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 hereFORCE_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 avalancheFORCE_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;//----------// bodyconst 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;}//----------// tailconst 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;};//----------// finalizationh1 ^= 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;//----------// bodyconst 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;}//----------// tailconst 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;};//----------// finalizationh1 ^= 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);//----------// bodyconst 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;}//----------// tailconst 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;};//----------// finalizationh1 ^= 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 0x97c29b3aint 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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