markdown_syntax/

serialize.rs

//! AST to Markdown. The verbs live on [`Document`]
//! ([`to_markdown`](Document::to_markdown) /
//! [`to_markdown_with`](Document::to_markdown_with)); [`SerializeOptions`] tunes
//! the output style. The document is validated first, so serialization can fail
//! with a [`SerializeError`].
//!
//! A document with its source is written partly from it (`reuse`, decision
//! 0017): each node that corresponds to the node its source parses to is
//! written as its source, an edited one with its own syntax copied around
//! what changed, and the rest is rendered. Rendering only renders: each node
//! is written in the spelling the AST records for it, or else by one fixed
//! rule, with text, escapes, and character references as recorded. The output
//! is parsed back only to check the blocks written partly from the source.

use alloc::{
    format,
    string::{String, ToString},
    vec::Vec,
};

use alloc::collections::BTreeMap;

use crate::{
    ast::*, diagnostic::Diagnostic, leaves, parse::Definitions, parse::InlineSource, span::Span,
    validate::validate_document,
};

mod reuse;

/// The newline style emitted by the serializer.
#[derive(Clone, Copy, Debug, Eq, PartialEq)]
pub enum LineEnding {
    /// Unix `\n`.
    Lf,
    /// Windows `\r\n`.
    CrLf,
}

impl LineEnding {
    fn as_str(self) -> &'static str {
        match self {
            Self::Lf => "\n",
            Self::CrLf => "\r\n",
        }
    }
}

/// The bullet char [`SerializeOptions::bullet`] writes unordered lists with.
#[derive(Clone, Copy, Debug, Eq, PartialEq)]
pub enum BulletMarker {
    /// `-`.
    Dash,
    /// `*`.
    Asterisk,
    /// `+`.
    Plus,
}

/// The delimiter [`SerializeOptions::ordered_delimiter`] writes ordered-list
/// markers with.
#[derive(Clone, Copy, Debug, Eq, PartialEq)]
pub enum OrderedDelimiter {
    /// `.`, as in `1.`.
    Period,
    /// `)`, as in `1)`.
    Paren,
}

/// Output-style options for serialization. Defaults: LF, trailing newline, and
/// the list markers and code fences the AST records.
#[derive(Clone, Debug, Eq, PartialEq)]
#[non_exhaustive]
pub struct SerializeOptions {
    /// Newline style to emit.
    pub line_ending: LineEnding,
    /// Whether to end the output with a trailing newline.
    pub final_newline: bool,
    /// The bullet for unordered lists: `None`, the default, keeps the marker
    /// each list records; `Some` writes every unordered list with it.
    pub bullet: Option<BulletMarker>,
    /// The delimiter for ordered-list markers: `None`, the default, keeps the
    /// delimiter each list records; `Some` writes every ordered list with it.
    pub ordered_delimiter: Option<OrderedDelimiter>,
    /// The fence char for fenced code blocks: `None`, the default, keeps the
    /// fence each block records; `Some` writes every fenced block with it,
    /// except that an info string holding a backtick always takes tildes.
    pub fence_marker: Option<FenceMarker>,
}

impl Default for SerializeOptions {
    fn default() -> Self {
        Self {
            line_ending: LineEnding::Lf,
            final_newline: true,
            bullet: None,
            ordered_delimiter: None,
            fence_marker: None,
        }
    }
}

/// Why serialization failed.
#[derive(Clone, Debug, Eq, PartialEq)]
pub enum SerializeError {
    /// The AST failed validation; carries the validation diagnostics.
    InvalidDocument(Vec<Diagnostic>),
}

impl Document {
    /// Write this document as Markdown with default options: the source of
    /// every unchanged node, the source of its own syntax around a node whose
    /// children changed, and canonical Markdown for the rest.
    pub fn to_markdown(&self) -> Result<String, SerializeError> {
        self.to_markdown_with(&SerializeOptions::default())
    }

    /// Write this document as Markdown with explicit options. A document
    /// with a source reuses it for each node that corresponds to the node
    /// the source parses to at the same span, and for the own syntax of one
    /// whose children changed; the options apply to the rest.
    pub fn to_markdown_with(&self, options: &SerializeOptions) -> Result<String, SerializeError> {
        Ok(self.write(options)?.0)
    }

