/*
 * %W% %E%
 *
 * Copyright 2004 Sun Microsystems, Inc. All rights reserved.
 * SUN PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
 */

package java.util;
import java.util.concurrent.atomic.AtomicInteger;

/**
 * A facility for threads to schedule tasks for future execution in a
 * background thread.  Tasks may be scheduled for one-time execution, or for
 * repeated execution at regular intervals.
 *
 * <p>Corresponding to each <tt>Timer</tt> object is a single background
 * thread that is used to execute all of the timer's tasks, sequentially.
 * Timer tasks should complete quickly.  If a timer task takes excessive time
 * to complete, it "hogs" the timer's task execution thread.  This can, in
 * turn, delay the execution of subsequent tasks, which may "bunch up" and
 * execute in rapid succession when (and if) the offending task finally
 * completes.
 *
 * <p>After the last live reference to a <tt>Timer</tt> object goes away
 * <i>and</i> all outstanding tasks have completed execution, the timer's task
 * execution thread terminates gracefully (and becomes subject to garbage
 * collection).  However, this can take arbitrarily long to occur.  By
 * default, the task execution thread does not run as a <i>daemon thread</i>,
 * so it is capable of keeping an application from terminating.  If a caller
 * wants to terminate a timer's task execution thread rapidly, the caller
 * should invoke the the timer's <tt>cancel</tt> method.
 *
 * <p>If the timer's task execution thread terminates unexpectedly, for
 * example, because its <tt>stop</tt> method is invoked, any further
 * attempt to schedule a task on the timer will result in an
 * <tt>IllegalStateException</tt>, as if the timer's <tt>cancel</tt>
 * method had been invoked.
 *
 * <p>This class is thread-safe: multiple threads can share a single
 * <tt>Timer</tt> object without the need for external synchronization.
 *
 * <p>This class does <i>not</i> offer real-time guarantees: it schedules
 * tasks using a timed version of the <tt>Object.wait()</tt> method.
 *
 * <p>Time values passed to the various <tt>schedule</tt> methods are subject
 * to certain limitations (none of which should represent a practical problem).
 * All relative time values (execution periods and delays) must be less than
 * 2<sup>42</sup> ms (approximately 140 years).  Absolute execution times
 * must be within 2<sup>42</sup> ms of the time this class was initialized.
 *
 * <p>Implementation note: This class scales to large numbers of concurrently
 * scheduled tasks (thousands should present no problem).  Internally,
 * it uses a binary heap to represent its task queue, so the cost to schedule
 * a task is O(log n), where n is the number of concurrently scheduled tasks.
 *
 * @author  Josh Bloch
 * @version %I%, %G%
 * @see     TimerTask
 * @see     Object#wait(long)
 * @since   1.3
 */

public class Timer {

    /**
     * The timer task queue.  This data structure is shared with the timer
     * thread.  The timer produces tasks, via its various schedule calls,
     * and the timer thread consumes, executing timer tasks as appropriate,
     * and removing them from the queue when they're obsolete.
     */
    private TaskQueue queue = new TaskQueue();

    /**
     * The timer thread.
     */
    private TimerThread thread = new TimerThread(queue);

    /**
     * This object causes the timer's task execution thread to exit
     * gracefully when there are no live references to the Timer object and no
     * tasks in the timer queue.  It is used in preference to a finalizer on
     * Timer as such a finalizer would be susceptible to a subclass's
     * finalizer forgetting to call it.
     */
    private Object threadReaper = new Object() {
        protected void finalize() throws Throwable {
            synchronized(queue) {
                thread.newTasksMayBeScheduled = false;
                queue.notify(); // In case queue is empty.
            }
        }
    };

    /**
     * This ID is used to generate thread names.  
     */
    private static AtomicInteger nextSerialNumber = new AtomicInteger(0);
    private static int serialNumber() {
        return nextSerialNumber.getAndIncrement();
    }

    /*
     * This class has been converted as of J2SE 1.5 to use nanosecond
     * timing resolution internally, while keeping the original
     * millisecond-based API.  The following constants and functions
     * perform conversions to and from external millisecond to
     * internal nanosecond times.
     */

    static final int NANOS_PER_MILLI = 1000000;

