org.h2.dev.hash

Class PerfectHash

java.lang.Object

org.h2.dev.hash.PerfectHash

public class PerfectHash
extends java.lang.Object

A perfect hash function tool. It needs about 1.4 bits per key, and the resulting hash table is about 79% full. The minimal perfect hash function needs about 2.3 bits per key.

Generating the hash function takes about 1 second per million keys for both perfect hash and minimal perfect hash.

The algorithm is recursive: sets that contain no or only one entry are not processed as no conflicts are possible. Sets that contain between 2 and 16 entries, up to 16 hash functions are tested to check if they can store the data without conflict. If no function was found, the same is tested on a larger bucket (except for the minimal perfect hash). If no hash function was found, and for larger buckets, the bucket is split into a number of smaller buckets (up to 32).

At the end of the generation process, the data is compressed using a general purpose compression tool (Deflate / Huffman coding). The uncompressed data is around 1.52 bits per key (perfect hash) and 3.72 (minimal perfect hash).

Please also note the MinimalPerfectHash class, which uses less space per key.

Constructor Summary

Constructors

Constructor and Description
PerfectHash(byte[] data) Create a hash object to convert keys to hashes.

Method Summary

Modifier and Type	Method and Description
static byte[]	generate(java.util.Set<java.lang.Integer> list, boolean minimal) Generate the perfect hash function data from the given set of integers.
int	get(int x) Calculate the hash from the key.

Methods inherited from class java.lang.Object

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

Constructor Detail

PerfectHash

public PerfectHash(byte[] data)

Create a hash object to convert keys to hashes.

Parameters:

data - the data returned by the generate method

Method Detail

get

public int get(int x)

Calculate the hash from the key.

Parameters:

x - the key

Returns:

the hash

generate

public static byte[] generate(java.util.Set<java.lang.Integer> list,
                              boolean minimal)

Generate the perfect hash function data from the given set of integers.

Parameters:

list - the set

minimal - whether the perfect hash function needs to be minimal

Returns:

the data