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/*
 * Original Guava code is copyright (C) 2015 The Guava Authors.
 * Modifications from Guava are copyright (C) 2016 DiffPlug.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 * http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */
package com.diffplug.common.util.concurrent;

import static java.util.concurrent.atomic.AtomicReferenceFieldUpdater.newUpdater;

import java.util.concurrent.atomic.AtomicReferenceFieldUpdater;

import com.diffplug.common.annotations.GwtCompatible;

@GwtCompatible(emulated = true)
abstract class InterruptibleTask implements Runnable {
	private static final AtomicReferenceFieldUpdater RUNNER = newUpdater(InterruptibleTask.class, Thread.class, "runner");

	// These two fields are used to interrupt running tasks.  The thread executing the task
	// publishes itself to the 'runner' field and the thread interrupting sets 'doneInterrupting'
	// when it has finished interrupting.
	private volatile Thread runner;
	private volatile boolean doneInterrupting;

	@Override
	public final void run() {
		if (!RUNNER.compareAndSet(this, null, Thread.currentThread())) {
			return; // someone else has run or is running.
		}
		try {
			runInterruptibly();
		} finally {
			if (wasInterrupted()) {
				// We were interrupted, it is possible that the interrupted bit hasn't been set yet.  Wait
				// for the interrupting thread to set 'doneInterrupting' to true. See interruptTask().
				// We want to wait so that we don't interrupt the _next_ thing run on the thread.
				// Note. We don't reset the interrupted bit, just wait for it to be set.
				// If this is a thread pool thread, the thread pool will reset it for us.  Otherwise, the
				// interrupted bit may have been intended for something else, so don't clear it.
				while (!doneInterrupting) {
					Thread.yield();
				}
			}
		}
	}

	abstract void runInterruptibly();

	abstract boolean wasInterrupted();

	final void interruptTask() {
		// interruptTask is guaranteed to be called at most once and if runner is non-null when that
		// happens then it must have been the first thread that entered run().  So there is no risk
		// that we are interrupting the wrong thread.
		Thread currentRunner = runner;
		if (currentRunner != null) {
			currentRunner.interrupt();
		}
		doneInterrupting = true;
	}
}