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adler32.c

/*
 *   DCAP - dCache Access Protocol client interface
 *
 *   Copyright (C) 2000,2004 DESY Hamburg DMG-Division.
 *
 *   AUTHOR: Tigran Mkrtchayn (tigran.mkrtchyan@desy.de)
 *
 *   This program can be distributed under the terms of the GNU LGPL.
 *   See the file COPYING.LIB
 *
 */
 
 
/* adler32.c -- compute the Adler-32 checksum of a data stream
 * Copyright (C) 1995-1996 Mark Adler
 * For conditions of distribution and use, see copyright notice in zlib.h 
 */

/* Id: adler32.c,v 1.3 1999/04/23 18:30:29 cgw Exp  */
/*
 * $Id: adler32.c,v 1.7 2005-04-25 07:56:37 tigran Exp $
 */

#define BASE 65521L /* 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)  {s1 += buf[i]; s2 += s1;}
#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);

/* ========================================================================= */
unsigned long update_adler32(unsigned long adler, const unsigned char *buf, unsigned int len)
{
    unsigned long s1 = adler & 0xffff;
    unsigned long s2 = (adler >> 16) & 0xffff;
    int k;

    if (buf == 0) return 1L;

    while (len > 0) {
        k = len < NMAX ? len : NMAX;
        len -= k;
        while (k >= 16) {
            DO16(buf);
          buf += 16;
            k -= 16;
        }
        if (k != 0) do {
            s1 += *buf++;
          s2 += s1;
        } while (--k);
        s1 %= BASE;
        s2 %= BASE;
    }
    return (s2 << 16) | s1;
}

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