    /** Scale a millisecond value to nanosecond units */
    static long millisToNanos(long millis) {
	return millis * NANOS_PER_MILLI;
    }

    /** Scale a nanosecond value to millisecond units */
    static long nanosToMillis(long nanos) {
	return nanos / NANOS_PER_MILLI;
    }

    /**
     * Bound on period values, delay values, and absolute times.
     */
    private static final long TIME_BOUND_MILLIS = 1L << 42;


    /**
     * Lower bound on legal times in Date.getTime() format.
     */
    private static final long MIN_MILLIS;

    /**
     * Upper bound on legal times in Date.getTime() format.
     */
    private static final long MAX_MILLIS;


    /** Origin for nanosecond times */
    private static final long ORIGIN_NANOS;

    /* We need to find a common base for millis and nanos in order to
     * convert absolute time readings.  So we perform initial readings
     * to find suitable origins. To improve accuracy, we sandwich a
     * millisecond reading around two nanosecond readings, and use
     * their average as matching time.
     */
    static {
	long nanos1 = System.nanoTime();
	long millis = System.currentTimeMillis();
	ORIGIN_NANOS = nanos1 + (System.nanoTime() - nanos1) / 2;
	MIN_MILLIS = millis - TIME_BOUND_MILLIS;
	MAX_MILLIS = millis + TIME_BOUND_MILLIS;
    }

    /**
     * Return nanosecond time relative to ORIGIN_NANOS
     */
    static long currentTimeNanos() {
	return System.nanoTime() - ORIGIN_NANOS;
    }

    /**
     * Convert absolute time represented as a date into internal
     * nanosecond-based format
     */
    static long dateToExecutionTime(Date time) {
	long cm = System.currentTimeMillis();
	long cn = System.nanoTime();
	long md = time.getTime() - cm;
	return cn - ORIGIN_NANOS + millisToNanos(md);
    }

    /**
     * Convert absolute time of internal form into the form used by
     * currentTimeMillis.
     */
    static long executionTimeToMillis(long x) {
	long cn = System.nanoTime();
	long cm = System.currentTimeMillis();
	long nd = cn - ORIGIN_NANOS - x;
	long round = (nd < 0)? -500000 : 500000;
	long md = nanosToMillis(nd + round);
	return cm - md;
    }

    /** 
     * Convenience method to perform a monitor wait for the given
     * nanoseconds
     */
    static void waitNanos(Object x, long nanos) throws InterruptedException {
	x.wait(       nanos / NANOS_PER_MILLI,
	       (int) (nanos % NANOS_PER_MILLI));
    }


    /**
     * Creates a new timer.  The associated thread does <i>not</i> run as
     * a daemon.
     *
     * @see Thread
     * @see #cancel()
     */
    public Timer() {
        this("Timer-" + serialNumber());
    }

    /**
     * Creates a new timer whose associated thread may be specified to 
     * run as a daemon.  A daemon thread is called for if the timer will
     * be used to schedule repeating "maintenance activities", which must
     * be performed as long as the application is running, but should not
     * prolong the lifetime of the application.
     *
     * @param isDaemon true if the associated thread should run as a daemon.
     *
     * @see Thread
     * @see #cancel()
     */
    public Timer(boolean isDaemon) {
        this("Timer-" + serialNumber(), isDaemon);
    }

    /**
     * Creates a new timer whose associated thread has the specified name.
     * The associated thread does <i>not</i> run as a daemon.
     *
     * @param name the name of the associated thread
     * @throws NullPointerException if name is null
     * @see Thread#getName()
     * @see Thread#isDaemon()
     * @since 1.5
     */
    public Timer(String name) {
        thread.setName(name);
        thread.start();
    }
 
    /**
     * Creates a new timer whose associated thread has the specified name,
     * and may be specified to run as a daemon.
     *
     * @param name the name of the associated thread
     * @param isDaemon true if the associated thread should run as a daemon
     * @throws NullPointerException if name is null
     * @see Thread#getName()
     * @see Thread#isDaemon()
     * @since 1.5
     */
    public Timer(String name, boolean isDaemon) {
        thread.setName(name);
        thread.setDaemon(isDaemon);
        thread.start();
    }

