moli/moli-html2md/src/machine.rs

use crate::output::Output;
use crate::table::Table;
use crate::writer::{Style, Writer, longest_run};
use crate::{Dom, NodeKind, Options};

enum Task<'a, Id> {
    Visit(Id, usize),
    Children(Option<Id>, usize),
    Boundary,
    PopStyle,
    EndLink(usize),
    EndQuote,
    EndHeading(usize),
    EndList(usize),
    EndItem(String),
    RawChildren(Option<Id>, usize, bool),
    EndRawBlock,
    EndCode,
    EndPre(Option<&'a str>),
    TableCell,
    EndTableCell,
}

struct List {
    output_depth: usize,
    ordered: bool,
    next: i64,
    loose: bool,
}

struct Machine<'a, D: Dom + ?Sized> {
    dom: &'a D,
    options: &'a Options,
    tasks: Vec<Task<'a, D::NodeId>>,
    writers: Vec<Writer<'a>>,
    lists: Vec<List>,
    tables: Vec<Table<D::NodeId>>,
    raw: String,
    serial: usize,
}

pub(crate) fn convert<D: Dom + ?Sized>(dom: &D, root: D::NodeId, options: &Options) -> String {
    let mut machine = Machine {
        dom,
        options,
        tasks: vec![Task::Visit(root, 0)],
        writers: vec![Writer::default()],
        lists: Vec::new(),
        tables: Vec::new(),
        raw: String::new(),
        serial: 0,
    };
    machine.run();
    machine.take_writer().into_string()
}

impl<'a, D: Dom + ?Sized> Machine<'a, D> {
    fn run(&mut self) {
        while let Some(task) = self.tasks.pop() {
            match task {
                Task::Visit(node, depth) => self.visit(node, depth),
                Task::Children(Some(node), depth) => {
                    // Keep one sibling continuation, not a vector of every
                    // child. Traversal storage is bounded by depth, not width.
                    self.tasks
                        .push(Task::Children(self.dom.next_sibling(node), depth));
                    self.tasks.push(Task::Visit(node, depth));
                }
                Task::Children(None, _) | Task::RawChildren(None, _, _) => {}
                Task::Boundary => self.writer().boundary(2),
                Task::PopStyle => self.writer().pop_style(),
                Task::EndLink(serial) => self.writer().end_link(serial),
                Task::EndQuote => {
                    let content = self.take_writer().into_string();
                    let mut quote = String::new();
                    for line in content.lines() {
                        if !quote.is_empty() {
                            quote.push('\n');
                        }
                        quote.push('>');
                        if !line.is_empty() {
                            quote.push(' ');
                            quote.push_str(line);
                        }
                    }
                    self.writer().block(quote.into(), 2, 2);
                }
                Task::EndHeading(level) => {
                    let content = self.take_writer().into_string();
                    if !content.is_empty() {
                        let heading =
                            format!("{} {}", "#".repeat(level), content.replace('\n', " "));
                        self.writer().block(heading.into(), 2, 2);
                    } else {
                        self.writer().boundary(2);
                    }
                }
                Task::EndList(before) => {
                    self.lists.pop();
                    let content = self.take_writer();
                    let has_blocks = self.writer().has_blocks;
                    self.writer().block(content, before, 2);
                    if before == 1 {
                        // A nested list alone does not make its parent item loose.
                        self.writer().has_blocks = has_blocks;
                    }
                }
                Task::EndItem(marker) => {
                    let loose = self.writer().has_blocks;
                    let content = self.take_writer().into_string();
                    if content.is_empty() {
                        continue;
                    }
                    let mut item = String::new();
                    for (index, line) in content.lines().enumerate() {
                        if index == 0 {
                            item.push_str(&marker);
                        } else {
                            item.push('\n');
                            if !line.is_empty() {
                                for _ in 0..marker.len() {
                                    item.push(' ');
                                }
                            }
                        }
                        item.push_str(line);
                    }
                    let list = self.lists.last_mut().expect("list item belongs to a list");
                    let separation = if loose || list.loose { 2 } else { 1 };
                    list.loose |= loose;
                    self.writer().block(item.into(), separation, separation);
                }
                Task::RawChildren(Some(node), depth, inline) => self.raw_node(node, depth, inline),
                Task::EndRawBlock => {
                    if !self.raw.is_empty() && !self.raw.ends_with('\n') {
                        self.raw.push('\n');
                    }
                }
                Task::EndCode => {
                    let text = std::mem::take(&mut self.raw);
                    let preformatted = self.options.preformatted_code;
                    self.writer().code_with_edges(&text, preformatted);
                }
                Task::EndPre(language) => self.end_pre(language),
                Task::TableCell => self.table_cell(),
                Task::EndTableCell => {
                    let content = self.take_writer();
                    let table = self.tables.last_mut().expect("cell belongs to a table");
                    table.finish_cell(content);
                }
            }
        }
    }

