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## DEFINITIONS

This source file includes following definitions.
```   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 /* ========================================================================= */
59     const Bytef *buf;
60     uInt len;
61 {
62     unsigned long sum2;
63     unsigned n;
64
65     /* split Adler-32 into component sums */
66     sum2 = (adler >> 16) & 0xffff;
67     adler &= 0xffff;
68
69     /* in case user likes doing a byte at a time, keep it fast */
70     if (len == 1) {
71         adler += buf[0];
72         if (adler >= BASE)
73             adler -= BASE;
74         sum2 += adler;
75         if (sum2 >= BASE)
76             sum2 -= BASE;
77         return adler | (sum2 << 16);
78     }
79
80     /* initial Adler-32 value (deferred check for len == 1 speed) */
81     if (buf == Z_NULL)
82         return 1L;
83
84     /* in case short lengths are provided, keep it somewhat fast */
85     if (len < 16) {
86         while (len--) {
87             adler += *buf++;
88             sum2 += adler;
89         }
90         if (adler >= BASE)
91             adler -= BASE;
92         MOD4(sum2);             /* only added so many BASE's */
93         return adler | (sum2 << 16);
94     }
95
96     /* do length NMAX blocks -- requires just one modulo operation */
97     while (len >= NMAX) {
98         len -= NMAX;
99         n = NMAX / 16;          /* NMAX is divisible by 16 */
100         do {
101             DO16(buf);          /* 16 sums unrolled */
102             buf += 16;
103         } while (--n);
105         MOD(sum2);
106     }
107
108     /* do remaining bytes (less than NMAX, still just one modulo) */
109     if (len) {                  /* avoid modulos if none remaining */
110         while (len >= 16) {
111             len -= 16;
112             DO16(buf);
113             buf += 16;
114         }
115         while (len--) {
116             adler += *buf++;
117             sum2 += adler;
118         }
120         MOD(sum2);
121     }
122
123     /* return recombined sums */
124     return adler | (sum2 << 16);
125 }
126
127 /* ========================================================================= */
131     z_off_t len2;
132 {
133     unsigned long sum1;
134     unsigned long sum2;
135     unsigned rem;
136
137     /* the derivation of this formula is left as an exercise for the reader */
138     rem = (unsigned)(len2 % BASE);
139     sum1 = adler1 & 0xffff;
140     sum2 = rem * sum1;
141     MOD(sum2);
142     sum1 += (adler2 & 0xffff) + BASE - 1;
143     sum2 += ((adler1 >> 16) & 0xffff) + ((adler2 >> 16) & 0xffff) + BASE - rem;
144     if (sum1 > BASE) sum1 -= BASE;
145     if (sum1 > BASE) sum1 -= BASE;
146     if (sum2 > (BASE << 1)) sum2 -= (BASE << 1);
147     if (sum2 > BASE) sum2 -= BASE;
148     return sum1 | (sum2 << 16);
149 }
```

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