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
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 的位置在 jenkins_hash.c .
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
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)<<(k)) ^ ((x)>>(32-(k))))
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 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); \
}
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])<<24;
a += ((uint32_t)k[1])<<16;
a += ((uint32_t)k[2])<<8;
a += ((uint32_t)k[3]);
b += ((uint32_t)k[4])<<24;
b += ((uint32_t)k[5])<<16;
b += ((uint32_t)k[6])<<8;
b += ((uint32_t)k[7]);
c += ((uint32_t)k[8])<<24;
c += ((uint32_t)k[9])<<16;
c += ((uint32_t)k[10])<<8;
c += ((uint32_t)k[11]);
mix(a,b,c);
length -= 12;
k += 12;
}
switch(length) {
case 12:
c+=k[11];
case 11:
c+=((uint32_t)k[10])<<8;
case 10:
c+=((uint32_t)k[9])<<16;
case 9 :
c+=((uint32_t)k[8])<<24;
case 8 :
b+=k[7];
case 7 :
b+=((uint32_t)k[6])<<8;
case 6 :
b+=((uint32_t)k[5])<<16;
case 5 :
b+=((uint32_t)k[4])<<24;
case 4 :
a+=k[3];
case 3 :
a+=((uint32_t)k[2])<<8;
case 2 :
a+=((uint32_t)k[1])<<16;
case 1 :
a+=((uint32_t)k[0])<<24;
break;
case 0 :
return c;
}
final(a,b,c);
return c;
}
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])<<8;
case 10:
c+=((uint32_t)k[9])<<16;
case 9 :
c+=((uint32_t)k[8])<<24;
case 8 :
b += *((uint32_t*)(k+4));
a += *((uint32_t*)(k+0));
break;
case 7 :
b+=((uint32_t)k[6])<<8;
case 6 :
b+=((uint32_t)k[5])<<16;
case 5 :
b+=((uint32_t)k[4])<<24;
case 4 :
a += *((uint32_t*)(k+0));
break;
case 3 :
a+=((uint32_t)k[2])<<8;
case 2 :
a+=((uint32_t)k[1])<<16;
case 1 :
a+=((uint32_t)k[0])<<24;
}
final(a,b,c);
return c;
}
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 << r) | (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] << 16;
case 2:
k1 ^= tail[1] << 8;
case 1:
k1 ^= tail[0];
k1 *= c1;
k1 = ROTL32(k1,15);
k1 *= c2;
h1 ^= k1;
};
h1 ^= length;
h1 = fmix32(h1);
return h1;
}
ub4 additive(char *key, ub4 len, ub4 prime){
ub4 hash, i;
for (hash=len, i=0; i<len; ++i)
hash += key[i];
return (hash % prime);
}
ub4 rotating(char *key, ub4 len, ub4 prime){
ub4 hash, i;
for (hash=len, i=0; i<len; ++i)
hash = (hash<<4)^(hash>>28)^key[i];
return (hash % prime);
}
ub4 one_at_a_time(char *key, ub4 len){
ub4 hash, i;
for (hash=0, i=0; i<len; ++i){
hash += key[i];
hash += (hash << 10);
hash ^= (hash >> 6);
}
hash += (hash << 3);
hash ^= (hash >> 11);
hash += (hash << 15);
return (hash & mask);
}
ub4 bernstein(ub1 *key, ub4 len, ub4 level){
ub4 hash = level;
ub4 i;
for (i=0; i<len; ++i) hash = 33*hash + key[i];
return 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<len; ++u ) {
h += g_table0[ cp[u] ];
h ^= (h << 3) ^ (h >> 29);
h += g_table1[ h >> 25 ];
h ^= (h << 14) ^ (h >> 18);
h += 1783936964UL;
}
return h;
}
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] << 16;
case 2:
h += data[1] << 8;
case 1:
h += data[0];
h *= m;
h ^= h >> r;
};
//----------
h *= m;
h ^= h >> 10;
h *= m;
h ^= h >> 17;
return h;
}
//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; i<len; ++i)
hash=tab[hash^key[i]];
return (hash);
}
ub4 crc(char *key, ub4 len, ub4 mask, ub4 tab[256]){
ub4 hash, i;
for (hash=len, i=0; i<len; ++i)
hash = (hash >> 8) ^ tab[(hash & 0xff) ^ key[i]];
return (hash & mask);
}
//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<(len<<3); i+=8){
register char k = key[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);
}
ub4 zobrist( char *key, ub4 len, ub4 mask, ub4 tab[MAXBYTES][256]){
ub4 hash, i;
for (hash=len, i=0; i<len; ++i)
hash ^= tab[i][key[i]];
return (hash & mask);
}
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。
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