    /* 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);
    }