generateprimes.java

来自「著名的uncle Bob的Agile software development的」· Java 代码 · 共 81 行

/**
 * This class Generates prime numbers up to a user specified maximum.
 * the algorithm used is the Sieve of Eratosthenes.
 * <p>
 * Eratosthenes of Cyrene, b. c. 276 BC, Cyrene, Libya --
 * d. c. 194, Alexandria.  The first man to calculate the circumference
 * of the Earth.  Also known for working on calendars with leap
 * years and ran the library at Alexandria.
 * <p>
 * The algorithm is quite simple.  Given an array of integers starting
 * at 2.  Cross out all multiples of 2.  Find the next uncrossed
 * integer, and cross out all of its multiples.  Repeat until
 * you have passed the square root of the maximum value.
 * 
 * @author Robert C. Martin
 * @version 9 Dec 1999 rcm
 */
import java.util.*;

/**
 * Class declaration
 * 
 * 
 * @author Robert C. Martin
 * @version %I%, %G%
 */
public class GeneratePrimes
{
  /**
   * @param maxValue is the generation limit.
   */
  public static int[] generatePrimes(int maxValue)
  {
    if (maxValue >= 2) // the only valid case
    {
      // declarations
      int s = maxValue + 1; // size of array
      boolean[] f = new boolean[s];
      int i;

      // initialize array to true.
      for (i = 0; i < s; i++)
        f[i] = true;

      // get rid of known non-primes
      f[0] = f[1] = false;

      // sieve
      int j;
      for (i = 2; i < Math.sqrt(s) + 1; i++)
      {
        for (j = 2 * i; j < s; j += i)
          f[j] = false; // multiple is not prime
      }

      // how many primes are there?
      int count = 0;
      for (i = 0; i < s; i++)
      {
        if (f[i])
          count++; // bump count.
      }

      int[] primes = new int[count];

      // move the primes into the result
      for (i = 0, j = 0; i < s; i++)
      {
        if (f[i])             // if prime
          primes[j++] = i;
      }
	  
      return primes;  // return the primes
    }
    else // maxValue < 2
      return new int[0]; // return null array if bad input.
  }
}