www.pudn.com > zlibsource.zip > adler32.c

/* adler32.c -- compute the Adler-32 checksum of a data stream 
 * Copyright (C) 1995-2002 Mark Adler 
 * For conditions of distribution and use, see copyright notice in zlib.h  
 */ 
 
/* @(#) $Id: adler32.c,v 1.0 2002-05-07 00:56:32+02 jdehalle Exp $ */ 
 
#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); 
 
/* ========================================================================= */ 
uLong ZEXPORT adler32(adler, buf, len) 
    uLong adler; 
    const Bytef *buf; 
    uInt len; 
{ 
    unsigned long s1 = adler & 0xffff; 
    unsigned long 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; 
}