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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.math.BigInteger;
import java.util.concurrent.ThreadLocalRandom;
import java.util.stream.IntStream;

/**
 * This demo shows how to use the parallel mode to find several number for given
 * length prime number. The process easily use a parallel stream to generate
 * a number of desired length probable prime number. Following with filter
 * operation to make sure we will get desirable length prime numbers.
 *
 * @author tyan
 */
public class ProbablePrimeNumber {
    /**
     * Try to get length from command line. Use the length to create random long
     * stream, that integer's digit is length.
     *
     * @param args This has to be a 1-element or 2-elements array, The first
     * element must be a positive number, which equals to length of generated
     * prime number stream. Second element is optional, it will be used as how
     * many probable prime number that will be generated if it's set.
     */
    private static int DEFAULT_NUM = 1 << 10;

    public static void main(String[] args) {
        if (args.length != 1 && args.length != 2) {
            usage();
            return;
        }
        try {
            int expectedLength = Integer.parseInt(args[0]);
            int size = (args.length == 2) ?
                    Integer.parseInt(args[1]) :
                    DEFAULT_NUM;
            IntStream.range(0, size).
                    parallel().
                    mapToObj(i ->
                        BigInteger.probablePrime(expectedLength * 10 / 3 -
                            ThreadLocalRandom.current().nextInt(3),
                            ThreadLocalRandom.current())).
                    filter(c -> c.toString().length() == expectedLength).
                    forEach(prime ->
                            System.out.println("Found a probable prime number :"
                                    + prime));
        } catch (NumberFormatException nfe) {
            usage();
        }
    }

    /**
     * Usage of this program
     */
    public static void usage() {
        System.out.println("Usage: java ProbablePrimeNumber lentgh [size]");
        System.out.println("size is a positive integer");
    }
}