345 lines
6.6 KiB
Java
345 lines
6.6 KiB
Java
/*
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* This file is part of the L2J Mobius project.
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*
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* This program is free software: you can redistribute it and/or modify
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* it under the terms of the GNU General Public License as published by
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* the Free Software Foundation, either version 3 of the License, or
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* (at your option) any later version.
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*
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* This program is distributed in the hope that it will be useful,
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* but WITHOUT ANY WARRANTY; without even the implied warranty of
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* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
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* General Public License for more details.
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*
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* You should have received a copy of the GNU General Public License
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* along with this program. If not, see <http://www.gnu.org/licenses/>.
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*/
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package com.l2jmobius.util;
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import java.util.Arrays;
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/*
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* Modified for Trove to use the java.util.Arrays sort/search
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* algorithms instead of those provided with colt.
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*/
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/**
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* Used to keep hash table capacities prime numbers. Not of interest for users; only for implementors of hashtables.
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* <p>
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* Choosing prime numbers as hash table capacities is a good idea to keep them working fast, particularly under hash table expansions.
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* <p>
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* 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.
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* <p>
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* Choosing a prime is <tt>O(log 300)</tt> (binary search in a list of 300 ints). Memory requirements: 1 KB static memory.
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* @author wolfgang.hoschek@cern.ch
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* @version 1.0, 09/24/99
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*/
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public final class PrimeFinder
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{
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/**
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* The largest prime this class can generate; currently equal to <tt>Integer.MAX_VALUE</tt>.
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*/
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public static final int largestPrime = Integer.MAX_VALUE; // yes, it is prime.
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/**
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* 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
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* 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
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* 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
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* 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.
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*/
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private static final int[] primeCapacities =
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{
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// chunk #0
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largestPrime,
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// chunk #1
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5,
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11,
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23,
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47,
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97,
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197,
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397,
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797,
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1597,
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3203,
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6421,
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12853,
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25717,
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51437,
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102877,
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205759,
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411527,
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823117,
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1646237,
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3292489,
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6584983,
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13169977,
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26339969,
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52679969,
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105359939,
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210719881,
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421439783,
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842879579,
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1685759167,
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// chunk #2
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433,
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877,
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1759,
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3527,
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7057,
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14143,
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28289,
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56591,
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113189,
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226379,
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452759,
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905551,
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1811107,
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3622219,
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7244441,
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14488931,
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28977863,
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57955739,
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115911563,
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231823147,
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463646329,
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927292699,
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1854585413,
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// chunk #3
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953,
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1907,
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3821,
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7643,
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15287,
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30577,
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61169,
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122347,
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244703,
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489407,
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978821,
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1957651,
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3915341,
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7830701,
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15661423,
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31322867,
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62645741,
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125291483,
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250582987,
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501165979,
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1002331963,
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2004663929,
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// chunk #4
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1039,
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2081,
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4177,
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8363,
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16729,
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33461,
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66923,
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133853,
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267713,
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535481,
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1070981,
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2141977,
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4283963,
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8567929,
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17135863,
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34271747,
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68543509,
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137087021,
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274174111,
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548348231,
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1096696463,
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// chunk #5
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31,
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67,
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137,
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277,
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557,
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1117,
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2237,
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4481,
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8963,
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17929,
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35863,
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71741,
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143483,
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286973,
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573953,
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1147921,
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2295859,
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4591721,
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9183457,
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18366923,
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36733847,
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73467739,
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146935499,
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293871013,
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587742049,
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1175484103,
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// chunk #6
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599,
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1201,
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2411,
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4831,
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9677,
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19373,
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38747,
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77509,
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155027,
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310081,
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620171,
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1240361,
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2480729,
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4961459,
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9922933,
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19845871,
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39691759,
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79383533,
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158767069,
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317534141,
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635068283,
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1270136683,
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// chunk #7
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311,
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631,
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1277,
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2557,
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5119,
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10243,
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20507,
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41017,
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82037,
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164089,
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328213,
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656429,
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1312867,
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2625761,
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5251529,
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10503061,
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21006137,
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42012281,
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84024581,
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168049163,
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336098327,
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672196673,
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1344393353,
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// chunk #8
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3,
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7,
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17,
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37,
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79,
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163,
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331,
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673,
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1361,
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2729,
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5471,
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10949,
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21911,
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43853,
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87719,
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175447,
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350899,
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701819,
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1403641,
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2807303,
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5614657,
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11229331,
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22458671,
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44917381,
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89834777,
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179669557,
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359339171,
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718678369,
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1437356741,
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// chunk #9
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43,
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89,
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179,
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359,
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719,
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1439,
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2879,
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5779,
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11579,
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23159,
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46327,
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92657,
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185323,
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370661,
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741337,
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1482707,
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2965421,
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5930887,
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11861791,
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23723597,
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47447201,
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94894427,
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189788857,
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379577741,
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759155483,
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1518310967,
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// chunk #10
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379,
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761,
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1523,
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3049,
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6101,
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12203,
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24407,
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48817,
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97649,
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195311,
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390647,
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781301,
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1562611,
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3125257,
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6250537,
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12501169,
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25002389,
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50004791,
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100009607,
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200019221,
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400038451,
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800076929,
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1600153859
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};
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static
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{ // initializer
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// The above prime numbers are formatted for human readability.
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// To find numbers fast, we sort them once and for all.
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Arrays.sort(primeCapacities);
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}
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/**
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* Returns a prime number which is <code>>= desiredCapacity</code> and very close to <code>desiredCapacity</code> (within 11% if <code>desiredCapacity >= 1000</code>).
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* @param desiredCapacity the capacity desired by the user.
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* @return the capacity which should be used for a hashtable.
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*/
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public static final int nextPrime(int desiredCapacity)
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{
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int i = Arrays.binarySearch(primeCapacities, desiredCapacity);
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if (i < 0)
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{
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// desired capacity not found, choose next prime greater
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// than desired capacity
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i = -i - 1; // remember the semantics of binarySearch...
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}
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return primeCapacities[i];
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}
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} |