
/**
 * A program that starts several threads, each of which performs the
 * same computation.  The user specifies the number of threads.  The
 * point is to see that the threads finish in an indeterminate order.
 */
public class ThreadTest1 {
    
    private final static int MAX = 5000000;


    /**
     * When a thread belonging to this class is run it will count the
     * number of primes between 2 and MAX.  It will print the result
     * to standard output, along with its id number and the elapsed
     * time between the start and the end of the computation.
     */
    private static class CountPrimesThread extends Thread {
        int id;  // An id number for this thread; specified in the constructor.
        public CountPrimesThread(int id) {
            this.id = id;
        }
        public void run() {
            long startTime = System.currentTimeMillis();
            int count = countPrimes(2,MAX);
            long elapsedTime = System.currentTimeMillis() - startTime;
            System.out.println("Thread " + id + " counted " + 
                    count + " primes in " + (elapsedTime/1000.0) + " seconds.");
        }
    }


    /**
     * Start several CountPrimesThreads.  The number of threads, between 1 and 25,
     * is specified by the user.
     */
    public static void main(String[] args) {
        int numberOfThreads = 0;
        while (numberOfThreads < 1 || numberOfThreads > 25) {
            System.out.print("How many threads do you want to use  (from 1 to 25) ?  ");
            numberOfThreads = TextIO.getlnInt();
            if (numberOfThreads < 1 || numberOfThreads > 25)
                System.out.println("Please enter a number between 1 and 25 !");
        }
        System.out.println("\nCreating " + numberOfThreads + " prime-counting threads...");
        CountPrimesThread[] worker = new CountPrimesThread[numberOfThreads];
        for (int i = 0; i < numberOfThreads; i++)
            worker[i] = new CountPrimesThread( i );
        for (int i = 0; i < numberOfThreads; i++)
            worker[i].start();
        System.out.println("Threads have been created and started.");
    }


    /**
     * Compute and return the number of prime numbers in the range
     * min to max, inclusive.
     */
    private static int countPrimes(int min, int max) {
        int count = 0;
        for (int i = min; i <= max; i++)
            if (isPrime(i))
                count++;
        return count;
    }


    /**
     * Test whether x is a prime number.  
     * x is assumed to be greater than 1.
     */
    private static boolean isPrime(int x) {
        assert x > 1;
        int top = (int)Math.sqrt(x);
        for (int i = 2; i <= top; i++)
            if ( x % i == 0 )
                return false;
        return true;
    }


} // end class ThreadTest1

