/*
 * Copyright 2002-2018 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.IOException;
import java.io.StringWriter;
import java.nio.charset.Charset;
import java.nio.charset.StandardCharsets;
import java.util.ArrayList;
import java.util.List;
import java.util.SortedSet;
import java.util.TreeSet;
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.util.FileCopyUtils;
import org.springframework.util.StringUtils;

A ResourceTransformer implementation that modifies links in a CSS file to match the public URL paths that should be exposed to clients (e.g. with an MD5 content-based hash inserted in the URL). 
The implementation looks for links in CSS @import statements and also inside CSS url() functions. All links are then passed through the ResourceResolverChain and resolved relative to the location of the containing CSS file. If successfully resolved, the link is modified, otherwise the original link is preserved. 
Author:
Rossen Stoyanchev
Since:
4.1/**
 * A {@link ResourceTransformer} implementation that modifies links in a CSS
 * file to match the public URL paths that should be exposed to clients (e.g.
 * with an MD5 content-based hash inserted in the URL).
 *
 * <p>The implementation looks for links in CSS {@code @import} statements and
 * also inside CSS {@code url()} functions. All links are then passed through the
 * {@link ResourceResolverChain} and resolved relative to the location of the
 * containing CSS file. If successfully resolved, the link is modified, otherwise
 * the original link is preserved.
 *
 * @author Rossen Stoyanchev
 * @since 4.1
 */
public class CssLinkResourceTransformer extends ResourceTransformerSupport {

	private static final Charset DEFAULT_CHARSET = StandardCharsets.UTF_8;

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

	private final List<LinkParser> linkParsers = new ArrayList<>(2);


	public CssLinkResourceTransformer() {
		this.linkParsers.add(new ImportStatementLinkParser());
		this.linkParsers.add(new UrlFunctionLinkParser());
	}


	@SuppressWarnings("deprecation")
	@Override
	public Resource transform(HttpServletRequest request, Resource resource, ResourceTransformerChain transformerChain)
			throws IOException {

		resource = transformerChain.transform(request, resource);

		String filename = resource.getFilename();
		if (!"css".equals(StringUtils.getFilenameExtension(filename)) ||
				resource instanceof EncodedResourceResolver.EncodedResource ||
				resource instanceof GzipResourceResolver.GzippedResource) {
			return resource;
		}

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

		SortedSet<ContentChunkInfo> links = new TreeSet<>();
		for (LinkParser parser : this.linkParsers) {
			parser.parse(content, links);
		}

		if (links.isEmpty()) {
			return resource;
		}

		int index = 0;
		StringWriter writer = new StringWriter();
		for (ContentChunkInfo linkContentChunkInfo : links) {
			writer.write(content.substring(index, linkContentChunkInfo.getStart()));
			String link = content.substring(linkContentChunkInfo.getStart(), linkContentChunkInfo.getEnd());
			String newLink = null;
			if (!hasScheme(link)) {
				String absolutePath = toAbsolutePath(link, request);
				newLink = resolveUrlPath(absolutePath, request, resource, transformerChain);
			}
			writer.write(newLink != null ? newLink : link);
			index = linkContentChunkInfo.getEnd();
		}
		writer.write(content.substring(index));

		return new TransformedResource(resource, writer.toString().getBytes(DEFAULT_CHARSET));
	}

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


	
Extract content chunks that represent links.
/**
	 * Extract content chunks that represent links.
	 */
	@FunctionalInterface
	protected interface LinkParser {

		void parse(String content, SortedSet<ContentChunkInfo> result);

	}


	
Abstract base class for LinkParser implementations. /**
	 * Abstract base class for {@link LinkParser} implementations.
	 */
	protected abstract static class AbstractLinkParser implements LinkParser {

		
Return the keyword to use to search for links, e.g. "@import", "url(" /** Return the keyword to use to search for links, e.g. "@import", "url(" */
		protected abstract String getKeyword();

		@Override
		public void parse(String content, SortedSet<ContentChunkInfo> result) {
			int position = 0;
			while (true) {
				position = content.indexOf(getKeyword(), position);
				if (position == -1) {
					return;
				}
				position += getKeyword().length();
				while (Character.isWhitespace(content.charAt(position))) {
					position++;
				}
				if (content.charAt(position) == '\'') {
					position = extractLink(position, "'", content, result);
				}
				else if (content.charAt(position) == '"') {
					position = extractLink(position, "\"", content, result);
				}
				else {
					position = extractLink(position, content, result);

				}
			}
		}

		protected int extractLink(int index, String endKey, String content, SortedSet<ContentChunkInfo> linksToAdd) {
			int start = index + 1;
			int end = content.indexOf(endKey, start);
			linksToAdd.add(new ContentChunkInfo(start, end));
			return end + endKey.length();
		}

		
Invoked after a keyword match, after whitespaces removed, and when
the next char is neither a single nor double quote.
/**
		 * Invoked after a keyword match, after whitespaces removed, and when
		 * the next char is neither a single nor double quote.
		 */
		protected abstract int extractLink(int index, String content, SortedSet<ContentChunkInfo> linksToAdd);
	}


	private static class ImportStatementLinkParser extends AbstractLinkParser {

		@Override
		protected String getKeyword() {
			return "@import";
		}

		@Override
		protected int extractLink(int index, String content, SortedSet<ContentChunkInfo> linksToAdd) {
			if (content.substring(index, index + 4).equals("url(")) {
				// Ignore, UrlLinkParser will take care
			}
			else if (logger.isTraceEnabled()) {
				logger.trace("Unexpected syntax for @import link at index " + index);
			}
			return index;
		}
	}


	private static class UrlFunctionLinkParser extends AbstractLinkParser {

		@Override
		protected String getKeyword() {
			return "url(";
		}

		@Override
		protected int extractLink(int index, String content, SortedSet<ContentChunkInfo> linksToAdd) {
			// A url() function without unquoted
			return extractLink(index - 1, ")", content, linksToAdd);
		}
	}


	private static class ContentChunkInfo implements Comparable<ContentChunkInfo> {

		private final int start;

		private final int end;

		ContentChunkInfo(int start, int end) {
			this.start = start;
			this.end = end;
		}

		public int getStart() {
			return this.start;
		}

		public int getEnd() {
			return this.end;
		}

		@Override
		public int compareTo(ContentChunkInfo other) {
			return Integer.compare(this.start, other.start);
		}

		@Override
		public boolean equals(Object other) {
			if (this == other) {
				return true;
			}
			if (!(other instanceof ContentChunkInfo)) {
				return false;
			}
			ContentChunkInfo otherCci = (ContentChunkInfo) other;
			return (this.start == otherCci.start && this.end == otherCci.end);
		}

		@Override
		public int hashCode() {
			return this.start * 31 + this.end;
		}
	}

}