data stream
  * Copyright (C) 1995-2011 Mark Adler
  * For conditions of distribution and use, see copyright notice in zlib.h
  */
 
 /* @(#) $Id$ */
 
 #include "zutil.h"
 
 #define local static
 
 local uLong adler32_combine_ OF((uLong adler1, uLong adler2, z_off64_t len2));
 
 #define BASE 65521      /* largest prime smaller than 65536 */
 #define NMAX 5552
 /* NMAX is the largest n such that 255n(n+1)/2 + (n+1)(BASE-1) <= 2^32-1 */
 
 #define DO1(buf,i)  {adler += (buf)[i]; sum2 += adler;}
 #define DO2(buf,i)  DO1(buf,i); DO1(buf,i+1);
 #define DO4(buf,i)  DO2(buf,i); DO2(buf,i+2);
 #define DO8(buf,i)  DO4(buf,i); DO4(buf,i+4);
 #define DO16(buf)   DO8(buf,0); DO8(buf,8);
 
 /* use NO_DIVIDE if your processor does not do division in hardware --
    try it both ways to see which is faster */
 #ifdef NO_DIVIDE
 /* note that this assumes BASE is 65521, where 65536 % 65521 == 15
    (thank you to John Reiser for pointing this out) */
 #  define CHOP(a) \
     do { \
         unsigned long tmp = a >> 16; \
         a &= 0xffffUL; \
         a += (tmp << 4) - tmp; \
     } while (0)
 #  define MOD28(a) \
     do { \
         CHOP(a); \
         if (a >= BASE) a -= BASE; \
     } while (0)
 #  define MOD(a) \
     do { \
         CHOP(a); \
         MOD28(a); \
     } while (0)
 #  define MOD63(a) \
     do { /* this assumes a is not negative */ \
         z_off64_t tmp = a >> 32; \
         a &= 0xffffffffL; \
         a += (tmp << 8) - (tmp << 5) + tmp; \
         tmp = a >> 16; \
         a &= 0xffffL; \
         a += (tmp << 4) - tmp; \
         tmp = a >> 16; \
         a &= 0xffffL; \
         a += (tmp << 4) - tmp; \
         if (a >= BASE) a -= BASE; \
     } while (0)
 #else
 #  define MOD(a) a %= BASE
 #  define MOD28(a) a %= BASE
 #  define MOD63(a) a %= BASE
 #endif
 
 /* ========================================================================= */
 uLong ZEXPORT adler32(adler, buf, len)
     uLong adler;
     const Bytef *buf;
     uInt len;
 {
     unsigned long sum2;
     unsigned n;
 
     /* split Adler-32 into component sums */
     sum2 = (adler >> 16) & 0xffff;
     adler &= 0xffff;
 
     /* in case user likes doing a byte at a time, keep it fast */
     if (len == 1) {
         adler += buf[0];
         if (adler >= BASE)
             adler -= BASE;
         sum2 += adler;
         if (sum2 >= BASE)
             sum2 -= BASE;
         return adler | (sum2 << 16);
     }
 
     /* initial Adler-32 value (deferred check for len == 1 speed) */
     if (buf == Z_NULL)
         return 1L;
 
     /* in case short lengths are provided, keep it somewhat fast */
     if (len < 16) {
         while (len--) {
             adler += *buf++;
             sum2 += adler;
         }
         if (adler >= BASE)
             adler -= BASE;
         MOD28(sum2);            /* only added so many BASE's */
         return adler | (sum2 << 16);
     }
 
     /* do length NMAX blocks -- requires just one modulo operation */
     while (len >= NMAX) {
         len -= NMAX;
         n = NMAX / 16;          /* NMAX is divisible by 16 */
         do {
             DO16(buf);          /* 16 sums unrolled */
             buf += 16;
         } while (--n);
         MOD(adler);
         MOD(sum2);
     }
 
     /* do remaining bytes (less than NMAX, still just one modulo) */
     if (len) {                  /* avoid modulos if none remaining */
         while (len >= 16) {
             len -= 16;
             DO16(buf);
             buf += 16;
         }
         while (len--) {
             adler += *buf++;
             sum2 += adler;
         }
         MOD(adler);
         MOD(sum2);
     }
 
     /* return recombined sums */
     return adler | (sum2 << 16);
 }
 
 /* ========================================================================= */
 local uLong adler32_combine_(adler1, adler2, len2)
     uLong adler1;
     uLong adler2;
     z_off64_t len2;
 {
     unsigned long sum1;
     unsigned long sum2;
     unsigned rem;
 
     /* for negative len, return invalid adler32 as a clue for debugging */
     if (len2 < 0)
         return 0xffffffffUL;
 
     /* the derivation of this formula is left as an exercise for the reader */
     MOD63(len2);                /* assumes len2 >= 0 */
     rem = (unsigned)len2;
     sum1 = adler1 & 0xffff;
     sum2 = rem * sum1;
     MOD(sum2);
     sum1 += (adler2 & 0xffff) + BASE - 1;
     sum2 += ((adler1 >> 16) & 0xffff) + ((adler2 >> 16) & 0xffff) + BASE - rem;
     if (sum1 >= BASE) sum1 -= BASE;
     if (sum1 >= BASE) sum1 -= BASE;
     if (sum2 >= (BASE << 1)) sum2 -= (BASE << 1);
     if (sum2 >= BASE) sum2 -= BASE;
     return sum1 | (sum2 << 16);
 }
 
 /* ========================================================================= */
 uLong ZEXPORT adler32_combine(adler1, adler2, len2)
     uLong adler1;
     uLong adler2;
     z_off_t len2;
 {
     return adler32_combine_(adler1, adler2, len2);
 }
 
 uLong ZEXPORT adler32_combine64(adler1, adler2, len2)
     uLong adler1;
     uLong adler2;
     z_off64_t len2;
 {
     return adler32_combine_(adler1, adler2, len2);
 }