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 pierre@demartines.com 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); // }}?