gnu.trove.PrimeFinder Maven / Gradle / Ivy
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// Copyright (c) 1999 CERN - European Organization for Nuclear Research.
// Permission to use, copy, modify, distribute and sell this software
// and its documentation for any purpose is hereby granted without fee,
// provided that the above copyright notice appear in all copies and
// that both that copyright notice and this permission notice appear in
// supporting documentation. CERN makes no representations about the
// suitability of this software for any purpose. It is provided "as is"
// without expressed or implied warranty.
package gnu.trove;
import java.util.Arrays;
/*
* Modified for Trove to use the java.util.Arrays sort/search
* algorithms instead of those provided with colt.
*/
/**
* 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.
*
* @author [email protected]
* @version 1.0, 09/24/99
*/
public final class PrimeFinder {
/**
* The largest prime this class can generate; currently equal to
* Integer.MAX_VALUE.
*/
public static final int largestPrime = Integer.MAX_VALUE; //yes, it is prime.
/**
* 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.
*/
private static final int[] primeCapacities = {
//chunk #0
largestPrime,
//chunk #1
5,11,23,47,97,197,397,797,1597,3203,6421,12853,25717,51437,102877,205759,
411527,823117,1646237,3292489,6584983,13169977,26339969,52679969,105359939,
210719881,421439783,842879579,1685759167,
//chunk #2
433,877,1759,3527,7057,14143,28289,56591,113189,226379,452759,905551,1811107,
3622219,7244441,14488931,28977863,57955739,115911563,231823147,463646329,927292699,
1854585413,
//chunk #3
953,1907,3821,7643,15287,30577,61169,122347,244703,489407,978821,1957651,3915341,
7830701,15661423,31322867,62645741,125291483,250582987,501165979,1002331963,
2004663929,
//chunk #4
1039,2081,4177,8363,16729,33461,66923,133853,267713,535481,1070981,2141977,4283963,
8567929,17135863,34271747,68543509,137087021,274174111,548348231,1096696463,
//chunk #5
31,67,137,277,557,1117,2237,4481,8963,17929,35863,71741,143483,286973,573953,
1147921,2295859,4591721,9183457,18366923,36733847,73467739,146935499,293871013,
587742049,1175484103,
//chunk #6
599,1201,2411,4831,9677,19373,38747,77509,155027,310081,620171,1240361,2480729,
4961459,9922933,19845871,39691759,79383533,158767069,317534141,635068283,1270136683,
//chunk #7
311,631,1277,2557,5119,10243,20507,41017,82037,164089,328213,656429,1312867,
2625761,5251529,10503061,21006137,42012281,84024581,168049163,336098327,672196673,
1344393353,
//chunk #8
3,7,17,37,79,163,331,673,1361,2729,5471,10949,21911,43853,87719,175447,350899,
701819,1403641,2807303,5614657,11229331,22458671,44917381,89834777,179669557,
359339171,718678369,1437356741,
//chunk #9
43,89,179,359,719,1439,2879,5779,11579,23159,46327,92657,185323,370661,741337,
1482707,2965421,5930887,11861791,23723597,47447201,94894427,189788857,379577741,
759155483,1518310967,
//chunk #10
379,761,1523,3049,6101,12203,24407,48817,97649,195311,390647,781301,1562611,
3125257,6250537,12501169,25002389,50004791,100009607,200019221,400038451,800076929,
1600153859
};
static { //initializer
// The above prime numbers are formatted for human readability.
// To find numbers fast, we sort them once and for all.
Arrays.sort(primeCapacities);
}
/**
* Returns a prime number which is >= desiredCapacity
* and very close to desiredCapacity (within 11% if
* desiredCapacity >= 1000).
*
* @param desiredCapacity the capacity desired by the user.
* @return the capacity which should be used for a hashtable.
*/
public static final int nextPrime(int desiredCapacity) {
int i = Arrays.binarySearch(primeCapacities, desiredCapacity);
if (i<0) {
// desired capacity not found, choose next prime greater
// than desired capacity
i = -i -1; // remember the semantics of binarySearch...
}
return primeCapacities[i];
}
}