zlib121/adler32.c
author Ryan C. Gordon <icculus@icculus.org>
Wed, 29 Sep 2004 06:03:44 +0000
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/* adler32.c -- compute the Adler-32 checksum of a data stream
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 * Copyright (C) 1995-2003 Mark Adler
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 * For conditions of distribution and use, see copyright notice in zlib.h
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 */
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/* @(#) $Id: adler32.c,v 1.1 2003/12/07 05:29:20 icculus Exp $ */
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#define ZLIB_INTERNAL
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#include "zlib.h"
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#define BASE 65521UL    /* largest prime smaller than 65536 */
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#define NMAX 5552
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/* NMAX is the largest n such that 255n(n+1)/2 + (n+1)(BASE-1) <= 2^32-1 */
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#define DO1(buf,i)  {s1 += buf[i]; s2 += s1;}
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#define DO2(buf,i)  DO1(buf,i); DO1(buf,i+1);
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#define DO4(buf,i)  DO2(buf,i); DO2(buf,i+2);
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#define DO8(buf,i)  DO4(buf,i); DO4(buf,i+4);
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#define DO16(buf)   DO8(buf,0); DO8(buf,8);
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#ifdef NO_DIVIDE
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#  define MOD(a) \
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    do { \
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        if (a >= (BASE << 16)) a -= (BASE << 16); \
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        if (a >= (BASE << 15)) a -= (BASE << 15); \
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        if (a >= (BASE << 14)) a -= (BASE << 14); \
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        if (a >= (BASE << 13)) a -= (BASE << 13); \
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        if (a >= (BASE << 12)) a -= (BASE << 12); \
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        if (a >= (BASE << 11)) a -= (BASE << 11); \
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        if (a >= (BASE << 10)) a -= (BASE << 10); \
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        if (a >= (BASE << 9)) a -= (BASE << 9); \
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        if (a >= (BASE << 8)) a -= (BASE << 8); \
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        if (a >= (BASE << 7)) a -= (BASE << 7); \
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        if (a >= (BASE << 6)) a -= (BASE << 6); \
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        if (a >= (BASE << 5)) a -= (BASE << 5); \
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        if (a >= (BASE << 4)) a -= (BASE << 4); \
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        if (a >= (BASE << 3)) a -= (BASE << 3); \
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        if (a >= (BASE << 2)) a -= (BASE << 2); \
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        if (a >= (BASE << 1)) a -= (BASE << 1); \
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        if (a >= BASE) a -= BASE; \
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    } while (0)
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#else
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#  define MOD(a) a %= BASE
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#endif
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/* ========================================================================= */
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uLong ZEXPORT adler32(adler, buf, len)
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    uLong adler;
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    const Bytef *buf;
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    uInt len;
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{
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    unsigned long s1 = adler & 0xffff;
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    unsigned long s2 = (adler >> 16) & 0xffff;
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    int k;
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    if (buf == Z_NULL) return 1L;
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    while (len > 0) {
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        k = len < NMAX ? (int)len : NMAX;
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        len -= k;
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        while (k >= 16) {
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            DO16(buf);
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            buf += 16;
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            k -= 16;
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        }
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        if (k != 0) do {
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            s1 += *buf++;
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            s2 += s1;
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        } while (--k);
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        MOD(s1);
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        MOD(s2);
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    }
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    return (s2 << 16) | s1;
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}