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memcached 源码阅读之 字符串 hash 与 搜集的一些 字符串 hash

程序员文章站 2022-05-25 21:58:52
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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

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; i

Rotating 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; i

Goulburn 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; i

CRC 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