PrimeEx.java

import java.math.BigInteger;
import java.util.Random;

public class PrimeEx {

	/**
	 * @param args
	 */
	public static void main(String[] args) {
		printTest(10, 4);
		printTest(2, 2);
		printTest(54161329, 4);
		printTest(1882341361, 2);
		printTest(36, 9);

		System.out.println(isPrime(54161329) + " expect false");
		System.out.println(isPrime(1882341361) + " expect true");
		System.out.println(isPrime(2) + " expect true");
		int numPrimes = 0;
		Stopwatch s = new Stopwatch();
		s.start();
		for(int i = 2; i < 10000000; i++) {
			if(isPrime(i)) {
				numPrimes++;
			}
		}
		s.stop();
		System.out.println(numPrimes + " " + s);
		s.start();
		boolean[] primes = getPrimes(10000000);
		int np = 0;
		for(boolean b : primes)
			if(b)
				np++;
		s.stop();
		System.out.println(np + " " + s);

		System.out.println(new BigInteger(1024, 10, new Random()));
	}

	public static boolean[] getPrimes(int max) {
		boolean[] result = new boolean[max + 1];
		for(int i = 2; i < result.length; i++)
			result[i] = true;
		final double LIMIT = Math.sqrt(max);
		for(int i = 2; i <= LIMIT; i++) {
			if(result[i]) {
				// cross out all multiples;
				int index = 2 * i;
				while(index < result.length){
					result[index] = false;
					 index += i;
				}
			}
		}
		return result;
	}


	public static void printTest(int num, int expectedFactors) {
		Stopwatch st = new Stopwatch();
		st.start();
		int actualFactors = numFactors(num);
		st.stop();
		System.out.println("Testing " + num + " expect " + expectedFactors + ", " +
				"actual " + actualFactors);
		if(actualFactors == expectedFactors)
			System.out.println("PASSED");
		else
			System.out.println("FAILED");
		System.out.println(st.time());
	}

	// pre: num >= 2
	public static boolean isPrime(int num) {
		assert num >= 2 : "failed precondition. num must be >= 2. num: " + num;
		final double LIMIT = Math.sqrt(num);
		boolean isPrime = (num == 2) ? true : num % 2 != 0;
		int div = 3;
		while(div <= LIMIT && isPrime) {
			isPrime = num % div != 0;
			div += 2;
		}
		return isPrime;
	}

	// pre: num >= 2
	public static int numFactors(int num) {
		assert num >= 2 : "failed precondition. num must be >= 2. num: " + num;
		int result = 0;
		final double SQRT = Math.sqrt(num);
		for(int i = 1; i < SQRT; i++) {
			if(num % i == 0) {
				result += 2;
			}
		}
		if(num % SQRT == 0)
			result++;
		return result;
	}

}