/* 
MODIFIED AND FROM:
adler32.c -- compute the Adler-32 checksum of a data stream
 * Copyright (C) 1995-2011 Mark Adler
 Copyright (C) 1995-2013 Jean-loup Gailly and Mark Adler

 This software is provided 'as-is', without any express or implied
 warranty.  In no event will the authors be held liable for any damages
 arising from the use of this software.

 Permission is granted to anyone to use this software for any purpose,
 including commercial applications, and to alter it and redistribute it
 freely, subject to the following restrictions:

 1. The origin of this software must not be misrepresented; you must not
 claim that you wrote the original software. If you use this software
 in a product, an acknowledgment in the product documentation would be
 appreciated but is not required.
 2. Altered source versions must be plainly marked as such, and must not be
 misrepresented as being the original software.
 3. This notice may not be removed or altered from any source distribution.

 Jean-loup Gailly        Mark Adler
 jloup@gzip.org          madler@alumni.caltech.edu


 The data format used by the zlib library is described by RFCs (Request for
 Comments) 1950 to 1952 in the files http://tools.ietf.org/html/rfc1950
 (zlib format), rfc1951 (deflate format) and rfc1952 (gzip format).
 */

/* @(#) $Id$ */

#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);


#  define MOD(a) a %= BASE
#  define MOD28(a) a %= BASE
#  define MOD63(a) a %= BASE

/* ========================================================================= */
unsigned int urb_adler32(unsigned int adler, const char* pbuf, unsigned int len)
{
	const unsigned char* buf = reinterpret_cast<const unsigned char*>(pbuf);
    unsigned int sum2;
    unsigned int 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 == nullptr)
        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);
}

unsigned int urb_adler32_combine(unsigned int adler1, unsigned int adler2, unsigned int len2)
{
	unsigned long sum1;
	unsigned long sum2;
	unsigned rem;

	/* 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);
}