/*
 * ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
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/*
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 * Written by Doug Lea with assistance from members of JCP JSR-166
 * Expert Group and released to the public domain, as explained at
 * http://creativecommons.org/publicdomain/zero/1.0/
 */

package java.util.concurrent;

import java.lang.invoke.MethodHandles;
import java.lang.invoke.VarHandle;
import java.util.concurrent.locks.LockSupport;

/**
 * A cancellable asynchronous computation. This class provides a base
 * implementation of {@link Future}, with methods to start and cancel
 * a computation, query to see if the computation is complete, and
 * retrieve the result of the computation. The result can only be
 * retrieved when the computation has completed; the {@code get}
 * methods will block if the computation has not yet completed. Once
 * the computation has completed, the computation cannot be restarted
 * or cancelled (unless the computation is invoked using
 * {@link #runAndReset}).
 *
 * <p>A {@code FutureTask} can be used to wrap a {@link Callable} or
 * {@link Runnable} object. Because {@code FutureTask} implements
 * {@code Runnable}, a {@code FutureTask} can be submitted to an
 * {@link Executor} for execution.
 *
 * <p>In addition to serving as a standalone class, this class provides
 * {@code protected} functionality that may be useful when creating
 * customized task classes.
 *
 * @since 1.5
 * @author Doug Lea
 * @param <V> The result type returned by this FutureTask's {@code get} methods
 */
public class FutureTask<V> implements RunnableFuture<V> {
 /*
 * Revision notes: This differs from previous versions of this
 * class that relied on AbstractQueuedSynchronizer, mainly to
 * avoid surprising users about retaining interrupt status during
 * cancellation races. Sync control in the current design relies
 * on a "state" field updated via CAS to track completion, along
 * with a simple Treiber stack to hold waiting threads.
 */

 /**
 * The run state of this task, initially NEW. The run state
 * transitions to a terminal state only in methods set,
 * setException, and cancel. During completion, state may take on
 * transient values of COMPLETING (while outcome is being set) or
 * INTERRUPTING (only while interrupting the runner to satisfy a
 * cancel(true)). Transitions from these intermediate to final
 * states use cheaper ordered/lazy writes because values are unique
 * and cannot be further modified.
 *
 * Possible state transitions:
 * NEW -> COMPLETING -> NORMAL
 * NEW -> COMPLETING -> EXCEPTIONAL
 * NEW -> CANCELLED
 * NEW -> INTERRUPTING -> INTERRUPTED
 */
 private volatile int state;
 private static final int NEW = 0;
 private static final int COMPLETING = 1;
 private static final int NORMAL = 2;
 private static final int EXCEPTIONAL = 3;
 private static final int CANCELLED = 4;
 private static final int INTERRUPTING = 5;
 private static final int INTERRUPTED = 6;

 /** The underlying callable; nulled out after running */
 private Callable<V> callable;
 /** The result to return or exception to throw from get() */
 private Object outcome; // non-volatile, protected by state reads/writes
 /** The thread running the callable; CASed during run() */
 private volatile Thread runner;
 /** Treiber stack of waiting threads */
 private volatile WaitNode waiters;

 /**
 * Returns result or throws exception for completed task.
 *
 * @param s completed state value
 */
 @SuppressWarnings("unchecked")
 private V report(int s) throws ExecutionException {
 Object x = outcome;
 if (s == NORMAL)
 return (V)x;
 if (s >= CANCELLED)
 throw new CancellationException();
 throw new ExecutionException((Throwable)x);
 }

 /**
 * Creates a {@code FutureTask} that will, upon running, execute the
 * given {@code Callable}.
 *
 * @param callable the callable task
 * @throws NullPointerException if the callable is null
 */
 public FutureTask(Callable<V> callable) {
 if (callable == null)
 throw new NullPointerException();
 this.callable = callable;
 this.state = NEW; // ensure visibility of callable
 }

 /**
 * Creates a {@code FutureTask} that will, upon running, execute the
 * given {@code Runnable}, and arrange that {@code get} will return the
 * given result on successful completion.
 *
 * @param runnable the runnable task
 * @param result the result to return on successful completion. If
 * you don't need a particular result, consider using
 * constructions of the form:
 * {@code Future<?> f = new FutureTask<Void>(runnable, null)}
 * @throws NullPointerException if the runnable is null
 */
 public FutureTask(Runnable runnable, V result) {
 this.callable = Executors.callable(runnable, result);
 this.state = NEW; // ensure visibility of callable
 }

 public boolean isCancelled() {
 return state >= CANCELLED;
 }

 public boolean isDone() {
 return state != NEW;
 }

 public boolean cancel(boolean mayInterruptIfRunning) {
 if (!(state == NEW && STATE.compareAndSet
 (this, NEW, mayInterruptIfRunning ? INTERRUPTING : CANCELLED)))
 return false;
 try { // in case call to interrupt throws exception
 if (mayInterruptIfRunning) {
 try {
 Thread t = runner;
 if (t != null)
 t.interrupt();
 } finally { // final state
 STATE.setRelease(this, INTERRUPTED);
 }
 }
 } finally {
 finishCompletion();
 }
 return true;
 }