    /// The text `to_markdown_with` writes, and how many top-level blocks it
    /// wrote canonically because, written partly from the source, they read
    /// back as something else.
    pub(crate) fn write(
        &self,
        options: &SerializeOptions,
    ) -> Result<(String, usize), SerializeError> {
        let diagnostics = validate_document(self);
        if !diagnostics.is_empty() {
            return Err(SerializeError::InvalidDocument(diagnostics));
        }
        // A leaf keeps its text only while its inlines are the ones the text
        // reads as.
        let current;
        let definitions = Definitions::of(self);
        let blocks = if leaves::any_kept(&self.children) {
            let mut blocks = self.children.clone();
            leaves::visit_mut(&mut blocks, &mut |leaf| {
                if let Some(text) = leaf.source.as_deref() {
                    if crate::parse::parse_inline_tree(text, &definitions).inlines != *leaf.inlines
                    {
                        *leaf.source = None;
                    }
                }
            });
            current = blocks;
            &current
        } else {
            &self.children
        };
        let Some(source) = self.source() else {
            return Ok((write_canonical(blocks, options)?, 0));
        };
        // A block written partly from the source stays so only where the
        // output reads back as it at its place.
        let base = crate::parse(source).document;
        let write =
            |whole: &[bool]| write_reusing(blocks, &base, source, options, &definitions, whole);
        // A `---` break kept opening the document stays only while no later
        // line closes it as frontmatter, which every write is checked for.
        let opens_with_break = matches!(blocks.first(), Some(Block::ThematicBreak(_)));
        let mut whole = alloc::vec![false; blocks.len()];
        let (mut output, mut partly) = write(&whole)?;
        if opens_with_break && opens_frontmatter(&output) {
            whole[0] = true;
            (output, partly) = write(&whole)?;
        }
        if partly.is_empty() {
            return Ok((output, 0));
        }
        let misread = misread(blocks, &output, &partly);
        if misread.is_empty() {
            return Ok((output, 0));
        }
        for index in &misread {
            whole[*index] = true;
        }
        output = write(&whole)?.0;
        if opens_with_break && !whole[0] && opens_frontmatter(&output) {
            whole[0] = true;
            output = write(&whole)?.0;
        }
        Ok((output, misread.len()))
    }
}

/// Whether `output` opens with frontmatter: its first line is a fence that
/// a later line closes.
fn opens_frontmatter(output: &str) -> bool {
    let mut lines = output.split(['\n', '\r']);
    let opening = lines.next().and_then(crate::parse::frontmatter_fence_kind);
    opening.is_some() && lines.any(|line| crate::parse::frontmatter_fence_kind(line) == opening)
}

/// The canonical Markdown of a document's top-level blocks.
fn write_canonical(blocks: &[Block], options: &SerializeOptions) -> Result<String, SerializeError> {
    let mut output = write_blocks(blocks, options, Join::Gap, true, None)?;
    if options.line_ending == LineEnding::CrLf {
        output = lf_to_crlf(&output);
    }
    // Blocks end without a line ending, except an HTML block whose last
    // line is empty (the final newline ends that line too) and an indented
    // code block that keeps its value's `\r` or `\r\n` ending.
    // A last blank line always takes its line ending.
    let ends_with_blank_line = matches!(blocks.last(), Some(Block::BlankLine(_)));
    if options.final_newline
        && !blocks.is_empty()
        && (ends_with_blank_line || !ends_with_carriage_return_ending(&output))
    {
        output.push_str(options.line_ending.as_str());
    }
    Ok(output)
}

