/* * 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; /** * 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 wolfgang.hoschek@cern.ch * @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 LARGEST_PRIME = 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[] 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 >= 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(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]; } }