Annotation of sys/lib/libz/adler32.c, Revision 1.1.1.1
1.1 nbrk 1: /* $OpenBSD: adler32.c,v 1.9 2006/07/20 11:14:10 mickey Exp $ */
2: /* adler32.c -- compute the Adler-32 checksum of a data stream
3: * Copyright (C) 1995-2004 Mark Adler
4: * For conditions of distribution and use, see copyright notice in zlib.h
5: */
6:
7: #define ZLIB_INTERNAL
8: #include "zlib.h"
9:
10: #define BASE 65521UL /* largest prime smaller than 65536 */
11: #define NMAX 5552
12: /* NMAX is the largest n such that 255n(n+1)/2 + (n+1)(BASE-1) <= 2^32-1 */
13:
14: #define DO1(buf,i) {adler += (buf)[i]; sum2 += adler;}
15: #define DO2(buf,i) DO1(buf,i); DO1(buf,i+1);
16: #define DO4(buf,i) DO2(buf,i); DO2(buf,i+2);
17: #define DO8(buf,i) DO4(buf,i); DO4(buf,i+4);
18: #define DO16(buf) DO8(buf,0); DO8(buf,8);
19:
20: /* use NO_DIVIDE if your processor does not do division in hardware */
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: # define MOD4(a) \
43: do { \
44: if (a >= (BASE << 4)) a -= (BASE << 4); \
45: if (a >= (BASE << 3)) a -= (BASE << 3); \
46: if (a >= (BASE << 2)) a -= (BASE << 2); \
47: if (a >= (BASE << 1)) a -= (BASE << 1); \
48: if (a >= BASE) a -= BASE; \
49: } while (0)
50: #else
51: # define MOD(a) a %= BASE
52: # define MOD4(a) a %= BASE
53: #endif
54:
55: /* ========================================================================= */
56: uLong ZEXPORT adler32(adler, buf, len)
57: uLong adler;
58: const Bytef *buf;
59: uInt len;
60: {
61: unsigned long sum2;
62: unsigned n;
63:
64: /* split Adler-32 into component sums */
65: sum2 = (adler >> 16) & 0xffff;
66: adler &= 0xffff;
67:
68: /* in case user likes doing a byte at a time, keep it fast */
69: if (len == 1) {
70: adler += buf[0];
71: if (adler >= BASE)
72: adler -= BASE;
73: sum2 += adler;
74: if (sum2 >= BASE)
75: sum2 -= BASE;
76: return adler | (sum2 << 16);
77: }
78:
79: /* initial Adler-32 value (deferred check for len == 1 speed) */
80: if (buf == Z_NULL)
81: return 1L;
82:
83: /* in case short lengths are provided, keep it somewhat fast */
84: if (len < 16) {
85: while (len--) {
86: adler += *buf++;
87: sum2 += adler;
88: }
89: if (adler >= BASE)
90: adler -= BASE;
91: MOD4(sum2); /* only added so many BASE's */
92: return adler | (sum2 << 16);
93: }
94:
95: /* do length NMAX blocks -- requires just one modulo operation */
96: while (len >= NMAX) {
97: len -= NMAX;
98: n = NMAX / 16; /* NMAX is divisible by 16 */
99: do {
100: DO16(buf); /* 16 sums unrolled */
101: buf += 16;
102: } while (--n);
103: MOD(adler);
104: MOD(sum2);
105: }
106:
107: /* do remaining bytes (less than NMAX, still just one modulo) */
108: if (len) { /* avoid modulos if none remaining */
109: while (len >= 16) {
110: len -= 16;
111: DO16(buf);
112: buf += 16;
113: }
114: while (len--) {
115: adler += *buf++;
116: sum2 += adler;
117: }
118: MOD(adler);
119: MOD(sum2);
120: }
121:
122: /* return recombined sums */
123: return adler | (sum2 << 16);
124: }
125:
126: #ifdef ADLER32_COMBINE
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: }
150: #endif /* ADLER32_COMBINE */
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