### Files

``````/* adler32.c -- compute the Adler-32 checksum of a data stream
* For conditions of distribution and use, see copyright notice in zlib.h
*/

/* @(#) \$Id\$ */

#include "zutil.h"

#define BASE 65521U     /* 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 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

/* ========================================================================= */
const Bytef *buf,
z_size_t len)
{
unsigned long sum2;
unsigned n;

/* split Adler-32 into component sums */
sum2 = (adler >> 16) & 0xffff;

/* in case user likes doing a byte at a time, keep it fast */
if (len == 1) {
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--) {
}
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(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--) {
}
MOD(sum2);
}

/* return recombined sums */
return adler | (sum2 << 16);
}

/* ========================================================================= */
const Bytef *buf,
uInt len)
{
}

/* ========================================================================= */
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;
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 >= ((unsigned long)BASE << 1)) sum2 -= ((unsigned long)BASE << 1);
if (sum2 >= BASE) sum2 -= BASE;
return sum1 | (sum2 << 16);
}

/* ========================================================================= */