各种字符串Hash函数比较

拼搏现实的明天。 2022-05-25 14:28 132阅读 0赞

[http://www.byvoid.com/blog/string-hash-compare/][http_www.byvoid.com_blog_string-hash-compare]

常用的字符串Hash函数还有ELFHash，APHash等等，都是十分简单有效的方法。这些函数使用位运算使得每一个字符都对最后的函数值产生影响。另外还有以MD5和SHA1为代表的杂凑函数，这些函数几乎不可能找到碰撞。

常用字符串哈希函数有BKDRHash，APHash，DJBHash，JSHash，RSHash，SDBMHash，PJWHash，ELFHash等等。对于以上几种哈希函数，我对其进行了一个小小的评测。

<table style="border-collapse:collapse; border-spacing:0px; border-width:1px; margin-top:0px; margin-bottom:10px; border-style:solid; color:rgb(51,51,51); font-family:Arial,MingLiU,Meiryo,'MS Gothic',serif; font-size:15px; line-height:26px">    
 <tbody> 
  <tr style="padding:2px; border:1px dashed gray"> 
   <td style="padding:2px; border:1px dashed gray">Hash函数</td> 
   <td style="padding:2px; border:1px dashed gray">数据1</td> 
   <td style="padding:2px; border:1px dashed gray">数据2</td> 
   <td style="padding:2px; border:1px dashed gray">数据3</td> 
   <td style="padding:2px; border:1px dashed gray">数据4</td> 
   <td style="padding:2px; border:1px dashed gray">数据1得分</td> 
   <td style="padding:2px; border:1px dashed gray">数据2得分</td> 
   <td style="padding:2px; border:1px dashed gray">数据3得分</td> 
   <td style="padding:2px; border:1px dashed gray">数据4得分</td> 
   <td style="padding:2px; border:1px dashed gray">平均分</td> 
  </tr> 
  <tr style="padding:2px; border:1px dashed gray"> 
   <td style="padding:2px; border:1px dashed gray">BKDRHash</td> 
   <td align="right" style="padding:2px; border:1px dashed gray">2</td> 
   <td align="right" style="padding:2px; border:1px dashed gray">0</td> 
   <td align="right" style="padding:2px; border:1px dashed gray">4774</td> 
   <td align="right" style="padding:2px; border:1px dashed gray">481</td> 
   <td align="right" style="padding:2px; border:1px dashed gray">96.55</td> 
   <td align="right" style="padding:2px; border:1px dashed gray">100</td> 
   <td align="right" style="padding:2px; border:1px dashed gray">90.95</td> 
   <td align="right" style="padding:2px; border:1px dashed gray">82.05</td> 
   <td align="right" style="padding:2px; border:1px dashed gray">92.64</td> 
  </tr> 
  <tr style="padding:2px; border:1px dashed gray"> 
   <td style="padding:2px; border:1px dashed gray">APHash</td> 
   <td align="right" style="padding:2px; border:1px dashed gray">2</td> 
   <td align="right" style="padding:2px; border:1px dashed gray">3</td> 
   <td align="right" style="padding:2px; border:1px dashed gray">4754</td> 
   <td align="right" style="padding:2px; border:1px dashed gray">493</td> 
   <td align="right" style="padding:2px; border:1px dashed gray">96.55</td> 
   <td align="right" style="padding:2px; border:1px dashed gray">88.46</td> 
   <td align="right" style="padding:2px; border:1px dashed gray">100</td> 
   <td align="right" style="padding:2px; border:1px dashed gray">51.28</td> 
   <td align="right" style="padding:2px; border:1px dashed gray">86.28</td> 
  </tr> 
  <tr style="padding:2px; border:1px dashed gray"> 
   <td style="padding:2px; border:1px dashed gray">DJBHash</td> 
   <td align="right" style="padding:2px; border:1px dashed gray">2</td> 
   <td align="right" style="padding:2px; border:1px dashed gray">2</td> 
   <td align="right" style="padding:2px; border:1px dashed gray">4975</td> 
   <td align="right" style="padding:2px; border:1px dashed gray">474</td> 
   <td align="right" style="padding:2px; border:1px dashed gray">96.55</td> 
   <td align="right" style="padding:2px; border:1px dashed gray">92.31</td> 
   <td align="right" style="padding:2px; border:1px dashed gray">0</td> 
   <td align="right" style="padding:2px; border:1px dashed gray">100</td> 
   <td align="right" style="padding:2px; border:1px dashed gray">83.43</td> 
  </tr> 
  <tr style="padding:2px; border:1px dashed gray"> 
   <td style="padding:2px; border:1px dashed gray">JSHash</td> 
   <td align="right" style="padding:2px; border:1px dashed gray">1</td> 