/// Writes `blocks` against `base_document`, the parse of `source`: each
/// block matched to the base block at its span as its source when it
/// corresponds to it, partly from the source when only what it holds
/// changed (`reuse`), and canonically otherwise; returns the output and the
/// indices of the blocks written partly from the source, which `whole`
/// writes canonically instead. Between two blocks written from the source
/// and adjacent there the source's own bytes are kept where they read the
/// same (`reuse::keeps_gap`), as are the bytes before the source's first
/// block and after its last when those blocks are written from it there;
/// every other boundary is a blank line.
///
/// A block is written canonically instead where its source would read
/// differently in its new place: one a blank line and a paragraph after it
/// would change, as they extend an unclosed fence, when it comes before a
/// block other than its successor in the source; one the block before it
/// would take in (`moves_into_previous`); and a thematic break that comes to
/// open the document, where it would open frontmatter.
fn write_reusing(
    blocks: &[Block],
    base_document: &Document,
    source: &str,
    options: &SerializeOptions,
    definitions: &Definitions,
    whole: &[bool],
) -> Result<(String, Vec<usize>), SerializeError> {
    let base = &base_document.children;
    let mut correspondence = Correspondence {
        base: base_document,
        sources: None,
    };
    let base_definitions = Definitions::of(base_document);
    let mut reuse = reuse::Reuse {
        source,
        options,
        correspondence: &mut correspondence,
        definitions,
        base_definitions: &base_definitions,
    };
    let span_of = |block: &Block| block.span().expect("a reused block has a span");
    let mut outs = Vec::with_capacity(blocks.len());
    // Whether each block written partly from the source is an open list.
    let mut open_lists = alloc::vec![false; blocks.len()];
    for (index, block) in blocks.iter().enumerate() {
        let found = block.span().and_then(|span| {
            let at = base
                .binary_search_by_key(&span.start, |base| {
                    base.span().map_or(usize::MAX, |span| span.start)
                })
                .ok()?;
            (base[at].span() == Some(span)
                && core::mem::discriminant(&base[at]) == core::mem::discriminant(block))
            .then_some(at)
        });
        outs.push(match found {
            Some(at) => match reuse.block(block, &base[at])? {
                reuse::Out::Kept(_) => (Some(at), None),
                reuse::Out::Lines(_) | reuse::Out::OpenList(_) if whole[index] => (None, None),
                reuse::Out::Lines(lines) => (None, Some((at, lines))),
                reuse::Out::OpenList(lines) => {
                    open_lists[index] = true;
                    (None, Some((at, lines)))
                }
                reuse::Out::Render => (None, None),
            },
            None => (None, None),
        });
    }
    let mut reused: Vec<Option<usize>> = outs.iter().map(|(at, _)| *at).collect();
    // The blocks written partly from the source, with their place in it.
    let mut partly: Vec<Option<(usize, Vec<reuse::Line>)>> =
        outs.into_iter().map(|(_, lines)| lines).collect();
    // Where each block written from the source, whole or partly, stands in
    // the source.
    let placed =
        |reused: &[Option<usize>], partly: &[Option<(usize, Vec<reuse::Line>)>], index: usize| {
            reused[index].or(partly[index].as_ref().map(|(at, _)| *at))
        };
    if blocks.len() == base.len()
        && reused
            .iter()
            .enumerate()
            .all(|(index, at)| *at == Some(index))
    {
        return Ok((source.into(), Vec::new()));
    }
    // Whether the boundary before each block holds the blank line that
    // separates two blocks; the source between two blocks adjacent in the
    // source is kept only where it still does or does not.
    let separated = separated_boundaries(blocks);
    let base_separated = separated_boundaries(base);
    // Whether the source between the block at `index`, placed at `at`, and
    // the one before it, placed at `at - 1`, is kept: the boundary holds a
    // separating blank line exactly when it did, and the source reads the
    // same between them (`reuse::keeps_gap`).
    let keeps_gap = |partly: &[Option<(usize, Vec<reuse::Line>)>], index: usize, at: usize| {
        if index == 0 || at == 0 || separated[index] != base_separated[at] {
            return false;
        }
        let left = base[at - 1].span().expect("a parsed block has a span");
        let right = base[at].span().expect("a parsed block has a span");
        let left_end = left.start + strip_line_ending(&source[left.start..left.end]).len();
        reuse::keeps_gap(
            source,
            &source[left_end..right.start],
            (
                &blocks[index - 1],
                left,
                partly[index - 1]
                    .as_ref()
                    .map(|(_, lines)| lines.as_slice()),
            ),
            (
                &blocks[index],
                right,
                partly[index].as_ref().map(|(_, lines)| lines.as_slice()),
            ),
            open_lists[index - 1],
            true,
        )
    };
    // Backward, so that each block knows whether its successor stays reused,
    // with the source between them.
    for index in (0..blocks.len().saturating_sub(1)).rev() {
        let Some(at) = placed(&reused, &partly, index) else {
            continue;
        };
        if placed(&reused, &partly, index + 1) == Some(at + 1)
            && keeps_gap(&partly, index + 1, at + 1)
        {
            continue;
        }
        match &partly[index] {
            Some((_, lines)) => {
                let mut text = reuse::join(lines, "\n");
                text.push('\n');
                if !closed_by_a_blank_line(&text) {
                    partly[index] = None;
                }
            }
            None => {
                let span = span_of(&blocks[index]);
                if !closed_by_a_blank_line(&source[span.start..span.end]) {
                    reused[index] = None;
                }
            }
        }
    }

