/*
 * Licensed to the Apache Software Foundation (ASF) under one or more
 * contributor license agreements.  See the NOTICE file distributed with
 * this work for additional information regarding copyright ownership.
 * The ASF licenses this file to You 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 org.apache.wicket.util.thread;

import java.time.Duration;
import java.time.Instant;

import org.slf4j.Logger;
import org.slf4j.LoggerFactory;


/**
 * Runs a block of code periodically. A <code>Task</code> can be started at a given time in the
 * future and can be a daemon. The block of code will be passed a <code>Log</code> object each time
 * it is run through its <code>ICode</code> interface.
 * <p>
 * If the code block takes longer than the period to run, the next task invocation will occur
 * immediately. In this case, tasks will not occur at precise multiples of the period. For example,
 * if you run a task every 30 seconds, and the first run takes 40 seconds but the second takes 20
 * seconds, your task will be invoked at 0 seconds, 40 seconds and 70 seconds (40 seconds + 30
 * seconds), which is not an even multiple of 30 seconds.
 * <p>
 * In general, this is a simple task class designed for polling activities. If you need precise
 * guarantees, you probably should be using a different task class.
 * 
 * @author Jonathan Locke
 * @since 1.2.6
 */
public final class Task
{
        /** <code>true</code> if the task's thread should be a daemon */
        private boolean isDaemon = true;

        /** <code>true</code> if the task's thread has already started executing */
        private boolean isStarted = false;

        /** the <code>log</code> to give to the user's code */
        private Logger log = null;

        /** the name of this <code>Task</code> */
        private final String name;

        /** the <code>Instant</code> at which the task should start */
        private Instant startTime = Instant.now();

        /** When set the task will stop as soon as possible */
        private boolean stop;

        /** each <code>Task</code> has an associated <code>Thread</code> */
        private Thread thread;

        /**
         * Constructor.
         * 
         * @param name
         *            the name of this <code>Task</code>
         */
        public Task(final String name)
        {
                this.name = name;
        }

        /**
         * Runs this <code>Task</code> at the given frequency. You may only call this method if the task
         * has not yet been started. If the task is already running, an
         * <code>IllegalStateException</code> will be thrown.
         * 
         * @param frequency
         *            the frequency at which to run the code
         * @param code
         *            the code to run
         * @throws IllegalStateException
         *             thrown if task is already running
         */
        public synchronized final void run(final Duration frequency, final ICode code)
        {
                if (!isStarted)
                {
                        final Runnable runnable = new Runnable()
                        {
                                @Override
                                public void run()
                                {
                                        // Sleep until start time
                                        Duration untilStart = Duration.between(startTime, Instant.now());

                                        final Logger log = getLog();

                                        if (!untilStart.isNegative())
                                        {
                                                try
                                                {
                                                        Thread.sleep(untilStart.toMillis());
                                                }
                                                catch (InterruptedException e)
                                                {
                                                        log.error("An error occurred during sleeping phase.", e);
                                                }
                                        }

                                        try
                                        {
                                                while (!stop)
                                                {
                                                        // Get the start of the current period
                                                        final Instant startOfPeriod = Instant.now();

                                                        if (log.isTraceEnabled())
                                                        {
                                                                log.trace("Run the job: '{}'", code);
                                                        }

                                                        try
                                                        {
                                                                // Run the user's code
                                                                code.run(getLog());
                                                        }
                                                        catch (Exception e)
                                                        {
                                                                log.error(
                                                                        "Unhandled exception thrown by user code in task " + name, e);
                                                        }

                                                        if (log.isTraceEnabled())
                                                        {
                                                                log.trace("Finished with job: '{}'", code);
                                                        }

                                                        // Sleep until the period is over (or not at all if it's
                                                        // already passed)
                                                        Instant nextExecution = startOfPeriod.plus(frequency);
                                                        
                                                        Duration timeToNextExecution = Duration.between(Instant.now(), nextExecution);
                                    
                                                        if (!timeToNextExecution.isNegative())
                                                        {
                                                                Thread.sleep(timeToNextExecution.toMillis());
                                                        }
                                                        
                                                }
                                        }
                                        catch (Exception x)
                                        {
                                                log.error("Task '{}' terminated", name, x);
                                        }
                                        finally
                                        {
                                                isStarted = false;
                                        }
                                }
                        };

                        // Start the thread
                        thread = new Thread(runnable, name + " Task");
                        thread.setDaemon(isDaemon);
                        thread.start();

                        // We're started all right!
                        isStarted = true;
                }
                else
                {
                        throw new IllegalStateException("Attempt to start task that has already been started");
                }
        }

        /**
         * Sets daemon or not. For obvious reasons, this value can only be set before the task starts
         * running. If you attempt to set this value after the task starts running, an
         * <code>IllegalStateException</code> will be thrown.
         * 
         * @param daemon
         *            <code>true</code> if this <code>Task</code>'s <code>Thread</code> should be a
         *            daemon
         * @throws IllegalStateException
         *             thrown if task is already running
         */
        public synchronized void setDaemon(final boolean daemon)
        {
                if (isStarted)
                {
                        throw new IllegalStateException(
                                "Attempt to set daemon state of a task that has already been started");
                }

                isDaemon = daemon;
        }

        /**
         * Sets log for user code to log to when task runs.
         * 
         * @param log
         *            the log
         */
        public synchronized void setLog(final Logger log)
        {
                this.log = log;
        }

        /**
         * Sets start time for this task. You cannot set the start time for a task which is already
         * running. If you attempt to, an IllegalStateException will be thrown.
         * 
         * @param startTime
         *            The time this task should start running
         * @throws IllegalStateException
         *             Thrown if task is already running
         */
        public synchronized void setStartTime(final Instant startTime)
        {
                if (isStarted)
                {
                        throw new IllegalStateException(
                                "Attempt to set start time of task that has already been started");
                }

                this.startTime = startTime;
        }

        /**
         * @see java.lang.Object#toString()
         */
        @Override
        public String toString()
        {
                return "[name=" + name + ", startTime=" + startTime + ", isDaemon=" + isDaemon +
                        ", isStarted=" + isStarted + ", codeListener=" + log + "]";
        }

        /**
         * Gets the log for this <code>Task</code>.
         * 
         * @return the log
         */
        protected synchronized Logger getLog()
        {
                if (log == null)
                {
                        log = LoggerFactory.getLogger(Task.class);
                }
                return log;
        }

        /**
         * Stops this <code>Task</code> as soon as it has the opportunity.
         */
        public void stop()
        {
                stop = true;
        }

        /**
         * Interrupts the <code>Task</code> as soon as it has the opportunity.
         */
        public void interrupt()
        {
                stop();
                if (thread != null)
                {
                        thread.interrupt();
                }
        }

        /**
         * Sets the priority of the thread
         * 
         * @param prio
         */
        public void setPriority(int prio)
        {
                if (prio < Thread.MIN_PRIORITY)
                {
                        prio = Thread.MIN_PRIORITY;
                }
                else if (prio > Thread.MAX_PRIORITY)
                {
                        prio = Thread.MAX_PRIORITY;
                }
                thread.setPriority(prio);
        }

        /**
         * Gets the thread priority
         * 
         * @return priority
         */
        public int getPriority()
        {
                return thread.getPriority();
        }
}