   <td align="right" style="padding:2px; border:1px dashed gray">4</td> 
   <td align="right" style="padding:2px; border:1px dashed gray">4761</td> 
   <td align="right" style="padding:2px; border:1px dashed gray">506</td> 
   <td align="right" style="padding:2px; border:1px dashed gray">100</td> 
   <td align="right" style="padding:2px; border:1px dashed gray">84.62</td> 
   <td align="right" style="padding:2px; border:1px dashed gray">96.83</td> 
   <td align="right" style="padding:2px; border:1px dashed gray">17.95</td> 
   <td align="right" style="padding:2px; border:1px dashed gray">81.94</td> 
  </tr> 
  <tr style="padding:2px; border:1px dashed gray"> 
   <td style="padding:2px; border:1px dashed gray">RSHash</td> 
   <td align="right" style="padding:2px; border:1px dashed gray">1</td> 
   <td align="right" style="padding:2px; border:1px dashed gray">0</td> 
   <td align="right" style="padding:2px; border:1px dashed gray">4861</td> 
   <td align="right" style="padding:2px; border:1px dashed gray">505</td> 
   <td align="right" style="padding:2px; border:1px dashed gray">100</td> 
   <td align="right" style="padding:2px; border:1px dashed gray">100</td> 
   <td align="right" style="padding:2px; border:1px dashed gray">51.58</td> 
   <td align="right" style="padding:2px; border:1px dashed gray">20.51</td> 
   <td align="right" style="padding:2px; border:1px dashed gray">75.96</td> 
  </tr> 
  <tr style="padding:2px; border:1px dashed gray"> 
   <td style="padding:2px; border:1px dashed gray">SDBMHash</td> 
   <td align="right" style="padding:2px; border:1px dashed gray">3</td> 
   <td align="right" style="padding:2px; border:1px dashed gray">2</td> 
   <td align="right" style="padding:2px; border:1px dashed gray">4849</td> 
   <td align="right" style="padding:2px; border:1px dashed gray">504</td> 
   <td align="right" style="padding:2px; border:1px dashed gray">93.1</td> 
   <td align="right" style="padding:2px; border:1px dashed gray">92.31</td> 
   <td align="right" style="padding:2px; border:1px dashed gray">57.01</td> 
   <td align="right" style="padding:2px; border:1px dashed gray">23.08</td> 
   <td align="right" style="padding:2px; border:1px dashed gray">72.41</td> 
  </tr> 
  <tr style="padding:2px; border:1px dashed gray"> 
   <td style="padding:2px; border:1px dashed gray">PJWHash</td> 
   <td align="right" style="padding:2px; border:1px dashed gray">30</td> 
   <td align="right" style="padding:2px; border:1px dashed gray">26</td> 
   <td align="right" style="padding:2px; border:1px dashed gray">4878</td> 
   <td align="right" style="padding:2px; border:1px dashed gray">513</td> 
   <td align="right" style="padding:2px; border:1px dashed gray">0</td> 
   <td align="right" style="padding:2px; border:1px dashed gray">0</td> 
   <td align="right" style="padding:2px; border:1px dashed gray">43.89</td> 
   <td align="right" style="padding:2px; border:1px dashed gray">0</td> 
   <td align="right" style="padding:2px; border:1px dashed gray">21.95</td> 
  </tr> 
  <tr style="padding:2px; border:1px dashed gray"> 
   <td style="padding:2px; border:1px dashed gray">ELFHash</td> 
   <td align="right" style="padding:2px; border:1px dashed gray">30</td> 
   <td align="right" style="padding:2px; border:1px dashed gray">26</td> 
   <td align="right" style="padding:2px; border:1px dashed gray">4878</td> 
   <td align="right" style="padding:2px; border:1px dashed gray">513</td> 
   <td align="right" style="padding:2px; border:1px dashed gray">0</td> 
   <td align="right" style="padding:2px; border:1px dashed gray">0</td> 
   <td align="right" style="padding:2px; border:1px dashed gray">43.89</td> 
   <td align="right" style="padding:2px; border:1px dashed gray">0</td> 
   <td align="right" style="padding:2px; border:1px dashed gray">21.95</td> 
  </tr> 
 </tbody> 
</table>

