gnu.trove

Class PrimeFinder

java.lang.Object

gnu.trove.PrimeFinder

public final class PrimeFinder
extends java.lang.Object

Used to keep hash table capacities prime numbers. Not of interest for users; only for implementors of hashtables.

Choosing prime numbers as hash table capacities is a good idea to keep them working fast, particularly under hash table expansions.

However, JDK 1.2, JGL 3.1 and many other toolkits do nothing to keep capacities prime. This class provides efficient means to choose prime capacities.

Choosing a prime is O(log 300) (binary search in a list of 300 ints). Memory requirements: 1 KB static memory.

Version:

1.0, 09/24/99

Author:

wolfgang.hoschek@cern.ch

Field Summary

Fields

Modifier and Type	Field and Description
static int	largestPrime The largest prime this class can generate; currently equal to Integer.MAX_VALUE.
private static int[]	primeCapacities The prime number list consists of 11 chunks.

Constructor Summary

Constructors

Constructor and Description
PrimeFinder()

Method Summary

Methods

Modifier and Type	Method and Description
static int	nextPrime(int desiredCapacity) Returns a prime number which is >= desiredCapacity and very close to desiredCapacity (within 11% if desiredCapacity >= 1000).

Methods inherited from class java.lang.Object

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

Field Detail

largestPrime

public static final int largestPrime

The largest prime this class can generate; currently equal to Integer.MAX_VALUE.

See Also:

Constant Field Values

primeCapacities

private static final int[] primeCapacities

The prime number list consists of 11 chunks. Each chunk contains prime numbers. A chunk starts with a prime P1. The next element is a prime P2. P2 is the smallest prime for which holds: P2 >= 2*P1. The next element is P3, for which the same holds with respect to P2, and so on. Chunks are chosen such that for any desired capacity >= 1000 the list includes a prime number <= desired capacity * 1.11. Therefore, primes can be retrieved which are quite close to any desired capacity, which in turn avoids wasting memory. For example, the list includes 1039,1117,1201,1277,1361,1439,1523,1597,1759,1907,2081. So if you need a prime >= 1040, you will find a prime <= 1040*1.11=1154. Chunks are chosen such that they are optimized for a hashtable growthfactor of 2.0; If your hashtable has such a growthfactor then, after initially "rounding to a prime" upon hashtable construction, it will later expand to prime capacities such that there exist no better primes. In total these are about 32*10=320 numbers -> 1 KB of static memory needed. If you are stingy, then delete every second or fourth chunk.

Constructor Detail

PrimeFinder

public PrimeFinder()

Method Detail

nextPrime

public static final int nextPrime(int desiredCapacity)

Returns a prime number which is >= desiredCapacity and very close to desiredCapacity (within 11% if desiredCapacity >= 1000).

Parameters:

desiredCapacity - the capacity desired by the user.

Returns:

the capacity which should be used for a hashtable.