cern.colt.map.PrimeFinder Maven / Gradle / Ivy
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package cern.colt.map;
/**
* Not of interest for users; only for implementors of hashtables.
* Used to keep hash table capacities prime numbers.
*
* 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 int's).
* Memory requirements: 1 KB static memory.
*
* @author [email protected]
* @version 1.0, 09/24/99
*/
public class PrimeFinder extends Object {
/**
* 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 (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);
//new cern.colt.list.IntArrayList(primeCapacities).mergeSort(); // for debug only, TODO
}
/**
* Makes this class non instantiable, but still let's others inherit from it.
*/
protected PrimeFinder() {
}
/**
* Tests correctness. Try
* from=1000, to=10000
* from=200, to=1000
* from=16, to=1000
* from=1000, to=Integer.MAX_VALUE
*/
protected static void main(String args[]) {
int from = Integer.parseInt(args[0]);
int to = Integer.parseInt(args[1]);
statistics(from, to);
}
/**
* 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 int nextPrime(int desiredCapacity) {
int i = java.util.Arrays.binarySearch(primeCapacities, desiredCapacity);
//int i = new cern.colt.list.IntArrayList(primeCapacities).binarySearch(desiredCapacity); // for debug only TODO
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];
}
/**
* Tests correctness.
*/
protected static void statistics(int from, int to) {
// check that primes contain no accidental errors
for (int i = 0; i < primeCapacities.length - 1; i++) {
if (primeCapacities[i] >= primeCapacities[i + 1])
throw new RuntimeException("primes are unsorted or contain duplicates; detected at " + i + "@" + primeCapacities[i]);
}
double accDeviation = 0.0;
double maxDeviation = -1.0;
for (int i = from; i <= to; i++) {
int primeCapacity = nextPrime(i);
//System.out.println(primeCapacity);
double deviation = (primeCapacity - i) / (double) i;
if (deviation > maxDeviation) {
maxDeviation = deviation;
System.out.println("new maxdev @" + i + "@dev=" + maxDeviation);
}
accDeviation += deviation;
}
long width = 1 + (long) to - (long) from;
double meanDeviation = accDeviation / width;
System.out.println("Statistics for [" + from + "," + to + "] are as follows");
System.out.println("meanDeviation = " + (float) meanDeviation * 100 + " %");
System.out.println("maxDeviation = " + (float) maxDeviation * 100 + " %");
}
}