/* adler32.c -- compute the Adler-32 checksum of a data stream
 * Copyright (C) 1995-2004 Mark Adler
* SPDX-License-Identifier:	GPL-2.0+
 * 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 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 */
#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

/* ========================================================================= */
uLong ZEXPORT adler32(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;
		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(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);
}