 /**
 * @throws CancellationException {@inheritDoc}
 */
 public V get() throws InterruptedException, ExecutionException {
 int s = state;
 if (s <= COMPLETING)
 s = awaitDone(false, 0L);
 return report(s);
 }

 /**
 * @throws CancellationException {@inheritDoc}
 */
 public V get(long timeout, TimeUnit unit)
 throws InterruptedException, ExecutionException, TimeoutException {
 if (unit == null)
 throw new NullPointerException();
 int s = state;
 if (s <= COMPLETING &&
 (s = awaitDone(true, unit.toNanos(timeout))) <= COMPLETING)
 throw new TimeoutException();
 return report(s);
 }

 /**
 * Protected method invoked when this task transitions to state
 * {@code isDone} (whether normally or via cancellation). The
 * default implementation does nothing. Subclasses may override
 * this method to invoke completion callbacks or perform
 * bookkeeping. Note that you can query status inside the
 * implementation of this method to determine whether this task
 * has been cancelled.
 */
 protected void done() { }

 /**
 * Sets the result of this future to the given value unless
 * this future has already been set or has been cancelled.
 *
 * <p>This method is invoked internally by the {@link #run} method
 * upon successful completion of the computation.
 *
 * @param v the value
 */
 protected void set(V v) {
 if (STATE.compareAndSet(this, NEW, COMPLETING)) {
 outcome = v;
 STATE.setRelease(this, NORMAL); // final state
 finishCompletion();
 }
 }

 /**
 * Causes this future to report an {@link ExecutionException}
 * with the given throwable as its cause, unless this future has
 * already been set or has been cancelled.
 *
 * <p>This method is invoked internally by the {@link #run} method
 * upon failure of the computation.
 *
 * @param t the cause of failure
 */
 protected void setException(Throwable t) {
 if (STATE.compareAndSet(this, NEW, COMPLETING)) {
 outcome = t;
 STATE.setRelease(this, EXCEPTIONAL); // final state
 finishCompletion();
 }
 }

 public void run() {
 if (state != NEW ||
 !RUNNER.compareAndSet(this, null, Thread.currentThread()))
 return;
 try {
 Callable<V> c = callable;
 if (c != null && state == NEW) {
 V result;
 boolean ran;
 try {
 result = c.call();
 ran = true;
 } catch (Throwable ex) {
 result = null;
 ran = false;
 setException(ex);
 }
 if (ran)
 set(result);
 }
 } finally {
 // runner must be non-null until state is settled to
 // prevent concurrent calls to run()
 runner = null;
 // state must be re-read after nulling runner to prevent
 // leaked interrupts
 int s = state;
 if (s >= INTERRUPTING)
 handlePossibleCancellationInterrupt(s);
 }
 }

 /**
 * Executes the computation without setting its result, and then
 * resets this future to initial state, failing to do so if the
 * computation encounters an exception or is cancelled. This is
 * designed for use with tasks that intrinsically execute more
 * than once.
 *
 * @return {@code true} if successfully run and reset
 */
 protected boolean runAndReset() {
 if (state != NEW ||
 !RUNNER.compareAndSet(this, null, Thread.currentThread()))
 return false;
 boolean ran = false;
 int s = state;
 try {
 Callable<V> c = callable;
 if (c != null && s == NEW) {
 try {
 c.call(); // don't set result
 ran = true;
 } catch (Throwable ex) {
 setException(ex);
 }
 }
 } finally {
 // runner must be non-null until state is settled to
 // prevent concurrent calls to run()
 runner = null;
 // state must be re-read after nulling runner to prevent
 // leaked interrupts
 s = state;
 if (s >= INTERRUPTING)
 handlePossibleCancellationInterrupt(s);
 }
 return ran && s == NEW;
 }

 /**
 * Ensures that any interrupt from a possible cancel(true) is only
 * delivered to a task while in run or runAndReset.
 */
 private void handlePossibleCancellationInterrupt(int s) {
 // It is possible for our interrupter to stall before getting a
 // chance to interrupt us. Let's spin-wait patiently.
 if (s == INTERRUPTING)
 while (state == INTERRUPTING)
 Thread.yield(); // wait out pending interrupt

 // assert state == INTERRUPTED;

 // We want to clear any interrupt we may have received from
 // cancel(true). However, it is permissible to use interrupts
 // as an independent mechanism for a task to communicate with
 // its caller, and there is no way to clear only the
 // cancellation interrupt.
 //
 // Thread.interrupted();
 }

 /**
 * Simple linked list nodes to record waiting threads in a Treiber
 * stack. See other classes such as Phaser and SynchronousQueue
 * for more detailed explanation.
 */
 static final class WaitNode {
 volatile Thread thread;
 volatile WaitNode next;
 WaitNode() { thread = Thread.currentThread(); }
 }