    let mut output = String::with_capacity(source.len());
    // The block written before the current one: its index in the source's
    // parse, where its text ends there, and its line ending there when it was
    // reused, and its marker when it is a list.
    let mut previous_at: Option<usize> = None;
    let mut placement = reuse::Placement::default();
    let mut previous_ending = "";
    let mut previous_body_end = 0;
    // Whether the output ends with the source's own trailing bytes, which
    // take the place of the final newline.
    let mut ends_with_source_tail = false;
    // Whether every block since the last one other than a blank line follows
    // its predecessor in the source.
    let mut run_follows = true;
    for (index, block) in blocks.iter().enumerate() {
        // A list written partly from the source keeps its recorded marker,
        // which a list before it may already use.
        if let (Block::List(list), Some(_)) = (block, &partly[index]) {
            if placement.previous_list() == Some(written_marker(list)) {
                partly[index] = None;
            }
        }
        let follows_predecessor = matches!(
            (previous_at, placed(&reused, &partly, index)),
            (Some(previous), Some(at)) if previous + 1 == at && keeps_gap(&partly, index, at)
        );
        // Blank lines between them do not end a list or a footnote
        // definition before an indented line, so a block follows its
        // predecessor only where every block since the last other than a
        // blank line did.
        let follows_run = follows_predecessor && run_follows;
        run_follows = index == 0 || !matches!(block, Block::BlankLine(_)) || follows_run;
        // A block whose source the block before it could take in stays only
        // after its predecessor in the source, written as it was there.
        let taken_in = |text: &str| placement.takes_in(block, text, follows_run);
        if let Some((_, lines)) = &partly[index] {
            if taken_in(&lines[0].body) {
                partly[index] = None;
            }
        }
        let mut at = reused[index];
        if let Some(position) = at {
            let start = span_of(block).start;
            // A break that comes to open the document could open
            // frontmatter; one that opened it there stays a break while no
            // later line closes it as frontmatter (`Document::write`), which
            // `whole` marks.
            let rewritten = matches!(block, Block::ThematicBreak(_))
                && index == 0
                && (position != 0 || whole[0] && source[start..].starts_with("---"))
                || taken_in(&source[start..]);
            if rewritten {
                at = None;
            }
        }
        if index > 0 {
            if follows_predecessor {
                let start = span_of(block).start;
                output.push_str(&source[previous_body_end..start]);
            } else if separated[index] {
                // A reused block keeps its own line ending; one more makes
                // the blank line.
                output.push_str(previous_ending);
                match previous_ending {
                    "" => push_gap(&mut output, options),
                    "\r" => output.push('\r'),
                    _ => output.push_str(options.line_ending.as_str()),
                }
            } else {
                // A blank line is a line of its own, without a separator.
                output.push_str(match previous_ending {
                    "" => options.line_ending.as_str(),
                    own => own,
                });
            }
        }
        previous_ending = "";
        match at {
            Some(position) => {
                let span = span_of(block);
                let body = strip_line_ending(&source[span.start..span.end]);
                let body_end = span.start + body.len();
                if index == 0 && position == 0 {
                    output.push_str(&source[..span.start]);
                }
                output.push_str(body);
                if index + 1 == blocks.len() && position + 1 == base.len() {
                    output.push_str(&source[body_end..]);
                    ends_with_source_tail = true;
                }
                previous_ending = &source[body_end..span.end];
                previous_body_end = body_end;
                let marker = match block {
                    Block::List(list) => Some(written_marker(list)),
                    _ => None,
                };
                placement.wrote(block, marker, false);
            }
            None if partly[index].is_some() => {
                let (position, lines) = partly[index].as_ref().expect("written partly");
                let span = base[*position].span().expect("a parsed block has a span");
                let body_end = span.start + strip_line_ending(&source[span.start..span.end]).len();
                if index == 0 && *position == 0 {
                    output.push_str(&source[..span.start]);
                }
                output.push_str(&reuse::join(lines, options.line_ending.as_str()));
                if index + 1 == blocks.len() && *position + 1 == base.len() {
                    output.push_str(&source[body_end..]);
                    ends_with_source_tail = true;
                }
                previous_body_end = body_end;
                let marker = match block {
                    Block::List(list) => Some(written_marker(list)),
                    _ => None,
                };
                placement.wrote(block, marker, open_lists[index]);
                at = Some(*position);
            }
            None => {
                let mut text = write_blocks(
                    core::slice::from_ref(block),
                    options,
                    Join::Gap,
                    index == 0,
                    placement.previous_list(),
                )?;
                if options.line_ending == LineEnding::CrLf {
                    text = lf_to_crlf(&text);
                }
                output.push_str(&text);
                let marker = match block {
                    Block::List(list) => {
                        Some(list_marker(list, options, placement.previous_list()))
                    }
                    _ => None,
                };
                placement.wrote(block, marker, false);
            }
        }
        previous_at = at;
    }
    let ends_with_blank_line = matches!(blocks.last(), Some(Block::BlankLine(_)));
    if !ends_with_source_tail
        && options.final_newline
        && !blocks.is_empty()
        && (ends_with_blank_line || !ends_with_carriage_return_ending(&output))
    {
        // A reused last block keeps its own line ending, which can be part
        // of its value, as in indented code.
        output.push_str(match previous_ending {
            "" => options.line_ending.as_str(),
            own => own,
        });
    }
    let partly = partly
        .iter()
        .enumerate()
        .filter_map(|(index, lines)| lines.as_ref().map(|_| index))
        .collect();
    Ok((output, partly))
}

