/*
 * Copyright 2002-2017 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.springframework.web.servlet.resource;

import java.io.ByteArrayOutputStream;
import java.io.IOException;
import java.io.StringWriter;
import java.nio.charset.Charset;
import java.nio.charset.StandardCharsets;
import java.util.Arrays;
import java.util.Collection;
import java.util.Collections;
import java.util.Scanner;
import javax.servlet.http.HttpServletRequest;

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

import org.springframework.core.io.Resource;
import org.springframework.lang.Nullable;
import org.springframework.util.DigestUtils;
import org.springframework.util.FileCopyUtils;
import org.springframework.util.StringUtils;

A ResourceTransformer implementation that helps handling resources within HTML5 AppCache manifests for HTML5 offline applications. 
This transformer:

modifies links to match the public URL paths that should be exposed to clients, using configured ResourceResolver strategies 
appends a comment in the manifest, containing a Hash (e.g. "# Hash: 9de0f09ed7caf84e885f1f0f11c7e326"),
thus changing the content of the manifest in order to trigger an appcache reload in the browser.

All files that have the ".appcache" file extension, or the extension given in the constructor,
will be transformed by this class. This hash is computed using the content of the appcache manifest
and the content of the linked resources; so changing a resource linked in the manifest
or the manifest itself should invalidate the browser cache.
In order to serve manifest files with the proper "text/manifest" content type, it is required to configure it with contentNegotiationConfigurer.mediaType("appcache", MediaType.valueOf("text/manifest") in a WebMvcConfigurer. 
Author:
Brian Clozel
See Also:
HTML5 offline applications spec
Since:
4.1/**
 * A {@link ResourceTransformer} implementation that helps handling resources
 * within HTML5 AppCache manifests for HTML5 offline applications.
 *
 * <p>This transformer:
 * <ul>
 * <li>modifies links to match the public URL paths that should be exposed to clients,
 * using configured {@code ResourceResolver} strategies
 * <li>appends a comment in the manifest, containing a Hash (e.g. "# Hash: 9de0f09ed7caf84e885f1f0f11c7e326"),
 * thus changing the content of the manifest in order to trigger an appcache reload in the browser.
 * </ul>
 *
 * <p>All files that have the ".appcache" file extension, or the extension given in the constructor,
 * will be transformed by this class. This hash is computed using the content of the appcache manifest
 * and the content of the linked resources; so changing a resource linked in the manifest
 * or the manifest itself should invalidate the browser cache.
 *
 * <p>In order to serve manifest files with the proper {@code "text/manifest"} content type,
 * it is required to configure it with
 * {@code contentNegotiationConfigurer.mediaType("appcache", MediaType.valueOf("text/manifest")}
 * in a {@code WebMvcConfigurer}.
 *
 * @author Brian Clozel
 * @since 4.1
 * @see <a href="https://html.spec.whatwg.org/multipage/browsers.html#offline">HTML5 offline applications spec</a>
 */
public class AppCacheManifestTransformer extends ResourceTransformerSupport {

	private static final Collection<String> MANIFEST_SECTION_HEADERS =
			Arrays.asList("CACHE MANIFEST", "NETWORK:", "FALLBACK:", "CACHE:");

	private static final String MANIFEST_HEADER = "CACHE MANIFEST";

	private static final String CACHE_HEADER = "CACHE:";

	private static final Charset DEFAULT_CHARSET = StandardCharsets.UTF_8;

	private static final Log logger = LogFactory.getLog(AppCacheManifestTransformer.class);


	private final String fileExtension;


	
Create an AppCacheResourceTransformer that transforms files with extension ".appcache".
/**
	 * Create an AppCacheResourceTransformer that transforms files with extension ".appcache".
	 */
	public AppCacheManifestTransformer() {
		this("appcache");
	}

	
Create an AppCacheResourceTransformer that transforms files with the extension
given as a parameter.
/**
	 * Create an AppCacheResourceTransformer that transforms files with the extension
	 * given as a parameter.
	 */
	public AppCacheManifestTransformer(String fileExtension) {
		this.fileExtension = fileExtension;
	}


