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[simantics/platform.git] / bundles / org.simantics.scenegraph / src / gnu / trove / PrimeFinder.java
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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.
+ *
+ * <p>Choosing prime numbers as hash table capacities is a good idea
+ * to keep them working fast, particularly under hash table
+ * expansions.
+ *
+ * <p>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.
+ *
+ * <p>Choosing a prime is <tt>O(log 300)</tt> (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
+     * <tt>Integer.MAX_VALUE</tt>.
+     */
+    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 <code>&gt;= desiredCapacity</code>
+     * and very close to <code>desiredCapacity</code> (within 11% if
+     * <code>desiredCapacity &gt;= 1000</code>).
+     *
+     * @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];
+    }
+}