adler32.c
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上传日期:2020-08-09
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文件大小:5k
- /* adler32.c -- compute the Adler-32 checksum of a data stream
- * Copyright (C) 1995-2004 Mark Adler
- * For conditions of distribution and use, see copyright notice in zlib.h
- */
- /* @(#) $Id$ */
- #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) {adler += (buf)[i]; sum2 += adler;}
- #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);
- /* use NO_DIVIDE if your processor does not do division in hardware */
- #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)
- # define MOD4(a)
- do {
- 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
- # define MOD4(a) a %= BASE
- #endif
- /* ========================================================================= */
- uLong ZEXPORT adler32(uLong adler, const Bytef *buf, uInt len)
- {
- unsigned long sum2 = 0;
- unsigned n = 0;
- /* split Adler-32 into component sums */
- sum2 = (adler >> 16) & 0xffff;
- adler &= 0xffff;
- /* in case user likes doing a byte at a time, keep it fast */
- if (len == 1) {
- adler += buf[0];
- if (adler >= BASE)
- adler -= BASE;
- sum2 += adler;
- if (sum2 >= BASE)
- sum2 -= BASE;
- return adler | (sum2 << 16);
- }
- /* initial Adler-32 value (deferred check for len == 1 speed) */
- if (buf == Z_NULL)
- return 1L;
- /* in case short lengths are provided, keep it somewhat fast */
- if (len < 16) {
- while (len--) {
- adler += *buf++;
- sum2 += adler;
- }
- if (adler >= BASE)
- adler -= BASE;
- MOD4(sum2); /* only added so many BASE's */
- return adler | (sum2 << 16);
- }
- /* do length NMAX blocks -- requires just one modulo operation */
- while (len >= NMAX) {
- len -= NMAX;
- n = NMAX / 16; /* NMAX is divisible by 16 */
- do {
- DO16(buf); /* 16 sums unrolled */
- buf += 16;
- } while (--n);
- MOD(adler);
- MOD(sum2);
- }
- /* do remaining bytes (less than NMAX, still just one modulo) */
- if (len) { /* avoid modulos if none remaining */
- while (len >= 16) {
- len -= 16;
- DO16(buf);
- buf += 16;
- }
- while (len--) {
- adler += *buf++;
- sum2 += adler;
- }
- MOD(adler);
- MOD(sum2);
- }
- /* return recombined sums */
- return adler | (sum2 << 16);
- }
- /* ========================================================================= */
- uLong ZEXPORT adler32_combine(uLong adler1, uLong adler2, z_off_t len2)
- {
- unsigned long sum1 = 0;
- unsigned long sum2 = 0;
- unsigned rem = 0;
- /* the derivation of this formula is left as an exercise for the reader */
- rem = (unsigned)(len2 % BASE);
- sum1 = adler1 & 0xffff;
- sum2 = rem * sum1;
- MOD(sum2);
- sum1 += (adler2 & 0xffff) + BASE - 1;
- sum2 += ((adler1 >> 16) & 0xffff) + ((adler2 >> 16) & 0xffff) + BASE - rem;
- if (sum1 > BASE) sum1 -= BASE;
- if (sum1 > BASE) sum1 -= BASE;
- if (sum2 > (BASE << 1)) sum2 -= (BASE << 1);
- if (sum2 > BASE) sum2 -= BASE;
- return sum1 | (sum2 << 16);
- }