【Hash】字符串哈希

Hash 的核心思想在于，将输入映射到一个值域较小、可以方便比较的范围，典型的用法就是将资源紧张的设备中的不定长字符串转化为定长整数，以达到节省空间的目的

如：printf("This is a string.")  =>  printf("0x12345678")   // 理想哈希算法可将不同的字符串转化为唯一的整数

常用hash算法：

• 00 - RS 哈希函数

unsigned int RSHash(const char* str, unsigned int length)
{
   unsigned int b    = 378551;
   unsigned int a    = 63689;
   unsigned int hash = 0;
   unsigned int i    = 0;

   for (i = 0; i < length; ++str, ++i)
   {
      hash = hash * a + (*str);
      a    = a * b;
   }

   return hash;
}

• 01 - JS 哈希函数

unsigned int JSHash(const char* str, unsigned int length)
{
   unsigned int hash = 1315423911;
   unsigned int i    = 0;

   for (i = 0; i < length; ++str, ++i)
   {
      hash ^= ((hash << 5) + (*str) + (hash >> 2));
   }

   return hash;
}

• 02 - PJW 哈希函数

unsigned int PJWHash(const char* str, unsigned int length)
{
   const unsigned int BitsInUnsignedInt = (unsigned int)(sizeof(unsigned int) * 8);
   const unsigned int ThreeQuarters     = (unsigned int)((BitsInUnsignedInt  * 3) / 4);
   const unsigned int OneEighth         = (unsigned int)(BitsInUnsignedInt / 8);
   const unsigned int HighBits          = (unsigned int)(0xFFFFFFFF) << (BitsInUnsignedInt - OneEighth);
   unsigned int hash = 0;
   unsigned int test = 0;
   unsigned int i    = 0;

   for (i = 0; i < length; ++str, ++i)
   {
      hash = (hash << OneEighth) + (*str);

      if ((test = hash & HighBits) != 0)
      {
         hash = (( hash ^ (test >> ThreeQuarters)) & (~HighBits));
      }
   }

   return hash;
}

• 03 - ELF哈希函数

unsigned int ELFHash(const char* str, unsigned int length)
{
   unsigned int hash = 0;
   unsigned int x    = 0;
   unsigned int i    = 0;

   for (i = 0; i < length; ++str, ++i)
   {
      hash = (hash << 4) + (*str);

      if ((x = hash & 0xF0000000L) != 0)
      {
         hash ^= (x >> 24);
      }

      hash &= ~x;
   }

   return hash;
}

• 04 - BKDR 哈希函数

unsigned int BKDRHash(const char* str, unsigned int length)
{
   unsigned int seed = 131; /* 31 131 1313 13131 131313 etc.. */
   unsigned int hash = 0;
   unsigned int i    = 0;

   for (i = 0; i < length; ++str, ++i)
   {
      hash = (hash * seed) + (*str);
   }

   return hash;
}

• 05 - SDBM哈希函数

unsigned int SDBMHash(const char* str, unsigned int length)
{
   unsigned int hash = 0;
   unsigned int i    = 0;

   for (i = 0; i < length; ++str, ++i)
   {
      hash = (*str) + (hash << 6) + (hash << 16) - hash;
   }

   return hash;
}

• 06 - DJB哈希函数

unsigned int DJBHash(const char* str, unsigned int length)
{
   unsigned int hash = 5381;
   unsigned int i    = 0;

   for (i = 0; i < length; ++str, ++i)
   {
      hash = ((hash << 5) + hash) + (*str);
   }

   return hash;
}

• 07 - DEK哈希函数

unsigned int DEKHash(const char* str, unsigned int length)
{
   unsigned int hash = len;
   unsigned int i    = 0;

   for (i = 0; i < length; ++str, ++i)
   {
      hash = ((hash << 5) ^ (hash >> 27)) ^ (*str);
   }

   return hash;
}

• 08 - AP 哈希函数

unsigned int APHash(const char* str, unsigned int length)
{
   unsigned int hash = 0xAAAAAAAA;
   unsigned int i    = 0;

   for (i = 0; i < length; ++str, ++i)
   {
      hash ^= ((i & 1) == 0) ? (  (hash <<  7) ^ (*str) * (hash >> 3)) :
                               (~((hash << 11) + ((*str) ^ (hash >> 5))));
   }

   return hash;
}

各hash算法冲突率测评

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
BKDRHash	2	0	4774	481	96.55	100	90.95	82.05	92.64
APHash	2	3	4754	493	96.55	88.46	100	51.28	86.28
DJBHash	2	2	4975	474	96.55	92.31	0	100	83.43
JSHash	1	4	4761	506	100	84.62	96.83	17.95	81.94
RSHash	1	0	4861	505	100	100	51.58	20.51	75.96
SDBMHash	3	2	4849	504	93.1	92.31	57.01	23.08	72.41
PJWHash	30	26	4878	513	0	0	43.89	0	21.95
ELFHash	30	26	4878	513	0	0	43.89	0	21.95

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。

参考：

http://www.partow.net/programming/hashfunctions/

    https://www.cnblogs.com/uvsjoh/archive/2012/03/27/2420120.html