mdpreviewer 0.1.2

//! Markdown to HTML rendering.
//!
//! Fenced ```mermaid blocks are left as ordinary code blocks
//! (`<pre><code class="language-mermaid">`) and turned into diagrams by the
//! browser client, so no raw-HTML injection is needed here.

use comrak::html::escape;
use comrak::nodes::{NodeValue, Sourcepos};
use comrak::{Arena, Options, format_html, parse_document};

/// Render a markdown document to an HTML fragment suitable for insertion into
/// the preview shell's `#content` element.
pub fn render_markdown(markdown: &str) -> String {
    let arena = Arena::new();
    let mut options = Options::default();

    // GitHub Flavored Markdown niceties.
    options.extension.table = true;
    options.extension.strikethrough = true;
    options.extension.autolink = true;
    options.extension.tasklist = true;
    options.extension.footnotes = true;

    // These files are the user's own, served on 127.0.0.1, so pass literal HTML
    // (for example <details>, <img>, <kbd>) through untouched for a faithful
    // preview instead of replacing it with "<!-- raw HTML omitted -->".
    options.render.r#unsafe = true;

    // Tag block elements with `data-sourcepos="L:C-L:C"` so the client can
    // scroll to the block under the editor's cursor line.
    options.render.sourcepos = true;

    // Recognise a leading `---` YAML block so it is not parsed as a rule plus a
    // setext heading. comrak's HTML output drops it; we render it ourselves.
    options.extension.front_matter_delimiter = Some(FRONT_MATTER_DELIMITER.to_owned());

    let root = parse_document(&arena, markdown, &options);

    let mut html = String::new();
    if let Some(node) = root.first_child()
        && let NodeValue::FrontMatter(raw) = &node.data().value
    {
        render_front_matter(raw, node.data().sourcepos, &mut html);
    }
    format_html(root, &options, &mut html).expect("writing to a String cannot fail");
    html
}

const FRONT_MATTER_DELIMITER: &str = "---";

/// Render raw front matter (delimiter lines included, as comrak stores it) as a
/// collapsed YAML code block, tagged with the front matter's source lines.
fn render_front_matter(raw: &str, sourcepos: Sourcepos, html: &mut String) {
    let yaml = raw
        .trim()
        .strip_prefix(FRONT_MATTER_DELIMITER)
        .and_then(|rest| rest.strip_suffix(FRONT_MATTER_DELIMITER))
        .unwrap_or(raw)
        .trim_matches('\n');

    html.push_str(&format!(
        "<details class=\"frontmatter\" data-sourcepos=\"{}:{}-{}:{}\">",
        sourcepos.start.line, sourcepos.start.column, sourcepos.end.line, sourcepos.end.column
    ));
    html.push_str("<summary>Front matter</summary>\n");
    html.push_str("<pre><code class=\"language-yaml\">");
    escape(html, yaml).expect("writing to a String cannot fail");
    html.push_str("</code></pre></details>\n");
}

#[cfg(test)]
mod tests {
    use super::render_markdown;

    #[test]
    fn front_matter_renders_as_collapsed_yaml_block() {
        let html = render_markdown("---\ntitle: Hello\ntags: [a, b]\n---\n# Body\n");
        assert!(
            html.starts_with("<details class=\"frontmatter\" data-sourcepos=\"1:1-4:3\">"),
            "{html}"
        );
        assert!(
            html.contains("<code class=\"language-yaml\">title: Hello\ntags: [a, b]</code>"),
            "{html}"
        );
        // Line numbers count the front matter, matching Helix's cursor line.
        assert!(
            html.contains("<h1 data-sourcepos=\"5:1-5:6\">Body</h1>"),
            "{html}"
        );
        // The closing fence must not turn the YAML into a setext heading.
        assert!(!html.contains("<h2"), "{html}");
        assert!(!html.contains("---"), "{html}");
    }

    #[test]
    fn front_matter_is_escaped() {
        let html = render_markdown("---\ndesc: \"<b>&</b>\"\n---\n");
        assert!(
            html.contains("desc: &quot;&lt;b&gt;&amp;&lt;/b&gt;&quot;"),
            "{html}"
        );
    }

    #[test]
    fn documents_without_front_matter_are_unchanged() {
        let html = render_markdown("# Title\n\ntext\n");
        assert_eq!(
            html,
            "<h1 data-sourcepos=\"1:1-1:7\">Title</h1>\n<p data-sourcepos=\"3:1-3:4\">text</p>\n"
        );
    }

    #[test]
    fn mid_document_rule_is_not_front_matter() {
        let html = render_markdown("intro\n\n---\n\nmore\n");
        assert!(!html.contains("frontmatter"), "{html}");
        assert!(html.contains("<hr data-sourcepos=\"3:1-3:3\" />"), "{html}");
    }

    #[test]
    fn mermaid_fences_keep_their_source_position() {
        let html = render_markdown("text\n\n```mermaid\nflowchart LR\n```\n");
        assert!(
            html.contains("<pre data-sourcepos=\"3:1-5:3\"><code class=\"language-mermaid\">"),
            "{html}"
        );
    }
}