ref: 66b6185845e85258f1408271d5f705aacfa6ffdb
dir: /sys/src/cmd/gs/zlib/adler32.c/
/* adler32.c -- compute the Adler-32 checksum of a data stream
* Copyright (C) 1995-2003 Mark Adler
* For conditions of distribution and use, see copyright notice in zlib.h
*/
/* @(#) $Id: adler32.c,v 1.1.1.1 2005/04/24 21:39:38 giles Exp $ */
#define ZLIB_INTERNAL
#include "zlib.h"
#define BASE 65521UL /* 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);
#ifdef NO_DIVIDE
# define MOD(a) \
do { \
if (a >= (BASE << 16)) a -= (BASE << 16); \
if (a >= (BASE << 15)) a -= (BASE << 15); \
if (a >= (BASE << 14)) a -= (BASE << 14); \
if (a >= (BASE << 13)) a -= (BASE << 13); \
if (a >= (BASE << 12)) a -= (BASE << 12); \
if (a >= (BASE << 11)) a -= (BASE << 11); \
if (a >= (BASE << 10)) a -= (BASE << 10); \
if (a >= (BASE << 9)) a -= (BASE << 9); \
if (a >= (BASE << 8)) a -= (BASE << 8); \
if (a >= (BASE << 7)) a -= (BASE << 7); \
if (a >= (BASE << 6)) a -= (BASE << 6); \
if (a >= (BASE << 5)) a -= (BASE << 5); \
if (a >= (BASE << 4)) a -= (BASE << 4); \
if (a >= (BASE << 3)) a -= (BASE << 3); \
if (a >= (BASE << 2)) a -= (BASE << 2); \
if (a >= (BASE << 1)) a -= (BASE << 1); \
if (a >= BASE) a -= BASE; \
} while (0)
#else
# define MOD(a) a %= BASE
#endif
/* ========================================================================= */
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 ? (int)len : NMAX;
len -= k;
while (k >= 16) {
DO16(buf);
buf += 16;
k -= 16;
}
if (k != 0) do {
s1 += *buf++;
s2 += s1;
} while (--k);
MOD(s1);
MOD(s2);
}
return (s2 << 16) | s1;
}