各种字符串Hash函数比较
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 |
| 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 |
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。
附：各种哈希函数的C语言
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
);
}