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
12 local uLong adler32_combine_ OF((uLong adler1, uLong adler2, z_off64_t len2));
14 #define BASE 65521 /* largest prime smaller than 65536 */
16 /* NMAX is the largest n such that 255n(n+1)/2 + (n+1)(BASE-1) <= 2^32-1 */
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);
24 /* use NO_DIVIDE if your processor does not do division in hardware --
25 try it both ways to see which is faster */
27 /* note that this assumes BASE is 65521, where 65536 % 65521 == 15
28 (thank you to John Reiser for pointing this out) */
31 unsigned long tmp = a >> 16; \
33 a += (tmp << 4) - tmp; \
38 if (a >= BASE) a -= BASE; \
46 do { /* this assumes a is not negative */ \
47 z_off64_t tmp = a >> 32; \
49 a += (tmp << 8) - (tmp << 5) + tmp; \
52 a += (tmp << 4) - tmp; \
55 a += (tmp << 4) - tmp; \
56 if (a >= BASE) a -= BASE; \
59 # define MOD(a) a %= BASE
60 # define MOD28(a) a %= BASE
61 # define MOD63(a) a %= BASE
64 /* ========================================================================= */
65 uLong ZEXPORT adler32(adler, buf, len)
73 /* split Adler-32 into component sums */
74 sum2 = (adler >> 16) & 0xffff;
77 /* in case user likes doing a byte at a time, keep it fast */
85 return adler | (sum2 << 16);
88 /* initial Adler-32 value (deferred check for len == 1 speed) */
92 /* in case short lengths are provided, keep it somewhat fast */
100 MOD28(sum2); /* only added so many BASE's */
101 return adler | (sum2 << 16);
104 /* do length NMAX blocks -- requires just one modulo operation */
105 while (len >= NMAX) {
107 n = NMAX / 16; /* NMAX is divisible by 16 */
109 DO16(buf); /* 16 sums unrolled */
116 /* do remaining bytes (less than NMAX, still just one modulo) */
117 if (len) { /* avoid modulos if none remaining */
131 /* return recombined sums */
132 return adler | (sum2 << 16);
135 /* ========================================================================= */
136 local uLong adler32_combine_(adler1, adler2, len2)
145 /* for negative len, return invalid adler32 as a clue for debugging */
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;
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);
164 /* ========================================================================= */
165 uLong ZEXPORT adler32_combine(adler1, adler2, len2)
170 return adler32_combine_(adler1, adler2, len2);
173 uLong ZEXPORT adler32_combine64(adler1, adler2, len2)
178 return adler32_combine_(adler1, adler2, len2);