ahtml-from-markdown 0.2.0

//! Convert markdown to HTML.

use std::{
    collections::HashMap,
    fmt::{Debug, Display},
    fs::read_to_string,
    panic::RefUnwindSafe,
    path::PathBuf,
    str::Utf8Error,
    string::FromUtf8Error,
};

use anyhow::Context;
use html5gum::{HtmlString, Token};
use kstring::KString;
use pulldown_cmark::{Event, HeadingLevel, LinkType, Options, Parser, Tag};

use ahtml::{
    att, flat::Flat, AId, ASlice, AVec, Element, HtmlAllocator, Node, Print, BLOCKQUOTE_META,
    DIV_META, EM_META, H1_META, H2_META, H3_META, H4_META, H5_META, H6_META, LI_META, METADB,
    OL_META, PRE_META, P_META, STRONG_META, S_META, TABLE_META, TD_META, TH_META, TITLE_META,
    TR_META, UL_META,
};
use ahtml_html::meta::ElementMeta;

use chj_util::{nodt as dt, nowarn as warn, nowarn_todo as warn_todo};

use crate::{
    myfrom::kstring_myfrom2,
    try_option,
    util::{autovivify_last, enum_name, infinite_sequence},
    webutils_simple::email_url,
};

#[derive(thiserror::Error, Debug)]
pub enum MarkdownFileError {
    #[error("not an HTML5 tag name in {} tag: {:?}",
            if *is_opening { "opening" } else { "closing" },
            name.as_str())]
    NotAnHTML5TagName { name: KString, is_opening: bool },
    #[error("non-balanced tags: <{opening}> ending as </{closing}>")]
    UnbalancedTags { opening: String, closing: String },

    #[error("multiple definitions of a footnote with the label {label:?}")]
    MultipleFootnoteWithLabel { label: KString },
    #[error("unused footnote {:?}", label.as_str())]
    UnusedFootnote { label: KString },
    #[error("missing definition for footnote {:?}", label.as_str())]
    MissingFootnoteDefinition { label: KString },

    #[error("multiple <title> elements")]
    MultipleTitleElements,
    // Really should record the location, not the input!
    #[error("HTML5 parsing error: {error} for {input:?}")]
    HTML5ParsingError {
        error: html5gum::Error,
        input: Box<str>,
    },
    #[error("{n} non-closed context(s) at end of markdown document: {msg}")]
    NonClosedContexts { n: usize, msg: String },
    #[error("can't shift header levels by {diff} because getting out of range")]
    HeaderLevelShiftOutOfRange { diff: i32 },

    #[error("anyhow: {0}")]
    Anyhow(#[from] anyhow::Error),
    #[error("UTF-8 decoding error: {0}")]
    Utf8Error(#[from] Utf8Error),
    #[error("UTF-8 decoding error: {0}")]
    FromUtf8Error(#[from] FromUtf8Error),
}

/// This can't be replaced with `att` or the MyFrom trait, because it
/// can fail.
fn kstring(s: HtmlString) -> Result<KString, FromUtf8Error> {
    Ok(KString::from_string(String::from_utf8(s.0)?))
}

// ------------------------------------------------------------------
// Formatting parametrization

pub trait StylingInterface: Send + Sync + RefUnwindSafe {
    fn new_context<'c>(
        &'c self,
        html: &HtmlAllocator,
    ) -> anyhow::Result<Box<dyn StylingContextInterface<'c> + 'c>>;
}

pub trait StylingContextInterface<'c> {
    fn format_footnote_definition(
        &self,
        html: &HtmlAllocator,
        reference: &Footnoteref,
        backreferences: &[Backref],
        clean_slice: &ASlice<Node>,
    ) -> anyhow::Result<Flat<Node>>;

    fn format_footnotes(
        &self,
        body: ASlice<Node>,
        html: &HtmlAllocator,
    ) -> anyhow::Result<AId<Node>>;
}

// ------------------------------------------------------------------

fn elementmeta_from_headinglevel(level: HeadingLevel) -> &'static ElementMeta {
    match level {
        HeadingLevel::H1 => *H1_META,
        HeadingLevel::H2 => *H2_META,
        HeadingLevel::H3 => *H3_META,
        HeadingLevel::H4 => *H4_META,
        HeadingLevel::H5 => *H5_META,
        HeadingLevel::H6 => *H6_META,
    }
}

fn elementmeta_from_num(level: i32) -> Option<&'static ElementMeta> {
    match level {
        1 => Some(*H1_META),
        2 => Some(*H2_META),
        3 => Some(*H3_META),
        4 => Some(*H4_META),
        5 => Some(*H5_META),
        6 => Some(*H6_META),
        _ => None,
    }
}

