/*
 * Copyright 2006 the original author or authors.
 * 
 * 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 org.wamblee.io;

import java.io.File;
import java.io.FileFilter;
import java.util.ArrayList;
import java.util.Date;
import java.util.HashMap;
import java.util.HashSet;
import java.util.List;
import java.util.Map;
import java.util.Set;

import org.apache.commons.logging.Log;
import org.apache.commons.logging.LogFactory;

/**
 * Monitors a directory for changes.
 * 
 * @author Erik Brakkee
 */
public class DirectoryMonitor {

	private static final Log LOG = LogFactory.getLog(DirectoryMonitor.class);

	public static interface Listener {
		
		void fileChanged(File aFile);

		void fileCreated(File aFile);

		void fileDeleted(File aFile);
	};

	private File _directory;
	private FileFilter _filter;
	private Listener _listener;
	private Map<File, Date> _contents;

	public DirectoryMonitor(File aDirectory, FileFilter aFilefilter,
			Listener aListener) {
		_directory = aDirectory;
		if (!_directory.isDirectory()) {
			throw new IllegalArgumentException("Directory '" + _directory
					+ "' does not exist");
		}
		_filter = aFilefilter;
		_listener = aListener;
		_contents = new HashMap<File, Date>();
	}

	/**
	 * Polls the directory for changes and notifies the listener of any changes. 
	 * In case of any exceptions thrown by the listener while handling the changes, 
	 * the next call to this method will invoked the listeners again for the same changes. 
	 */
	public void poll() {
		LOG.debug("Polling " + _directory);
		Map<File, Date> newContents = new HashMap<File, Date>();
		File[] files = _directory.listFiles(_filter);

		// Check deleted files. 
		Set<File> deletedFiles = new HashSet<File>(_contents.keySet());
		for (File file : files) {
			if (file.isFile()) {
				if (_contents.containsKey(file)) {
					deletedFiles.remove(file);
				}
			}
		}
		for (File file : deletedFiles) {
			_listener.fileDeleted(file);
		}
		
		for (File file : files) {
			if (file.isFile()) {
				if (_contents.containsKey(file)) {
					Date oldDate = _contents.get(file);
					if (file.lastModified() != oldDate.getTime()) {
						_listener.fileChanged(file);
					} else {
						// No change.
					}
					newContents.put(file, new Date(file.lastModified()));
				} else {
					_listener.fileCreated(file);
					newContents.put(file, new Date(file.lastModified()));
				}
			}
		}
		
		_contents = newContents;
	}

}