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adler32.c (4604B)
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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: adler32.c 1925 2005-10-06 19:32:02Z baford $ */
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 /* ========================================================================= */
57 uLong ZEXPORT adler32(adler, buf, len)
58 uLong adler;
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);
104 MOD(adler);
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 }
119 MOD(adler);
120 MOD(sum2);
121 }
122
123 /* return recombined sums */
124 return adler | (sum2 << 16);
125 }
126
127 /* ========================================================================= */
128 uLong ZEXPORT adler32_combine(adler1, adler2, len2)
129 uLong adler1;
130 uLong adler2;
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 }