来自哈希函数的相同键的不同值和良好的哈希值
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【中文标题】来自哈希函数的相同键的不同值和良好的哈希值【英文标题】：Different values for the same key from a hash function and good sort of hash values 【发布时间】：2017-07-18 14:10:36 【问题描述】：我必须执行 MurmurHash3，一个哈希函数。但我得到了奇怪的结果。 我想知道我是不是用不好这个功能：
MurmurHash3.cpp
#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 uint32_t rotl32 ( uint32_t x, int8_t r )
return (x << r) | (x >> (32 - r));
inline 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_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;
MurmurHash3.h
#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_
Testing.cpp
#include "MurmurHash3.h"
#include <time.h>
#include <string.h>
#include <iostream>
using namespace std;
int main ( int argc, char ** argv )
const char * hashToTest = "murmur3a";
char out[128] ;
uint32_t seed = time(0);
MurmurHash3_x64_128( hashToTest, strlen(hashToTest) , seed, out );
for(int i=0 ; i<128 ;i++)
cout<<(int) out[i]<<" ";
cout<<endl;
我调用函数 MurmurHash3_x64_128 因为我在 64 位系统上。 如果你是 32 位的，你应该调用 MurmurHash3_x86_128
但是，例如，我进入 out
-34 -106 32 -60 34 44 -30 -128 -127 -10 -75 25 73 -64 -50 31 -120 32 96 0 0 0 0 0 41 116 50 -56 7 127 0 0 1 0 0 0 0 0 0 0 -32 -71 12 29 -3 127 0 0 -8 29 96 0 0 0 0 0 68 24 64 0 0 0 0 0 -8 -79 47 -56 7 127 0 0 -1 -1 0 0 1 0 0 0 -16 -71 12 29 -3 127 0 0 89 24 64 0 0 0 0 0 2 0 0 0 0 0 0 0 -83 24 64 0 0 0 0 0 1 0 0 0 -3 127 0 0 0 0 0 0 0 0 0 0
也有负值。我不是哈希函数的专家。这种行为正常吗？ 此外，我需要对从更多函数调用返回的哈希值进行排序。如何有效地比较哈希值？用异或？
让我认为我错了的另一件事是该函数为同一执行返回 2 个不同的哈希值。即看这段代码
Testing.cpp
#include "MurmurHash3.h"
#include <time.h>
#include <string.h>
#include <iostream>
using namespace std;
int main ( int argc, char ** argv )
const char * hashToTest = "murmur3a";
char out[128] ;
char out2[128] ;
uint32_t seed = time(0);
MurmurHash3_x64_128( hashToTest, strlen(hashToTest) , seed, out );
MurmurHash3_x64_128( hashToTest, strlen(hashToTest) , seed, out2 );
for(int i=0 ; i<128 ;i++)
cout<<(int) out[i]<<" ";
cout<<endl;
for(int i=0 ; i<128 ;i++)
cout<<(int) out2[i]<<" ";
cout<<endl;
我得到 2 个不同的哈希值：
-93 -105 98 -119 -121 125 76 -5 -48 -108 51 -50 18 -74 -72 2 -24 -68 37 32 -4 127 0 0 1 0 0 0 0 0 0 0 -80 -69 37 32 -4 127 0 0 -9 102 56 -80 99 127 0 0 1 0 0 0 99 127 0 0 0 0 0 0 0 0 0 0 -80 -81 53 -80 99 127 0 0 -40 40 -53 -81 99 127 0 0 1 0 0 0 0 0 0 0 -80 -69 37 32 -4 127 0 0 -8 29 96 0 0 0 0 0 -91 -42 56 -80 99 127 0 0 0 0 0 0 0 0 0 0 6 0 0 0 0 0 0 0
-93 -105 98 -119 -121 125 76 -5 -48 -108 51 -50 18 -74 -72 2 -128 32 96 0 0 0 0 0 41 100 -50 -81 99 127 0 0 1 0 0 0 0 0 0 0 -80 -69 37 32 -4 127 0 0 -8 29 96 0 0 0 0 0 79 24 64 0 0 0 0 0 -8 -95 -53 -81 99 127 0 0 -1 -1 0 0 1 0 0 0 -64 -69 37 32 -4 127 0 0 100 24 64 0 0 0 0 0 2 0 0 0 0 0 0 0 -67 24 64 0 0 0 0 0 1 0 0 0 -4 127 0 0 0 0 0 0 0 0 0 0
（我使用的是 C++11）
【问题讨论】：
【参考方案1】：我得到 2 个不同的哈希值：
一个问题是您要打印 128 个字节的数据，但 MurmurHash3() 只输出 128 个位。这意味着只有输出的每行的前 (128/8)=16 个字节是有效的哈希数据；您打印出来的剩余字节是没有意义的未初始化/随机数据。
MurmurHash3() Wikipedia 页面上提到了第二个原因：
使用 128 位时，x86 和 x64 版本产生的结果不同 值，因为算法针对它们各自的进行了优化 平台。
进入下一部分...
也有负值。我不是哈希函数的专家。 这种行为正常吗？
值是否为负取决于数据打印机制如何解释字节。目前，您正在打印这些值，就好像它们是ints，而int 是有符号数据类型，这意味着任何设置了最高有效位的值都将打印为负值。如果您想查看打印为无符号的值，则应在将它们传递给 cout 之前将它们转换为 (unsigned int)。
如何有效地比较哈希值？
memcmp() 是比较两个任意内存缓冲区内容的一种常用方法。
【讨论】：
他在调用MurmurHash之前只调用了一次time(0)，并且对两者都使用了相同的seed值。
是的，我明白了——我会修正我的答案。
你回答了我所有的问题。谢谢。我犯了2个愚蠢的错误。我忘了一件事。如果我想在 32 或 64 位系统上建立代码，我可以使用： if(sizeof(void *) ==8) ??还是有更聪明的方法？以上是关于来自哈希函数的相同键的不同值和良好的哈希值的主要内容，如果未能解决你的问题，请参考以下文章