adler校验算法

Adler-32校验算法

Adler-32是Mark Adler发明的校验和算法，和32位CRC校验算法一样，都是保护数据防止意外更改的算法，但是这个算法较容易被伪造，所以是不安全的保护措施。但是比CRC好点的是，它计算的很快。这个算法那是从Fletcher校验和算法中修改过来的，原始的算法形式略快，但是可依赖性并不高。

Adler-32的一种滚动哈希版本被用在了rsync工具中

Adler-32通过求解两个16位的数值A、B实现，并将结果连结成一个32位整数.

A就是字符串中每个字节的和，而B是A在相加时每一步的阶段值之和。在Adler-32开始运行时，A初始化为1，B初始化为0，最后的校验和要模上65521(继216之后的最小素数)。

其实Zlib源码里面也有实现。

/* adler32.c -- compute the Adler-32 checksum of a data stream * Copyright (C) 1995-2011 Mark Adler * For conditions of distribution and use, see copyright notice in zlib.h *//* @(#) $Id$ */#include "zutil.h"#define local staticlocal uLong adler32_combine_ OF((uLong adler1, uLong adler2, z_off64_t len2));#define BASE 65521      /* largest prime smaller than 65536 */#define NMAX 5552/* NMAX is the largest n such that 255n(n+1)/2 + (n+1)(BASE-1) <= 2^32-1 */#define DO1(buf,i)  {adler += (buf)[i]; sum2 += adler;}#define DO2(buf,i)  DO1(buf,i); DO1(buf,i+1);#define DO4(buf,i)  DO2(buf,i); DO2(buf,i+2);#define DO8(buf,i)  DO4(buf,i); DO4(buf,i+4);#define DO16(buf)   DO8(buf,0); DO8(buf,8);/* use NO_DIVIDE if your processor does not do division in hardware --   try it both ways to see which is faster */#ifdef NO_DIVIDE/* note that this assumes BASE is 65521, where 65536 % 65521 == 15   (thank you to John Reiser for pointing this out) */#  define CHOP(a) \    do { \        unsigned long tmp = a >> 16; \        a &= 0xffffUL; \        a += (tmp << 4) - tmp; \    } while (0)#  define MOD28(a) \    do { \        CHOP(a); \        if (a >= BASE) a -= BASE; \    } while (0)#  define MOD(a) \    do { \        CHOP(a); \        MOD28(a); \    } while (0)#  define MOD63(a) \    do { /* this assumes a is not negative */ \        z_off64_t tmp = a >> 32; \        a &= 0xffffffffL; \        a += (tmp << 8) - (tmp << 5) + tmp; \        tmp = a >> 16; \        a &= 0xffffL; \        a += (tmp << 4) - tmp; \        tmp = a >> 16; \        a &= 0xffffL; \        a += (tmp << 4) - tmp; \        if (a >= BASE) a -= BASE; \    } while (0)#else#  define MOD(a) a %= BASE#  define MOD28(a) a %= BASE#  define MOD63(a) a %= BASE#endif/* ========================================================================= */uLong ZEXPORT adler32(adler, buf, len)    uLong adler;    const Bytef *buf;    uInt len;{    unsigned long sum2;    unsigned n;    /* split Adler-32 into component sums */    sum2 = (adler >> 16) & 0xffff;    adler &= 0xffff;    /* in case user likes doing a byte at a time, keep it fast */    if (len == 1) {        adler += buf[0];        if (adler >= BASE)            adler -= BASE;        sum2 += adler;        if (sum2 >= BASE)            sum2 -= BASE;        return adler | (sum2 << 16);    }    /* initial Adler-32 value (deferred check for len == 1 speed) */    if (buf == Z_NULL)        return 1L;    /* in case short lengths are provided, keep it somewhat fast */    if (len < 16) {        while (len--) {            adler += *buf++;            sum2 += adler;        }        if (adler >= BASE)            adler -= BASE;        MOD28(sum2);            /* only added so many BASE's */        return adler | (sum2 << 16);    }    /* do length NMAX blocks -- requires just one modulo operation */    while (len >= NMAX) {        len -= NMAX;        n = NMAX / 16;          /* NMAX is divisible by 16 */        do {            DO16(buf);          /* 16 sums unrolled */            buf += 16;        } while (--n);        MOD(adler);        MOD(sum2);    }    /* do remaining bytes (less than NMAX, still just one modulo) */    if (len) {                  /* avoid modulos if none remaining */        while (len >= 16) {            len -= 16;            DO16(buf);            buf += 16;        }        while (len--) {            adler += *buf++;            sum2 += adler;        }        MOD(adler);        MOD(sum2);    }    /* return recombined sums */    return adler | (sum2 << 16);}/* ========================================================================= */local uLong adler32_combine_(adler1, adler2, len2)    uLong adler1;    uLong adler2;    z_off64_t len2;{    unsigned long sum1;    unsigned long sum2;    unsigned rem;    /* for negative len, return invalid adler32 as a clue for debugging */    if (len2 < 0)        return 0xffffffffUL;    /* the derivation of this formula is left as an exercise for the reader */    MOD63(len2);                /* assumes len2 >= 0 */    rem = (unsigned)len2;    sum1 = adler1 & 0xffff;    sum2 = rem * sum1;    MOD(sum2);    sum1 += (adler2 & 0xffff) + BASE - 1;    sum2 += ((adler1 >> 16) & 0xffff) + ((adler2 >> 16) & 0xffff) + BASE - rem;    if (sum1 >= BASE) sum1 -= BASE;    if (sum1 >= BASE) sum1 -= BASE;    if (sum2 >= (BASE << 1)) sum2 -= (BASE << 1);    if (sum2 >= BASE) sum2 -= BASE;    return sum1 | (sum2 << 16);}/* ========================================================================= */uLong ZEXPORT adler32_combine(adler1, adler2, len2)    uLong adler1;    uLong adler2;    z_off_t len2;{    return adler32_combine_(adler1, adler2, len2);}uLong ZEXPORT adler32_combine64(adler1, adler2, len2)    uLong adler1;    uLong adler2;    z_off64_t len2;{    return adler32_combine_(adler1, adler2, len2);}

简单的实现：

#define BASE 65521      /* largest prime smaller than 65536 */
#define NMAX 5552

//adler32 校验
int32 adler32(int32 adler, const byte *buf, int32 len);

int32 adler32( int32 adler, const byte *buf, int32 len )
{
unsigned long uladler = (unsigned long)adler;
unsigned long s1 = uladler & 0xffff;
unsigned long s2 = (uladler >> 16) & 0xffff;


int i;
for (i = 0; i < len; i++) 
{
s1 = (s1 + buf[i]) % BASE;
s2 = (s2 + s1) % BASE;
}
return (s2 << 16) + s1;
}