PrimeFinder.java

/*
 * Carrot2 project.
 *
 * Copyright (C) 2002-2016, Dawid Weiss, Stanisław Osiński.
 * All rights reserved.
 *
 * Refer to the full license file "carrot2.LICENSE"
 * in the root folder of the repository checkout or at:
 * http://www.carrot2.org/carrot2.LICENSE
 */

package org.carrot2.mahout.math.map;

import java.util.Arrays;


public class PrimeFinder {

  
  public static final int largestPrime = Integer.MAX_VALUE; //yes, it is prime.

  

  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);
  }

  
  private PrimeFinder() {
  }

  
  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];
  }
}