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annotate libs/zlib/adler32.c @ 0:b2f14e535253

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author John Tsiombikas <nuclear@member.fsf.org>
date Sat, 01 Feb 2014 19:58:19 +0200
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nuclear@0 1 /* adler32.c -- compute the Adler-32 checksum of a data stream
nuclear@0 2 * Copyright (C) 1995-2004 Mark Adler
nuclear@0 3 * For conditions of distribution and use, see copyright notice in zlib.h
nuclear@0 4 */
nuclear@0 5
nuclear@0 6 /* @(#) $Id$ */
nuclear@0 7
nuclear@0 8 #define ZLIB_INTERNAL
nuclear@0 9 #include "zlib.h"
nuclear@0 10
nuclear@0 11 #define BASE 65521UL /* largest prime smaller than 65536 */
nuclear@0 12 #define NMAX 5552
nuclear@0 13 /* NMAX is the largest n such that 255n(n+1)/2 + (n+1)(BASE-1) <= 2^32-1 */
nuclear@0 14
nuclear@0 15 #define DO1(buf,i) {adler += (buf)[i]; sum2 += adler;}
nuclear@0 16 #define DO2(buf,i) DO1(buf,i); DO1(buf,i+1);
nuclear@0 17 #define DO4(buf,i) DO2(buf,i); DO2(buf,i+2);
nuclear@0 18 #define DO8(buf,i) DO4(buf,i); DO4(buf,i+4);
nuclear@0 19 #define DO16(buf) DO8(buf,0); DO8(buf,8);
nuclear@0 20
nuclear@0 21 /* use NO_DIVIDE if your processor does not do division in hardware */
nuclear@0 22 #ifdef NO_DIVIDE
nuclear@0 23 # define MOD(a) \
nuclear@0 24 do { \
nuclear@0 25 if (a >= (BASE << 16)) a -= (BASE << 16); \
nuclear@0 26 if (a >= (BASE << 15)) a -= (BASE << 15); \
nuclear@0 27 if (a >= (BASE << 14)) a -= (BASE << 14); \
nuclear@0 28 if (a >= (BASE << 13)) a -= (BASE << 13); \
nuclear@0 29 if (a >= (BASE << 12)) a -= (BASE << 12); \
nuclear@0 30 if (a >= (BASE << 11)) a -= (BASE << 11); \
nuclear@0 31 if (a >= (BASE << 10)) a -= (BASE << 10); \
nuclear@0 32 if (a >= (BASE << 9)) a -= (BASE << 9); \
nuclear@0 33 if (a >= (BASE << 8)) a -= (BASE << 8); \
nuclear@0 34 if (a >= (BASE << 7)) a -= (BASE << 7); \
nuclear@0 35 if (a >= (BASE << 6)) a -= (BASE << 6); \
nuclear@0 36 if (a >= (BASE << 5)) a -= (BASE << 5); \
nuclear@0 37 if (a >= (BASE << 4)) a -= (BASE << 4); \
nuclear@0 38 if (a >= (BASE << 3)) a -= (BASE << 3); \
nuclear@0 39 if (a >= (BASE << 2)) a -= (BASE << 2); \
nuclear@0 40 if (a >= (BASE << 1)) a -= (BASE << 1); \
nuclear@0 41 if (a >= BASE) a -= BASE; \
nuclear@0 42 } while (0)
nuclear@0 43 # define MOD4(a) \
nuclear@0 44 do { \
nuclear@0 45 if (a >= (BASE << 4)) a -= (BASE << 4); \
nuclear@0 46 if (a >= (BASE << 3)) a -= (BASE << 3); \
nuclear@0 47 if (a >= (BASE << 2)) a -= (BASE << 2); \
nuclear@0 48 if (a >= (BASE << 1)) a -= (BASE << 1); \
nuclear@0 49 if (a >= BASE) a -= BASE; \
nuclear@0 50 } while (0)
nuclear@0 51 #else
nuclear@0 52 # define MOD(a) a %= BASE
nuclear@0 53 # define MOD4(a) a %= BASE
nuclear@0 54 #endif
nuclear@0 55
nuclear@0 56 /* ========================================================================= */
nuclear@0 57 uLong ZEXPORT adler32(adler, buf, len)
nuclear@0 58 uLong adler;
nuclear@0 59 const Bytef *buf;
nuclear@0 60 uInt len;
nuclear@0 61 {
nuclear@0 62 unsigned long sum2;
