Inserting YAML front matter

[djole21]

What would be the best way to insert yaml front matter into markdown document if it does not exist?

[marad]

@djole21 did you find the way to do this? Can you share?

[djole21]

It was something like this. I hope it helps.

public String addOrUpdateFrontMatter(String markdownContent) {
    String frontMatterDelimiter = "---";
    Map<String, Object> data = new LinkedHashMap<>();

    Map<String, List<String>> frontMatter = getYamlFrontDataMatter(markdownContent);

    Yaml yaml = createYamlParser();
    Matcher matcher = getMatcher(markdownContent);
    String body;
    boolean matcherFound = matcher.find();
    // add edge case, document can have empty yaml front matter,
    // or it can be located somewhere in document but not at the beginning
    if (matcherFound && !frontMatter.isEmpty() || matcherFound && markdownContent.startsWith("---")) {
        String yamlFrontMatter = matcher.group(1);
        body = matcher.replaceFirst("").trim();
        data = yaml.load(yamlFrontMatter);
        // yaml.load() returns null if there are no items
        data = data == null ? new LinkedHashMap<>() : data;
        data.merge("header", "headerValue", (old, newValue) -> newValue);
    } else {
        data.put("header", "headerValue");
        body = markdownContent;
    }

    StringWriter writer = new StringWriter();
    yaml.dump(data, writer);

    return frontMatterDelimiter + "\n" + writer.toString().trim() + "\n" + frontMatterDelimiter + "\n" + body;
}
private Map<String, List<String>> getYamlFrontDataMatter(String markdownContent) {
    MutableDataSet options = new MutableDataSet();
    options.set(Parser.EXTENSIONS, List.of(YamlFrontMatterExtension.create()));
    Parser parser = Parser.builder(options).build();

    Node document = parser.parse(markdownContent);

    AbstractYamlFrontMatterVisitor visitor = new AbstractYamlFrontMatterVisitor();
    visitor.visit(document);
    return visitor.getData();
}

private static Yaml createYamlParser() {
    DumperOptions dumperOptions = new DumperOptions();
    //        dumperOptions.setIndent(2);
    dumperOptions.setPrettyFlow(true);
    dumperOptions.setDefaultFlowStyle(DumperOptions.FlowStyle.BLOCK);
    return new Yaml(dumperOptions);
}

private Matcher getMatcher(String markdownContent) {
    Pattern pattern = Pattern.compile("^---(.*)^---", Pattern.DOTALL | Pattern.MULTILINE);
    return pattern.matcher(markdownContent);
}