/*
    * 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.commons.io.monitor;
  
  import java.time.Duration;
  import java.util.ArrayList;
  import java.util.Collection;
  import java.util.Collections;
  import java.util.List;
  import java.util.Optional;
  import java.util.concurrent.CopyOnWriteArrayList;
  import java.util.concurrent.ThreadFactory;
  import java.util.stream.Stream;
  
  import org.apache.commons.io.ThreadUtils;
  
  /**
   * A runnable that spawns a monitoring thread triggering any
   * registered {@link FileAlterationObserver} at a specified interval.
   *
   * @see FileAlterationObserver
   * @since 2.0
   */
  public final class FileAlterationMonitor implements Runnable {
  
      private static final FileAlterationObserver[] EMPTY_ARRAY = {};
  
      private final long intervalMillis;
      private final List<FileAlterationObserver> observers = new CopyOnWriteArrayList<>();
      private Thread thread;
      private ThreadFactory threadFactory;
      private volatile boolean running;
  
      /**
       * Constructs a monitor with a default interval of 10 seconds.
       */
      public FileAlterationMonitor() {
          this(10_000);
      }
  
      /**
       * Constructs a monitor with the specified interval.
       *
       * @param intervalMillis The amount of time in milliseconds to wait between
       * checks of the file system.
       */
      public FileAlterationMonitor(final long intervalMillis) {
          this.intervalMillis = intervalMillis;
      }
  
      /**
       * Constructs a monitor with the specified interval and collection of observers.
       *
       * @param interval The amount of time in milliseconds to wait between
       * checks of the file system.
       * @param observers The collection of observers to add to the monitor.
       * @since 2.9.0
       */
      public FileAlterationMonitor(final long interval, final Collection<FileAlterationObserver> observers) {
          // @formatter:off
          this(interval,
              Optional
                  .ofNullable(observers)
                  .orElse(Collections.emptyList())
                  .toArray(EMPTY_ARRAY)
          );
          // @formatter:on
      }
  
      /**
       * Constructs a monitor with the specified interval and set of observers.
       *
       * @param interval The amount of time in milliseconds to wait between
       * checks of the file system.
       * @param observers The set of observers to add to the monitor.
       */
      public FileAlterationMonitor(final long interval, final FileAlterationObserver... observers) {
          this(interval);
          if (observers != null) {
              Stream.of(observers).forEach(this::addObserver);
          }
      }
  
      /**
       * Adds a file system observer to this monitor.
      *
      * @param observer The file system observer to add
      */
     public void addObserver(final FileAlterationObserver observer) {
         if (observer != null) {
             observers.add(observer);
         }
     }
 
     /**
      * Returns the interval.
      *
      * @return the interval
      */
     public long getInterval() {
         return intervalMillis;
     }
 
     /**
      * Returns the set of {@link FileAlterationObserver} registered with
      * this monitor.
      *
      * @return The set of {@link FileAlterationObserver}
      */
     public Iterable<FileAlterationObserver> getObservers() {
         return new ArrayList<>(observers);
     }
 
     /**
      * Removes a file system observer from this monitor.
      *
      * @param observer The file system observer to remove
      */
     public void removeObserver(final FileAlterationObserver observer) {
         if (observer != null) {
             observers.removeIf(observer::equals);
         }
     }
 
     /**
      * Runs this monitor.
      */
     @Override
     public void run() {
         while (running) {
             observers.forEach(FileAlterationObserver::checkAndNotify);
             if (!running) {
                 break;
             }
             try {
                 ThreadUtils.sleep(Duration.ofMillis(intervalMillis));
             } catch (final InterruptedException ignored) {
                 // ignore
             }
         }
     }
 
     /**
      * Sets the thread factory.
      *
      * @param threadFactory the thread factory
      */
     public synchronized void setThreadFactory(final ThreadFactory threadFactory) {
         this.threadFactory = threadFactory;
     }
 
     /**
      * Starts monitoring.
      *
      * @throws Exception if an error occurs initializing the observer
      */
     public synchronized void start() throws Exception {
         if (running) {
             throw new IllegalStateException("Monitor is already running");
         }
         for (final FileAlterationObserver observer : observers) {
             observer.initialize();
         }
         running = true;
         if (threadFactory != null) {
             thread = threadFactory.newThread(this);
         } else {
             thread = new Thread(this);
         }
         thread.start();
     }
 
     /**
      * Stops monitoring.
      *
      * @throws Exception if an error occurs initializing the observer
      */
     public synchronized void stop() throws Exception {
         stop(intervalMillis);
     }
 
     /**
      * Stops monitoring.
      *
      * @param stopInterval the amount of time in milliseconds to wait for the thread to finish.
      * A value of zero will wait until the thread is finished (see {@link Thread#join(long)}).
      * @throws Exception if an error occurs initializing the observer
      * @since 2.1
      */
     public synchronized void stop(final long stopInterval) throws Exception {
         if (!running) {
             throw new IllegalStateException("Monitor is not running");
         }
         running = false;
         try {
             thread.interrupt();
             thread.join(stopInterval);
         } catch (final InterruptedException e) {
             Thread.currentThread().interrupt();
         }
         for (final FileAlterationObserver observer : observers) {
             observer.destroy();
         }
     }
 }