    /**
     * Schedules the specified task for execution after the specified delay.
     *
     * @param task  task to be scheduled.
     * @param delay delay in milliseconds before task is to be executed.
     * @throws IllegalArgumentException if <tt>delay</tt> is negative or is
     *     greater than or equal to 2<sup>42</sup> (approximately 140 years).
     * @throws IllegalStateException if task was already scheduled or
     *     cancelled, or timer was cancelled.
     */
    public void schedule(TimerTask task, long delay) {
        if (delay < 0 || delay >= TIME_BOUND_MILLIS)
            throw new IllegalArgumentException("Delay out of range: " + delay);
        sched(task, currentTimeNanos() + millisToNanos(delay), 0);
    }

    /**
     * Schedules the specified task for execution at the specified time.  If
     * the time is in the past, the task is scheduled for immediate execution.
     *
     * @param task task to be scheduled.
     * @param time time at which task is to be executed.
     * @throws IllegalArgumentException if <tt>time</tt> is not within
     *     2<sup>42</sup> ms (approximately 140 years) of the time this
     *     class was initialized.
     * @throws IllegalStateException if task was already scheduled or
     *         cancelled, timer was cancelled, or timer thread terminated.
     */
    public void schedule(TimerTask task, Date time) {
        long millis = time.getTime();
        if (millis <= MIN_MILLIS || millis >= MAX_MILLIS)
            throw new IllegalArgumentException("Time out of range: " + time);
        sched(task, dateToExecutionTime(time), 0);
    }

    /**
     * Schedules the specified task for repeated <i>fixed-delay execution</i>,
     * beginning after the specified delay.  Subsequent executions take place
     * at approximately regular intervals separated by the specified period.
     *
     * <p>In fixed-delay execution, each execution is scheduled relative to
     * the actual execution time of the previous execution.  If an execution
     * is delayed for any reason (such as garbage collection or other
     * background activity), subsequent executions will be delayed as well.
     * In the long run, the frequency of execution will generally be slightly
     * lower than the reciprocal of the specified period (assuming the system
     * clock underlying <tt>Object.wait(long)</tt> is accurate).
     *
     * <p>Fixed-delay execution is appropriate for recurring activities
     * that require "smoothness."  In other words, it is appropriate for
     * activities where it is more important to keep the frequency accurate
     * in the short run than in the long run.  This includes most animation
     * tasks, such as blinking a cursor at regular intervals.  It also includes
     * tasks wherein regular activity is performed in response to human
     * input, such as automatically repeating a character as long as a key
     * is held down.
     *
     * @param task   task to be scheduled.
     * @param delay  delay in milliseconds before task is to be executed.
     * @param period time in milliseconds between successive task executions.
     * @throws IllegalArgumentException if <tt>delay</tt> is negative or is
     *     greater than or equal to 2<sup>42</sup> (approximately 140 years).
     * @throws IllegalArgumentException if <tt>period</tt> is non-positive
     *     or is greater than or equal to 2<sup>42</sup>.
     * @throws IllegalStateException if task was already scheduled or
     *         cancelled, timer was cancelled, or timer thread terminated.
     */
    public void schedule(TimerTask task, long delay, long period) {
        if (delay < 0 || delay >=  TIME_BOUND_MILLIS)
            throw new IllegalArgumentException("Delay out of range: " + delay);
        if (period <= 0 || period >=  TIME_BOUND_MILLIS)
            throw new IllegalArgumentException("Period out of range: "+period);
        sched(task, 
	      currentTimeNanos() + millisToNanos(delay), 
	      millisToNanos(-period));
    }

