/* adler32.c -- compute the Adler-32 checksum of a data stream
 * Copyright (C) 1995-2007 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_(uLong adler1, uLong adler2, z_off64_t len2);

#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(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;
		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);
}

/* ========================================================================= */
local uLong adler32_combine_(adler1, adler2, len2)
uLong adler1;
uLong adler2;
z_off64_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);
	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);
}
