/*
 * Copyright 2002-2019 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
 *
 *      https://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.File;
import java.io.IOException;
import java.io.InputStream;
import java.net.URI;
import java.net.URL;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.Collections;
import java.util.LinkedHashMap;
import java.util.List;
import java.util.Map;

import javax.servlet.http.HttpServletRequest;

import org.springframework.core.io.AbstractResource;
import org.springframework.core.io.Resource;
import org.springframework.http.HttpHeaders;
import org.springframework.lang.Nullable;
import org.springframework.util.Assert;

Resolver that delegates to the chain, and if a resource is found, it then
attempts to find an encoded (e.g. gzip, brotli) variant that is acceptable
based on the "Accept-Encoding" request header.
The list of supported contentCodings can be configured, in order of preference, and each coding must be associated with extensions. 
Note that this resolver must be ordered ahead of a VersionResourceResolver with a content-based, version strategy to ensure the version calculation is not impacted by the encoding. 
Author:
Rossen Stoyanchev
Since:
5.1/**
 * Resolver that delegates to the chain, and if a resource is found, it then
 * attempts to find an encoded (e.g. gzip, brotli) variant that is acceptable
 * based on the "Accept-Encoding" request header.
 *
 * <p>The list of supported {@link #setContentCodings(List) contentCodings} can
 * be configured, in order of preference, and each coding must be associated
 * with {@link #setExtensions(Map) extensions}.
 *
 * <p>Note that this resolver must be ordered ahead of a
 * {@link VersionResourceResolver} with a content-based, version strategy to
 * ensure the version calculation is not impacted by the encoding.
 *
 * @author Rossen Stoyanchev
 * @since 5.1
 */
public class EncodedResourceResolver extends AbstractResourceResolver {

	
The default content codings.
/**
	 * The default content codings.
	 */
	public static final List<String> DEFAULT_CODINGS = Arrays.asList("br", "gzip");


	private final List<String> contentCodings = new ArrayList<>(DEFAULT_CODINGS);

	private final Map<String, String> extensions = new LinkedHashMap<>();


	public EncodedResourceResolver() {
		this.extensions.put("gzip", ".gz");
		this.extensions.put("br", ".br");
	}


	
Configure the supported content codings in order of preference. The first coding that is present in the "Accept-Encoding" header for a given request, and that has a file present with the associated extension, is used. 
Note: Each coding must be associated with a file extension via registerExtension or setExtensions. Also customizations to the list of codings here should be matched by customizations to the same list in CachingResourceResolver to ensure encoded variants of a resource are cached under separate keys. 
By default this property is set to ["br", "gzip"]. 
Params:
codings – one or more supported content codings/**
	 * Configure the supported content codings in order of preference. The first
	 * coding that is present in the {@literal "Accept-Encoding"} header for a
	 * given request, and that has a file present with the associated extension,
	 * is used.
	 * <p><strong>Note:</strong> Each coding must be associated with a file
	 * extension via {@link #registerExtension} or {@link #setExtensions}. Also
	 * customizations to the list of codings here should be matched by
	 * customizations to the same list in {@link CachingResourceResolver} to
	 * ensure encoded variants of a resource are cached under separate keys.
	 * <p>By default this property is set to {@literal ["br", "gzip"]}.
	 * @param codings one or more supported content codings
	 */
	public void setContentCodings(List<String> codings) {
		Assert.notEmpty(codings, "At least one content coding expected");
		this.contentCodings.clear();
		this.contentCodings.addAll(codings);
	}

	
Return a read-only list with the supported content codings.
/**
	 * Return a read-only list with the supported content codings.
	 */
	public List<String> getContentCodings() {
		return Collections.unmodifiableList(this.contentCodings);
	}

	
Configure mappings from content codings to file extensions. A dot "."
will be prepended in front of the extension value if not present.
By default this is configured with ["br" -> ".br"] and ["gzip" -> ".gz"]. 
Params:
extensions – the extensions to use.
See Also:
registerExtension(String, String)/**
	 * Configure mappings from content codings to file extensions. A dot "."
	 * will be prepended in front of the extension value if not present.
	 * <p>By default this is configured with {@literal ["br" -> ".br"]} and
	 * {@literal ["gzip" -> ".gz"]}.
	 * @param extensions the extensions to use.
	 * @see #registerExtension(String, String)
	 */
	public void setExtensions(Map<String, String> extensions) {
		extensions.forEach(this::registerExtension);
	}

