//! Markdown to sanitised HTML: pulldown-cmark, then ammonia with an
//! allow-list (hub spec §15.2). Relative links and images are rebased on
//! `base`, so a README's `docs/x.md` opens in the hub.

use pulldown_cmark::{html, Event, Options, Parser, Tag};

/// `base_file` and `base_raw` are URL prefixes (ending in `/`) for links to
/// files and for images.
pub fn render(text: &str, base_file: &str, base_raw: &str) -> String {
    let mut opts = Options::empty();
    opts.insert(Options::ENABLE_TABLES);
    opts.insert(Options::ENABLE_STRIKETHROUGH);
    opts.insert(Options::ENABLE_TASKLISTS);
    opts.insert(Options::ENABLE_FOOTNOTES);
    opts.insert(Options::ENABLE_HEADING_ATTRIBUTES);
    let parser = Parser::new_ext(text, opts).map(|ev| match ev {
        Event::Start(Tag::Link { link_type, dest_url, title, id }) => {
            Event::Start(Tag::Link { link_type, dest_url: rebase(&dest_url, base_file).into(), title, id })
        }
        Event::Start(Tag::Image { link_type, dest_url, title, id }) => {
            Event::Start(Tag::Image { link_type, dest_url: rebase(&dest_url, base_raw).into(), title, id })
        }
        other => other,
    });
    let mut out = String::with_capacity(text.len() * 2);
    html::push_html(&mut out, parser);
    let mut builder = ammonia::Builder::default();
    builder.add_generic_attributes(["id", "class"]);
    builder.add_tag_attributes("input", ["type", "checked", "disabled"]);
    builder.add_tags(["input"]);
    builder.link_rel(Some("noopener noreferrer"));
    builder.clean(&out).to_string()
}

/// Leave absolute URLs, fragments and mail links alone; rebase the rest.
fn rebase(url: &str, base: &str) -> String {
    if url.starts_with('#') || url.contains("://") || url.starts_with("mailto:") || url.starts_with('/') || url.starts_with("data:") {
        return url.to_string();
    }
    let url = url.strip_prefix("./").unwrap_or(url);
    format!("{base}{url}")
}

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

    #[test]
    fn renders_and_sanitises() {
        let html = render(
            "# Hi\n\nSee [docs](docs/x.md) and ![i](img.png) <script>alert(1)</script>\n\n- [ ] task\n",
            "/a/b/file/main/",
            "/a/b/file/main/",
        );
        assert!(html.contains("<h1>Hi</h1>"));
        assert!(html.contains("href=\"/a/b/file/main/docs/x.md\""));
        assert!(html.contains("src=\"/a/b/file/main/img.png\""));
        assert!(!html.contains("<script"));
        assert!(html.contains("type=\"checkbox\""));
        assert!(render("[x](https://e.com) [y](#frag)", "/p/", "/p/").contains("href=\"https://e.com\""));
    }
}