/// The blocks among `partly`, indices of `blocks` written partly from the
/// source, that `output` reads back as something else in their place: each
/// one that differs, or every one when the output holds a different number
/// of top-level blocks.
fn misread(blocks: &[Block], output: &str, partly: &[usize]) -> Vec<usize> {
    let read = crate::parse(output).document.children;
    if read.len() != blocks.len() {
        return partly.to_vec();
    }
    partly
        .iter()
        .copied()
        .filter(|index| !reads_as(&read[*index], &blocks[*index]))
        .collect()
}

/// Whether `read`, a block of the parsed output, reads as `block` as
/// "Round-trip stability" compares them, an ordered item without a number
/// reading as the number it is written with.
fn reads_as(read: &Block, block: &Block) -> bool {
    let mut read = crate::compare::normalized_blocks(core::slice::from_ref(read));
    let mut block = crate::compare::normalized_blocks(core::slice::from_ref(block));
    number_items(&mut read);
    number_items(&mut block);
    read == block
}

/// Gives each ordered item without a number the one it is written with.
fn number_items(blocks: &mut [Block]) {
    for block in blocks {
        match block {
            Block::List(list) => {
                let mut previous = None;
                for item in &mut list.children {
                    if list.ordered {
                        item.number = Some(item_number(item, &mut previous));
                    }
                    number_items(&mut item.children);
                }
            }
            Block::BlockQuote(node) => number_items(&mut node.children),
            Block::Alert(node) => number_items(&mut node.children),
            Block::FootnoteDefinition(node) => number_items(&mut node.children),
            Block::ContainerDirective(node) => number_items(&mut node.children),
            Block::HtmlContainer(node) => {
                if let HtmlContainerContent::Blocks(children) = &mut node.content {
                    number_items(children);
                }
            }
            Block::Paragraph(_)
            | Block::Heading(_)
            | Block::ThematicBreak(_)
            | Block::CodeBlock(_)
            | Block::HtmlBlock(_)
            | Block::Definition(_)
            | Block::Table(_)
            | Block::MathBlock(_)
            | Block::Frontmatter(_)
            | Block::LeafDirective(_)
            | Block::BlankLine(_) => {}
        }
    }
}