    /**
     * Schedules the specified task for repeated <i>fixed-delay execution</i>,
     * beginning at the specified time. Subsequent executions take place at
     * approximately regular intervals, separated by the specified period.
     *
     * <p>In fixed-delay execution, each execution is scheduled relative to
     * the actual execution time of the previous execution.  If an execution
     * is delayed for any reason (such as garbage collection or other
     * background activity), subsequent executions will be delayed as well.
     * In the long run, the frequency of execution will generally be slightly
     * lower than the reciprocal of the specified period (assuming the system
     * clock underlying <tt>Object.wait(long)</tt> is accurate).
     *
     * <p>Fixed-delay execution is appropriate for recurring activities
     * that require "smoothness."  In other words, it is appropriate for
     * activities where it is more important to keep the frequency accurate
     * in the short run than in the long run.  This includes most animation
     * tasks, such as blinking a cursor at regular intervals.  It also includes
     * tasks wherein regular activity is performed in response to human
     * input, such as automatically repeating a character as long as a key
     * is held down.
     *
     * @param task task to be scheduled.
     * @param firstTime First time at which task is to be executed.
     * @param period time in milliseconds between successive task executions.
     * @throws IllegalArgumentException if <tt>firstTime</tt> is not within
     *     2<sup>42</sup> ms (approximately 140 years) of the time this
     *     class was initialized.
     * @throws IllegalArgumentException if <tt>period</tt> is non-positive
     *     or is greater than or equal to 2<sup>42</sup>.
     * @throws IllegalStateException if task was already scheduled or
     *         cancelled, timer was cancelled, or timer thread terminated.
     */
    public void schedule(TimerTask task, Date firstTime, long period) {
        long millis = firstTime.getTime();
        if (millis <= MIN_MILLIS || millis >= MAX_MILLIS)
            throw new IllegalArgumentException("Time out of range: " + millis);
        if (period <= 0 || period >=  TIME_BOUND_MILLIS)
            throw new IllegalArgumentException("Period out of range: "+period);
        sched(task, 
	      dateToExecutionTime(firstTime), 
	      millisToNanos(-period));
    }

    /**
     * Schedules the specified task for repeated <i>fixed-rate execution</i>,
     * beginning after the specified delay.  Subsequent executions take place
     * at approximately regular intervals, separated by the specified period.
     *
     * <p>In fixed-rate execution, each execution is scheduled relative to the
     * scheduled execution time of the initial execution.  If an execution is
     * delayed for any reason (such as garbage collection or other background
     * activity), two or more executions will occur in rapid succession to
     * "catch up."  In the long run, the frequency of execution will be
     * exactly the reciprocal of the specified period (assuming the system
     * clock underlying <tt>Object.wait(long)</tt> is accurate).
     *
     * <p>Fixed-rate execution is appropriate for recurring activities that
     * are sensitive to <i>absolute</i> time, such as ringing a chime every
     * hour on the hour, or running scheduled maintenance every day at a
     * particular time.  It is also appropriate for for recurring activities
     * where the total time to perform a fixed number of executions is
     * important, such as a countdown timer that ticks once every second for
     * ten seconds.  Finally, fixed-rate execution is appropriate for
     * scheduling multiple repeating timer tasks that must remain synchronized
     * with respect to one another.
     *
     * @param task   task to be scheduled.
     * @param delay  delay in milliseconds before task is to be executed.
     * @param period time in milliseconds between successive task executions.
     * @throws IllegalArgumentException if <tt>delay</tt> is negative or is
     *     greater than or equal to 2<sup>42</sup> (approximately 140 years).
     * @throws IllegalArgumentException if <tt>period</tt> is non-positive
     *     or is greater than or equal to 2<sup>42</sup>.
     * @throws IllegalStateException if task was already scheduled or
     *         cancelled, timer was cancelled, or timer thread terminated.
     */
    public void scheduleAtFixedRate(TimerTask task, long delay, long period) {
        if (delay < 0 || delay >=  TIME_BOUND_MILLIS)
            throw new IllegalArgumentException("Delay out of range: " + delay);
        if (period <= 0 || period >=  TIME_BOUND_MILLIS)
            throw new IllegalArgumentException("Period out of range: "+period);
        sched(task, 
	      currentTimeNanos() + millisToNanos(delay), 
	      millisToNanos(period));
    }

