
import java.awt.*;
import java.awt.event.*;
import javax.swing.*;

/**
 * A panel that can show a demonstration of Quicksort in action.
 * The items to be sorted are the hues of a set of vertical bars.
 * When sorted, the bars form a spectrum from red to violet.
 * Initially, the bars are sorted.  There is a Start button.  When
 * the user clicks this button, the order of the bars is randomized
 * and then Quicksort is applied.  During the sort, a black bar
 * marks the location of an empty space from which the "pivot" element
 * has been removed.  The user can abort the sort by clicking the
 * button again.
 * <p>The main point of this program is to demonstrate threads, with
 * very simple inter-thread communication.  The recursive Quicksort
 * algorithm is run in a separate thread.  The abort operation is
 * implemented by setting the value of a volatile variable that
 * is checked periodically by the thread.  When the user aborts
 * the sort before it finishes, the value of the variable changes;
 * the thread sees the change and exits.
 * <p>This class contains a main() routine that allows the demo to
 * be run as a stand-alone application.
 */
public class QuicksortThreadDemo extends JPanel {

    /**
     * This main routine just shows a panel of type QuicksortThreadDemo.
     */
    public static void main(String[] args) {
        JFrame window = new JFrame("Demo: Recursion in a Thread");
        QuicksortThreadDemo content = new QuicksortThreadDemo();
        window.setContentPane(content);
        window.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        window.pack();
        Dimension screenSize = Toolkit.getDefaultToolkit().getScreenSize();
        window.setLocation( (screenSize.width - window.getWidth()) / 2,
                (screenSize.height - window.getHeight()) / 2 );
        window.setVisible(true);
    }

    private final static int ARRAY_SIZE = 150;  // The number of colored bars.

    private int[] hue = new int[ARRAY_SIZE];  // The array that will be sorted.
    private Color[] palette = new Color[ARRAY_SIZE]; // Colors in spectral order.
    private Display display;     // The panel that displays the colored bars.
    private JButton startButton; // The button that starts and stops the demo.

    private Runner runner; // The thread that runs the recursion.

    private volatile boolean running;  // Set to true while recursion is running;
                                       // This is set to false as a signal to the
                                       // thread to abort.



    /**
     * When the user aborts the recursion before it finishes, an exception of
     * this type is thrown to end the recursion cleanly.
     */
    private class ThreadTerminationException extends RuntimeException {
    }


    /**
     * A subpanel of type Display shows the colored bars that are being sorted.
     * The current pivot, if any, is shown in black.  A 3-pixel gray border is
     * left around the bars.
     */
    private class Display extends JPanel {
        Display() {
            setPreferredSize(new Dimension(606,206));
            setBackground(Color.GRAY);
        }
        protected void paintComponent(Graphics g) {
            super.paintComponent(g);
            double barWidth = (double)(getWidth() - 6) / hue.length;
            int h = getHeight() - 6;
            for (int i = 0; i < hue.length; i++) {
                int x1 = 3 + (int)(i*barWidth + 0.49);
                int x2 = 3 + (int)((i+1)*barWidth + 0.49);
                int w = x2 - x1;
                if (hue[i] == -1)
                    g.setColor(Color.BLACK);
                else
                    g.setColor(palette[hue[i]]);
                g.fillRect(x1,3,w,h);
            }
        }
    }


    /**
     * The constructor sets up the panel, containing the Display and the
     * Start button below it.
     */
    public QuicksortThreadDemo() {
        for (int i = 0; i < ARRAY_SIZE; i++) {
            palette[i] = Color.getHSBColor((i*230)/(ARRAY_SIZE*255.0F), 1, 1);
            hue[i] = i;
        }
        setLayout(new BorderLayout());
        display = new Display();
        add(display, BorderLayout.CENTER);
        startButton = new JButton("Start");
        JPanel bottom = new JPanel();
        bottom.add(startButton);
        bottom.setBackground(Color.WHITE);
        add(bottom,BorderLayout.SOUTH);
        startButton.addActionListener(new ActionListener() {
            public void actionPerformed(ActionEvent e) {
                if (running)
                    stop();
                else
                    start();
            }
        });
    }


    /**
     * This method is called when the user clicks the Start button,
     * while no thread is running.  It starts a new thread and
     * sets the signaling variable, running, to true;  Also changes
     * the text on the Start button to "Finish".
     */
    private void start() {
        startButton.setText("Finish");
        runner = new Runner();
        running = true;  // Set the signal before starting the thread!
        runner.start();
    }


