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Simple implementation of basic algorithm.
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/******************************************************************************
 *  Compilation:  javac StopwatchCPU.java
 *  Execution:    java StopwtachCPU n
 *  Dependencies: none
 *
 *  A version of Stopwatch.java that measures CPU time on a single
 *  core or processor (instead of wall clock time).
 *
 *  % java8 StopwatchCPU 100000000
 *  6.666667e+11 (1.05 seconds)
 *  6.666667e+11 (7.50 seconds)
 *
 ******************************************************************************/

package edu.princeton.cs.algs4;

import java.lang.management.ThreadMXBean;
import java.lang.management.ManagementFactory;

/**
 *  The {@code StopwatchCPU} data type is for measuring
 *  the CPU time used during a programming task.
 *
 *  See {@link Stopwatch} for a version that measures wall-clock time
 *  (the real time that elapses).
 *
 *  @author Josh Hug
 *  @author Robert Sedgewick
 *  @author Kevin Wayne
 */

public class StopwatchCPU {
    private static final double NANOSECONDS_PER_SECOND = 1000000000;

    private final ThreadMXBean threadTimer;
    private final long start;
            
    /**
     * Initializes a new stopwatch.
     */
    public StopwatchCPU() {  
        threadTimer = ManagementFactory.getThreadMXBean();
        start = threadTimer.getCurrentThreadCpuTime();
    }   
        
    /**
     * Returns the elapsed CPU time (in seconds) since the stopwatch was created.
     *
     * @return elapsed CPU time (in seconds) since the stopwatch was created
     */
    public double elapsedTime() {
        long now = threadTimer.getCurrentThreadCpuTime();
        return (now - start) / NANOSECONDS_PER_SECOND;
    }

    /**
     * Unit tests the {@code StopwatchCPU} data type.
     * Takes a command-line argument {@code n} and computes the 
     * sum of the square roots of the first {@code n} positive integers,
     * first using {@code Math.sqrt()}, then using {@code Math.pow()}.
     * It prints to standard output the sum and the amount of time to
     * compute the sum. Note that the discrete sum can be approximated by
     * an integral - the sum should be approximately 2/3 * (n^(3/2) - 1).
     *
     * @param args the command-line arguments
     */
    public static void main(String[] args) {
        int n = Integer.parseInt(args[0]);

        // sum of square roots of integers from 1 to n using Math.sqrt(x).
        StopwatchCPU timer1 = new StopwatchCPU();
        double sum1 = 0.0;
        for (int i = 1; i <= n; i++) {
            sum1 += Math.sqrt(i);
        }
        double time1 = timer1.elapsedTime();
        StdOut.printf("%e (%.2f seconds)\n", sum1, time1);

        // sum of square roots of integers from 1 to n using Math.pow(x, 0.5).
        StopwatchCPU timer2 = new StopwatchCPU();
        double sum2 = 0.0;
        for (int i = 1; i <= n; i++) {
            sum2 += Math.pow(i, 0.5);
        }
        double time2 = timer2.elapsedTime();
        StdOut.printf("%e (%.2f seconds)\n", sum2, time2);
    }
}

/******************************************************************************
 *  Copyright 2002-2018, Robert Sedgewick and Kevin Wayne.
 *
 *  This file is part of algs4.jar, which accompanies the textbook
 *
 *      Algorithms, 4th edition by Robert Sedgewick and Kevin Wayne,
 *      Addison-Wesley Professional, 2011, ISBN 0-321-57351-X.
 *      http://algs4.cs.princeton.edu
 *
 *
 *  algs4.jar is free software: you can redistribute it and/or modify
 *  it under the terms of the GNU General Public License as published by
 *  the Free Software Foundation, either version 3 of the License, or
 *  (at your option) any later version.
 *
 *  algs4.jar is distributed in the hope that it will be useful,
 *  but WITHOUT ANY WARRANTY; without even the implied warranty of
 *  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 *  GNU General Public License for more details.
 *
 *  You should have received a copy of the GNU General Public License
 *  along with algs4.jar.  If not, see http://www.gnu.org/licenses.
 ******************************************************************************/