//! 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\""));

    }

}