1 /* adler32.c -- compute the Adler-32 checksum of a data stream
2  * Copyright (C) 1995-2007 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_(uLong adler1, uLong adler2, z_off64_t len2);
13 
14 #define BASE 65521UL    /* 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 #ifdef NO_DIVIDE
26 #  define MOD(a) \
27     do { \
28         if (a >= (BASE << 16)) a -= (BASE << 16); \
29         if (a >= (BASE << 15)) a -= (BASE << 15); \
30         if (a >= (BASE << 14)) a -= (BASE << 14); \
31         if (a >= (BASE << 13)) a -= (BASE << 13); \
32         if (a >= (BASE << 12)) a -= (BASE << 12); \
33         if (a >= (BASE << 11)) a -= (BASE << 11); \
34         if (a >= (BASE << 10)) a -= (BASE << 10); \
35         if (a >= (BASE << 9)) a -= (BASE << 9); \
36         if (a >= (BASE << 8)) a -= (BASE << 8); \
37         if (a >= (BASE << 7)) a -= (BASE << 7); \
38         if (a >= (BASE << 6)) a -= (BASE << 6); \
39         if (a >= (BASE << 5)) a -= (BASE << 5); \
40         if (a >= (BASE << 4)) a -= (BASE << 4); \
41         if (a >= (BASE << 3)) a -= (BASE << 3); \
42         if (a >= (BASE << 2)) a -= (BASE << 2); \
43         if (a >= (BASE << 1)) a -= (BASE << 1); \
44         if (a >= BASE) a -= BASE; \
45     } while (0)
46 #  define MOD4(a) \
47     do { \
48         if (a >= (BASE << 4)) a -= (BASE << 4); \
49         if (a >= (BASE << 3)) a -= (BASE << 3); \
50         if (a >= (BASE << 2)) a -= (BASE << 2); \
51         if (a >= (BASE << 1)) a -= (BASE << 1); \
52         if (a >= BASE) a -= BASE; \
53     } while (0)
54 #else
55 #  define MOD(a) a %= BASE
56 #  define MOD4(a) a %= BASE
57 #endif
58 
59 /* ========================================================================= */
adler32(adler,buf,len)60 uLong ZEXPORT adler32(adler, buf, len)
61     uLong adler;
62     const Bytef *buf;
63     uInt len;
64 {
65     unsigned long sum2;
66     unsigned n;
67 
68     /* split Adler-32 into component sums */
69     sum2 = (adler >> 16) & 0xffff;
70     adler &= 0xffff;
71 
72     /* in case user likes doing a byte at a time, keep it fast */
73     if (len == 1) {
74         adler += buf[0];
75         if (adler >= BASE)
76             adler -= BASE;
77         sum2 += adler;
78         if (sum2 >= BASE)
79             sum2 -= BASE;
80         return adler | (sum2 << 16);
81     }
82 
83     /* initial Adler-32 value (deferred check for len == 1 speed) */
84     if (buf == Z_NULL)
85         return 1L;
86 
87     /* in case short lengths are provided, keep it somewhat fast */
88     if (len < 16) {
89         while (len--) {
90             adler += *buf++;
91             sum2 += adler;
92         }
93         if (adler >= BASE)
94             adler -= BASE;
95         MOD4(sum2);             /* only added so many BASE's */
96         return adler | (sum2 << 16);
97     }
98 
99     /* do length NMAX blocks -- requires just one modulo operation */
100     while (len >= NMAX) {
101         len -= NMAX;
102         n = NMAX / 16;          /* NMAX is divisible by 16 */
103         do {
104             DO16(buf);          /* 16 sums unrolled */
105             buf += 16;
106         } while (--n);
107         MOD(adler);
108         MOD(sum2);
109     }
110 
111     /* do remaining bytes (less than NMAX, still just one modulo) */
112     if (len) {                  /* avoid modulos if none remaining */
113         while (len >= 16) {
114             len -= 16;
115             DO16(buf);
116             buf += 16;
117         }
118         while (len--) {
119             adler += *buf++;
120             sum2 += adler;
121         }
122         MOD(adler);
123         MOD(sum2);
124     }
125 
126     /* return recombined sums */
127     return adler | (sum2 << 16);
128 }
129 
130 /* ========================================================================= */
adler32_combine_(adler1,adler2,len2)131 local uLong adler32_combine_(adler1, adler2, len2)
132     uLong adler1;
133     uLong adler2;
134     z_off64_t len2;
135 {
136     unsigned long sum1;
137     unsigned long sum2;
138     unsigned rem;
139 
140     /* the derivation of this formula is left as an exercise for the reader */
141     rem = (unsigned)(len2 % BASE);
142     sum1 = adler1 & 0xffff;
143     sum2 = rem * sum1;
144     MOD(sum2);
145     sum1 += (adler2 & 0xffff) + BASE - 1;
146     sum2 += ((adler1 >> 16) & 0xffff) + ((adler2 >> 16) & 0xffff) + BASE - rem;
147     if (sum1 >= BASE) sum1 -= BASE;
148     if (sum1 >= BASE) sum1 -= BASE;
149     if (sum2 >= (BASE << 1)) sum2 -= (BASE << 1);
150     if (sum2 >= BASE) sum2 -= BASE;
151     return sum1 | (sum2 << 16);
152 }
153 
154 /* ========================================================================= */
adler32_combine(adler1,adler2,len2)155 uLong ZEXPORT adler32_combine(adler1, adler2, len2)
156     uLong adler1;
157     uLong adler2;
158     z_off_t len2;
159 {
160     return adler32_combine_(adler1, adler2, len2);
161 }
162 
adler32_combine64(adler1,adler2,len2)163 uLong ZEXPORT adler32_combine64(adler1, adler2, len2)
164     uLong adler1;
165     uLong adler2;
166     z_off64_t len2;
167 {
168     return adler32_combine_(adler1, adler2, len2);
169 }
170