把string映射到64位的hashcode值方法
可以参考 xmemcached 的实现，源码如下：
public enum HashAlgorithm {
/**
* Native hash (String.hashCode()).
*/
NATIVE_HASH,
/**
* CRC32_HASH as used by the perl API. This will be more consistent both
* across multiple API users as well as java versions, but is mostly likely
* significantly slower.
*/
CRC32_HASH,
/**
* FNV hashes are designed to be fast while maintaining a low collision
* rate. The FNV speed allows one to quickly hash lots of data while
* maintaining a reasonable collision rate.
*
* @see http://www.isthe.com/chongo/tech/comp/fnv/
* @see http://en.wikipedia.org/wiki/Fowler_Noll_Vo_hash
*/
FNV1_64_HASH,
/**
* Variation of FNV.
*/
FNV1A_64_HASH,
/**
* 32-bit FNV1.
*/
FNV1_32_HASH,
/**
* 32-bit FNV1a.
*/
FNV1A_32_HASH,
/**
* MD5-based hash algorithm used by ketama.
*/
KETAMA_HASH,
/**
* From mysql source
*/
MYSQL_HASH,
ELF_HASH,
RS_HASH,
/**
* From lua source,it is used for long key
*/
LUA_HASH,
ELECTION_HASH,
/**
* The Jenkins One-at-a-time hash ,please see
* http://www.burtleburtle.net/bob/hash/doobs.html
*/
ONE_AT_A_TIME;
private static final long FNV_64_INIT = 0xcbf29ce484222325L;
private static final long FNV_64_PRIME = 0x100000001b3L;
private static final long FNV_32_INIT = 2166136261L;
private static final long FNV_32_PRIME = 16777619;
/**
* Compute the hash for the given key.
*
* @return a positive integer hash
*/
public long hash(final String k) {
long rv = 0;
switch (this) {
case NATIVE_HASH:
rv = k.hashCode();
break;
case CRC32_HASH:
// return (crc32(shift) >> 16) & 0x7fff;
CRC32 crc32 = new CRC32();
crc32.update(ByteUtils.getBytes(k));
rv = crc32.getValue() >> 16 & 0x7fff;
break;
case FNV1_64_HASH: {
// Thanks to [email protected] for the pointer
rv = FNV_64_INIT;
int len = k.length();
for (int i = 0; i < len; i++) {
rv *= FNV_64_PRIME;
rv ^= k.charAt(i);
}
}
break;
case FNV1A_64_HASH: {
rv = FNV_64_INIT;
int len = k.length();
for (int i = 0; i < len; i++) {
rv ^= k.charAt(i);
rv *= FNV_64_PRIME;
}
}
break;
case FNV1_32_HASH: {
rv = FNV_32_INIT;
int len = k.length();
for (int i = 0; i < len; i++) {
rv *= FNV_32_PRIME;
rv ^= k.charAt(i);
}
}
break;
case FNV1A_32_HASH: {
rv = FNV_32_INIT;
int len = k.length();
for (int i = 0; i < len; i++) {
rv ^= k.charAt(i);
rv *= FNV_32_PRIME;
}
}
break;
case ELECTION_HASH:
case KETAMA_HASH:
byte[] bKey = computeMd5(k);
rv = (long) (bKey[3] & 0xFF) << 24 | (long) (bKey[2] & 0xFF) << 16
| (long) (bKey[1] & 0xFF) << 8 | bKey[0] & 0xFF;
break;
case MYSQL_HASH:
int nr2 = 4;
for (int i = 0; i < k.length(); i++) {
rv ^= ((rv & 63) + nr2) * k.charAt(i) + (rv << 8);
nr2 += 3;
}
break;
case ELF_HASH:
long x = 0;
for (int i = 0; i < k.length(); i++) {
rv = (rv << 4) + k.charAt(i);
if ((x = rv & 0xF0000000L) != 0) {
rv ^= x >> 24;
rv &= ~x;
}
}
rv = rv & 0x7FFFFFFF;
break;
case RS_HASH:
long b = 378551;
long a = 63689;
for (int i = 0; i < k.length(); i++) {
rv = rv * a + k.charAt(i);
a *= b;
}
rv = rv & 0x7FFFFFFF;
break;
case LUA_HASH:
int step = (k.length() >> 5) + 1;
rv = k.length();
for (int len = k.length(); len >= step; len -= step) {
rv = rv ^ (rv << 5) + (rv >> 2) + k.charAt(len - 1);
}
case ONE_AT_A_TIME:
try {
int hash = 0;
for (byte bt : k.getBytes("utf-8")) {
hash += (bt & 0xFF);
hash += (hash << 10);
hash ^= (hash >>> 6);
}
hash += (hash << 3);
hash ^= (hash >>> 11);
hash += (hash << 15);
return hash;
} catch (UnsupportedEncodingException e) {
throw new IllegalStateException("Hash function error", e);
}
default:
assert false;
}
return rv & 0xffffffffL; /* Truncate to 32-bits */
}
/**
* Get the md5 of the given key.
*/
public static byte[] computeMd5(String k) {
MessageDigest md5;
try {
md5 = MessageDigest.getInstance("MD5");
} catch (NoSuchAlgorithmException e) {
throw new RuntimeException("MD5 not supported", e);
}
md5.reset();
md5.update(ByteUtils.getBytes(k));
return md5.digest();
}
// public static void main(String[] args) {
// HashAlgorithm alg=HashAlgorithm.CRC32_HASH;
// long h=0;
// long start=System.currentTimeMillis();
// for(int i=0;i<100000;i++)
// h=alg.hash("MYSQL_HASH");
// System.out.println(System.currentTimeMillis()-start);
// }
}