150 lines
4.5 KiB
C
150 lines
4.5 KiB
C
/* adler32.c -- compute the Adler-32 checksum of a data stream
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* Copyright (C) 1995-2004 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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/* @(#) $Id$ */
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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) {adler += (buf)[i]; sum2 += adler;}
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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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/* use NO_DIVIDE if your processor does not do division in hardware */
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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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# define MOD4(a) \
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do { \
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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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# define MOD4(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 sum2;
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unsigned n;
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/* split Adler-32 into component sums */
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sum2 = (adler >> 16) & 0xffff;
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adler &= 0xffff;
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/* in case user likes doing a byte at a time, keep it fast */
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if (len == 1) {
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adler += buf[0];
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if (adler >= BASE)
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adler -= BASE;
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sum2 += adler;
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if (sum2 >= BASE)
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sum2 -= BASE;
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return adler | (sum2 << 16);
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}
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/* initial Adler-32 value (deferred check for len == 1 speed) */
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if (buf == Z_NULL)
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return 1L;
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/* in case short lengths are provided, keep it somewhat fast */
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if (len < 16) {
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while (len--) {
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adler += *buf++;
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sum2 += adler;
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}
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if (adler >= BASE)
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adler -= BASE;
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MOD4(sum2); /* only added so many BASE's */
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return adler | (sum2 << 16);
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}
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/* do length NMAX blocks -- requires just one modulo operation */
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while (len >= NMAX) {
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len -= NMAX;
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n = NMAX / 16; /* NMAX is divisible by 16 */
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do {
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DO16(buf); /* 16 sums unrolled */
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buf += 16;
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} while (--n);
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MOD(adler);
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MOD(sum2);
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}
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/* do remaining bytes (less than NMAX, still just one modulo) */
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if (len) { /* avoid modulos if none remaining */
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while (len >= 16) {
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len -= 16;
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DO16(buf);
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buf += 16;
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}
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while (len--) {
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adler += *buf++;
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sum2 += adler;
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}
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MOD(adler);
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MOD(sum2);
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}
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/* return recombined sums */
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return adler | (sum2 << 16);
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}
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/* ========================================================================= */
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uLong ZEXPORT adler32_combine(adler1, adler2, len2)
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uLong adler1;
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uLong adler2;
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z_off_t len2;
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{
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unsigned long sum1;
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unsigned long sum2;
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unsigned rem;
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/* the derivation of this formula is left as an exercise for the reader */
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rem = (unsigned)(len2 % BASE);
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sum1 = adler1 & 0xffff;
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sum2 = rem * sum1;
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MOD(sum2);
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sum1 += (adler2 & 0xffff) + BASE - 1;
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sum2 += ((adler1 >> 16) & 0xffff) + ((adler2 >> 16) & 0xffff) + BASE - rem;
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if (sum1 > BASE) sum1 -= BASE;
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if (sum1 > BASE) sum1 -= BASE;
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if (sum2 > (BASE << 1)) sum2 -= (BASE << 1);
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if (sum2 > BASE) sum2 -= BASE;
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return sum1 | (sum2 << 16);
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}
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