    /**
     * Schedules the specified task for repeated <i>fixed-rate execution</i>,
     * beginning at the specified time. Subsequent executions take place at
     * approximately regular intervals, separated by the specified period.
     *
     * <p>In fixed-rate execution, each execution is scheduled relative to the
     * scheduled execution time of the initial execution.  If an execution is
     * delayed for any reason (such as garbage collection or other background
     * activity), two or more executions will occur in rapid succession to
     * "catch up."  In the long run, the frequency of execution will be
     * exactly the reciprocal of the specified period (assuming the system
     * clock underlying <tt>Object.wait(long)</tt> is accurate).
     *
     * <p>Fixed-rate execution is appropriate for recurring activities that
     * are sensitive to <i>absolute</i> time, such as ringing a chime every
     * hour on the hour, or running scheduled maintenance every day at a
     * particular time.  It is also appropriate for for recurring activities
     * where the total time to perform a fixed number of executions is
     * important, such as a countdown timer that ticks once every second for
     * ten seconds.  Finally, fixed-rate execution is appropriate for
     * scheduling multiple repeating timer tasks that must remain synchronized
     * with respect to one another.
     *
     * @param task   task to be scheduled.
     * @param firstTime First time at which task is to be executed.
     * @param period time in milliseconds between successive task executions.
     * @throws IllegalArgumentException if <tt>firstTime</tt> is not within
     *     2<sup>42</sup> ms (approximately 140 years) of the time this
     *     class was initialized.
     * @throws IllegalArgumentException if <tt>period</tt> is non-positive
     *     or is greater than or equal to 2<sup>42</sup>.
     * @throws IllegalStateException if task was already scheduled or
     *         cancelled, timer was cancelled, or timer thread terminated.
     */
    public void scheduleAtFixedRate(TimerTask task, Date firstTime,
                                    long period) {
        long millis = firstTime.getTime();
        if (millis <= MIN_MILLIS || millis >= MAX_MILLIS)
            throw new IllegalArgumentException("Time out of range: " + millis);
        if (period <= 0)
            throw new IllegalArgumentException("Non-positive period.");
        sched(task, 
	      dateToExecutionTime(firstTime), 
	      millisToNanos(period));
    }

    /**
     * Schedule the specified timer task for execution at the specified
     * time with the specified period, in milliseconds.  If period is
     * positive, the task is scheduled for repeated execution; if period is
     * zero, the task is scheduled for one-time execution. Time is specified
     * in Date.getTime() format.  This method checks timer state and task
     * state, but not initial execution time or period.
     *
     * @throws IllegalStateException if task was already scheduled or
     *         cancelled, timer was cancelled, or timer thread terminated.
     */
    private void sched(TimerTask task, long time, long period) {
        synchronized(queue) {
            if (!thread.newTasksMayBeScheduled)
                throw new IllegalStateException("Timer already cancelled.");

            synchronized(task.lock) {
                if (task.state != TimerTask.VIRGIN)
                    throw new IllegalStateException(
                        "Task already scheduled or cancelled");
                task.nextExecutionTime = time;
                task.period = period;
                task.state = TimerTask.SCHEDULED;
            }

            queue.add(task);
            if (queue.getMin() == task)
                queue.notify();
        }
    }

    /**
     * Terminates this timer, discarding any currently scheduled tasks.
     * Does not interfere with a currently executing task (if it exists).
     * Once a timer has been terminated, its execution thread terminates
     * gracefully, and no more tasks may be scheduled on it.
     *
     * <p>Note that calling this method from within the run method of a
     * timer task that was invoked by this timer absolutely guarantees that
     * the ongoing task execution is the last task execution that will ever
     * be performed by this timer.
     *
     * <p>This method may be called repeatedly; the second and subsequent 
     * calls have no effect.
     */
    public void cancel() {
        synchronized(queue) {
            thread.newTasksMayBeScheduled = false;
            queue.clear();
            queue.notify();  // In case queue was already empty.
        }
    }

