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/Modules/zlib/adler32.c

http://unladen-swallow.googlecode.com/
C | 149 lines | 117 code | 15 blank | 17 comment | 42 complexity | a6e767699d7b6c1c28a2730530aaf1fa MD5 | raw file
  1/* adler32.c -- compute the Adler-32 checksum of a data stream
  2 * Copyright (C) 1995-2004 Mark Adler
  3 * For conditions of distribution and use, see copyright notice in zlib.h
  4 */
  5
  6/* @(#) $Id$ */
  7
  8#define ZLIB_INTERNAL
  9#include "zlib.h"
 10
 11#define BASE 65521UL    /* largest prime smaller than 65536 */
 12#define NMAX 5552
 13/* NMAX is the largest n such that 255n(n+1)/2 + (n+1)(BASE-1) <= 2^32-1 */
 14
 15#define DO1(buf,i)  {adler += (buf)[i]; sum2 += adler;}
 16#define DO2(buf,i)  DO1(buf,i); DO1(buf,i+1);
 17#define DO4(buf,i)  DO2(buf,i); DO2(buf,i+2);
 18#define DO8(buf,i)  DO4(buf,i); DO4(buf,i+4);
 19#define DO16(buf)   DO8(buf,0); DO8(buf,8);
 20
 21/* use NO_DIVIDE if your processor does not do division in hardware */
 22#ifdef NO_DIVIDE
 23#  define MOD(a) \
 24    do { \
 25        if (a >= (BASE << 16)) a -= (BASE << 16); \
 26        if (a >= (BASE << 15)) a -= (BASE << 15); \
 27        if (a >= (BASE << 14)) a -= (BASE << 14); \
 28        if (a >= (BASE << 13)) a -= (BASE << 13); \
 29        if (a >= (BASE << 12)) a -= (BASE << 12); \
 30        if (a >= (BASE << 11)) a -= (BASE << 11); \
 31        if (a >= (BASE << 10)) a -= (BASE << 10); \
 32        if (a >= (BASE << 9)) a -= (BASE << 9); \
 33        if (a >= (BASE << 8)) a -= (BASE << 8); \
 34        if (a >= (BASE << 7)) a -= (BASE << 7); \
 35        if (a >= (BASE << 6)) a -= (BASE << 6); \
 36        if (a >= (BASE << 5)) a -= (BASE << 5); \
 37        if (a >= (BASE << 4)) a -= (BASE << 4); \
 38        if (a >= (BASE << 3)) a -= (BASE << 3); \
 39        if (a >= (BASE << 2)) a -= (BASE << 2); \
 40        if (a >= (BASE << 1)) a -= (BASE << 1); \
 41        if (a >= BASE) a -= BASE; \
 42    } while (0)
 43#  define MOD4(a) \
 44    do { \
 45        if (a >= (BASE << 4)) a -= (BASE << 4); \
 46        if (a >= (BASE << 3)) a -= (BASE << 3); \
 47        if (a >= (BASE << 2)) a -= (BASE << 2); \
 48        if (a >= (BASE << 1)) a -= (BASE << 1); \
 49        if (a >= BASE) a -= BASE; \
 50    } while (0)
 51#else
 52#  define MOD(a) a %= BASE
 53#  define MOD4(a) a %= BASE
 54#endif
 55
 56/* ========================================================================= */
 57uLong ZEXPORT adler32(adler, buf, len)
 58    uLong adler;
 59    const Bytef *buf;
 60    uInt len;
 61{
 62    unsigned long sum2;
 63    unsigned n;
 64
 65    /* split Adler-32 into component sums */
 66    sum2 = (adler >> 16) & 0xffff;
 67    adler &= 0xffff;
 68
 69    /* in case user likes doing a byte at a time, keep it fast */
 70    if (len == 1) {
 71        adler += buf[0];
 72        if (adler >= BASE)
 73            adler -= BASE;
 74        sum2 += adler;
 75        if (sum2 >= BASE)
 76            sum2 -= BASE;
 77        return adler | (sum2 << 16);
 78    }
 79
 80    /* initial Adler-32 value (deferred check for len == 1 speed) */
 81    if (buf == Z_NULL)
 82        return 1L;
 83
 84    /* in case short lengths are provided, keep it somewhat fast */
 85    if (len < 16) {
 86        while (len--) {
 87            adler += *buf++;
 88            sum2 += adler;
 89        }
 90        if (adler >= BASE)
 91            adler -= BASE;
 92        MOD4(sum2);             /* only added so many BASE's */
 93        return adler | (sum2 << 16);
 94    }
 95
 96    /* do length NMAX blocks -- requires just one modulo operation */
 97    while (len >= NMAX) {
 98        len -= NMAX;
 99        n = NMAX / 16;          /* NMAX is divisible by 16 */
100        do {
101            DO16(buf);          /* 16 sums unrolled */
102            buf += 16;
103        } while (--n);
104        MOD(adler);
105        MOD(sum2);
106    }
107
108    /* do remaining bytes (less than NMAX, still just one modulo) */
109    if (len) {                  /* avoid modulos if none remaining */
110        while (len >= 16) {
111            len -= 16;
112            DO16(buf);
113            buf += 16;
114        }
115        while (len--) {
116            adler += *buf++;
117            sum2 += adler;
118        }
119        MOD(adler);
120        MOD(sum2);
121    }
122
123    /* return recombined sums */
124    return adler | (sum2 << 16);
125}
126
127/* ========================================================================= */
128uLong 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}