// Returning a signed integer so that calculating with differences is
// easy.
fn headinglevel_num(level: HeadingLevel) -> i32 {
    match level {
        HeadingLevel::H1 => 1,
        HeadingLevel::H2 => 2,
        HeadingLevel::H3 => 3,
        HeadingLevel::H4 => 4,
        HeadingLevel::H5 => 5,
        HeadingLevel::H6 => 6,
    }
}

fn level_from_elementmeta(meta: &'static ElementMeta) -> Option<i32> {
    if meta == *H1_META {
        Some(1)
    } else if meta == *H2_META {
        Some(2)
    } else if meta == *H3_META {
        Some(3)
    } else if meta == *H4_META {
        Some(4)
    } else if meta == *H5_META {
        Some(5)
    } else if meta == *H6_META {
        Some(6)
    } else {
        None
    }
}

fn text_to_anchor(s: &str, res: &mut String) {
    let mut last_was_space = false;
    for c in s.chars() {
        if c.is_ascii_alphanumeric() {
            res.push(c.to_ascii_lowercase());
            last_was_space = false;
        } else if c.is_whitespace() {
            if !last_was_space {
                res.push('-');
            }
        } else {
            res.push('_');
            last_was_space = false;
        }
    }
}

pub struct MarkdownFile {
    path: PathBuf,
}

pub struct MarkdownHeader {
    /// The body of the <hX> element
    html: ASlice<Node>,
    anchor_name: KString,
}

pub struct MarkdownHeading {
    /// Original level as per .md document, used for building up (won't
    /// correspond to the HTML any more after fixing that up).
    level: HeadingLevel,
    header: Option<MarkdownHeader>,
    subheadings: Vec<MarkdownHeading>,
}

impl MarkdownHeading {
    fn append_heading(&mut self, our_level: u32, h: MarkdownHeading) {
        if h.level as u32 == our_level {
            self.subheadings.push(h)
        } else {
            autovivify_last(&mut self.subheadings, || MarkdownHeading {
                level: HeadingLevel::try_from(our_level as usize)
                    .expect("must exist because h.level is yet larger"),
                header: None,
                subheadings: Vec::new(),
            })
            .append_heading(our_level + 1, h)
        }
    }

    fn to_toc_html_fragment(&self, html: &HtmlAllocator) -> anyhow::Result<AId<Node>> {
        let mut body = html.new_vec();
        for subheading in &self.subheadings {
            body.push(subheading.to_toc_html_fragment(html)?)?;
        }
        html.dl(
            [],
            [
                if let Some(header) = &self.header {
                    let mut anchor = String::new(); // cache?
                    anchor.push_str("#");
                    anchor.push_str(&header.anchor_name);
                    html.dt(
                        [],
                        [html.a(
                            [att("href", anchor)],
                            // Should we actually strip HTML markup?
                            &header.html,
                        )?],
                    )?
                } else {
                    html.dt([], [])?
                },
                html.dd([], body)?,
            ],
        )
    }

    // Again duplication with method in MarkdownMeta. Stupid. todo clean up?
    fn top_heading_level(&self) -> Option<HeadingLevel> {
        if self.header.is_some() {
            Some(self.level)
        } else {
            self.subheadings
                .iter()
                .filter_map(|heading| heading.top_heading_level())
                .max()
        }
    }
}

#[derive(Copy, Clone, Eq, PartialEq, Ord, PartialOrd)]
pub struct Backref(pub u32);

impl Backref {
    pub fn to_kstring(&self, with_hash: bool) -> KString {
        KString::from_string(format!(
            "{}footnoteref-{}",
            if with_hash { "#" } else { "" },
            self.0
        ))
    }
}

#[derive(Copy, Clone, Eq, PartialEq, Ord, PartialOrd)]
pub struct Footnoteref(pub u32);

impl Footnoteref {
    pub fn to_kstring(&self, with_hash: bool) -> KString {
        KString::from_string(format!(
            "{}footnote-{}",
            if with_hash { "#" } else { "" },
            self.0
        ))
    }
}

struct FootnoteDefinition {
    reference: Option<Footnoteref>,
    text: Option<ASlice<Node>>,
    /// places with references to this definition, in order of
    /// appearance in document
    backreferences: Vec<Backref>,
}

pub struct MarkdownMeta {
    /// contents of <title> tag only (deriving from headers happens
    /// outside)
    title: Option<ASlice<Node>>,
    headings: Vec<MarkdownHeading>,
    /// footnote label to definition
    footnotes: HashMap<KString, FootnoteDefinition>,
}
impl MarkdownMeta {
    fn new() -> MarkdownMeta {
        MarkdownMeta {
            title: None,
            headings: Vec::new(),
            footnotes: HashMap::new(),
        }
    }

    fn push_heading(&mut self, h: MarkdownHeading) {
        match h.level {
            HeadingLevel::H1 => self.headings.push(h),
            _ => autovivify_last(&mut self.headings, || MarkdownHeading {
                level: HeadingLevel::H1,
                header: None,
                subheadings: Vec::new(),
            })
            .append_heading(2, h),
        }
    }