	@Override
	public Resource transform(HttpServletRequest request, Resource resource,
			ResourceTransformerChain chain) throws IOException {

		resource = chain.transform(request, resource);
		if (!this.fileExtension.equals(StringUtils.getFilenameExtension(resource.getFilename()))) {
			return resource;
		}

		byte[] bytes = FileCopyUtils.copyToByteArray(resource.getInputStream());
		String content = new String(bytes, DEFAULT_CHARSET);

		if (!content.startsWith(MANIFEST_HEADER)) {
			if (logger.isTraceEnabled()) {
				logger.trace("Skipping " + resource + ": Manifest does not start with 'CACHE MANIFEST'");
			}
			return resource;
		}

		@SuppressWarnings("resource")
		Scanner scanner = new Scanner(content);
		LineInfo previous = null;
		LineAggregator aggregator = new LineAggregator(resource, content);

		while (scanner.hasNext()) {
			String line = scanner.nextLine();
			LineInfo current = new LineInfo(line, previous);
			LineOutput lineOutput = processLine(current, request, resource, chain);
			aggregator.add(lineOutput);
			previous = current;
		}

		return aggregator.createResource();
	}

	private static byte[] getResourceBytes(Resource resource) throws IOException {
		return FileCopyUtils.copyToByteArray(resource.getInputStream());
	}

	private LineOutput processLine(LineInfo info, HttpServletRequest request,
			Resource resource, ResourceTransformerChain transformerChain) {

		if (!info.isLink()) {
			return new LineOutput(info.getLine(), null);
		}

		Resource appCacheResource = transformerChain.getResolverChain()
				.resolveResource(null, info.getLine(), Collections.singletonList(resource));

		String path = info.getLine();
		String absolutePath = toAbsolutePath(path, request);
		String newPath = resolveUrlPath(absolutePath, request, resource, transformerChain);

		return new LineOutput((newPath != null ? newPath : path), appCacheResource);
	}


	private static class LineInfo {

		private final String line;

		private final boolean cacheSection;

		private final boolean link;

		public LineInfo(String line, @Nullable LineInfo previous) {
			this.line = line;
			this.cacheSection = initCacheSectionFlag(line, previous);
			this.link = iniLinkFlag(line, this.cacheSection);
		}

		private static boolean initCacheSectionFlag(String line, @Nullable LineInfo previousLine) {
			if (MANIFEST_SECTION_HEADERS.contains(line.trim())) {
				return line.trim().equals(CACHE_HEADER);
			}
			else if (previousLine != null) {
				return previousLine.isCacheSection();
			}
			throw new IllegalStateException(
					"Manifest does not start with " + MANIFEST_HEADER + ": " + line);
		}

		private static boolean iniLinkFlag(String line, boolean isCacheSection) {
			return (isCacheSection && StringUtils.hasText(line) && !line.startsWith("#")
					&& !line.startsWith("//") && !hasScheme(line));
		}

		private static boolean hasScheme(String line) {
			int index = line.indexOf(':');
			return (line.startsWith("//") || (index > 0 && !line.substring(0, index).contains("/")));
		}

		public String getLine() {
			return this.line;
		}

		public boolean isCacheSection() {
			return this.cacheSection;
		}

		public boolean isLink() {
			return this.link;
		}
	}


	private static class LineOutput {

		private final String line;

		@Nullable
		private final Resource resource;

		public LineOutput(String line, @Nullable Resource resource) {
			this.line = line;
			this.resource = resource;
		}

		public String getLine() {
			return this.line;
		}

		@Nullable
		public Resource getResource() {
			return this.resource;
		}
	}


	private static class LineAggregator {

		private final StringWriter writer = new StringWriter();

		private final ByteArrayOutputStream baos;

		private final Resource resource;

		public LineAggregator(Resource resource, String content) {
			this.resource = resource;
			this.baos = new ByteArrayOutputStream(content.length());
		}

		public void add(LineOutput lineOutput) throws IOException {
			this.writer.write(lineOutput.getLine() + "\n");
			byte[] bytes = (lineOutput.getResource() != null ?
					DigestUtils.md5Digest(getResourceBytes(lineOutput.getResource())) :
					lineOutput.getLine().getBytes(DEFAULT_CHARSET));
			this.baos.write(bytes);
		}

		public TransformedResource createResource() {
			String hash = DigestUtils.md5DigestAsHex(this.baos.toByteArray());
			this.writer.write("\n" + "# Hash: " + hash);
			byte[] bytes = this.writer.toString().getBytes(DEFAULT_CHARSET);
			return new TransformedResource(this.resource, bytes);
		}
	}

}