    fn writer(&mut self) -> &mut Writer<'a> {
        self.writers.last_mut().expect("conversion has an output")
    }

    fn capture(&mut self) {
        let writer = self.writer().child();
        self.writers.push(writer);
    }

    fn take_writer(&mut self) -> Output {
        let writer = self.writers.pop().expect("capture has an output");
        if let Some(parent) = self.writers.last_mut() {
            parent.last_link = parent.last_link.max(writer.last_link);
        }
        writer.finish()
    }

    fn children(&mut self, node: D::NodeId, depth: usize) {
        if depth < self.options.max_depth {
            self.tasks
                .push(Task::Children(self.dom.first_child(node), depth));
        }
    }

    fn style(&mut self, style: Style<'a>) {
        if self.writer().push_style(style) {
            self.tasks.push(Task::PopStyle);
        }
    }

    fn visit(&mut self, node: D::NodeId, depth: usize) {
        if depth >= self.options.max_depth {
            return;
        }
        let tag = match self.dom.node_kind(node) {
            NodeKind::Document => {
                self.children(node, depth + 1);
                return;
            }
            NodeKind::Text(text) => {
                self.writer().text(text);
                return;
            }
            NodeKind::Other => return,
            NodeKind::Element(tag) => tag,
        };
        if let Some(table) = self.tables.last_mut() {
            table.visit_element(tag);
        }
        match tag {
            "head" | "script" | "style" | "noscript" | "template" => return,
            "br" => {
                self.writer().hard_break();
                return;
            }
            "hr" => {
                self.writer().block("---".into(), 2, 2);
                return;
            }
            "strong" | "b" => self.style(Style::Strong),
            "em" | "i" => self.style(Style::Emphasis),
            "del" | "s" | "strike" => self.style(Style::Strike),
            "a" => {
                if let Some(href) = self
                    .dom
                    .attribute(node, "href")
                    .filter(|href| !href.is_empty())
                {
                    self.serial += 1;
                    let serial = self.serial;
                    let style = Style::Link {
                        serial,
                        href,
                        title: self.dom.attribute(node, "title"),
                    };
                    if self.writer().push_style(style) {
                        self.tasks.push(Task::EndLink(serial));
                    }
                }
            }
            "img" => {
                let alt = self.dom.attribute(node, "alt").unwrap_or_default();
                let title = self.dom.attribute(node, "title");
                if let Some(src) = self
                    .dom
                    .attribute(node, "src")
                    .filter(|src| !src.is_empty())
                {
                    self.writer().image(alt, src, title);
                }
                return;
            }
            "code" | "pre" => {
                self.raw.clear();
                let end = if tag == "pre" {
                    Task::EndPre(self.language(node))
                } else {
                    Task::EndCode
                };
                self.tasks.push(end);
                if depth + 1 < self.options.max_depth {
                    self.tasks.push(Task::RawChildren(
                        self.dom.first_child(node),
                        depth + 1,
                        tag == "code",
                    ));
                }
                return;
            }
            "h1" | "h2" | "h3" | "h4" | "h5" | "h6" => {
                self.capture();
                self.writer().heading();
                self.tasks
                    .push(Task::EndHeading((tag.as_bytes()[1] - b'0') as usize));
            }
            "blockquote" => {
                self.capture();
                self.tasks.push(Task::EndQuote);
            }
            "ul" | "ol" => self.start_list(node, tag == "ol"),
            "li" => self.start_item(),
            "table" => {
                if let Some(mut table) =
                    Table::from_dom(self.dom, node, depth, self.options.max_depth)
                {
                    let captions = table.take_captions();
                    self.tables.push(table);
                    self.tasks.push(Task::TableCell);
                    for caption in captions.into_iter().rev() {
                        self.tasks.push(Task::Visit(caption.node, caption.depth));
                    }
                    return;
                }
                self.writer().boundary(2);
                self.tasks.push(Task::Boundary);
            }
            _ if is_block(tag) => {
                self.writer().boundary(2);
                self.tasks.push(Task::Boundary);
            }
            _ => {}
        }
        self.children(node, depth + 1);
    }

    fn table_cell(&mut self) {
        let table = self.tables.last_mut().expect("table conversion is active");
        if let Some(cell) = table.next_cell() {
            self.capture();
            self.writer().table_cell();
            self.tasks.push(Task::TableCell);
            self.tasks.push(Task::EndTableCell);
            self.children(cell.node, cell.depth + 1);
        } else {
            let table = self.tables.pop().expect("table conversion is active");
            self.writer().block(table.finish(), 2, 2);
        }
    }

    fn start_list(&mut self, node: D::NodeId, ordered: bool) {
        let before = if self
            .lists
            .last()
            .is_some_and(|list| self.writers.len() == list.output_depth + 1)
        {
            1
        } else {
            2
        };
        let next = self
            .dom
            .attribute(node, "start")
            .and_then(|s| s.trim().parse().ok())
            .unwrap_or(1);
        self.capture();
        self.lists.push(List {
            output_depth: self.writers.len(),
            ordered,
            next,
            loose: false,
        });
        self.tasks.push(Task::EndList(before));
    }

