/*
 * Adler-32 checksum
 *
 * Copyright (c) 2003-2019 Joergen Ibsen
 *
 * 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.
 */

/*
 * Adler-32 algorithm taken from the zlib source, which is
 * Copyright (C) 1995-1998 Jean-loup Gailly and Mark Adler
 */

#include "tinf.h"

#define A32_BASE 65521
#define A32_NMAX 5552

unsigned int tinf_adler32(const void *data, unsigned int length)
{
	const unsigned char *buf = (const unsigned char *) data;

	unsigned int s1 = 1;
	unsigned int s2 = 0;

	while (length > 0) {
		int k = length < A32_NMAX ? length : A32_NMAX;
		int i;

		for (i = k / 16; i; --i, buf += 16) {
			s1 += buf[0];
			s2 += s1;
			s1 += buf[1];
			s2 += s1;
			s1 += buf[2];
			s2 += s1;
			s1 += buf[3];
			s2 += s1;
			s1 += buf[4];
			s2 += s1;
			s1 += buf[5];
			s2 += s1;
			s1 += buf[6];
			s2 += s1;
			s1 += buf[7];
			s2 += s1;

			s1 += buf[8];
			s2 += s1;
			s1 += buf[9];
			s2 += s1;
			s1 += buf[10];
			s2 += s1;
			s1 += buf[11];
			s2 += s1;
			s1 += buf[12];
			s2 += s1;
			s1 += buf[13];
			s2 += s1;
			s1 += buf[14];
			s2 += s1;
			s1 += buf[15];
			s2 += s1;
		}

		for (i = k % 16; i; --i) {
			s1 += *buf++;
			s2 += s1;
		}

		s1 %= A32_BASE;
		s2 %= A32_BASE;

		length -= k;
	}

	return (s2 << 16) | s1;
}