memcached 源码阅读之 字符串 hash 与 搜集的一些 字符串 hash
前言 在 memcached 源码阅读之 hash table文章的最后我说了,要研究一下 memcached 的 字符串 hash 方法的。 现在就开始记录下研究的结果。 Jenkins hash jenkins 的位置在 jenkins_hash.c . 大端小端 Little-Endian就是低位字节排放在内存的低地址端,高位
前言
在 memcached 源码阅读之 hash table文章的最后我说了,要研究一下 memcached 的 字符串 hash 方法的。
现在就开始记录下研究的结果。
Jenkins hash
jenkins 的位置在 jenkins_hash.c .
大端小端
Little-Endian就是低位字节排放在内存的低地址端,高位字节排放在内存的高地址端。
Big-Endian就是高位字节排放在内存的低地址端,低位字节排放在内存的高地址端。
举一个例子,比如数字0x12 34 56 78在内存中的表示形式为:
1)大端模式: 低地址 -----------------> 高地址 0x12 | 0x34 | 0x56 | 0x78 2)小端模式: 低地址 ------------------> 高地址 0x78 | 0x56 | 0x34 | 0x12
#if ENDIAN_BIG == 1 # define HASH_LITTLE_ENDIAN 0 # define HASH_BIG_ENDIAN 1 #else # if ENDIAN_LITTLE == 1 # define HASH_LITTLE_ENDIAN 1 # define HASH_BIG_ENDIAN 0 # else # define HASH_LITTLE_ENDIAN 0 # define HASH_BIG_ENDIAN 0 # endif #endif
rot 宏
看到的第一个是 rot 宏。
这个宏的作用是循环左移若干位。
#define rot(x,k) (((x)>(32-(k))))
mix 宏
一个可逆的加密。
This is reversible, so any information in (a,b,c) before mix() is still in (a,b,c) after mix().
#define mix(a,b,c) \ { \ a -= c; a ^= rot(c, 4); c += b; \ b -= a; b ^= rot(a, 6); a += c; \ c -= b; c ^= rot(b, 8); b += a; \ a -= c; a ^= rot(c,16); c += b; \ b -= a; b ^= rot(a,19); a += c; \ c -= b; c ^= rot(b, 4); b += a; \ }
final 宏
final mixing of 3 32-bit values (a,b,c) into c
将 a,b,c 合并到 c中。
#define final(a,b,c) \ { \ c ^= b; c -= rot(b,14); \ a ^= c; a -= rot(c,11); \ b ^= a; b -= rot(a,25); \ c ^= b; c -= rot(b,16); \ a ^= c; a -= rot(c,4); \ b ^= a; b -= rot(a,14); \ c ^= b; c -= rot(b,24); \ }
hash 算法
源代码中大端小端,而且还分是 0x3 还是 0x1,这个目前就不知道干什么了。
uint32_t jenkins_hash( const void *key, size_t length) { uint32_t a,b,c; a = b = c = 0xdeadbeef + ((uint32_t)length) + 0; const char *k = (const char *)key; while (length > 12) { a += ((uint32_t)k[0])看完这个代码,我们可以给他缩短一下。
uint32_t jenkins_hash( const void *key, size_t length) { uint32_t a,b,c; a = b = c = 0xdeadbeef + ((uint32_t)length) + 0; const char *k = (const char *)key; while (length >= 12) { a += *((uint32_t*)(k+0)); b += *((uint32_t*)(k+4)); c += *((uint32_t*)(k+8)); mix(a,b,c); length -= 12; k += 12; } if(length == 0) { return c; } switch(length) { case 11: c+=((uint32_t)k[10])murmur3 hash
murmur3 hash 的位置在 murmur3_hash.c .