	
Return a read-only map with coding-to-extension mappings.
/**
	 * Return a read-only map with coding-to-extension mappings.
	 */
	public Map<String, String> getExtensions() {
		return Collections.unmodifiableMap(this.extensions);
	}

	
Java config friendly alternative to setExtensions(Map<String,String>). 
Params:
coding – the content coding
extension – the associated file extension/**
	 * Java config friendly alternative to {@link #setExtensions(Map)}.
	 * @param coding the content coding
	 * @param extension the associated file extension
	 */
	public void registerExtension(String coding, String extension) {
		this.extensions.put(coding, (extension.startsWith(".") ? extension : "." + extension));
	}


	@Override
	protected Resource resolveResourceInternal(@Nullable HttpServletRequest request, String requestPath,
			List<? extends Resource> locations, ResourceResolverChain chain) {

		Resource resource = chain.resolveResource(request, requestPath, locations);
		if (resource == null || request == null) {
			return resource;
		}

		String acceptEncoding = getAcceptEncoding(request);
		if (acceptEncoding == null) {
			return resource;
		}

		for (String coding : this.contentCodings) {
			if (acceptEncoding.contains(coding)) {
				try {
					String extension = getExtension(coding);
					Resource encoded = new EncodedResource(resource, coding, extension);
					if (encoded.exists()) {
						return encoded;
					}
				}
				catch (IOException ex) {
					if (logger.isTraceEnabled()) {
						logger.trace("No " + coding + " resource for [" + resource.getFilename() + "]", ex);
					}
				}
			}
		}

		return resource;
	}

	@Nullable
	private String getAcceptEncoding(HttpServletRequest request) {
		String header = request.getHeader(HttpHeaders.ACCEPT_ENCODING);
		return (header != null ? header.toLowerCase() : null);
	}

	private String getExtension(String coding) {
		String extension = this.extensions.get(coding);
		if (extension == null) {
			throw new IllegalStateException("No file extension associated with content coding " + coding);
		}
		return extension;
	}

	@Override
	protected String resolveUrlPathInternal(String resourceUrlPath,
			List<? extends Resource> locations, ResourceResolverChain chain) {

		return chain.resolveUrlPath(resourceUrlPath, locations);
	}


	
An encoded HttpResource. /**
	 * An encoded {@link HttpResource}.
	 */
	static final class EncodedResource extends AbstractResource implements HttpResource {

		private final Resource original;

		private final String coding;

		private final Resource encoded;

		EncodedResource(Resource original, String coding, String extension) throws IOException {
			this.original = original;
			this.coding = coding;
			this.encoded = original.createRelative(original.getFilename() + extension);
		}


		@Override
		public InputStream getInputStream() throws IOException {
			return this.encoded.getInputStream();
		}

		@Override
		public boolean exists() {
			return this.encoded.exists();
		}

		@Override
		public boolean isReadable() {
			return this.encoded.isReadable();
		}

		@Override
		public boolean isOpen() {
			return this.encoded.isOpen();
		}

		@Override
		public boolean isFile() {
			return this.encoded.isFile();
		}

		@Override
		public URL getURL() throws IOException {
			return this.encoded.getURL();
		}

		@Override
		public URI getURI() throws IOException {
			return this.encoded.getURI();
		}

		@Override
		public File getFile() throws IOException {
			return this.encoded.getFile();
		}

		@Override
		public long contentLength() throws IOException {
			return this.encoded.contentLength();
		}

		@Override
		public long lastModified() throws IOException {
			return this.encoded.lastModified();
		}

		@Override
		public Resource createRelative(String relativePath) throws IOException {
			return this.encoded.createRelative(relativePath);
		}

		@Override
		@Nullable
		public String getFilename() {
			return this.original.getFilename();
		}

		@Override
		public String getDescription() {
			return this.encoded.getDescription();
		}

		@Override
		public HttpHeaders getResponseHeaders() {
			HttpHeaders headers;
			if (this.original instanceof HttpResource) {
				headers = ((HttpResource) this.original).getResponseHeaders();
			}
			else {
				headers = new HttpHeaders();
			}
			headers.add(HttpHeaders.CONTENT_ENCODING, this.coding);
			headers.add(HttpHeaders.VARY, HttpHeaders.ACCEPT_ENCODING);
			return headers;
		}
	}

}