/* 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);

}