/*
    * 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.maven.doxia.module.markdown;
  
  import javax.inject.Inject;
  import javax.inject.Named;
  import javax.inject.Singleton;
  
  import java.io.IOException;
  import java.io.Reader;
  import java.util.Arrays;
  import java.util.Collections;
  import java.util.LinkedHashMap;
  import java.util.List;
  import java.util.Map;
  import java.util.Map.Entry;
  import java.util.regex.Matcher;
  import java.util.regex.Pattern;
  import java.util.stream.Collectors;
  
  import com.vladsch.flexmark.ast.Heading;
  import com.vladsch.flexmark.ast.HtmlCommentBlock;
  import com.vladsch.flexmark.ast.util.TextCollectingVisitor;
  import com.vladsch.flexmark.ext.abbreviation.AbbreviationExtension;
  import com.vladsch.flexmark.ext.autolink.AutolinkExtension;
  import com.vladsch.flexmark.ext.definition.DefinitionExtension;
  import com.vladsch.flexmark.ext.escaped.character.EscapedCharacterExtension;
  import com.vladsch.flexmark.ext.gfm.strikethrough.StrikethroughExtension;
  import com.vladsch.flexmark.ext.tables.TablesExtension;
  import com.vladsch.flexmark.ext.typographic.TypographicExtension;
  import com.vladsch.flexmark.ext.wikilink.WikiLinkExtension;
  import com.vladsch.flexmark.ext.yaml.front.matter.YamlFrontMatterExtension;
  import com.vladsch.flexmark.html.HtmlRenderer;
  import com.vladsch.flexmark.util.ast.Node;
  import com.vladsch.flexmark.util.data.MutableDataSet;
  import org.apache.commons.io.IOUtils;
  import org.apache.maven.doxia.markup.HtmlMarkup;
  import org.apache.maven.doxia.markup.TextMarkup;
  import org.apache.maven.doxia.module.xhtml5.Xhtml5Parser;
  import org.apache.maven.doxia.parser.AbstractTextParser;
  import org.apache.maven.doxia.parser.ParseException;
  import org.apache.maven.doxia.sink.Sink;
  import org.apache.maven.doxia.util.HtmlTools;
  import org.codehaus.plexus.util.xml.pull.XmlPullParser;
  
  /**
   * <p>
   * Implementation of {@link org.apache.maven.doxia.parser.Parser} for Markdown documents.
   * </p>
   * <p>
   * Defers effective parsing to the <a href="https://github.com/vsch/flexmark-java">flexmark-java library</a>,
   * which generates HTML content then delegates parsing of this content to a slightly modified Doxia Xhtml5 parser.
   * (before 1.8, the <a href="http://pegdown.org">PegDown library</a> was used)
   * </p>
   *
   * @author Vladimir Schneider
   * @author Julien Nicoulaud
   * @since 1.3
   */
  @Singleton
  @Named("markdown")
  public class MarkdownParser extends AbstractTextParser implements TextMarkup {
  
      /**
       * Regex that identifies a multimarkdown-style metadata section at the start of the document
       *
       * In order to ensure that we have minimal risk of false positives when slurping metadata sections, the
       * first key in the metadata section must be one of these standard keys or else the entire metadata section is
       * ignored.
       * @see <a href="https://fletcher.github.io/MultiMarkdown-5/metadata.html">Multimarkdown Metadata</a>
       */
      private static final Pattern METADATA_SECTION_PATTERN = Pattern.compile(
              "\\A^"
                      + "(?:title|author|date|address|affiliation|copyright|email|keywords|language|phone|subtitle)"
                      + "[ \\t]*:[\\S\\s]+?^[ \\t]*$",
              Pattern.MULTILINE | Pattern.CASE_INSENSITIVE);
  
      /**
       * Regex that captures the key and value of a multimarkdown-style metadata entry.
       * Group 1 captures the key, group 2 captures the value. Multivalues are not supported in the syntax!
       * Multiline values need to be normalized
       * @see <a href="https://fletcher.github.io/MultiMarkdown-5/metadata.html">Multimarkdown Metadata</a>
       *
      */
     private static final Pattern METADATA_ENTRY_PATTERN = Pattern.compile(
             "^([^:\\r\\n]+?)[ \\t]*:([\\S\\s]+?)(?=(?:^(?:[^:\\r\\n]+?)[ \\t]*:)|^[ \\t]*$)", Pattern.MULTILINE);
 
     /**
      * The parser of the HTML produced by Flexmark, that we will
      * use to convert this HTML to Sink events
      */
     @Inject
     private MarkdownHtmlParser parser;
 
     /**
      * Flexmark's Markdown parser (one static instance fits all)
      */
     private static final com.vladsch.flexmark.parser.Parser FLEXMARK_PARSER;
 
