package prefuse.util.collections;
/**
* Not of interest for users; only for implementors of hashtables. Used to keep
* hash table capacities prime numbers.
*
* <p>
* Choosing prime numbers as hash table capacities is a good idea to keep them
* working fast, particularly under hash table expansions.
* </p>
*
* <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>
*
* <p>
* Choosing a prime is <tt>O(log 300)</tt> (binary search in a list of 300
* int's). Memory requirements: 1 KB static memory.
* </p>
*
* This class has been adapted from the corresponding class in the COLT
* library for scientfic computing.
*
* @author wolfgang.hoschek@cern.ch
* @version 1.0, 09/24/99
*/
public class PrimeFinder extends Object {
/**
* 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 (11%). For any desired
* capacity >= 200 the list includes a prime number <= desired capacity *
* 1.16 (16%). For any desired capacity >= 16 the list includes a prime
* number <= desired capacity * 1.21 (21%).
*
* 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
/*
* // some more chunks for the low range [3..1000] //chunk #11
* 13,29,59,127,257,521,1049,2099,4201,8419,16843,33703,67409,134837,269683,
* 539389,1078787,2157587,4315183,8630387,17260781,34521589,69043189,138086407,
* 276172823,552345671,1104691373,
*
* //chunk #12 19,41,83,167,337,677,
* //1361,2729,5471,10949,21911,43853,87719,175447,350899,
* //701819,1403641,2807303,5614657,11229331,22458671,44917381,89834777,179669557,
* //359339171,718678369,1437356741,
*
* //chunk #13 53,107,223,449,907,1823,3659,7321,14653,29311,58631,117269,
* 234539,469099,938207,1876417,3752839,7505681,15011389,30022781,
* 60045577,120091177,240182359,480364727,960729461,1921458943
*/
};
static { // initializer
// The above prime numbers are formatted for human readability.
// To find numbers fast, we sort them once and for all.
java.util.Arrays.sort(primeCapacities);
}
/**
* Makes this class non instantiable, but still let's others inherit from
* it.
*/
protected PrimeFinder() {
}
/**
* 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 = java.util.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];
}
} // end of class PrimeFinder