org.apache.hadoop.util.hash
Class JenkinsHash

java.lang.Object
  org.apache.hadoop.util.hash.Hash
      org.apache.hadoop.util.hash.JenkinsHash

public class JenkinsHash
extends Hash

Produces 32-bit hash for hash table lookup.

lookup3.c, by Bob Jenkins, May 2006, Public Domain.

 You can use this free for any purpose.  It's in the public domain.
 It has no warranty.
 

See Also:

lookup3.c, Hash Functions (and how this function compares to others such as CRC, MD?, etc, Has update on the Dr. Dobbs Article

Field Summary

Fields inherited from class org.apache.hadoop.util.hash.Hash

INVALID_HASH, JENKINS_HASH, MURMUR_HASH

Constructor Summary

JenkinsHash()

Method Summary

static Hash	getInstance()
int	hash(byte[] key, int nbytes, int initval) taken from hashlittle() -- hash a variable-length key into a 32-bit value
static void	main(String[] args) Compute the hash of the specified file

Methods inherited from class org.apache.hadoop.util.hash.Hash

getHashType, getInstance, getInstance, hash, hash, parseHashType

Methods inherited from class java.lang.Object

clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

Constructor Detail

JenkinsHash

public JenkinsHash()

Method Detail

getInstance

public static Hash getInstance()

hash

public int hash(byte[] key,
                int nbytes,
                int initval)

taken from hashlittle() -- hash a variable-length key into a 32-bit value

Specified by:

hash in class Hash

Parameters:

key - the key (the unaligned variable-length array of bytes)

nbytes - number of bytes to include in hash

initval - can be any integer value

Returns:

a 32-bit value. Every bit of the key affects every bit of the return value. Two keys differing by one or two bits will have totally different hash values.

The best hash table sizes are powers of 2. There is no need to do mod a prime (mod is sooo slow!). If you need less than 32 bits, use a bitmask. For example, if you need only 10 bits, do h = (h & hashmask(10)); In which case, the hash table should have hashsize(10) elements.

If you are hashing n strings byte[][] k, do it like this: for (int i = 0, h = 0; i < n; ++i) h = hash( k[i], h);

By Bob Jenkins, 2006. bob_jenkins@burtleburtle.net. You may use this code any way you wish, private, educational, or commercial. It's free.

Use for hash table lookup, or anything where one collision in 2^^32 is acceptable. Do NOT use for cryptographic purposes.

main

public static void main(String[] args)
                 throws IOException

Compute the hash of the specified file

Parameters:

args - name of file to compute hash of.

Throws:

IOException