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

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

#define ZLIB_INTERNAL
#include "zlib.h"

#define BASE 65521UL    /* 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 */
#ifdef NO_DIVIDE
#  define MOD(a) \
do { \
if (a >= (BASE << 16)) a -= (BASE << 16); \
if (a >= (BASE << 15)) a -= (BASE << 15); \
if (a >= (BASE << 14)) a -= (BASE << 14); \
if (a >= (BASE << 13)) a -= (BASE << 13); \
if (a >= (BASE << 12)) a -= (BASE << 12); \
if (a >= (BASE << 11)) a -= (BASE << 11); \
if (a >= (BASE << 10)) a -= (BASE << 10); \
if (a >= (BASE << 9)) a -= (BASE << 9); \
if (a >= (BASE << 8)) a -= (BASE << 8); \
if (a >= (BASE << 7)) a -= (BASE << 7); \
if (a >= (BASE << 6)) a -= (BASE << 6); \
if (a >= (BASE << 5)) a -= (BASE << 5); \
if (a >= (BASE << 4)) a -= (BASE << 4); \
if (a >= (BASE << 3)) a -= (BASE << 3); \
if (a >= (BASE << 2)) a -= (BASE << 2); \
if (a >= (BASE << 1)) a -= (BASE << 1); \
if (a >= BASE) a -= BASE; \
} while (0)
#  define MOD4(a) \
do { \
if (a >= (BASE << 4)) a -= (BASE << 4); \
if (a >= (BASE << 3)) a -= (BASE << 3); \
if (a >= (BASE << 2)) a -= (BASE << 2); \
if (a >= (BASE << 1)) a -= (BASE << 1); \
if (a >= BASE) a -= BASE; \
} while (0)
#else
#  define MOD(a) a %= BASE
#  define MOD4(a) a %= BASE
#endif

/* ========================================================================= */
const Bytef *buf;
uInt 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--) {
}
MOD4(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);
}

/* ========================================================================= */
z_off_t len2;
{
unsigned long sum1;
unsigned long sum2;
unsigned rem;

/* the derivation of this formula is left as an exercise for the reader */
rem = (unsigned)(len2 % BASE);