//不检查数据越界问题,主要用于得到一些随机数字 #define FORCE_INLINE inline __attribute__((always_inline)) //循环左移 static inline uint32_t ROTL32 ( uint32_t x, int8_t r ) { return (x > (32 - r)); } //得到指针p位置的值,i可能为负数 static FORCE_INLINE uint32_t getblock32 ( const uint32_t * p, int i ) { return p[i]; } static FORCE_INLINE uint32_t fmix32 ( uint32_t h ) { h ^= h >> 16; h *= 0x85ebca6b; h ^= h >> 13; h *= 0xc2b2ae35; h ^= h >> 16; return h; } uint32_t MurmurHash3_x86_32 ( const void * key, size_t length) { const uint8_t * data = (const uint8_t*)key; const int nblocks = length / 4; uint32_t h1 = 0; uint32_t c1 = 0xcc9e2d51; uint32_t c2 = 0x1b873593; const uint32_t * blocks = (const uint32_t *)(data + nblocks*4); for(int i = -nblocks; i; i++) { uint32_t k1 = getblock32(blocks,i); k1 *= c1; k1 = ROTL32(k1,15); k1 *= c2; h1 ^= k1; h1 = ROTL32(h1,13); h1 = h1*5+0xe6546b64; } const uint8_t * tail = (const uint8_t*)(data + nblocks*4); uint32_t k1 = 0; switch(length & 3) { case 3: k1 ^= tail[2]Additive Hash
ub4 additive(char *key, ub4 len, ub4 prime){ ub4 hash, i; for (hash=len, i=0; iRotating Hash
ub4 rotating(char *key, ub4 len, ub4 prime){ ub4 hash, i; for (hash=len, i=0; i>28)^key[i]; return (hash % prime); } One-at-a-Time Hash
ub4 one_at_a_time(char *key, ub4 len){ ub4 hash, i; for (hash=0, i=0; i> 6); } hash += (hash > 11); hash += (hash Bernstein hash
ub4 bernstein(ub1 *key, ub4 len, ub4 level){ ub4 hash = level; ub4 i; for (i=0; iGoulburn Hash
u4 goulburn( const unsigned char *cp, size_t len, uint32_t last_value){ register u4 h = last_value; int u; for( u=0; u> 29); h += g_table1[ h >> 25 ]; h ^= (h > 18); h += 1783936964UL; } return h; } Murmur Hash
uint32_t MurmurHash1 ( const void * key, int len, uint32_t seed ){ const unsigned int m = 0xc6a4a793;
const int r = 16; unsigned int h = seed ^ (len * m); //---------- const unsigned char * data = (const unsigned char *)key; while(len >= 4){ unsigned int k = *(unsigned int *)data; h += k; h *= m; h ^= h >> 16; data += 4; len -= 4; } //---------- switch(len){ case 3: h += data[2] > r; }; //---------- h *= m; h ^= h >> 10; h *= m; h ^= h >> 17; return h;}
Pearson Hash
//This preinitializes tab[] to an arbitrary permutation of 0 .. 255. char pearson(char *key, ub4 len, char tab[256]){ char hash; ub4 i; for (hash=len, i=0; iCRC Hashing
ub4 crc(char *key, ub4 len, ub4 mask, ub4 tab[256]){ ub4 hash, i; for (hash=len, i=0; i> 8) ^ tab[(hash & 0xff) ^ key[i]]; return (hash & mask); } Generalized CRC Hashing
//The size of tab[] is the maximum number of input bits. //Values in tab[] are chosen at random. ub4 universal(char *key, ub4 len, ub4 mask, ub4 tab[MAXBITS]){ ub4 hash, i; for (hash=len, i=0; i>3]; if (k&0x01) hash ^= tab[i+0]; if (k&0x02) hash ^= tab[i+1]; if (k&0x04) hash ^= tab[i+2]; if (k&0x08) hash ^= tab[i+3]; if (k&0x10) hash ^= tab[i+4]; if (k&0x20) hash ^= tab[i+5]; if (k&0x40) hash ^= tab[i+6]; if (k&0x80) hash ^= tab[i+7]; } return (hash & mask); }Zobrist Hashing
ub4 zobrist( char *key, ub4 len, ub4 mask, ub4 tab[MAXBYTES][256]){ ub4 hash, i; for (hash=len, i=0; i本文出自:http://tiankonguse.github.io, 原文地址:http://github.tiankonguse.com//blog/2014/11/07/memcached-string-hash/, 感谢原作者分享。
上一篇: 阿里云CentOS 6.3 64位下搭建php运行环境
下一篇: php获取本地实际IP