    // Stupid modified copy-paste[aha, from to_toc_html_fragment],
    // "todo really unify the two DS;
    // actually easy just None header ?"-- but now ~happy
    // with it, OK? &Vec<MarkdownHeading> is now the thing to be generic on?
    // Alright, should then do function on *that* ^, todo?
    pub fn toc_html_fragment(&self, html: &HtmlAllocator) -> anyhow::Result<AId<Node>> {
        let headings = self.title_and_remaining_headings().1;
        let mut body = html.new_vec();
        for subheading in headings {
            body.push(subheading.to_toc_html_fragment(html)?)?;
        }
        // Using `div` here instead of `dl` is wrong in that multiple
        // toplevel entries will be separate now. But what would the
        // `dt`? Empty? It would indent the `dd` holding `body`. Do it
        // iff there are >1 body nodes? Perennial question about what
        // '#' header should mean in Markdown.
        if true {
            html.div([att("class", "toc_wrapper")], body)
        } else {
            html.dl([], [html.dt([], [])?, html.dd([], body)?])
        }
    }

    // XX why not just preserialize the individual footnote
    // definitions, and leave formatting of the rest to blog.rs?
    // Checking for missing definitions should perhaps still be done
    // in markdown.rs, though.
    pub fn footnotes_html_fragment(
        &self,
        html: &HtmlAllocator,
        style: &dyn StylingInterface,
    ) -> Result<(usize, AId<Node>), MarkdownFileError> {
        let mut footnotes: Vec<_> = self.footnotes.iter().collect();
        footnotes.sort_by_key(|f| f.1.reference);
        // dbg!(&footnotes);

        let context = style.new_context(html)?;
        let mut body = html.new_vec();
        for (label, fnd) in &footnotes {
            let reference = fnd
                .reference
                .ok_or_else(|| MarkdownFileError::UnusedFootnote {
                    label: (*label).clone(),
                })?;
            let slice = fnd
                .text
                .ok_or_else(|| MarkdownFileError::MissingFootnoteDefinition {
                    label: (*label).clone(),
                })?;
            let clean_slice = slice.unwrap_element(*P_META, true, html);
            body.push_flat(context.format_footnote_definition(
                html,
                &reference,
                &fnd.backreferences,
                &clean_slice,
            )?)?;
        }
        Ok((
            footnotes.len(),
            context.format_footnotes(body.as_slice(), html)?,
        ))
    }

    /// Split title/header hierarchy into title and rest; takes
    /// `<title>` if available by preference, otherwise the first
    /// heading if it's a '#' and there are no other '#' ones. The
    /// last returned value is true if a heading from the markdown
    /// file was skipped (i.e. it needs to be dropped from the
    /// generated HTML to avoid header duplication).
    pub fn title_and_remaining_headings(
        &self,
    ) -> (Option<&ASlice<Node>>, &Vec<MarkdownHeading>, bool) {
        if let Some(title) = &self.title {
            (Some(title), &self.headings, false)
        } else {
            if let Some(header) = try_option! {
                if self.headings.len() != 1 { return None; }
                self.headings[0].header.as_ref()
            } {
                (Some(&header.html), &self.headings[0].subheadings, true)
            } else {
                (None, &self.headings, false)
            }
        }
    }

    /// The contents of an optional single `<title>` element,
    /// or if missing, the first heading if it's a
    /// '#' and there are no other '#' ones.
    pub fn title(&self) -> Option<&ASlice<Node>> {
        self.title_and_remaining_headings().0
    }

    /// Like `title` but as a string with markup stripped, and falling
    /// back to `alternative` if not present.
    pub fn title_string(&self, html: &HtmlAllocator, alternative: &str) -> anyhow::Result<KString> {
        if let Some(sl) = self.title() {
            let mut v = String::new();
            sl.print_plain(&mut v, html)?;
            Ok(KString::from_string(v))
        } else {
            Ok(KString::from_ref(alternative))
        }
    }

    fn top_heading_level(&self) -> Option<HeadingLevel> {
        self.headings
            .iter()
            .filter_map(|heading| heading.top_heading_level())
            .max()
    }
}

