各种字符串哈希函数比较 转载

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。 

Hash函数数据1数据2数据3数据4数据1得分数据2得分数据3得分数据4得分平均分BKDRHash20477448196.5510090.9582.0592.64APHash23475449396.5588.4610051.2886.28DJBHash22497547496.5592.31010083.43JSHash14476150610084.6296.8317.9581.94RSHash10486150510010051.5820.5175.96SDBMHash32484950493.192.3157.0123.0872.41PJWHash302648785130043.89021.95ELFHash302648785130043.89021.95
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。


CmYkRgB123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


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 
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 
unsigned int JSHash(char *str)
{
     unsigned int hash = 1315423911;
 
     while (*str)
 {
               hash ^= ((hash << 5) + (*str++) + (hash >> 2));
        }
 
     return (hash & 0x7FFFFFFF);
}
 
// P. J. Weinberger Hash 
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 
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 
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 
unsigned int DJBHash(char *str)
{
   unsigned int hash = 5381;
 
   while (*str)
 {
               hash += (hash << 5) + (*str++);
 }
 
     return (hash & 0x7FFFFFFF);
}
 
// AP Hash 
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);
}