/*
 * numerical integration example, as discussed in textbook:  
 *
 * compute pi by approximating the area under the curve f(x) = 4 / (1 + x*x)
 * between 0 and 1.
 *
 * parallel version using java.util.concurrent.
 */
package csci3366.sample.numint;
import csci3366.sample.utility.Utility;

import java.util.concurrent.Callable;
import java.util.concurrent.ExecutionException;
import java.util.concurrent.Executors;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Future;
import java.util.ArrayList;

public class NumIntPar2 {

    /* variables to be used by all threads */

    private static long numSteps;
    private static double step;
    private static int numThreads;

    /* main method */

    public static void main(String[] args) {

        /* process command-line arguments */

        String usageMessage = "arguments:  number_of_steps number_of_threads";

        numSteps = Utility.getLongArg(args, 0, 1, 
                "number_of_steps", usageMessage);
        numThreads = Utility.getIntegerArg(args, 1, 1, 
                "number_of_threads", usageMessage);

        step = 1.0/(double) numSteps; 

        double sum = 0.0;
 
        /* start timing */
        long startTime = System.currentTimeMillis();

        /* create executor for threads */
        ExecutorService executor = Executors.newFixedThreadPool(numThreads);

        /* 
         * create tasks that return partial sums and send to executor, 
         * creating Future objects so we can wait for tasks to finish (and 
         * get the returned values.
         *
         * see ArrayList here instead of array?  see Hello5 example
         */
        ArrayList<Future<Double>> partialSums = new ArrayList<Future<Double>>();
        for (int i = 0; i < numThreads; ++i) {
            partialSums.add(executor.submit(new CodeForThread(i)));
        }

        /* shut down executor and wait for threads to finish */
        executor.shutdown();
        for (Future<Double> p : partialSums) {
            try {
                sum += p.get();
            }
            catch (ExecutionException e) {
                System.err.println("should not happen");
            }
            catch (InterruptedException e) {
                System.err.println("should not happen");
            }
        }

        /* finish computation */
        double pi = sum * step;

        /* end timing and print result */
        long endTime = System.currentTimeMillis();
        System.out.printf(
                "parallel program (v2) results with %d threads and %d steps\n",
                numThreads, numSteps);
        System.out.printf("computed pi = %17.15f\n" , pi);
        System.out.printf(
                "difference between estimated pi and Math.PI = %17.15f\n",
                Math.abs(pi - Math.PI));
        System.out.printf("time to compute = %g seconds\n",
                (double) (endTime - startTime) / 1000);
    }

    /* static inner class to contain code to run in each thread */

    private static class CodeForThread implements Callable<Double> {

        private int myID;

        public CodeForThread(int myID_) {
            myID = myID_;
        }

        public Double call() {
            double partsum = 0.0;
            for (int i=myID; i < numSteps; i += numThreads) {
                double x = (i+0.5)*step;
                partsum += 4.0/(1.0+x*x);
            }
            return new Double(partsum);
        }
    }
}