    /**
     * This method is called when the user clicks the button while
     * a thread is running.  A signal is sent to the thread to terminate,
     * by setting the value of the signaling variable, running, to false.
     */
    private void stop() {

        /* Set the value of the signaling variable to false as a signal
         * to the thread to terminate.
         */

        running = false; 

        /* Wake the thread, in case it is sleeping, to get a more
         * immediate reaction to the signal.
         */

        runner.interrupt(); 

        /* Wait for the thread to stop before setting runner = null.
         * One second should be plenty of time for this to happen, but
         * in case something goes wrong, it's better not to 
         */

        try {
            runner.join(1000);  // Wait for thread to stop.  One second should be plenty of time.
        }
        catch (InterruptedException e) {
        }

        runner = null;
    }


    /**
     * This method is called frequently by the thread that is running
     * the recursion, in order to insert delays.  It calls the repaint()
     * method of the display to allow the user to see what is going on;
     * the delay will give the system a chance to actually update the display.
     * Since this method is called regularly while the recursion is in 
     * progress, it is also used as a convenient place to check the value
     * of the signaling variable, running.  If the value of running has
     * been set to false, this method throws an exception of type
     * ThreadTerminationException.  This exception will cause all active
     * levels of the recursion to be terminated.  It is caught in the
     * run() method of the thread.
     * @param millis  The number of milliseconds to sleep.
     */
    private void delay(int millis) {
        if (! running)
            throw new ThreadTerminationException();
        display.repaint();
        try {
            Thread.sleep(millis);
        }
        catch (InterruptedException e) {
        }
        if (! running)
            throw new ThreadTerminationException();
    }


    /**
     * The basic non-recursive QuickSortStep algorithm, which
     * uses hue[lo] as a "pivot" and rearranges elements of the 
     * hue array from positions lo through hi so that positions
     * the pivot value is in its correct location, with smaller
     * items to the left and bigger items to the right.  The
     * position of the pivot is returned.  In this version,
     * we conceptually remove the pivot from the array, leaving
     * an empty space.  The space is marked by a -1, and it moves
     * around as the algorithm proceeds.  It is shown as a black
     * bar in the display. Every time a change is made, the
     * delay() method is called to insert a 1/10 second delay
     * to let the user see the change.
     */
    private int quickSortStep(int lo, int hi) {
        int pivot = hue[lo];  // Save pivot item.
        hue[lo] = -1;  // Mark location lo as empty.
        delay(100);
        while (true) {
            while (hi > lo  && hue[hi] > pivot)
                hi--;
            if (hi == lo)
                break;
            hue[lo] = hue[hi]; // Move hue[hi] into empty space.
            hue[hi]  = -1;     // Mark location hi as empty.
            delay(100);
            while (lo < hi && hue[lo] < pivot)
                lo++;
            if (hi == lo)
                break;
            hue[hi] = hue[lo];  // Move hue[lo] into empty space.
            hue[lo] = -1;       // Mark location lo as empty.
            delay(100);
        }
        hue[lo] = pivot;  // Move pivot item into the empty space.
        delay(100);
        return lo;
    }


    /**
     * The recursive quickSort algorithm, for sorting the hue
     * array from positions lo through hi into increasing order.
     * Most of the actual work is done in quickSortStep().
     */
    private void quickSort(int lo, int hi) {
        if (hi <= lo)
            return;
        int mid = quickSortStep(lo, hi);
        quickSort(lo, mid-1);
        quickSort(mid+1, hi);
    }


    /**
     * This class defines the threads that run the recursive
     * QuickSort algorithm.  The thread begins by randomizing the
     * array.  It then calls quickSort() to sort the entire array.
     * If quickSort() is aborted by a ThreadTerminationExcpetion,
     * which would be caused by the user clicking the Finish button,
     * then the thread will restore the array to sorted order before
     * terminating, so that whether or not the quickSort is aborted,
     * the array ends up sorted.
     */
    private class Runner extends Thread {
        public void run() {
            try {
                for (int i = hue.length-1; i > 0; i--) { // Randomize array.
                    int r = (int)((i+1)*Math.random());
                    int temp = hue[r];
                    hue[r] = hue[i];
                    hue[i] = temp;
                }
                delay(1000);  // Wait one second before starting the sort.
                quickSort(0,hue.length-1);  // Sort the whole array.
            }
            catch (ThreadTerminationException e) { // User aborted quickSort.
                for (int i = 0; i < hue.length; i++)
                    hue[i] = i;
            }
            finally {// Make sure running is false and button label is correct. 
                running = false; 
                startButton.setText("Start");
                display.repaint();
            }
        }
    }

} // end QuicksortThreadDemo
