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/*
 * This source code is provided to illustrate the usage of a given feature
 * or technique and has been deliberately simplified. Additional steps
 * required for a production-quality application, such as security checks,
 * input validation and proper error handling, might not be present in
 * this sample code.
 */
package stream.parallel;

import java.util.concurrent.ThreadLocalRandom;
import java.util.stream.LongStream;

/**
 * This demo shows how to use the parallel mode and the Monte Carlo method to
 * calculate the value of PI
 *
 * @author tyan
 */
public class MonteCarloPI {

    public static void main(String[] args) {
        System.out.println("This might take several minutes, "
                + "depending on your system");
        LongStream.iterate(10l, l -> 10 * l).
                limit(10).
                forEach(times -> {
                    System.out.printf(
                            "The value of PI is %1.14f after %,d times calculation\n",
                            pi(times), times);
                });
    }

    /**
     * Use the Monte Carlo method to calculate the value of PI. basic algorithm
     * is: 1. Draw a square on the ground, then inscribe a circle within it. 2.
     * Scatter some objects of uniform size (grains of rice or sand) over the
     * square. 3. Count the total number of objects inside the circle and the
     * total number of objects overall. 4. The ratio of the two total is an
     * estimate of the ratio of the two areas, which is PI/4. Multiply the
     * result by 4 to estimate PI.
     *
     * @param x how many times randomly selected a point
     * @return value of π by x times calculation
     */
    private static double pi(long N) {
        long M = LongStream.range(0, N).parallel().filter(sr -> {
            // Random picked up point to check if it's in the circle.
            double x = ThreadLocalRandom.current().nextDouble(-1, 1);
            double y = ThreadLocalRandom.current().nextDouble(-1, 1);

            return x * x + y * y < 1.0 * 1.0;  // Not using
        }).count();
        return 4.0 * M / N;
    }
}