/// The result of processing a markdown file.
pub struct ProcessedMarkdown {
    /// Conversion to html of the text, with the original heading
    /// levels translated to identical HTML levels (may need fixing up
    /// before serving).
    html: AId<Node>,
    /// Metadata extracted also during the conversion.
    meta: MarkdownMeta,
}

impl ProcessedMarkdown {
    pub fn html(&self) -> AId<Node> {
        self.html
    }
    pub fn meta(&self) -> &MarkdownMeta {
        &self.meta
    }

    pub fn fixed_html(&self, html: &HtmlAllocator) -> anyhow::Result<AId<Node>> {
        // Which is the top level we *want*?
        let (opt_title, _heading, do_drop_h1) = self.meta.title_and_remaining_headings();
        dt!(&format!(
            "fixed_html {:?}",
            opt_title.map_or_else(|| Ok(String::from("(no title)")), |t| t.to_string(html))
        ));
        // We want to either drop H1 in the document and not shift
        // anything (because H1 existed and was the only H1 header,
        // after dropping it the next level can only be H2 or less and
        // we leave it at what remains), or, shift them if necessary
        // so that the top level becomes H2. Unless it couldn't
        // extract a title, in which case we leave the document
        // untouched.
        if opt_title.is_none() {
            warn!("no title could be derived");
            return Ok(self.html);
        }
        let fixup: Box<dyn Fn(_) -> _> = if do_drop_h1 {
            warn!("do_drop_h1");
            Box::new(|id: AId<Node>| -> anyhow::Result<Option<AId<Node>>> {
                let node = html.get_node(id).expect("correct HtmlAllocator");
                if let Some(elt) = node.as_element() {
                    if elt.meta() == *H1_META {
                        Ok(None)
                    } else {
                        Ok(Some(id))
                    }
                } else {
                    Ok(Some(id))
                }
            })
        } else {
            if let Some(top_level_have) = self.meta.top_heading_level() {
                let top_level_want = 2; // HeadingLevel::H2;
                let diff = top_level_want - headinglevel_num(top_level_have);
                warn!("diff = {diff}");
                if diff == 0 {
                    return Ok(self.html);
                }
                Box::new(move |id: AId<Node>| -> anyhow::Result<Option<AId<Node>>> {
                    let node = html.get_node(id).expect("correct HtmlAllocator");
                    if let Some(elt) = node.as_element() {
                        if let Some(lvl) = level_from_elementmeta(elt.meta()) {
                            let lvl2 = lvl + diff;
                            let meta2 = elementmeta_from_num(lvl2).ok_or_else(|| {
                                MarkdownFileError::HeaderLevelShiftOutOfRange { diff }
                            })?;
                            let elt2 = Element {
                                meta: meta2,
                                attr: elt.attr().clone(),
                                body: elt.body().clone(),
                            };
                            Ok(Some(html.allocate_element(elt2)?))
                        } else {
                            Ok(Some(id))
                        }
                    } else {
                        Ok(Some(id))
                    }
                })
            } else {
                warn!("no headings, thus noop");
                return Ok(self.html);
            }
        };

        let node2 = {
            let elt = {
                let node = html.get_node(self.html).expect(
                    "ProcessedMarkdown to be used with the same HtmlAllocator it was created with",
                );
                // Bummer, Element is quite large (5 words?), but we have
                // to free up the borrow from get_node because
                // try_filter_map_body needs a writable one.
                (*node.try_element()?).clone()
            };
            elt.try_filter_map_body::<Node>(fixup, html)?
        };
        Ok(html.allocate_element(node2)?)
    }
}

// Internals for impl MarkdownFile:

#[derive(Debug)]
enum ContextTag<'t> {
    Markdown(Tag<'t>),
    Html(&'static ElementMeta),
}

impl<'t> Display for ContextTag<'t> {
    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
        match self {
            ContextTag::Markdown(tag) => {
                f.write_fmt(format_args!("Markdown {:?} scope", enum_name(tag)))
            }
            ContextTag::Html(meta) => {
                f.write_fmt(format_args!("HTML {:?} element", meta.tag_name.as_str()))
            }
        }
    }
}

impl<'t> PartialEq for ContextTag<'t> {
    fn eq(&self, other: &Self) -> bool {
        match (self, other) {
            (ContextTag::Markdown(a), ContextTag::Markdown(b)) => a == b,
            (ContextTag::Html(a), ContextTag::Html(b)) => std::ptr::eq(*a, *b),
            _ => false,
        }
    }
}

impl<'t> ContextTag<'t> {
    fn assert_eq(&self, closing: &ContextTag) -> Result<(), MarkdownFileError> {
        if *self == *closing {
            Ok(())
        } else {
            Err(MarkdownFileError::UnbalancedTags {
                opening: self.to_string(),
                closing: closing.to_string(),
            })
        }
    }
}

struct ContextFrame<'a, 't> {
    tag: ContextTag<'t>,
    // meta: &'static ElementMeta, -- no, given ad-hoc on closing
    // event.
    atts: AVec<'a, (KString, KString)>,
    body: AVec<'a, Node>,
    last_footnote_reference: Option<u32>, // last index into body holding one
}

