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adler32.c
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1 /* adler32.c -- compute the Adler-32 checksum of a data stream
2  * Copyright (C) 1995-2011 Mark Adler
3  * For conditions of distribution and use, see copyright notice in zlib.h
4  */
5 
6 /* @(#) $Id$ */
7 
8 #include "zutil.h"
9 
10 #define local static
11 
12 local uLong adler32_combine_ OF((uLong adler1, uLong adler2, z_off64_t len2));
13 
14 #define BASE 65521 /* largest prime smaller than 65536 */
15 #define NMAX 5552
16 /* NMAX is the largest n such that 255n(n+1)/2 + (n+1)(BASE-1) <= 2^32-1 */
17 
18 #define DO1(buf,i) {adler += (buf)[i]; sum2 += adler;}
19 #define DO2(buf,i) DO1(buf,i); DO1(buf,i+1);
20 #define DO4(buf,i) DO2(buf,i); DO2(buf,i+2);
21 #define DO8(buf,i) DO4(buf,i); DO4(buf,i+4);
22 #define DO16(buf) DO8(buf,0); DO8(buf,8);
23 
24 /* use NO_DIVIDE if your processor does not do division in hardware --
25  try it both ways to see which is faster */
26 #ifdef NO_DIVIDE
27 /* note that this assumes BASE is 65521, where 65536 % 65521 == 15
28  (thank you to John Reiser for pointing this out) */
29 # define CHOP(a) \
30  do { \
31  unsigned long tmp = a >> 16; \
32  a &= 0xffffUL; \
33  a += (tmp << 4) - tmp; \
34  } while (0)
35 # define MOD28(a) \
36  do { \
37  CHOP(a); \
38  if (a >= BASE) a -= BASE; \
39  } while (0)
40 # define MOD(a) \
41  do { \
42  CHOP(a); \
43  MOD28(a); \
44  } while (0)
45 # define MOD63(a) \
46  do { /* this assumes a is not negative */ \
47  z_off64_t tmp = a >> 32; \
48  a &= 0xffffffffL; \
49  a += (tmp << 8) - (tmp << 5) + tmp; \
50  tmp = a >> 16; \
51  a &= 0xffffL; \
52  a += (tmp << 4) - tmp; \
53  tmp = a >> 16; \
54  a &= 0xffffL; \
55  a += (tmp << 4) - tmp; \
56  if (a >= BASE) a -= BASE; \
57  } while (0)
58 #else
59 # define MOD(a) a %= BASE
60 # define MOD28(a) a %= BASE
61 # define MOD63(a) a %= BASE
62 #endif
63 
64 /* ========================================================================= */
65 uLong ZEXPORT adler32(adler, buf, len)
66  uLong adler;
67  const Bytef *buf;
68  uInt len;
69 {
70  unsigned long sum2;
71  unsigned n;
72 
73  /* split Adler-32 into component sums */
74  sum2 = (adler >> 16) & 0xffff;
75  adler &= 0xffff;
76 
77  /* in case user likes doing a byte at a time, keep it fast */
78  if (len == 1) {
79  adler += buf[0];
80  if (adler >= BASE)
81  adler -= BASE;
82  sum2 += adler;
83  if (sum2 >= BASE)
84  sum2 -= BASE;
85  return adler | (sum2 << 16);
86  }
87 
88  /* initial Adler-32 value (deferred check for len == 1 speed) */
89  if (buf == Z_NULL)
90  return 1L;
91 
92  /* in case short lengths are provided, keep it somewhat fast */
93  if (len < 16) {
94  while (len--) {
95  adler += *buf++;
96  sum2 += adler;
97  }
98  if (adler >= BASE)
99  adler -= BASE;
100  MOD28(sum2); /* only added so many BASE's */
101  return adler | (sum2 << 16);
102  }
103 
104  /* do length NMAX blocks -- requires just one modulo operation */
105  while (len >= NMAX) {
106  len -= NMAX;
107  n = NMAX / 16; /* NMAX is divisible by 16 */
108  do {
109  DO16(buf); /* 16 sums unrolled */
110  buf += 16;
111  } while (--n);
112  MOD(adler);
113  MOD(sum2);
114  }
115 
116  /* do remaining bytes (less than NMAX, still just one modulo) */
117  if (len) { /* avoid modulos if none remaining */
118  while (len >= 16) {
119  len -= 16;
120  DO16(buf);
121  buf += 16;
122  }
123  while (len--) {
124  adler += *buf++;
125  sum2 += adler;
126  }
127  MOD(adler);
128  MOD(sum2);
129  }
130 
131  /* return recombined sums */
132  return adler | (sum2 << 16);
133 }
134 
135 /* ========================================================================= */
136 local uLong adler32_combine_(adler1, adler2, len2)
137  uLong adler1;
138  uLong adler2;
139  z_off64_t len2;
140 {
141  unsigned long sum1;
142  unsigned long sum2;
143  unsigned rem;
144 
145  /* for negative len, return invalid adler32 as a clue for debugging */
146  if (len2 < 0)
147  return 0xffffffffUL;
148 
149  /* the derivation of this formula is left as an exercise for the reader */
150  MOD63(len2); /* assumes len2 >= 0 */
151  rem = (unsigned)len2;
152  sum1 = adler1 & 0xffff;
153  sum2 = rem * sum1;
154  MOD(sum2);
155  sum1 += (adler2 & 0xffff) + BASE - 1;
156  sum2 += ((adler1 >> 16) & 0xffff) + ((adler2 >> 16) & 0xffff) + BASE - rem;
157  if (sum1 >= BASE) sum1 -= BASE;
158  if (sum1 >= BASE) sum1 -= BASE;
159  if (sum2 >= (BASE << 1)) sum2 -= (BASE << 1);
160  if (sum2 >= BASE) sum2 -= BASE;
161  return sum1 | (sum2 << 16);
162 }
163 
164 /* ========================================================================= */
165 uLong ZEXPORT adler32_combine(adler1, adler2, len2)
166  uLong adler1;
167  uLong adler2;
168  z_off_t len2;
169 {
170  return adler32_combine_(adler1, adler2, len2);
171 }
172 
173 uLong ZEXPORT adler32_combine64(adler1, adler2, len2)
174  uLong adler1;
175  uLong adler2;
176  z_off64_t len2;
177 {
178  return adler32_combine_(adler1, adler2, len2);
179 }