/// Whether a block corresponds to a block of the document its source parses
/// to: their spans and own fields are equal and their children correspond,
/// where a leaf that keeps its text corresponds to the leaf at its span whose
/// inline source is that text.
struct Correspondence<'a> {
    base: &'a Document,
    /// The inline source of each leaf of `base`, by span, read once the
    /// first leaf that keeps its text needs it.
    sources: Option<BTreeMap<(usize, usize), InlineSource>>,
}

impl Correspondence<'_> {
    /// The inline source of each leaf of the base, by span.
    fn sources(&mut self) -> &BTreeMap<(usize, usize), InlineSource> {
        let base_document = self.base;
        self.sources.get_or_insert_with(|| {
            let mut sources = BTreeMap::new();
            base_document.each_leaf_source(|_, span, source| {
                sources.insert((span.start, span.end), source);
            });
            sources
        })
    }

    /// The inline source of the base leaf at `span`.
    fn source_of(&mut self, span: Span) -> Option<&InlineSource> {
        self.sources().get(&(span.start, span.end))
    }

    fn holds(&mut self, block: &Block, base: &Block) -> bool {
        if !leaves::any_kept(core::slice::from_ref(block)) {
            return block == base;
        }
        let sources = self.sources();
        // Each leaf whose text is its base leaf's inline source takes the
        // inlines that text reads as there.
        let mut block = block.clone();
        leaves::visit_mut(core::slice::from_mut(&mut block), &mut |leaf| {
            let (Some(text), Some(span)) = (leaf.source.as_deref(), leaf.span) else {
                return;
            };
            if let Some(source) = sources
                .get(&(span.start, span.end))
                .filter(|source| source.text == text)
            {
                crate::parse::inline_spans_to_source(source, leaf.inlines);
                *leaf.source = None;
            }
        });
        block == *base
    }
}

/// For each block, whether the boundary before it holds the blank line that
/// separates two blocks: one written after a block other than a blank line,
/// before a block or before the blank lines that precede one.
fn separated_boundaries(blocks: &[Block]) -> Vec<bool> {
    let mut separated = alloc::vec![false; blocks.len()];
    let mut block_follows = false;
    for index in (1..blocks.len()).rev() {
        block_follows |= !matches!(blocks[index], Block::BlankLine(_));
        separated[index] = block_follows && !matches!(blocks[index - 1], Block::BlankLine(_));
    }
    separated
}

/// Whether a block written from the source, whose text opens with `text`,
/// reads differently where it does not follow its predecessor in the
/// source, after `last`, the last block written before it other than a blank
/// line, and `previous_list`, the marker of a list written just before it:
/// indentation after a list or a footnote definition, which blank lines do
/// not end, continues them; four columns of it on a block other than
/// indented code read as indented code, as a paragraph opened by definitions
/// took them as a continuation; and a list after a list written with its
/// marker continues that list. A blank line reads as one wherever it stands.
/// `rewritten_after_an_edit` in `tests/support/edits.rs` states the same rule
/// for the simulated edits.
fn moves_into_previous(
    block: &Block,
    text: &str,
    last: Option<&Block>,
    previous_list: Option<char>,
) -> bool {
    match block {
        Block::BlankLine(_) => false,
        Block::List(list) if previous_list == Some(written_marker(list)) => true,
        _ if matches!(last, Some(Block::List(_) | Block::FootnoteDefinition(_))) => {
            text.starts_with([' ', '\t'])
        }
        Block::CodeBlock(_) => false,
        _ => opens_with_code_indent(text),
    }
}

/// Whether `text` opens with four columns of indentation, which starts
/// indented code where no paragraph continues.
fn opens_with_code_indent(text: &str) -> bool {
    let mut column = 0;
    for char in text.chars() {
        match char {
            ' ' => column += 1,
            '\t' => column += 4 - column % 4,
            _ => break,
        }
        if column >= 4 {
            return true;
        }
    }
    false
}

