adler32.c

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/****************************************************************************
**  Copyright  (c)  2004,  UTStarcom, Inc.
**          All Rights Reserved.
**
**  Subsystem  : all modules
**  File       : adler32.c
**  Created By : Pengliang
**  Created On : 2/2/2005
**
**  Purpose:
**
****************************************************************************/

/* adler32.c -- compute the Adler-32 checksum of a data stream
 */

#include "zlib.h"
#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);

/**********************************************************
*	Name:uLong adler32(adler, buf, len).
*
*	Desp: compute the Adler-32 checksum of a data stream.
*
*	Para:
*		uLong adler: the orginal crc value.
*		 const Bytef *buf: the data stream buffer.
*		 uInt len: the data stream's size used to count the crc value 
*	NOTE:
*		
******************************************************************/
uLong adler32(uLong adler, unzip_const Bytef *buf, uInt len)
{
    unsigned short s1 = adler & 0xffff;
    unsigned short s2 = (adler >> 16) & 0xffff;
    int k;

    if (buf == Z_NULL) 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;
}