adler32.c revision 146081 
1/* adler32.c -- compute the Adler-32 checksum of a data stream
2 * Copyright (C) 1995-2003 Mark Adler
3 * For conditions of distribution and use, see copyright notice in zlib.h
4 */
5
6/* @(#) $Id$ */
7
8#define ZLIB_INTERNAL
9#include "zlib.h"
10
11#define BASE 65521UL    /* largest prime smaller than 65536 */
12#define NMAX 5552
13/* NMAX is the largest n such that 255n(n+1)/2 + (n+1)(BASE-1) <= 2^32-1 */
14
15#define DO1(buf,i)  {s1 += buf[i]; s2 += s1;}
16#define DO2(buf,i)  DO1(buf,i); DO1(buf,i+1);
17#define DO4(buf,i)  DO2(buf,i); DO2(buf,i+2);
18#define DO8(buf,i)  DO4(buf,i); DO4(buf,i+4);
19#define DO16(buf)   DO8(buf,0); DO8(buf,8);
20
21#ifdef NO_DIVIDE
22#  define MOD(a) \
23    do { \
24        if (a >= (BASE << 16)) a -= (BASE << 16); \
25        if (a >= (BASE << 15)) a -= (BASE << 15); \
26        if (a >= (BASE << 14)) a -= (BASE << 14); \
27        if (a >= (BASE << 13)) a -= (BASE << 13); \
28        if (a >= (BASE << 12)) a -= (BASE << 12); \
29        if (a >= (BASE << 11)) a -= (BASE << 11); \
30        if (a >= (BASE << 10)) a -= (BASE << 10); \
31        if (a >= (BASE << 9)) a -= (BASE << 9); \
32        if (a >= (BASE << 8)) a -= (BASE << 8); \
33        if (a >= (BASE << 7)) a -= (BASE << 7); \
34        if (a >= (BASE << 6)) a -= (BASE << 6); \
35        if (a >= (BASE << 5)) a -= (BASE << 5); \
36        if (a >= (BASE << 4)) a -= (BASE << 4); \
37        if (a >= (BASE << 3)) a -= (BASE << 3); \
38        if (a >= (BASE << 2)) a -= (BASE << 2); \
39        if (a >= (BASE << 1)) a -= (BASE << 1); \
40        if (a >= BASE) a -= BASE; \
41    } while (0)
42#else
43#  define MOD(a) a %= BASE
44#endif
45
46/* ========================================================================= */
47uLong ZEXPORT adler32(adler, buf, len)
48    uLong adler;
49    const Bytef *buf;
50    uInt len;
51{
52    unsigned long s1 = adler & 0xffff;
53    unsigned long s2 = (adler >> 16) & 0xffff;
54    int k;
55
56    if (buf == Z_NULL) return 1L;
57
58    while (len > 0) {
59        k = len < NMAX ? (int)len : NMAX;
60        len -= k;
61        while (k >= 16) {
62            DO16(buf);
63            buf += 16;
64            k -= 16;
65        }
66        if (k != 0) do {
67            s1 += *buf++;
68            s2 += s1;
69        } while (--k);
70        MOD(s1);
71        MOD(s2);
72    }
73    return (s2 << 16) | s1;
74}
75