1 /* $NetBSD: adler32.c,v 1.1.1.1 2006/01/14 20:10:24 christos Exp $ */
2
3 /* adler32.c -- compute the Adler-32 checksum of a data stream
4 * Copyright (C) 1995-2004 Mark Adler
5 * For conditions of distribution and use, see copyright notice in zlib.h
6 */
7
8 /* @(#) Id */
9
10 #define ZLIB_INTERNAL
11 #include "zlib.h"
12
13 #define BASE 65521UL /* largest prime smaller than 65536 */
14 #define NMAX 5552
15 /* NMAX is the largest n such that 255n(n+1)/2 + (n+1)(BASE-1) <= 2^32-1 */
16
17 #define DO1(buf,i) {adler += (buf)[i]; sum2 += adler;}
18 #define DO2(buf,i) DO1(buf,i); DO1(buf,i+1);
19 #define DO4(buf,i) DO2(buf,i); DO2(buf,i+2);
20 #define DO8(buf,i) DO4(buf,i); DO4(buf,i+4);
21 #define DO16(buf) DO8(buf,0); DO8(buf,8);
22
23 /* use NO_DIVIDE if your processor does not do division in hardware */
24 #ifdef NO_DIVIDE
25 # define MOD(a) \
26 do { \
27 if (a >= (BASE << 16)) a -= (BASE << 16); \
28 if (a >= (BASE << 15)) a -= (BASE << 15); \
29 if (a >= (BASE << 14)) a -= (BASE << 14); \
30 if (a >= (BASE << 13)) a -= (BASE << 13); \
31 if (a >= (BASE << 12)) a -= (BASE << 12); \
32 if (a >= (BASE << 11)) a -= (BASE << 11); \
33 if (a >= (BASE << 10)) a -= (BASE << 10); \
34 if (a >= (BASE << 9)) a -= (BASE << 9); \
35 if (a >= (BASE << 8)) a -= (BASE << 8); \
36 if (a >= (BASE << 7)) a -= (BASE << 7); \
37 if (a >= (BASE << 6)) a -= (BASE << 6); \
38 if (a >= (BASE << 5)) a -= (BASE << 5); \
39 if (a >= (BASE << 4)) a -= (BASE << 4); \
40 if (a >= (BASE << 3)) a -= (BASE << 3); \
41 if (a >= (BASE << 2)) a -= (BASE << 2); \
42 if (a >= (BASE << 1)) a -= (BASE << 1); \
43 if (a >= BASE) a -= BASE; \
44 } while (0)
45 # define MOD4(a) \
46 do { \
47 if (a >= (BASE << 4)) a -= (BASE << 4); \
48 if (a >= (BASE << 3)) a -= (BASE << 3); \
49 if (a >= (BASE << 2)) a -= (BASE << 2); \
50 if (a >= (BASE << 1)) a -= (BASE << 1); \
51 if (a >= BASE) a -= BASE; \
52 } while (0)
53 #else
54 # define MOD(a) a %= BASE
55 # define MOD4(a) a %= BASE
56 #endif
57
58 /* ========================================================================= */
adler32(adler,buf,len)59 uLong ZEXPORT adler32(adler, buf, len)
60 uLong adler;
61 const Bytef *buf;
62 uInt len;
63 {
64 unsigned long sum2;
65 unsigned n;
66
67 /* split Adler-32 into component sums */
68 sum2 = (adler >> 16) & 0xffff;
69 adler &= 0xffff;
70
71 /* in case user likes doing a byte at a time, keep it fast */
72 if (len == 1) {
73 adler += buf[0];
74 if (adler >= BASE)
75 adler -= BASE;
76 sum2 += adler;
77 if (sum2 >= BASE)
78 sum2 -= BASE;
79 return adler | (sum2 << 16);
80 }
81
82 /* initial Adler-32 value (deferred check for len == 1 speed) */
83 if (buf == Z_NULL)
84 return 1L;
85
86 /* in case short lengths are provided, keep it somewhat fast */
87 if (len < 16) {
88 while (len--) {
89 adler += *buf++;
90 sum2 += adler;
91 }
92 if (adler >= BASE)
93 adler -= BASE;
94 MOD4(sum2); /* only added so many BASE's */
95 return adler | (sum2 << 16);
96 }
97
98 /* do length NMAX blocks -- requires just one modulo operation */
99 while (len >= NMAX) {
100 len -= NMAX;
101 n = NMAX / 16; /* NMAX is divisible by 16 */
102 do {
103 DO16(buf); /* 16 sums unrolled */
104 buf += 16;
105 } while (--n);
106 MOD(adler);
107 MOD(sum2);
108 }
109
110 /* do remaining bytes (less than NMAX, still just one modulo) */
111 if (len) { /* avoid modulos if none remaining */
112 while (len >= 16) {
113 len -= 16;
114 DO16(buf);
115 buf += 16;
116 }
117 while (len--) {
118 adler += *buf++;
119 sum2 += adler;
120 }
121 MOD(adler);
122 MOD(sum2);
123 }
124
125 /* return recombined sums */
126 return adler | (sum2 << 16);
127 }
128
129 /* ========================================================================= */
adler32_combine(adler1,adler2,len2)130 uLong ZEXPORT adler32_combine(adler1, adler2, len2)
131 uLong adler1;
132 uLong adler2;
133 z_off_t len2;
134 {
135 unsigned long sum1;
136 unsigned long sum2;
137 unsigned rem;
138
139 /* the derivation of this formula is left as an exercise for the reader */
140 rem = (unsigned)(len2 % BASE);
141 sum1 = adler1 & 0xffff;
142 sum2 = rem * sum1;
143 MOD(sum2);
144 sum1 += (adler2 & 0xffff) + BASE - 1;
145 sum2 += ((adler1 >> 16) & 0xffff) + ((adler2 >> 16) & 0xffff) + BASE - rem;
146 if (sum1 > BASE) sum1 -= BASE;
147 if (sum1 > BASE) sum1 -= BASE;
148 if (sum2 > (BASE << 1)) sum2 -= (BASE << 1);
149 if (sum2 > BASE) sum2 -= BASE;
150 return sum1 | (sum2 << 16);
151 }
152