最近在项目代码中看到了一种hash算法，以前没有遇见过，在此记录下来。  

一、介绍

  MurmurHash 是一种非加密型哈希函数，适用于一般的哈希检索操作。 由Austin Appleby在2008年发明， 并出现了多个变种，都已经发布到了公有领域(public domain)。与其它流行的哈希函数相比，对于规律性较强的key，MurmurHash的随机分布特征表现更良好。—摘自wiki

  Redis在实现字典时用到了两种不同的哈希算法，MurmurHash便是其中一种（另一种是djb），在Redis中应用十分广泛，包括数据库、集群、哈希键、阻塞操作等功能都用到了这个算法。发明算法的作者被邀到google工作，该算法最新版本是MurmurHash3，基于MurmurHash2改进了一些小瑕疵，使得速度更快，实现了32位（低延时）、128位HashKey，尤其对大块的数据，具有较高的平衡性与低碰撞率。  

二、代码

代码来自GitHub: https://github.com/aappleby/smhasher 因为Murmurhash最新版本是MurmurHash3，这里只简单测试了一下MurmurHash3

Murmurhash3.h

//----------------------------------------------------------------------------- // MurmurHash3 was written by Austin Appleby, and is placed in the public // domain. The author hereby disclaims copyright to this source code. #ifndef _MURMURHASH3_H_ #define _MURMURHASH3_H_ //----------------------------------------------------------------------------- // Platform-specific functions and macros // Microsoft Visual Studio #if defined(_MSC_VER) && (_MSC_VER < 1600) typedef unsigned char uint8_t; typedef unsigned int uint32_t; typedef unsigned __int64 uint64_t; // Other compilers #else // defined(_MSC_VER) #include <stdint.h> #endif // !defined(_MSC_VER) //----------------------------------------------------------------------------- void MurmurHash3_x86_32 ( const void * key, int len, uint32_t seed, void * out ); void MurmurHash3_x86_128 ( const void * key, int len, uint32_t seed, void * out ); void MurmurHash3_x64_128 ( const void * key, int len, uint32_t seed, void * out ); //----------------------------------------------------------------------------- #endif // _MURMURHASH3_H_