 /**
 * Removes and signals all waiting threads, invokes done(), and
 * nulls out callable.
 */
 private void finishCompletion() {
 // assert state > COMPLETING;
 for (WaitNode q; (q = waiters) != null;) {
 if (WAITERS.weakCompareAndSet(this, q, null)) {
 for (;;) {
 Thread t = q.thread;
 if (t != null) {
 q.thread = null;
 LockSupport.unpark(t);
 }
 WaitNode next = q.next;
 if (next == null)
 break;
 q.next = null; // unlink to help gc
 q = next;
 }
 break;
 }
 }

 done();

 callable = null; // to reduce footprint
 }

 /**
 * Awaits completion or aborts on interrupt or timeout.
 *
 * @param timed true if use timed waits
 * @param nanos time to wait, if timed
 * @return state upon completion or at timeout
 */
 private int awaitDone(boolean timed, long nanos)
 throws InterruptedException {
 // The code below is very delicate, to achieve these goals:
 // - call nanoTime exactly once for each call to park
 // - if nanos <= 0L, return promptly without allocation or nanoTime
 // - if nanos == Long.MIN_VALUE, don't underflow
 // - if nanos == Long.MAX_VALUE, and nanoTime is non-monotonic
 // and we suffer a spurious wakeup, we will do no worse than
 // to park-spin for a while
 long startTime = 0L; // Special value 0L means not yet parked
 WaitNode q = null;
 boolean queued = false;
 for (;;) {
 int s = state;
 if (s > COMPLETING) {
 if (q != null)
 q.thread = null;
 return s;
 }
 else if (s == COMPLETING)
 // We may have already promised (via isDone) that we are done
 // so never return empty-handed or throw InterruptedException
 Thread.yield();
 else if (Thread.interrupted()) {
 removeWaiter(q);
 throw new InterruptedException();
 }
 else if (q == null) {
 if (timed && nanos <= 0L)
 return s;
 q = new WaitNode();
 }
 else if (!queued)
 queued = WAITERS.weakCompareAndSet(this, q.next = waiters, q);
 else if (timed) {
 final long parkNanos;
 if (startTime == 0L) { // first time
 startTime = System.nanoTime();
 if (startTime == 0L)
 startTime = 1L;
 parkNanos = nanos;
 } else {
 long elapsed = System.nanoTime() - startTime;
 if (elapsed >= nanos) {
 removeWaiter(q);
 return state;
 }
 parkNanos = nanos - elapsed;
 }
 // nanoTime may be slow; recheck before parking
 if (state < COMPLETING)
 LockSupport.parkNanos(this, parkNanos);
 }
 else
 LockSupport.park(this);
 }
 }

 /**
 * Tries to unlink a timed-out or interrupted wait node to avoid
 * accumulating garbage. Internal nodes are simply unspliced
 * without CAS since it is harmless if they are traversed anyway
 * by releasers. To avoid effects of unsplicing from already
 * removed nodes, the list is retraversed in case of an apparent
 * race. This is slow when there are a lot of nodes, but we don't
 * expect lists to be long enough to outweigh higher-overhead
 * schemes.
 */
 private void removeWaiter(WaitNode node) {
 if (node != null) {
 node.thread = null;
 retry:
 for (;;) { // restart on removeWaiter race
 for (WaitNode pred = null, q = waiters, s; q != null; q = s) {
 s = q.next;
 if (q.thread != null)
 pred = q;
 else if (pred != null) {
 pred.next = s;
 if (pred.thread == null) // check for race
 continue retry;
 }
 else if (!WAITERS.compareAndSet(this, q, s))
 continue retry;
 }
 break;
 }
 }
 }

 /**
 * Returns a string representation of this FutureTask.
 *
 * @implSpec
 * The default implementation returns a string identifying this
 * FutureTask, as well as its completion state. The state, in
 * brackets, contains one of the strings {@code "Completed Normally"},
 * {@code "Completed Exceptionally"}, {@code "Cancelled"}, or {@code
 * "Not completed"}.
 *
 * @return a string representation of this FutureTask
 */
 public String toString() {
 final String status;
 switch (state) {
 case NORMAL:
 status = "[Completed normally]";
 break;
 case EXCEPTIONAL:
 status = "[Completed exceptionally: " + outcome + "]";
 break;
 case CANCELLED:
 case INTERRUPTING:
 case INTERRUPTED:
 status = "[Cancelled]";
 break;
 default:
 final Callable<?> callable = this.callable;
 status = (callable == null)
 ? "[Not completed]"
 : "[Not completed, task = " + callable + "]";
 }
 return super.toString() + status;
 }

 // VarHandle mechanics
 private static final VarHandle STATE;
 private static final VarHandle RUNNER;
 private static final VarHandle WAITERS;
 static {
 try {
 MethodHandles.Lookup l = MethodHandles.lookup();
 STATE = l.findVarHandle(FutureTask.class, "state", int.class);
 RUNNER = l.findVarHandle(FutureTask.class, "runner", Thread.class);
 WAITERS = l.findVarHandle(FutureTask.class, "waiters", WaitNode.class);
 } catch (ReflectiveOperationException e) {
 throw new ExceptionInInitializerError(e);
 }

 // Reduce the risk of rare disastrous classloading in first call to
 // LockSupport.park: https://bugs.openjdk.java.net/browse/JDK-8074773
 Class<?> ensureLoaded = LockSupport.class;
 }

}