nuclear@0 63 unsigned n;
nuclear@0 64
nuclear@0 65 /* split Adler-32 into component sums */
nuclear@0 66 sum2 = (adler >> 16) & 0xffff;
nuclear@0 67 adler &= 0xffff;
nuclear@0 68
nuclear@0 69 /* in case user likes doing a byte at a time, keep it fast */
nuclear@0 70 if (len == 1) {
nuclear@0 71 adler += buf[0];
nuclear@0 72 if (adler >= BASE)
nuclear@0 73 adler -= BASE;
nuclear@0 74 sum2 += adler;
nuclear@0 75 if (sum2 >= BASE)
nuclear@0 76 sum2 -= BASE;
nuclear@0 77 return adler | (sum2 << 16);
nuclear@0 78 }
nuclear@0 79
nuclear@0 80 /* initial Adler-32 value (deferred check for len == 1 speed) */
nuclear@0 81 if (buf == Z_NULL)
nuclear@0 82 return 1L;
nuclear@0 83
nuclear@0 84 /* in case short lengths are provided, keep it somewhat fast */
nuclear@0 85 if (len < 16) {
nuclear@0 86 while (len--) {
nuclear@0 87 adler += *buf++;
nuclear@0 88 sum2 += adler;
nuclear@0 89 }
nuclear@0 90 if (adler >= BASE)
nuclear@0 91 adler -= BASE;
nuclear@0 92 MOD4(sum2); /* only added so many BASE's */
nuclear@0 93 return adler | (sum2 << 16);
nuclear@0 94 }
nuclear@0 95
nuclear@0 96 /* do length NMAX blocks -- requires just one modulo operation */
nuclear@0 97 while (len >= NMAX) {
nuclear@0 98 len -= NMAX;
nuclear@0 99 n = NMAX / 16; /* NMAX is divisible by 16 */
nuclear@0 100 do {
nuclear@0 101 DO16(buf); /* 16 sums unrolled */
nuclear@0 102 buf += 16;
nuclear@0 103 } while (--n);
nuclear@0 104 MOD(adler);
nuclear@0 105 MOD(sum2);
nuclear@0 106 }
nuclear@0 107
nuclear@0 108 /* do remaining bytes (less than NMAX, still just one modulo) */
nuclear@0 109 if (len) { /* avoid modulos if none remaining */
nuclear@0 110 while (len >= 16) {
nuclear@0 111 len -= 16;
nuclear@0 112 DO16(buf);
nuclear@0 113 buf += 16;
nuclear@0 114 }
nuclear@0 115 while (len--) {
nuclear@0 116 adler += *buf++;
nuclear@0 117 sum2 += adler;
nuclear@0 118 }
nuclear@0 119 MOD(adler);
nuclear@0 120 MOD(sum2);
nuclear@0 121 }
nuclear@0 122
nuclear@0 123 /* return recombined sums */
nuclear@0 124 return adler | (sum2 << 16);
nuclear@0 125 }
nuclear@0 126
nuclear@0 127 /* ========================================================================= */
nuclear@0 128 uLong ZEXPORT adler32_combine(adler1, adler2, len2)
nuclear@0 129 uLong adler1;
nuclear@0 130 uLong adler2;
nuclear@0 131 z_off_t len2;
nuclear@0 132 {
nuclear@0 133 unsigned long sum1;
nuclear@0 134 unsigned long sum2;
nuclear@0 135 unsigned rem;
nuclear@0 136
nuclear@0 137 /* the derivation of this formula is left as an exercise for the reader */
nuclear@0 138 rem = (unsigned)(len2 % BASE);
nuclear@0 139 sum1 = adler1 & 0xffff;
nuclear@0 140 sum2 = rem * sum1;
nuclear@0 141 MOD(sum2);
nuclear@0 142 sum1 += (adler2 & 0xffff) + BASE - 1;
nuclear@0 143 sum2 += ((adler1 >> 16) & 0xffff) + ((adler2 >> 16) & 0xffff) + BASE - rem;
nuclear@0 144 if (sum1 > BASE) sum1 -= BASE;
nuclear@0 145 if (sum1 > BASE) sum1 -= BASE;
nuclear@0 146 if (sum2 > (BASE << 1)) sum2 -= (BASE << 1);
nuclear@0 147 if (sum2 > BASE) sum2 -= BASE;
nuclear@0 148 return sum1 | (sum2 << 16);
nuclear@0 149 }