FNV哈希算法（转）

由来：FNV哈希算法全名为Fowler-Noll-Vo算法，是以三位发明人Glenn Fowler，Landon Curt Noll，Phong Vo的名字来命名的，最早在1991年提出。

特点和用途：FNV能快速hash大量数据并保持较小的冲突率，它的高度分散使它适用于hash一些非常相近的字符串，比如URL，hostname，文件名，text，IP地址等。

算法版本：FNV算法有两个版本FNV-1和FNV-1a

算法描述：

相关变量：

hash值：一个n位的unsigned int型hash值

offset_basis：初始的哈希值

FNV_prime：FNV用于散列的质数

octet_of_data：8位数据（即一个字节）

FNV-1描述：

hash = offset_basis

for each octet_of_data to be hashed

hash = hash * FNV_prime

hash = hash xor octet_of_data

return hash

FNV-1a描述：

hash = offset_basis 

for each octet_of_data to be hashed

hash = hash xor octet_of_data

hash = hash * FNV_prime

return hash

FNV-1a和FNV-1的唯一区别就是xor和multiply的顺序不同，他们所采用的FNV_prime和offset_basis都相同，有人认为FNV-1a在进行小数据（小于4个字节）哈希时有更好的性能。

FNV_prime的取值:
32 bit FNV_prime = 2^24 + 2^8 + 0x93 = 16777619
64 bit FNV_prime = 2^40 + 2^8 + 0xb3 = 1099511628211
128 bit FNV_prime = 2^88 + 2^8 + 0x3b = 309485009821345068724781371
256 bit FNV_prime = 2^168 + 2^8 + 0x63 =374144419156711147060143317175368453031918731002211
512 bit FNV_prime = 2^344 + 2^8 + 0x57 = 35835915874844867368919076489095108449946327955754392558399825615420669938882575
126094039892345713852759
1024 bit FNV_prime = 2^680 + 2^8 + 0x8d =
50164565101131186554345988110352789550307653454047907443030175238311120551081474
51509157692220295382716162651878526895249385292291816524375083746691371804094271
873160484737966720260389217684476157468082573

offset_basis的取值:
32 bit offset_basis = 2166136261
64 bit offset_basis = 14695981039346656037
128 bit offset_basis = 144066263297769815596495629667062367629
256 bit offset_basis = 100029257958052580907070968620625704837092796014241193945225284501741471925557
512 bit offset_basis = 96593031294966694980094354007163104660904187456726378961083743294344626579945829
32197716438449813051892206539805784495328239340083876191928701583869517785
1024 bit offset_basis = 14197795064947621068722070641403218320880622795441933960878474914617582723252296
73230371772215086409652120235554936562817466910857181476047101507614802975596980
40773201576924585630032153049571501574036444603635505054127112859663616102678680
82893823963790439336411086884584107735010676915

算法实现：

//sheepdog中64位FNV-1a算法的实现
/*
 * 64 bit FNV-1a non-zero initial basis
 */
#define FNV1A_64_INIT ((uint64_t) 0xcbf29ce484222325ULL)
/*
 * 64 bit Fowler/Noll/Vo FNV-1a hash code
 */
// 调用时，hval的参数值为FNV1A_64_INT,即算法描述中的offset_basis
static inline uint64_t fnv_64a_buf(void *buf, size_t len, uint64_t hval)
{
unsigned char *bp = (unsigned char *) buf;
unsigned char *be = bp + len;
while (bp < be) {
hval ^= (uint64_t) *bp++;
hval += (hval << 1) + (hval << 4) + (hval << 5) +
(hval << 7) + (hval << 8) + (hval << 40);
}
return hval;
}

参考文献：点击打开链接（包含详细讲解和代码）

转自：https://blog.csdn.net/TCCaiWQ/article/details/7319739