/*
 * 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.request.resource.caching.version;

import java.time.Instant;
import java.util.regex.Pattern;
import org.apache.wicket.request.resource.caching.IStaticCacheableResource;
import org.apache.wicket.util.resource.IResourceStream;

/**
 * Uses the last modified timestamp of a {@link org.apache.wicket.request.resource.caching.IStaticCacheableResource} 
 * converted to milliseconds as a version string.
 *
 * @author Peter Ertl
 *
 * @since 1.5
 */
public class LastModifiedResourceVersion implements IResourceVersion
{
	/**
	 * A valid pattern is a sequence of digits
	 */
	private static final Pattern TIMESTAMP_PATTERN = Pattern.compile("[0-9]+");

	@Override
	public String getVersion(IStaticCacheableResource resource)
	{
		// get last modified timestamp of resource
		IResourceStream stream = resource.getResourceStream();

		// if resource stream can not be found do not cache
		if (stream == null)
		{
			return null;
		}

		final Instant lastModified = stream.lastModifiedTime();

		// if no timestamp is available we can not provide a version
		if (lastModified == null)
		{
			return null;
		}
		// version string = last modified timestamp converted to milliseconds
		return String.valueOf(lastModified.toEpochMilli()).intern();
	}

	@Override
	public Pattern getVersionPattern()
	{
		return TIMESTAMP_PATTERN;
	}
}