    fn start_item(&mut self) {
        let Some(list) = self
            .lists
            .last_mut()
            .filter(|list| list.output_depth == self.writers.len())
        else {
            self.writer().boundary(2);
            self.tasks.push(Task::Boundary);
            return;
        };
        let marker = if list.ordered {
            let marker = format!("{}. ", list.next);
            list.next = list.next.saturating_add(1);
            marker
        } else {
            "- ".to_owned()
        };
        self.capture();
        self.tasks.push(Task::EndItem(marker));
    }

    fn language(&self, node: D::NodeId) -> Option<&'a str> {
        let mut child = self.dom.first_child(node);
        while let Some(id) = child {
            if self.dom.node_kind(id) == NodeKind::Element("code")
                && let Some(language) = class_language(self.dom.attribute(id, "class"))
            {
                return Some(language);
            }
            child = self.dom.next_sibling(id);
        }
        class_language(self.dom.attribute(node, "class"))
            .or(self.options.default_code_language.as_deref())
    }

    fn raw_node(&mut self, node: D::NodeId, depth: usize, inline: bool) {
        self.tasks.push(Task::RawChildren(
            self.dom.next_sibling(node),
            depth,
            inline,
        ));
        if depth >= self.options.max_depth {
            return;
        }
        match self.dom.node_kind(node) {
            NodeKind::Text(text) => self.raw.push_str(text),
            // Inline code normalizes line endings to spaces. Keep explicit
            // breaks without converting descendant formatting into Markdown.
            NodeKind::Element("br") => self.raw.push('\n'),
            NodeKind::Document | NodeKind::Element(_) if depth + 1 < self.options.max_depth => {
                if !inline
                    && matches!(self.dom.node_kind(node), NodeKind::Element(tag) if is_structural_block(tag))
                {
                    if !self.raw.is_empty() && !self.raw.ends_with('\n') {
                        self.raw.push('\n');
                    }
                    self.tasks.push(Task::EndRawBlock);
                }
                self.tasks.push(Task::RawChildren(
                    self.dom.first_child(node),
                    depth + 1,
                    inline,
                ));
            }
            _ => {}
        }
    }

    fn end_pre(&mut self, language: Option<&str>) {
        let text = std::mem::take(&mut self.raw);
        if text.trim().is_empty() {
            self.writer().boundary(2);
            return;
        }
        // CommonMark code blocks normalize CRLF/CR and require a closing fence
        // on its own line. Preserve all other spaces and blank lines verbatim.
        let text = text.replace("\r\n", "\n").replace('\r', "\n");
        // The info string occupies one line. Tilde fences also allow language
        // names containing backticks without changing the name itself.
        let language = language
            .unwrap_or_default()
            .split(['\r', '\n'])
            .next()
            .unwrap();
        let marker = if language.contains('`') { '~' } else { '`' };
        let fence = marker
            .to_string()
            .repeat(3.max(longest_run(&text, marker) + 1));
        let mut block = fence.clone();
        for ch in language.chars() {
            match ch {
                '\\' => block.push_str("\\\\"),
                '&' => block.push_str("&amp;"),
                _ => block.push(ch),
            }
        }
        block.push('\n');
        block.push_str(&text);
        if !text.is_empty() && !text.ends_with('\n') {
            block.push('\n');
        }
        block.push_str(&fence);
        self.writer().block(block.into(), 2, 2);
    }
}

fn class_language(class: Option<&str>) -> Option<&str> {
    class?
        .split_ascii_whitespace()
        .find_map(|part| part.strip_prefix("language-").filter(|s| !s.is_empty()))
}

fn is_block(tag: &str) -> bool {
    is_structural_block(tag)
        || matches!(
            tag,
            "audio" | "canvas" | "frameset" | "isindex" | "noframes" | "output"
        )
}

fn is_structural_block(tag: &str) -> bool {
    matches!(
        tag,
        "address"
            | "article"
            | "aside"
            | "blockquote"
            | "body"
            | "caption"
            | "center"
            | "dd"
            | "details"
            | "dialog"
            | "dir"
            | "div"
            | "dl"
            | "dt"
            | "fieldset"
            | "figcaption"
            | "figure"
            | "footer"
            | "form"
            | "h1"
            | "h2"
            | "h3"
            | "h4"
            | "h5"
            | "h6"
            | "header"
            | "hgroup"
            | "html"
            | "legend"
            | "li"
            | "main"
            | "menu"
            | "nav"
            | "ol"
            | "p"
            | "pre"
            | "section"
            | "summary"
            | "table"
            | "tbody"
            | "td"
            | "tfoot"
            | "th"
            | "thead"
            | "tr"
            | "ul"
    )
}