1/* adler32.c -- compute the Adler-32 checksum of a data stream
2 * Copyright (C) 1995-2004 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)  {adler += (buf)[i]; sum2 += adler;}
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/* use NO_DIVIDE if your processor does not do division in hardware */
22#ifdef NO_DIVIDE
23#  define MOD(a) \
24    do { \
25        if (a >= (BASE << 16)) a -= (BASE << 16); \
26        if (a >= (BASE << 15)) a -= (BASE << 15); \
27        if (a >= (BASE << 14)) a -= (BASE << 14); \
28        if (a >= (BASE << 13)) a -= (BASE << 13); \
29        if (a >= (BASE << 12)) a -= (BASE << 12); \
30        if (a >= (BASE << 11)) a -= (BASE << 11); \
31        if (a >= (BASE << 10)) a -= (BASE << 10); \
32        if (a >= (BASE << 9)) a -= (BASE << 9); \
33        if (a >= (BASE << 8)) a -= (BASE << 8); \
34        if (a >= (BASE << 7)) a -= (BASE << 7); \
35        if (a >= (BASE << 6)) a -= (BASE << 6); \
36        if (a >= (BASE << 5)) a -= (BASE << 5); \
37        if (a >= (BASE << 4)) a -= (BASE << 4); \
38        if (a >= (BASE << 3)) a -= (BASE << 3); \
39        if (a >= (BASE << 2)) a -= (BASE << 2); \
40        if (a >= (BASE << 1)) a -= (BASE << 1); \
41        if (a >= BASE) a -= BASE; \
42    } while (0)
43#  define MOD4(a) \
44    do { \
45        if (a >= (BASE << 4)) a -= (BASE << 4); \
46        if (a >= (BASE << 3)) a -= (BASE << 3); \
47        if (a >= (BASE << 2)) a -= (BASE << 2); \
48        if (a >= (BASE << 1)) a -= (BASE << 1); \
49        if (a >= BASE) a -= BASE; \
50    } while (0)
51#else
52#  define MOD(a) a %= BASE
53#  define MOD4(a) a %= BASE
54#endif
55
56/* ========================================================================= */
57uLong ZEXPORT adler32(uLong adler, const Bytef *buf, uInt len)
58{
59    unsigned long sum2;
60    unsigned n;
61
62    /* split Adler-32 into component sums */
63    sum2 = (adler >> 16) & 0xffff;
64    adler &= 0xffff;
65
66    /* in case user likes doing a byte at a time, keep it fast */
67    if (len == 1) {
68        adler += buf[0];
69        if (adler >= BASE)
70            adler -= BASE;
71        sum2 += adler;
72        if (sum2 >= BASE)
73            sum2 -= BASE;
74        return adler | (sum2 << 16);
75    }
76
77    /* initial Adler-32 value (deferred check for len == 1 speed) */
78    if (buf == Z_NULL)
79        return 1L;
80
81    /* in case short lengths are provided, keep it somewhat fast */
82    if (len < 16) {
83        while (len--) {
84            adler += *buf++;
85            sum2 += adler;
86        }
87        if (adler >= BASE)
88            adler -= BASE;
89        MOD4(sum2);             /* only added so many BASE's */
90        return adler | (sum2 << 16);
91    }
92
93    /* do length NMAX blocks -- requires just one modulo operation */
94    while (len >= NMAX) {
95        len -= NMAX;
96        n = NMAX / 16;          /* NMAX is divisible by 16 */
97        do {
98            DO16(buf);          /* 16 sums unrolled */
99            buf += 16;
100        } while (--n);
101        MOD(adler);
102        MOD(sum2);
103    }
104
105    /* do remaining bytes (less than NMAX, still just one modulo) */
106    if (len) {                  /* avoid modulos if none remaining */
107        while (len >= 16) {
108            len -= 16;
109            DO16(buf);
110            buf += 16;
111        }
112        while (len--) {
113            adler += *buf++;
114            sum2 += adler;
115        }
116        MOD(adler);
117        MOD(sum2);
118    }
119
120    /* return recombined sums */
121    return adler | (sum2 << 16);
122}
123