    /**
     * Removes all cancelled tasks from this timer's task queue.  <i>Calling
     * this method has no effect on the behavior of the timer</i>, but
     * eliminates the references to the cancelled tasks from the queue.
     * If there are no external references to these tasks, they become
     * eligible for garbage collection.
     *
     * <p>Most programs will have no need to call this method.
     * It is designed for use by the rare application that cancels a large
     * number of of tasks.  Calling this method trades time for space: the
     * runtime of the method may be proportional to n + c log n, where n
     * is the number of tasks in the queue and c is the number of cancelled
     * tasks.
     *
     * <p>Note that it is permissible to call this method from within a
     * a task scheduled on this timer.
     *
     * @return the number of tasks removed from the queue.
     * @since 1.5
     */
     public int purge() {
         int result = 0;

         synchronized(queue) {
             for (int i = queue.size(); i > 0; i--) {
                 if (queue.get(i).state == TimerTask.CANCELLED) {
                     queue.quickRemove(i);
                     result++;
                 }
             }

             if (result != 0)
                 queue.heapify();
         }

         return result;
     }
}

/**
 * This "helper class" implements the timer's task execution thread, which
 * waits for tasks on the timer queue, executions them when they fire,
 * reschedules repeating tasks, and removes cancelled tasks and spent
 * non-repeating tasks from the queue.
 */
class TimerThread extends Thread {
    /**
     * This flag is set to false by the reaper to inform us that there
     * are no more live references to our Timer object.  Once this flag
     * is true and there are no more tasks in our queue, there is no
     * work left for us to do, so we terminate gracefully.  Note that
     * this field is protected by queue's monitor!
     */
    boolean newTasksMayBeScheduled = true;

    /**
     * Our Timer's queue.  We store this reference in preference to
     * a reference to the Timer so the reference graph remains acyclic.
     * Otherwise, the Timer would never be garbage-collected and this
     * thread would never go away.
     */
    private TaskQueue queue;

    TimerThread(TaskQueue queue) {
        this.queue = queue;
    }

    public void run() {
        try {
            mainLoop();
        } finally {
            // Someone killed this Thread, behave as if Timer cancelled
            synchronized(queue) {
                newTasksMayBeScheduled = false;
                queue.clear();  // Eliminate obsolete references
            }
        }
    }

    /**
     * The main timer loop.  (See class comment.)
     */
    private void mainLoop() {
        while (true) {
            try {
                TimerTask task;
                boolean taskFired;
                synchronized(queue) {
                    // Wait for queue to become non-empty
                    while (queue.isEmpty() && newTasksMayBeScheduled)
                        queue.wait();
                    if (queue.isEmpty())
                        break; // Queue is empty and will forever remain; die

                    // Queue nonempty; look at first evt and do the right thing
                    long currentTime, executionTime, delay;
                    task = queue.getMin();
                    synchronized(task.lock) {
                        if (task.state == TimerTask.CANCELLED) {
                            queue.removeMin();
                            continue;  // No action required, poll queue again
                        }
                        currentTime = Timer.currentTimeNanos();
                        executionTime = task.nextExecutionTime;
			delay = executionTime - currentTime;
                        if (taskFired = (delay <= 0)) {
                            if (task.period == 0) { // Non-repeating, remove
                                queue.removeMin();
                                task.state = TimerTask.EXECUTED;
                            } else { // Repeating task, reschedule
                                queue.rescheduleMin(
                                  task.period<0 ? currentTime   - task.period
                                                : executionTime + task.period);
                            }
                        }
                    }
                    if (!taskFired)  // Task hasn't yet fired; wait
			Timer.waitNanos(queue, delay);
                }
                if (taskFired)  // Task fired; run it, holding no locks
                    task.run();
            } catch(InterruptedException e) {
            }
        }
    }
}

/**
 * This class represents a timer task queue: a priority queue of TimerTasks,
 * ordered on nextExecutionTime.  Each Timer object has one of these, which it
 * shares with its TimerThread.  Internally this class uses a heap, which
 * offers log(n) performance for the add, removeMin and rescheduleMin
 * operations, and constant time performance for the the getMin operation.
 */
class TaskQueue {
    /**
     * Priority queue represented as a balanced binary heap: the two children
     * of queue[n] are queue[2*n] and queue[2*n+1].  The priority queue is
     * ordered on the nextExecutionTime field: The TimerTask with the lowest
     * nextExecutionTime is in queue[1] (assuming the queue is nonempty).  For
     * each node n in the heap, and each descendant of n, d,
     * n.nextExecutionTime <= d.nextExecutionTime. 
     */
    private TimerTask[] queue = new TimerTask[128];