Murmurhash3.c

//----------------------------------------------------------------------------- // MurmurHash3 was written by Austin Appleby, and is placed in the public // domain. The author hereby disclaims copyright to this source code. // Note - The x86 and x64 versions do _not_ produce the same results, as the // algorithms are optimized for their respective platforms. You can still // compile and run any of them on any platform, but your performance with the // non-native version will be less than optimal. // // github : https://github.com/aappleby/smhasher #include "MurmurHash3.h" //----------------------------------------------------------------------------- // Platform-specific functions and macros // Microsoft Visual Studio #if defined(_MSC_VER) #define FORCE_INLINE __forceinline #include <stdlib.h> #define ROTL32(x,y) _rotl(x,y) #define ROTL64(x,y) _rotl64(x,y) #define BIG_CONSTANT(x) (x) // Other compilers #else // defined(_MSC_VER) #define FORCE_INLINE inline __attribute__((always_inline)) inline static uint32_t rotl32 ( uint32_t x, int8_t r ) { return (x << r) | (x >> (32 - r)); } inline static uint64_t rotl64 ( uint64_t x, int8_t r ) { return (x << r) | (x >> (64 - r)); } #define ROTL32(x,y) rotl32(x,y) #define ROTL64(x,y) rotl64(x,y) #define BIG_CONSTANT(x) (x##LLU) #endif // !defined(_MSC_VER) //----------------------------------------------------------------------------- // Block read - if your platform needs to do endian-swapping or can only // handle aligned reads, do the conversion here FORCE_INLINE uint32_t getblock32 ( const uint32_t * p, int i ) { return p[i]; } FORCE_INLINE uint64_t getblock64 ( const uint64_t * p, int i ) { return p[i]; } //----------------------------------------------------------------------------- // Finalization mix - force all bits of a hash block to avalanche FORCE_INLINE uint32_t fmix32 ( uint32_t h ) { h ^= h >> 16; h *= 0x85ebca6b; h ^= h >> 13; h *= 0xc2b2ae35; h ^= h >> 16; return h; } //---------- FORCE_INLINE uint64_t fmix64 ( uint64_t k ) { k ^= k >> 33; k *= BIG_CONSTANT(0xff51afd7ed558ccd); k ^= k >> 33; k *= BIG_CONSTANT(0xc4ceb9fe1a85ec53); k ^= k >> 33; return k; } //----------------------------------------------------------------------------- void MurmurHash3_x86_32 ( const void * key, int len, ¦ uint32_t seed, void * out ) { const uint8_t * data = (const uint8_t*)key; const int nblocks = len / 4; uint32_t h1 = seed; const uint32_t c1 = 0xcc9e2d51; const uint32_t c2 = 0x1b873593; //---------- // body 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; } //---------- // tail const uint8_t * tail = (const uint8_t*)(data + nblocks*4); uint32_t k1 = 0; switch(len & 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; }; //---------- // finalization h1 ^= len; h1 = fmix32(h1); *(uint32_t*)out = h1; } //----------------------------------------------------------------------------- void MurmurHash3_x86_128 ( const void * key, const int len, uint32_t seed, void * out ) { const uint8_t * data = (const uint8_t*)key; const int nblocks = len / 16; uint32_t h1 = seed; uint32_t h2 = seed; uint32_t h3 = seed; uint32_t h4 = seed; const uint32_t c1 = 0x239b961b; const uint32_t c2 = 0xab0e9789; const uint32_t c3 = 0x38b34ae5; const uint32_t c4 = 0xa1e38b93; //---------- // body const uint32_t * blocks = (const uint32_t *)(data + nblocks*16); for(int i = -nblocks; i; i++) { uint32_t k1 = getblock32(blocks,i*4+0); uint32_t k2 = getblock32(blocks,i*4+1); uint32_t k3 = getblock32(blocks,i*4+2); uint32_t k4 = getblock32(blocks,i*4+3); k1 *= c1; k1 = ROTL32(k1,15); k1 *= c2; h1 ^= k1; h1 = ROTL32(h1,19); h1 += h2; h1 = h1*5+0x561ccd1b; k2 *= c2; k2 = ROTL32(k2,16); k2 *= c3; h2 ^= k2; h2 = ROTL32(h2,17); h2 += h3; h2 = h2*5+0x0bcaa747; k3 *= c3; k3 = ROTL32(k3,17); k3 *= c4; h3 ^= k3; h3 = ROTL32(h3,15); h3 += h4; h3 = h3*5+0x96cd1c35; k4 *= c4; k4 = ROTL32(k4,18); k4 *= c1; h4 ^= k4; h4 = ROTL32(h4,13); h4 += h1; h4 = h4*5+0x32ac3b17; } //---------- // tail const uint8_t * tail = (const uint8_t*)(data + nblocks*16); uint32_t k1 = 0; uint32_t k2 = 0; uint32_t k3 = 0; uint32_t k4 = 0; switch(len & 15) { case 15: k4 ^= tail[14] << 16; case 14: k4 ^= tail[13] << 8; case 13: k4 ^= tail[12] << 0; k4 *= c4; k4 = ROTL32(k4,18); k4 *= c1; h4 ^= k4; case 12: k3 ^= tail[11] << 24; case 11: k3 ^= tail[10] << 16; case 10: k3 ^= tail[ 9] << 8; case 9: k3 ^= tail[ 8] << 0; k3 *= c3; k3 = ROTL32(k3,17); k3 *= c4; h3 ^= k3; case 8: k2 ^= tail[ 7] << 24; case 7: k2 ^= tail[ 6] << 16; case 6: k2 ^= tail[ 5] << 8; case 5: k2 ^= tail[ 4] << 0; k2 *= c2; k2 = ROTL32(k2,16); k2 *= c3; h2 ^= k2; case 4: k1 ^= tail[ 3] << 24; case 3: k1 ^= tail[ 2] << 16; case 2: k1 ^= tail[ 1] << 8; case 1: k1 ^= tail[ 0] << 0; k1 *= c1; k1 = ROTL32(k1,15); k1 *= c2; h1 ^= k1; }; //---------- // finalization h1 ^= len; h2 ^= len; h3 ^= len; h4 ^= len; h1 += h2; h1 += h3; h1 += h4; h2 += h1; h3 += h1; h4 += h1; h1 = fmix32(h1); h2 = fmix32(h2); h3 = fmix32(h3); h4 = fmix32(h4); h1 += h2; h1 += h3; h1 += h4; h2 += h1; h3 += h1; h4 += h1; ((uint32_t*)out)[0] = h1; ((uint32_t*)out)[1] = h2; ((uint32_t*)out)[2] = h3; ((uint32_t*)out)[3] = h4; } //----------------------------------------------------------------------------- void MurmurHash3_x64_128 ( const void * key, const int len, const uint32_t seed, void * out ) { const uint8_t * data = (const uint8_t*)key; const int nblocks = len / 16; uint64_t h1 = seed; uint64_t h2 = seed; const uint64_t c1 = BIG_CONSTANT(0x87c37b91114253d5); const uint64_t c2 = BIG_CONSTANT(0x4cf5ad432745937f); //---------- // body const uint64_t * blocks = (const uint64_t *)(data); for(int i = 0; i < nblocks; i++) { uint64_t k1 = getblock64(blocks,i*2+0); uint64_t k2 = getblock64(blocks,i*2+1); k1 *= c1; k1 = ROTL64(k1,31); k1 *= c2; h1 ^= k1; h1 = ROTL64(h1,27); h1 += h2; h1 = h1*5+0x52dce729; k2 *= c2; k2 = ROTL64(k2,33); k2 *= c1; h2 ^= k2; h2 = ROTL64(h2,31); h2 += h1; h2 = h2*5+0x38495ab5; } //---------- // tail const uint8_t * tail = (const uint8_t*)(data + nblocks*16); uint64_t k1 = 0; uint64_t k2 = 0; switch(len & 15) { case 15: k2 ^= ((uint64_t)tail[14]) << 48; case 14: k2 ^= ((uint64_t)tail[13]) << 40; case 13: k2 ^= ((uint64_t)tail[12]) << 32; case 12: k2 ^= ((uint64_t)tail[11]) << 24; case 11: k2 ^= ((uint64_t)tail[10]) << 16; case 10: k2 ^= ((uint64_t)tail[ 9]) << 8; case 9: k2 ^= ((uint64_t)tail[ 8]) << 0; k2 *= c2; k2 = ROTL64(k2,33); k2 *= c1; h2 ^= k2; case 8: k1 ^= ((uint64_t)tail[ 7]) << 56; case 7: k1 ^= ((uint64_t)tail[ 6]) << 48; case 6: k1 ^= ((uint64_t)tail[ 5]) << 40; case 5: k1 ^= ((uint64_t)tail[ 4]) << 32; case 4: k1 ^= ((uint64_t)tail[ 3]) << 24; case 3: k1 ^= ((uint64_t)tail[ 2]) << 16; case 2: k1 ^= ((uint64_t)tail[ 1]) << 8; case 1: k1 ^= ((uint64_t)tail[ 0]) << 0; k1 *= c1; k1 = ROTL64(k1,31); k1 *= c2; h1 ^= k1; }; //---------- // finalization h1 ^= len; h2 ^= len; h1 += h2; h2 += h1; h1 = fmix64(h1); h2 = fmix64(h2); h1 += h2; h2 += h1; ((uint64_t*)out)[0] = h1; ((uint64_t*)out)[1] = h2; } //----------------------------------------------------------------------------- #if 1 #include <string.h> #include <stdio.h> #define SEED 0x97c29b3a int main() { const char *str="abcdefghijklmn"; uint32_t out1; MurmurHash3_x86_32(str, strlen(str), SEED, &out1); printf("%u\n", out1); uint32_t out2[4]; MurmurHash3_x86_128(str, strlen(str), SEED, out2); printf("%u, %u, %u, %u\n", out2[0], out2[1], out2[2], out2[3]); uint64_t out3[2]; MurmurHash3_x64_128(str, strlen(str), SEED, out3); printf("%lu, %lu\n", out3[0], out3[1]); return 0; } #endif

MurmurHash3_x86_32 将key 哈希32位的正整数 MurmurHash3_x86_128 将key 哈希128位的4个无符号位32整数，x86是32位的 MurmurHash3_x64_128 将key 哈希128位的2个无符号64位整数，x64是64位的

编译： gcc -std=c99 MurmurHash3.c MurmurHash3.h -o MurmurHash3

注意： 直接从GitHub拉取的代码在Linux可能编译不过去。 问题是在定义rotl32() 和rotl64()使用关键字inline,要么把inline去掉，要么在inline后面加关键字static.

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