adler32.c
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上传日期:2013-07-19
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- /* 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 1.1.1.1 2002/10/18 01:43:40 ping 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;
- }