/**
 * Copyright 2014 Netflix, Inc.
 *
 * 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 rx.schedulers;

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

import rx.*;
import rx.functions.Action0;
import rx.internal.schedulers.*;
import rx.plugins.RxJavaPlugins;
import rx.subscriptions.*;

/**
 * Scheduler that wraps an Executor instance and establishes the Scheduler contract upon it.
 * <p>
 * Note that thread-hopping is unavoidable with this kind of Scheduler as we don't know about the underlying
 * threading behavior of the executor.
 */
/* public */final class ExecutorScheduler extends Scheduler {
    final Executor executor;
    public ExecutorScheduler(Executor executor) {
        this.executor = executor;
    }

    /**
     * @warn javadoc missing
     * @return
     */
    @Override
    public Worker createWorker() {
        return new ExecutorSchedulerWorker(executor);
    }

    /** Worker that schedules tasks on the executor indirectly through a trampoline mechanism. */
    static final class ExecutorSchedulerWorker extends Scheduler.Worker implements Runnable {
        final Executor executor;
        // TODO: use a better performing structure for task tracking
        final CompositeSubscription tasks;
        // TODO: use MpscLinkedQueue once available
        final ConcurrentLinkedQueue<ScheduledAction> queue; 
        final AtomicInteger wip;
        
        public ExecutorSchedulerWorker(Executor executor) {
            this.executor = executor;
            this.queue = new ConcurrentLinkedQueue<ScheduledAction>();
            this.wip = new AtomicInteger();
            this.tasks = new CompositeSubscription();
        }

        @Override
        public Subscription schedule(Action0 action) {
            if (isUnsubscribed()) {
                return Subscriptions.unsubscribed();
            }
            ScheduledAction ea = new ScheduledAction(action, tasks);
            tasks.add(ea);
            queue.offer(ea);
            if (wip.getAndIncrement() == 0) {
                try {
                    // note that since we schedule the emission of potentially multiple tasks
                    // there is no clear way to cancel this schedule from individual tasks
                    // so even if executor is an ExecutorService, we can't associate the future
                    // returned by submit() with any particular ScheduledAction
                    executor.execute(this);
                } catch (RejectedExecutionException t) {
                    // cleanup if rejected
                    tasks.remove(ea);
                    wip.decrementAndGet();
                    // report the error to the plugin
                    RxJavaPlugins.getInstance().getErrorHandler().handleError(t);
                    // throw it to the caller
                    throw t;
                }
            }
            
            return ea;
        }

        @Override
        public void run() {
            do {
                ScheduledAction sa = queue.poll();
                if (!sa.isUnsubscribed()) {
                    sa.run();
                }
            } while (wip.decrementAndGet() > 0);
        }
        
        @Override
        public Subscription schedule(final Action0 action, long delayTime, TimeUnit unit) {
            if (delayTime <= 0) {
                return schedule(action);
            }
            if (isUnsubscribed()) {
                return Subscriptions.unsubscribed();
            }
            ScheduledExecutorService service;
            if (executor instanceof ScheduledExecutorService) {
                service = (ScheduledExecutorService)executor;
            } else {
                service = GenericScheduledExecutorService.getInstance();
            }
            
            final MultipleAssignmentSubscription first = new MultipleAssignmentSubscription();
            final MultipleAssignmentSubscription mas = new MultipleAssignmentSubscription();
            mas.set(first);
            tasks.add(mas);
            final Subscription removeMas = Subscriptions.create(new Action0() {
                @Override
                public void call() {
                    tasks.remove(mas);
                }
            });
            
            ScheduledAction ea = new ScheduledAction(new Action0() {
                @Override
                public void call() {
                    if (mas.isUnsubscribed()) {
                        return;
                    }
                    // schedule the real action untimed
                    Subscription s2 = schedule(action);
                    mas.set(s2);
                    // unless the worker is unsubscribed, we should get a new ScheduledAction
                    if (s2.getClass() == ScheduledAction.class) {
                        // when this ScheduledAction completes, we need to remove the
                        // MAS referencing the whole setup to avoid leaks
                        ((ScheduledAction)s2).add(removeMas);
                    }
                }
            });
            // This will make sure if ea.call() gets executed before this line
            // we don't override the current task in mas.
            first.set(ea);
            // we don't need to add ea to tasks because it will be tracked through mas/first
            
            
            try {
                Future<?> f = service.schedule(ea, delayTime, unit);
                ea.add(f);
            } catch (RejectedExecutionException t) {
                // report the rejection to plugins
                RxJavaPlugins.getInstance().getErrorHandler().handleError(t);
                throw t;
            }
            
            /*
             * This allows cancelling either the delayed schedule or the actual schedule referenced
             * by mas and makes sure mas is removed from the tasks composite to avoid leaks.
             */
            return removeMas;
        }

        @Override
        public boolean isUnsubscribed() {
            return tasks.isUnsubscribed();
        }

        @Override
        public void unsubscribe() {
            tasks.unsubscribe();
        }
        
    }
}
