/* PrimeCounter.java
 * Use Java threading to count the number of prime
 * numbers between 0 and a large number N
 * Compare the timing of using 2 threads vs
 * using just one thread.
 * 
 * The load balancing is not great in this version
 *
 * javac PrimeCounter.java
 * java PrimeCounter
 */
public class PrimeCounter implements Runnable {
	private int start;
	private int extent;
	private int count;
	
	public PrimeCounter (int s, int e) {
		start = s;
		extent = e;
		count = 0;
	}
	
	public boolean is_prime (int n) {
		if (n < 2) return false;
		if (n == 2) return true;
		if (n%2 == 0) return false;
		for (int i = 3; i < n; i += 2) {
            if (n % i == 0) return false;
		}
		return true;
	}
	
	public void run () {
		for (int i = start; i < start + extent; i++) {
			if (is_prime(i)) {
				count += 1;
			}
		}
	}
	
	public int count () {
		return count;
	}
	
	public static void main (String args[]) {
		int N = 1000000;
		
		PrimeCounter pc1 = new PrimeCounter(0, N/2);
		PrimeCounter pc2 = new PrimeCounter(N/2, N/2);
		PrimeCounter pc3 = new PrimeCounter(0, N);
		
		Thread t1 = new Thread(pc1);
		Thread t2 = new Thread(pc2);
		Thread t3 = new Thread(pc3);
		
		long start = System.currentTimeMillis();
		
		t1.start();
		t2.start();
		try {
			t1.join();
			t2.join();
		} catch (Exception e) {
			System.err.println(e);
		}
		
		int count = pc1.count() + pc2.count();
		
		long end = System.currentTimeMillis();
		System.out.printf("Two threads count %d primes in %d ms\n", count, end-start);
		
		// Time sequential solution, but using 1 thread
		start = System.currentTimeMillis();
		
		t3.start();
		try {
			t3.join();
		} catch (Exception e) {
			System.err.println(e);
		}
		
		count = pc3.count();
		
		end = System.currentTimeMillis();
		System.out.printf("One threads count %d primes in %d ms\n", count, end-start);
	}
}