     /**
      * Flexmark's Markdown Metadata parser
      */
     private static final com.vladsch.flexmark.parser.Parser FLEXMARK_METADATA_PARSER;
 
     /**
      * Flexmark's HTML renderer (its output will be re-parsed and converted to Sink events)
      */
     private static final HtmlRenderer FLEXMARK_HTML_RENDERER;
 
     // Initialize the Flexmark parser and renderer, once and for all
     static {
         MutableDataSet flexmarkOptions = new MutableDataSet();
 
         // Enable the extensions that we used to have in Pegdown
         flexmarkOptions.set(
                 com.vladsch.flexmark.parser.Parser.EXTENSIONS,
                 Arrays.asList(
                         EscapedCharacterExtension.create(),
                         AbbreviationExtension.create(),
                         AutolinkExtension.create(),
                         DefinitionExtension.create(),
                         TypographicExtension.create(),
                         TablesExtension.create(),
                         WikiLinkExtension.create(),
                         StrikethroughExtension.create()));
 
         // Disable wrong apostrophe replacement
         flexmarkOptions.set(TypographicExtension.SINGLE_QUOTE_UNMATCHED, "&apos;");
 
         // Additional options on the HTML rendering
         flexmarkOptions.set(HtmlRenderer.HTML_BLOCK_OPEN_TAG_EOL, false);
         flexmarkOptions.set(HtmlRenderer.HTML_BLOCK_CLOSE_TAG_EOL, false);
         flexmarkOptions.set(HtmlRenderer.MAX_TRAILING_BLANK_LINES, -1);
 
         // Build the Markdown parser
         FLEXMARK_PARSER =
                 com.vladsch.flexmark.parser.Parser.builder(flexmarkOptions).build();
 
         MutableDataSet flexmarkMetadataOptions = new MutableDataSet();
         flexmarkMetadataOptions.set(
                 com.vladsch.flexmark.parser.Parser.EXTENSIONS, Arrays.asList(YamlFrontMatterExtension.create()));
         FLEXMARK_METADATA_PARSER = com.vladsch.flexmark.parser.Parser.builder(flexmarkMetadataOptions)
                 .build();
 
         // Build the HTML renderer
         FLEXMARK_HTML_RENDERER = HtmlRenderer.builder(flexmarkOptions)
                 .linkResolverFactory(new FlexmarkDoxiaLinkResolver.Factory())
                 .build();
     }
 
     /** {@inheritDoc} */
     @Override
     public void parse(Reader source, Sink sink, String reference) throws ParseException {
         try {
             // Markdown to HTML (using flexmark-java library)
             String html = toHtml(source);
 
             // TODO: add locator for the markdown source (not the intermediate HTML format)
             // this requires writing a custom renderer not leveraging the XHTML parser
 
             // then HTML to Sink API
             parser.parse(html, getWrappedSink(sink), "Intermediate HTML from " + reference);
         } catch (IOException e) {
             throw new ParseException("Failed reading Markdown source document", e);
         }
     }
 
     private boolean processMetadataForHtml(StringBuilder html, StringBuilder source) {
         final Map<String, List<String>> metadata;
         final int endOffset; // end of metadata within source
         // support two types of metadata:
         if (source.toString().startsWith("---")) {
             // 1. YAML front matter (https://github.com/vsch/flexmark-java/wiki/Extensions#yaml-front-matter)
             Node documentRoot = FLEXMARK_METADATA_PARSER.parse(source.toString());
             YamlFrontMatterVisitor visitor = new YamlFrontMatterVisitor();
             visitor.visit(documentRoot);
             metadata = visitor.getData();
             endOffset = visitor.getEndOffset();
         } else {
             // 2. Multimarkdown metadata (https://fletcher.github.io/MultiMarkdown-5/metadata.html), not yet supported
             // by Flexmark (https://github.com/vsch/flexmark-java/issues/550)
             metadata = new LinkedHashMap<>();
             Matcher metadataMatcher = METADATA_SECTION_PATTERN.matcher(source);
             if (metadataMatcher.find()) {
                 String entry = metadataMatcher.group(0) + EOL;
                 Matcher entryMatcher = METADATA_ENTRY_PATTERN.matcher(entry);
                 while (entryMatcher.find()) {
                     String key = entryMatcher.group(1);
                     String value = normalizeMultilineValue(entryMatcher.group(2));
                     metadata.put(key, Collections.singletonList(value));
                 }
                 endOffset = metadataMatcher.end(0);
             } else {
                 endOffset = 0;
             }
         }
         if (endOffset > 0) {
             // Trim the metadata from the source
             source.delete(0, endOffset);
         }
         return writeHtmlMetadata(html, metadata);
     }
 
     static String normalizeMultilineValue(String value) {
         return value.trim().replaceAll("[ \\t]*[\\r\\n]+[ \\t]*", " ");
     }
 