/// Convert to HTML, and capture metainformation to allow for
/// creation of TOC and footnotes section.
pub fn markdown_to_html(
    s: &str,
    html: &HtmlAllocator,
) -> Result<ProcessedMarkdown, MarkdownFileError> {
    let mut options = Options::empty();
    options.insert(Options::ENABLE_TABLES);
    options.insert(Options::ENABLE_FOOTNOTES);
    options.insert(Options::ENABLE_STRIKETHROUGH);
    options.insert(Options::ENABLE_TASKLISTS);
    options.insert(Options::ENABLE_SMART_PUNCTUATION); // XX config
    options.insert(Options::ENABLE_HEADING_ATTRIBUTES);

    let mut parser = Parser::new_ext(&s, options);

    // Context
    let mut _context: Vec<ContextFrame> = Vec::new();
    let mut context = &mut _context;
    // Push a base frame (wrapper around everything):
    context.push(ContextFrame {
        tag: ContextTag::Markdown(Tag::Paragraph), // fake
        atts: AVec::new(html),
        body: AVec::new(html),
        last_footnote_reference: None,
    });
    macro_rules! new_contextframe {
        ($tag:expr) => {
            ContextFrame {
                tag: $tag,
                atts: AVec::new(html),
                body: AVec::new(html),
                last_footnote_reference: None,
            }
        };
    }

    // Opening a context
    macro_rules! mdopen {
        ($tag:expr) => {
            context.push(new_contextframe!(ContextTag::Markdown($tag)))
        };
    }

    // Closing a context
    let frame_to_element =
        |frame: ContextFrame, meta: &'static ElementMeta| -> Result<AId<Node>, MarkdownFileError> {
            Ok(html.new_element(meta, frame.atts.as_slice(), frame.body.as_slice())?)
        };
    let close = |context: &mut Vec<ContextFrame>,
                 tag: ContextTag,
                 meta: &'static ElementMeta|
     -> Result<(), MarkdownFileError> {
        let frame = context.pop().expect("start before end");
        frame.tag.assert_eq(&tag)?;
        let outerframe = context.last_mut().expect("at least base frame");
        outerframe.body.push(frame_to_element(frame, meta)?)?;
        Ok(())
    };
    macro_rules! mdclose {
        ($tag:expr, $meta:expr) => {
            close(&mut context, ContextTag::Markdown($tag), $meta)
        };
    }
    // Alternative approach:
    macro_rules! pop {
        ($tag:expr) => {{
            // XX minimize code via local function
            let frame = context.pop().expect("start before end");
            frame.tag.assert_eq(&$tag)?;
            let outerframe = context.last_mut().expect("at least base frame");
            (frame.atts, frame.body, outerframe)
        }};
    }
    macro_rules! mdpop {
        ($tag:expr) => {
            pop!(ContextTag::Markdown($tag))
        };
    }

    macro_rules! current_frame {
        () => {
            context
                .last_mut()
                .expect("At least base frame; at least bug in markdown lib?")
        };
    }

    let mut markdownmeta = MarkdownMeta::new();
    // let mut current_heading = None;
    let mut anchor_name = String::new();
    let mut tmp = String::new();
    // Anchor names to number of uses, acting as id
    let mut anchor_names: HashMap<KString, u32> = HashMap::new();

    let mut next_footnote_number = infinite_sequence(1, 1);
    let mut next_footnote_backreference = infinite_sequence(1, 1);

    while let Some(item) = parser.next() {
        match item {
            Event::Start(x) => match x {
                Tag::Paragraph => mdopen!(Tag::Paragraph),
                Tag::Heading(level, fragmentid, classes) => {
                    mdopen!(Tag::Heading(level, fragmentid, classes))
                }
                Tag::BlockQuote => mdopen!(Tag::BlockQuote),
                Tag::CodeBlock(kind) => mdopen!(Tag::CodeBlock(kind)),
                Tag::List(firstitemnum) => mdopen!(Tag::List(firstitemnum)),
                Tag::Item => mdopen!(Tag::Item),
                Tag::FootnoteDefinition(label) => mdopen!(Tag::FootnoteDefinition(label)),
                Tag::Table(alignments) => mdopen!(Tag::Table(alignments)),
                Tag::TableHead => mdopen!(Tag::TableHead),
                Tag::TableRow => mdopen!(Tag::TableRow),
                Tag::TableCell => mdopen!(Tag::TableCell),
                Tag::Emphasis => mdopen!(Tag::Emphasis),
                Tag::Strong => mdopen!(Tag::Strong),
                Tag::Strikethrough => mdopen!(Tag::Strikethrough),
                Tag::Link(linktype, url, title) => mdopen!(Tag::Link(linktype, url, title)),
                Tag::Image(linktype, url, title) => mdopen!(Tag::Image(linktype, url, title)),
            },
            Event::End(x) => match x {
                Tag::Paragraph => mdclose!(Tag::Paragraph, *P_META)?,
                Tag::Heading(level, fragmentid, classes) => {
                    {
                        // Store generated HTML for this
                        // heading in markdownmeta, too,
                        // and add a reference to the html
                        // element in the body.
                        let frame = current_frame!();
                        let bodyslice = frame.body.as_slice();
                        tmp.clear();
                        for node in bodyslice.iter_node(html) {
                            node.print_plain(&mut tmp, html)?;
                        }
                        anchor_name.clear();
                        text_to_anchor(&tmp, &mut anchor_name);

