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 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];
+ }
+}