     private boolean writeHtmlMetadata(StringBuilder html, Map<String, List<String>> data) {
         boolean containsTitle = false;
         for (Entry<String, List<String>> entry : data.entrySet()) {
             if (writeHtmlMetadata(html, entry.getKey(), entry.getValue())) {
                 containsTitle = true;
             }
         }
         return containsTitle;
     }
 
     private boolean writeHtmlMetadata(StringBuilder html, String key, List<String> values) {
         if ("title".equalsIgnoreCase(key)) {
             html.append("<title>");
             html.append(HtmlTools.escapeHTML(values.stream().collect(Collectors.joining(", ")), false));
             html.append("</title>");
             return true;
         } else {
             if (key.equalsIgnoreCase("author") && values.size() > 1) {
                 // for multiple authors emit multiple meta tags
                 for (String value : values) {
                     writeHtmlMetadata(html, key, Collections.singletonList(value));
                 }
             } else {
                 // every other multi-value should just be concatenated and emitted in a single meta tag
                 final String separator;
                 if (key.equalsIgnoreCase("keywords")) {
                     separator = ",";
                 } else {
                     separator = EOL;
                 }
                 html.append("<meta name='");
                 html.append(HtmlTools.escapeHTML(key));
                 html.append("' content='");
                 html.append(HtmlTools.escapeHTML(values.stream().collect(Collectors.joining(separator))));
                 html.append("' />");
             }
             return false;
         }
     }
 
     /**
      * uses flexmark-java library to parse content and generate HTML output.
      *
      * @param source the Markdown source
      * @return HTML content generated by flexmark-java
      * @throws IOException passed through
      */
     String toHtml(Reader source) throws IOException {
         // Read the source
         StringBuilder markdownText = new StringBuilder(IOUtils.toString(source));
 
         // Now, build the HTML document
         StringBuilder html = new StringBuilder(1000);
         html.append("<html>");
         html.append("<head>");
 
         boolean haveTitle = processMetadataForHtml(html, markdownText);
 
         // Now is the time to parse the Markdown document
         // (after we've trimmed out the metadatas, and before we check for its headings)
         Node documentRoot = FLEXMARK_PARSER.parse(markdownText.toString());
 
         // Special trick: if there is no title specified as a metadata in the header, we will use the first
         // heading as the document title
         if (!haveTitle && documentRoot.hasChildren()) {
             // Skip the comment nodes
             Node firstNode = documentRoot.getFirstChild();
             while (firstNode != null && firstNode instanceof HtmlCommentBlock) {
                 firstNode = firstNode.getNext();
             }
 
             // If this first non-comment node is a heading, we use it as the document title
             if (firstNode != null && firstNode instanceof Heading) {
                 html.append("<title>");
                 TextCollectingVisitor collectingVisitor = new TextCollectingVisitor();
                 String headingText = collectingVisitor.collectAndGetText(firstNode);
                 html.append(HtmlTools.escapeHTML(headingText, false));
                 html.append("</title>");
             }
         }
         html.append("</head>");
         html.append("<body>");
 
         // Convert our Markdown document to HTML and append it to our HTML
         FLEXMARK_HTML_RENDERER.render(documentRoot, html);
 
         html.append("</body>");
         html.append("</html>");
 
         return html.toString();
     }
 
     /**
      * Internal parser for HTML generated by the Markdown library.
      *
      * 2 special things:
      * <ul>
      * <li> DIV elements are translated as Unknown Sink events
      * </ul>
      * PRE elements need to be "source" because the Xhtml5Sink will surround the
      * corresponding verbatim() Sink event with a DIV element with class="source",
      * which is how most Maven Skin (incl. Fluido) recognize a block of code, which
      * needs to be highlighted accordingly.
      */
     @Named
     public static class MarkdownHtmlParser extends Xhtml5Parser {
         public MarkdownHtmlParser() {
             super();
         }
 
         @Override
         protected void init() {
             super.init();
         }
 
         @Override
         protected boolean baseEndTag(XmlPullParser parser, Sink sink) {
             boolean visited = super.baseEndTag(parser, sink);
             if (!visited) {
                 if (parser.getName().equals(HtmlMarkup.DIV.toString())) {
                     handleUnknown(parser, sink, TAG_TYPE_END);
                     visited = true;
                 }
             }
             return visited;
         }
 
         @Override
         protected boolean baseStartTag(XmlPullParser parser, Sink sink) {
             boolean visited = super.baseStartTag(parser, sink);
             if (!visited) {
                 if (parser.getName().equals(HtmlMarkup.DIV.toString())) {
                     handleUnknown(parser, sink, TAG_TYPE_START);
                     visited = true;
                 }
             }
             return visited;
         }
     }
 }