                        // Append number if necessary to avoid conflicts
                        // (XX should actually do a check like this on the whole
                        // generated page (uh, preserialized parts!))
                        let anchor_name_kstr;
                        'search: loop {
                            // loop bc labels on blocks are unstable
                            for _ in 0..10 {
                                if let Some(counter) = anchor_names.get_mut(&*anchor_name) {
                                    *counter += 1;
                                    anchor_name.push_str(&format!("-{}", *counter));
                                } else {
                                    anchor_name_kstr = KString::from(&anchor_name);
                                    anchor_names.insert(anchor_name_kstr.clone(), 1);
                                    break 'search;
                                }
                            }
                            warn!(
                                "more than 10 *levels* of conflicts trying to find \
                                       unallocated name; leaving it conflicting"
                            );
                            anchor_name_kstr = KString::from(&anchor_name);
                            break;
                        }

                        frame.atts.push(
                            // XX Should offer an `attribute`
                            // method that accepts 2 arguments
                            // which are ToKString. clone should
                            // be faster than from_str.
                            html.attribute("id", anchor_name_kstr.as_str())?,
                        )?;

                        markdownmeta.push_heading(MarkdownHeading {
                            level,
                            header: Some(MarkdownHeader {
                                html: bodyslice,
                                anchor_name: anchor_name_kstr,
                            }),
                            subheadings: Vec::new(),
                        });
                    }

                    let meta = elementmeta_from_headinglevel(level);
                    // XX todo: handle fragmentid, classes
                    mdclose!(Tag::Heading(level, fragmentid, classes), meta)?
                }
                Tag::BlockQuote => mdclose!(Tag::BlockQuote, *BLOCKQUOTE_META)?,
                Tag::CodeBlock(kind) =>
                // XX kind -> class="language-xxx", and do highlighting
                {
                    mdclose!(Tag::CodeBlock(kind), *PRE_META)?
                }

                Tag::List(firstitemnum) => mdclose!(
                    Tag::List(firstitemnum),
                    if firstitemnum.is_some() {
                        *OL_META
                    } else {
                        *UL_META
                    }
                )?,
                Tag::Item => mdclose!(Tag::Item, *LI_META)?,
                Tag::FootnoteDefinition(label) => {
                    // A footnote definition. The value contained is the footnote's
                    // label by which it can be referred to.
                    let frame = context.pop().expect("start before end");
                    if let Some(FootnoteDefinition {
                        text: footnote_text,
                        ..
                    }) = markdownmeta.footnotes.get_mut(&*label)
                    {
                        if let Some(_) = footnote_text {
                            return Err(MarkdownFileError::MultipleFootnoteWithLabel {
                                label: KString::from_ref(&*label),
                            });
                        } else {
                            *footnote_text = Some(frame.body.as_slice());
                            // XX what about atts?
                        }
                    } else {
                        // Definition before first use
                        markdownmeta.footnotes.insert(
                            KString::from_ref(&*label),
                            FootnoteDefinition {
                                reference: None,
                                text: Some(frame.body.as_slice()),
                                backreferences: Vec::new(),
                            },
                        );
                    }
                }
                Tag::Table(alignments) => mdclose!(
                    Tag::Table(alignments),
                    // XX todo: handle alignments
                    *TABLE_META
                )?,
                Tag::TableHead => mdclose!(Tag::TableHead, *TH_META)?,
                Tag::TableRow => mdclose!(Tag::TableRow, *TR_META)?,
                Tag::TableCell => mdclose!(Tag::TableCell, *TD_META)?,
                Tag::Emphasis => mdclose!(Tag::Emphasis, *EM_META)?,
                Tag::Strong => mdclose!(Tag::Strong, *STRONG_META)?,
                Tag::Strikethrough => mdclose!(Tag::Strikethrough, *S_META)?,
                Tag::Link(linktype, url, title) => {
                    let (mut atts, body, outerframe) = mdpop!(
                        // XX uh, need to clone just to verify. better?
                        Tag::Link(linktype, url.clone(), title)
                    );

