353 lines
6.6 KiB
Java
353 lines
6.6 KiB
Java
/*
|
|
* 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 com.l2jmobius.util;
|
|
|
|
import java.util.Arrays;
|
|
|
|
/**
|
|
* <b>Modified for Trove to use the java.util.Arrays sort/search<br>
|
|
* algorithms instead of those provided with colt.</b><br>
|
|
* Used to keep hash table capacities prime numbers.<br>
|
|
* Not of interest for users; only for implementors of hashtables.<br>
|
|
* <p>
|
|
* Choosing prime numbers as hash table capacities is a good idea<br>
|
|
* to keep them working fast, particularly under hash table expansions.<br>
|
|
* <p>
|
|
* However, JDK 1.2, JGL 3.1 and many other toolkits do nothing to keep capacities prime.<br>
|
|
* This class provides efficient means to choose prime capacities.
|
|
* <p>
|
|
* Choosing a prime is <tt>O(log 300)</tt> (binary search in a list of 300 ints).<br>
|
|
* Memory requirements: 1 KB static memory.<br>
|
|
* @author wolfgang.hoschek@cern.ch
|
|
* @version 1.0, 09/24/99
|
|
*/
|
|
public final class PrimeFinder
|
|
{
|
|
/**
|
|
* The largest prime this class can generate; currently equal to <tt>Integer.MAX_VALUE</tt>.
|
|
*/
|
|
public static final int LARGEST_PRIME = Integer.MAX_VALUE; // yes, it is prime.
|
|
|
|
/**
|
|
* The prime number list consists of 11 chunks.<br>
|
|
* Each chunk contains prime numbers.<br>
|
|
* A chunk starts with a prime P1.<br>
|
|
* The next element is a prime P2.<br>
|
|
* P2 is the smallest prime for which holds: P2 >= 2*P1.<br>
|
|
* 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<br>
|
|
* the list includes a prime number <= desired capacity * 1.11.<br>
|
|
* Therefore, primes can be retrieved which are quite close to any<br>
|
|
* desired capacity, which in turn avoids wasting memory.<br>
|
|
* For example, the list includes<br>
|
|
* 1039,1117,1201,1277,1361,1439,1523,1597,1759,1907,2081.<br>
|
|
* So if you need a prime >= 1040, you will find a prime <= 1040*1.11=1154.<br> Chunks are chosen such that they are optimized for a hashtable growthfactor of 2.0;<br>
|
|
* If your hashtable has such a growthfactor then, after initially<br>
|
|
* "rounding to a prime" upon hashtable construction, it will<br>
|
|
* later expand to prime capacities such that there exist no better primes.<br>
|
|
* In total these are about 32*10=320 numbers -> 1 KB of static memory needed.<br>
|
|
* If you are stingy, then delete every second or fourth chunk.
|
|
*/
|
|
|
|
private static final int[] PRIME_CAPACITIES =
|
|
{
|
|
// chunk #0
|
|
LARGEST_PRIME,
|
|
|
|
// 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(PRIME_CAPACITIES);
|
|
}
|
|
|
|
/**
|
|
* Returns a prime number which is <code>>= desiredCapacity</code> and very close to <code>desiredCapacity</code> (within 11% if <code>desiredCapacity >= 1000</code>).
|
|
* @param desiredCapacity the capacity desired by the user.
|
|
* @return the capacity which should be used for a hashtable.
|
|
*/
|
|
public static int nextPrime(int desiredCapacity)
|
|
{
|
|
int i = Arrays.binarySearch(PRIME_CAPACITIES, desiredCapacity);
|
|
if (i < 0)
|
|
{
|
|
// desired capacity not found, choose next prime greater
|
|
// than desired capacity
|
|
i = -i - 1; // remember the semantics of binarySearch...
|
|
}
|
|
return PRIME_CAPACITIES[i];
|
|
}
|
|
} |