    /**
     * The number of tasks in the priority queue.  (The tasks are stored in
     * queue[1] up to queue[size]).
     */
    private int size = 0;

    /**
     * Returns the number of tasks currently on the queue.
     */
    int size() {
        return size;
    }

    /**
     * Adds a new task to the priority queue.
     */
    void add(TimerTask task) {
        // Grow backing store if necessary
        if (++size == queue.length) {
            TimerTask[] newQueue = new TimerTask[2*queue.length];
            System.arraycopy(queue, 0, newQueue, 0, size);
            queue = newQueue;
        }

        queue[size] = task;
        fixUp(size);
    }

    /**
     * Return the "head task" of the priority queue.  (The head task is an
     * task with the lowest nextExecutionTime.)
     */
    TimerTask getMin() {
        return queue[1];
    }

    /**
     * Return the ith task in the priority queue, where i ranges from 1 (the
     * head task, which is returned by getMin) to the number of tasks on the
     * queue, inclusive.
     */
    TimerTask get(int i) {
        return queue[i];
    }

    /**
     * Remove the head task from the priority queue.
     */
    void removeMin() {
        queue[1] = queue[size];
        queue[size--] = null;  // Drop extra reference to prevent memory leak
        fixDown(1);
    }

    /**
     * Removes the ith element from queue without regard for maintaining
     * the heap invariant.  Recall that queue is one-based, so
     * 1 <= i <= size.
     */
    void quickRemove(int i) {
        assert i <= size;

        queue[i] = queue[size];
        queue[size--] = null;  // Drop extra ref to prevent memory leak
    }

    /**
     * Sets the nextExecutionTime associated with the head task to the 
     * specified value, and adjusts priority queue accordingly.
     */
    void rescheduleMin(long newTime) {
        queue[1].nextExecutionTime = newTime;
        fixDown(1);
    }

    /**
     * Returns true if the priority queue contains no elements.
     */
    boolean isEmpty() {
        return size==0;
    }

    /**
     * Removes all elements from the priority queue.
     */
    void clear() {
        // Null out task references to prevent memory leak
        for (int i=1; i<=size; i++)
            queue[i] = null;

        size = 0;
    }

    /**
     * Establishes the heap invariant (described above) assuming the heap
     * satisfies the invariant except possibly for the leaf-node indexed by k
     * (which may have a nextExecutionTime less than its parent's).
     *
     * This method functions by "promoting" queue[k] up the hierarchy
     * (by swapping it with its parent) repeatedly until queue[k]'s
     * nextExecutionTime is greater than or equal to that of its parent.
     */
    private void fixUp(int k) {
        while (k > 1) {
            int j = k >> 1;
            if (queue[j].nextExecutionTime <= queue[k].nextExecutionTime)
                break;
            TimerTask tmp = queue[j];  queue[j] = queue[k]; queue[k] = tmp;
            k = j;
        }
    }

    /**
     * Establishes the heap invariant (described above) in the subtree
     * rooted at k, which is assumed to satisfy the heap invariant except
     * possibly for node k itself (which may have a nextExecutionTime greater
     * than its children's).
     *
     * This method functions by "demoting" queue[k] down the hierarchy
     * (by swapping it with its smaller child) repeatedly until queue[k]'s
     * nextExecutionTime is less than or equal to those of its children.
     */
    private void fixDown(int k) {
        int j;
        while ((j = k << 1) <= size && j > 0) {
            if (j < size &&
                queue[j].nextExecutionTime > queue[j+1].nextExecutionTime)
                j++; // j indexes smallest kid
            if (queue[k].nextExecutionTime <= queue[j].nextExecutionTime)
                break;
            TimerTask tmp = queue[j];  queue[j] = queue[k]; queue[k] = tmp;
            k = j;
        }
    }

    /**
     * Establishes the heap invariant (described above) in the entire tree,
     * assuming nothing about the order of the elements prior to the call.
     */
    void heapify() {
        for (int i = size/2; i >= 1; i--)
            fixDown(i);
    }
}