                    let elt = match linktype {
                        // Inline link like `[foo](bar)`
                        LinkType::Inline => {
                            atts.push(html.attribute("href", kstring_myfrom2(url))?)?;
                            html.a(atts, body)
                        }
                        // Reference link like `[foo][bar]`
                        LinkType::Reference => {
                            warn_todo!(
                                "LinkType::Reference: \
                                            url, presumably?"
                            );
                            atts.push(html.attribute("href", kstring_myfrom2(url))?)?;
                            html.a(atts, body)
                        }
                        // Reference without destination in
                        // the document, but resolved by the
                        // broken_link_callback
                        LinkType::ReferenceUnknown => todo!(),
                        // Collapsed link like `[foo][]`
                        LinkType::Collapsed => todo!(),
                        // Collapsed link without destination
                        // in the document, but resolved by
                        // the broken_link_callback
                        LinkType::CollapsedUnknown => todo!(),
                        // Shortcut link like `[foo]`
                        LinkType::Shortcut => {
                            warn_todo!(
                                "LinkType::Shortcut: need to build \
                                            index and look up"
                            );
                            atts.push(html.attribute("href", kstring_myfrom2(url))?)?;
                            html.a(atts, body)
                        }
                        // Shortcut without destination in the
                        // document, but resolved by the
                        // broken_link_callback
                        LinkType::ShortcutUnknown => todo!(),
                        // Autolink like `<http://foo.bar/baz>`
                        LinkType::Autolink => html.a([att("href", kstring_myfrom2(url))], body),
                        // Email address in autolink like `<john@example.org>`
                        LinkType::Email => html.a([att("href", email_url(&url))], body),
                    };
                    outerframe.body.push(elt?)?;
                }
                Tag::Image(linktype, url, title) =>
                // Oh, almost COPYPASTE of Tag::Link
                {
                    let (mut atts, body, outerframe) = mdpop!(
                        // XX uh, need to clone just to verify. better?
                        Tag::Link(linktype, url.clone(), title)
                    );
                    let elt = match linktype {
                        LinkType::Inline => {
                            atts.push(html.attribute("src", kstring_myfrom2(url))?)?;
                            html.img(atts, body)
                        }
                        LinkType::Reference => todo!(),
                        LinkType::ReferenceUnknown => todo!(),
                        LinkType::Collapsed => todo!(),
                        LinkType::CollapsedUnknown => todo!(),
                        LinkType::Shortcut => todo!(),
                        LinkType::ShortcutUnknown => todo!(),
                        LinkType::Autolink => todo!(),
                        LinkType::Email => todo!(),
                    };
                    outerframe.body.push(elt?)?;
                }
            },
            Event::Text(s) => {
                let frame = current_frame!();
                frame.body.push(html.str(&s)?)?;
            }
            Event::Code(s) => {
                warn!("Event::Code({:?})", &*s);
                let frame = current_frame!();
                let elt = html.code([], [html.str(&s)?])?;
                frame.body.push(elt)?;
            }
            Event::Html(s) => {
                // I don't really want to put it all in here. This
                // function is horribly long. But working with
                // closures and hygienic macros in a way to re-use
                // them, move them outside, is too painful for me
                // right now, so I go.
                dt!(&format!("Event::Html({s:?})"));
                for token in html5gum::Tokenizer::new(&*s).infallible() {
                    match token {
                        Token::StartTag(starttag) => {
                            let name: &str = std::str::from_utf8(&**starttag.name)?;
                            let meta = METADB.elementmeta.get(name).ok_or_else(|| {
                                MarkdownFileError::NotAnHTML5TagName {
                                    name: KString::from_ref(name),
                                    is_opening: true,
                                }
                            })?;
                            let mut newframe = new_contextframe!(ContextTag::Html(meta));
                            for (k, v) in starttag.attributes {
                                newframe
                                    .atts
                                    .push(html.attribute(kstring(k)?, kstring(v)?)?)?;
                            }
                            if starttag.self_closing || !meta.has_closing_tag {
                                let cf = current_frame!();
                                // XX give context to errors,
                                // e.g. invalid attribute because,
                                // where was the element coming
                                // from? Or utf-8 conversion errors above, too.
                                cf.body.push(frame_to_element(newframe, meta)?)?;
                            } else {
                                context.push(newframe);
                            }
                        }
                        Token::EndTag(endtag) => {
                            let name: &str = std::str::from_utf8(&**endtag.name)?;
                            let meta = METADB.elementmeta.get(name).ok_or_else(|| {
                                MarkdownFileError::NotAnHTML5TagName {
                                    name: KString::from_ref(name),
                                    is_opening: false,
                                }
                            })?;
                            if meta.has_closing_tag {
                                let (atts, body, outerframe) =
                                    // XX error context. if only I had
                                    // location info? sigh?
                                    pop!(ContextTag::Html(meta));
                                // Special HTML tag treatments
                                if meta == *TITLE_META {
                                    if markdownmeta.title.is_some() {
                                        return Err(MarkdownFileError::MultipleTitleElements);
                                    }
                                    markdownmeta.title = Some(body.as_slice());
                                    // XX dropping atts OK?
                                } else {
                                    outerframe.body.push(html.new_element(
                                        meta,
                                        atts.as_slice(),
                                        body.as_slice(),
                                    )?)?;
                                }
                            } else {
                                // NOOP, we haven't made a frame for it.
                            }
                        }
                        Token::String(s) => {
                            let frame = current_frame!();
                            frame.body.push(html.kstring(kstring(s)?)?)?;
                        }
                        Token::Comment(_s) => {
                            // This happens only when <!-- and -->
                            // appear in the same markdown event,
                            // i.e. in the same paragraph.  todo:
                            // do something with _s?
                        }
                        Token::Doctype(_) => todo!(),
                        Token::Error(e) => {
                            if s.starts_with("<!--") {
                                // XX how to check `e` ? Should verify it's "eof-in-comment"
                                // let newframe = new_contextframe!(
                                //     ContextTag::HtmlComment);
                                // context.push(newframe);