其中数据1为100000个字母和数字组成的随机串哈希冲突个数。数据2为100000个有意义的英文句子哈希冲突个数。数据3为数据1的哈希值与1000003(大素数)求模后存储到线性表中冲突的个数。数据4为数据1的哈希值与10000019(更大素数)求模后存储到线性表中冲突的个数。

经过比较，得出以上平均得分。平均数为平方平均数。可以发现，BKDRHash无论是在实际效果还是编码实现中，效果都是最突出的。APHash也是较为优秀的算法。DJBHash,JSHash,RSHash与SDBMHash各有千秋。PJWHash与ELFHash效果最差，但得分相似，其算法本质是相似的。

在信息修竞赛中，要本着易于编码调试的原则，个人认为BKDRHash是最适合记忆和使用的。

BYVoid原创，欢迎建议、交流、批评和指正。

附：各种哈希函数的C语言程序代码

//SDBMHash
    unsigned int SDBMHash(char *str)
    {
    	unsigned int hash = 0;
    
    	while (*str)
    	{
    		// equivalent to: hash = 65599*hash + (*str++);
    		hash = (*str++) + (hash << 6) + (hash << 16) - hash;
    	}
    
    	return (hash & 0x7FFFFFFF);
    }
    
    // RS Hash Function
    unsigned int RSHash(char *str)
    {
    	unsigned int b = 378551;
    	unsigned int a = 63689;
    	unsigned int hash = 0;
    
    	while (*str)
    	{
    		hash = hash * a + (*str++);
    		a *= b;
    	}
    
    	return (hash & 0x7FFFFFFF);
    }
    
    // JS Hash Function
    unsigned int JSHash(char *str)
    {
    	unsigned int hash = 1315423911;
    
    	while (*str)
    	{
    		hash ^= ((hash << 5) + (*str++) + (hash >> 2));
    	}
       
    	return (hash & 0x7FFFFFFF);
    }
    
    // P. J. Weinberger Hash Function
    unsigned int PJWHash(char *str)
    {
    	unsigned int BitsInUnignedInt = (unsigned int)(sizeof(unsigned int) * 8);
    	unsigned int ThreeQuarters	= (unsigned int)((BitsInUnignedInt  * 3) / 4);
    	unsigned int OneEighth		= (unsigned int)(BitsInUnignedInt / 8);
    	unsigned int HighBits		 = (unsigned int)(0xFFFFFFFF) << (BitsInUnignedInt - OneEighth);
    	unsigned int hash			 = 0;
    	unsigned int test			 = 0;
    
    	while (*str)
    	{
    		hash = (hash << OneEighth) + (*str++);
    		if ((test = hash & HighBits) != 0)
    		{
    			hash = ((hash ^ (test >> ThreeQuarters)) & (~HighBits));
    		}
    	}
    
    	return (hash & 0x7FFFFFFF);
    }
    
    // ELF Hash Function
    unsigned int ELFHash(char *str)
    {
    	unsigned int hash = 0;
    	unsigned int x	= 0;
    
    	while (*str)
    	{
    		hash = (hash << 4) + (*str++);
    		if ((x = hash & 0xF0000000L) != 0)
    		{
    			hash ^= (x >> 24);
    			hash &= ~x;
    		}
    	}
    
    	return (hash & 0x7FFFFFFF);
    }
    
    // BKDR Hash Function
    unsigned int BKDRHash(char *str)
    {
    	unsigned int seed = 131; // 31 131 1313 13131 131313 etc..
    	unsigned int hash = 0;
    
    	while (*str)
    	{
    		hash = hash * seed + (*str++);
    	}
    
    	return (hash & 0x7FFFFFFF);
    }
    
    // DJB Hash Function
    unsigned int DJBHash(char *str)
    {
    	unsigned int hash = 5381;
    
    	while (*str)
    	{
    		hash += (hash << 5) + (*str++);
    	}
    
    	return (hash & 0x7FFFFFFF);
    }
    
    // AP Hash Function
    unsigned int APHash(char *str)
    {
    	unsigned int hash = 0;
    	int i;
    
    	for (i=0; *str; i++)
    	{
    		if ((i & 1) == 0)
    		{
    			hash ^= ((hash << 7) ^ (*str++) ^ (hash >> 3));
    		}
    		else
    		{
    			hash ^= (~((hash << 11) ^ (*str++) ^ (hash >> 5)));
    		}
    	}
    
    	return (hash & 0x7FFFFFFF);
    }

[http_www.byvoid.com_blog_string-hash-compare]: http://www.byvoid.com/blog/string-hash-compare/