                                // No, slurp up markdown
                                // events right here until -->
                                // appears.
                                while let Some(item) = parser.next() {
                                    match item {
                                        Event::Html(s) => {
                                            if s.starts_with("-->") {
                                                break;
                                            }
                                        }
                                        _ => (),
                                    }
                                }
                            } else {
                                return Err(MarkdownFileError::HTML5ParsingError {
                                    error: e,
                                    input: s.as_ref().into(),
                                });
                            }
                        }
                    }
                }
            }
            Event::FootnoteReference(label) => {
                // "A reference to a footnote with given label, which may or may
                // not be defined by an event with a `Tag::FootnoteDefinition`
                // tag. Definitions and references to them may occur in any
                // order."
                let backref = Backref(next_footnote_backreference());
                let reference = if let Some(fnd) = markdownmeta.footnotes.get_mut(&*label) {
                    let reference = if let Some(reference) = fnd.reference {
                        reference
                    } else {
                        let reference = Footnoteref(next_footnote_number());
                        fnd.reference = Some(reference);
                        reference
                    };
                    fnd.backreferences.push(backref.clone());
                    reference
                } else {
                    let reference = Footnoteref(next_footnote_number());
                    markdownmeta.footnotes.insert(
                        KString::from_ref(&*label),
                        FootnoteDefinition {
                            reference: Some(reference),
                            text: None,
                            backreferences: vec![backref.clone()],
                        },
                    );
                    reference
                };

                let frame = current_frame!();
                if let Some(i) = frame.last_footnote_reference {
                    if i == frame.body.len() {
                        // Separate the new reference from the
                        // last reference; todo?: ideally the 3
                        // `sup` would be merged.
                        frame.body.push(html.sup([], [html.str(",")?])?)?;
                    }
                }
                frame.body.push(html.sup(
                    [att("id", backref.to_kstring(false))],
                    [html.a(
                        [att("href", reference.to_kstring(true))],
                        [html.string(reference.0.to_string())?],
                    )?],
                )?)?;
                frame.last_footnote_reference = Some(frame.body.len());
            }
            Event::SoftBreak => {
                // a single \n in the input
                let frame = current_frame!();
                frame.body.push(html.str("\n")?)?;
            }
            Event::HardBreak => {
                // "  \n" in the input
                let frame = current_frame!();
                frame.body.push(html.br([], [])?)?;
            }
            Event::Rule => {
                let frame = current_frame!();
                frame.body.push(html.hr([], [])?)?;
            }
            Event::TaskListMarker(checked) => {
                let frame = current_frame!();
                let mut atts = html.new_vec();
                atts.push(html.attribute("type", "checkbox")?)?;
                atts.push(html.attribute("disabled", "")?)?;
                if checked {
                    atts.push(html.attribute("checked", "")?)?;
                }
                frame.body.push(html.input(atts, [])?)?;
            }
        }
    }

    match context.len() {
        0 => panic!("top-level context was dropped -- should be impossible?"),
        1 => (),
        n => {
            return Err(MarkdownFileError::NonClosedContexts {
                n: n - 1,
                msg: context[1..]
                    .iter()
                    .map(|c| c.tag.to_string())
                    .collect::<Vec<String>>()
                    .join(", "),
            })
        }
    }
    let baseframe = context.pop().unwrap();
    Ok(ProcessedMarkdown {
        html: frame_to_element(baseframe, *DIV_META)?,
        meta: markdownmeta,
    })
}

// (No point for this type, really, only holds the path for one method
// call.)
impl MarkdownFile {
    pub fn new(path: PathBuf) -> MarkdownFile {
        MarkdownFile { path }
    }

    pub fn path(&self) -> &PathBuf {
        &self.path
    }

    pub fn process_to_html(
        &self,
        html: &HtmlAllocator,
    ) -> Result<ProcessedMarkdown, MarkdownFileError> {
        // `Parser` is NOT supporting streaming. For reasons of
        // shining in (superficial) performance bencharks?
        // XX impose a size limit on the markdown file here?
        let s = read_to_string(&self.path)
            .with_context(|| anyhow::anyhow!("can't read file {:?}", self.path))?;

        markdown_to_html(&s, html)
    }
}