/// Whether the block parsed from `text`, its source with its line ending,
/// keeps that text when a blank line and a paragraph follow it, as a closed
/// fence does and an unclosed one does not. `tests/support/edits.rs` holds a
/// copy.
fn closed_by_a_blank_line(text: &str) -> bool {
    let gap = if text.ends_with('\n') {
        "\n"
    } else if text.ends_with('\r') {
        "\r"
    } else {
        "\n\n"
    };
    let probe = format!("{text}{gap}x");
    let children = crate::parse(&probe).document.children;
    let Some((Block::Paragraph(last), rest)) = children.split_last() else {
        return false;
    };
    last.meta
        .span
        .is_some_and(|span| span.start + 1 == probe.len())
        && rest.iter().all(|block| {
            matches!(block, Block::BlankLine(_))
                || block.span().is_some_and(|span| span.end <= text.len())
        })
}

/// Pushes the gap between two top-level blocks in the options' line ending.
fn push_gap(output: &mut String, options: &SerializeOptions) {
    let from = output.len();
    push_block_gap(output);
    if options.line_ending == LineEnding::CrLf {
        let gap = lf_to_crlf(&output[from..]);
        output.truncate(from);
        output.push_str(&gap);
    }
}

/// `text` without one trailing `\n`, `\r\n`, or `\r`.
fn strip_line_ending(text: &str) -> &str {
    text.strip_suffix("\r\n")
        .or_else(|| text.strip_suffix('\n'))
        .or_else(|| text.strip_suffix('\r'))
        .unwrap_or(text)
}

/// Rewrites each bare `\n` as `\r\n`; existing `\r\n` and `\r` endings that
/// verbatim values carry stay as they are.
fn lf_to_crlf(input: &str) -> String {
    let mut output = String::with_capacity(input.len());
    let mut previous = '\0';
    for ch in input.chars() {
        if ch == '\n' && previous != '\r' {
            output.push('\r');
        }
        output.push(ch);
        previous = ch;
    }
    output
}

fn ends_with_carriage_return_ending(input: &str) -> bool {
    input.ends_with('\r') || input.ends_with("\r\n")
}

/// Separates two blocks with one blank line. A block that already ends its
/// last line with `\r` or `\r\n` gets only the blank line, written so that it
/// cannot merge into that ending.
fn push_block_gap(output: &mut String) {
    if output.ends_with('\r') {
        output.push('\r');
    } else if output.ends_with("\r\n") {
        output.push('\n');
    } else {
        output.push_str("\n\n");
    }
}

/// How sibling blocks are separated.
#[derive(Clone, Copy, Eq, PartialEq)]
enum Join {
    /// By a blank line.
    Gap,
    /// By a line ending alone, as the blocks of a tight list item are.
    Line,
}

/// Writes a sequence of sibling blocks. `document_start` is true only for the
/// top-level document body, where the first block sits at byte 0 and a
/// contiguous `---` would open frontmatter.
///
/// Two blocks are separated by `join`, and each `BlankLine` is one more empty
/// line: between two blocks after their separator, and before the first
/// block or after the last on its own.
fn write_blocks(
    blocks: &[Block],
    options: &SerializeOptions,
    join: Join,
    document_start: bool,
    previous_list: Option<char>,
) -> Result<String, SerializeError> {
    let mut output = String::new();
    // The marker of the list written right before the current block; ordered
    // and unordered markers never share a char.
    let mut previous_list = previous_list;
    // Whether a block other than a blank line was written, and the blank
    // lines since the last one.
    let mut wrote_block = false;
    let mut blank_lines = 0;
    for (index, block) in blocks.iter().enumerate() {
        if let Block::BlankLine(_) = block {
            blank_lines += 1;
            continue;
        }
        if wrote_block {
            match join {
                Join::Gap => push_block_gap(&mut output),
                Join::Line => output.push('\n'),
            }
        }
        output.extend(core::iter::repeat_n('\n', blank_lines));
        blank_lines = 0;
        let after_paragraph_line =
            join == Join::Line && index > 0 && matches!(blocks[index - 1], Block::Paragraph(_));
        let written = match block {
            Block::List(list) => {
                let marker = list_marker(list, options, previous_list);
                previous_list = Some(marker);
                write_list(list, options, marker)?
            }
            Block::ThematicBreak(node) => {
                previous_list = None;
                write_thematic_break(node, document_start && index == 0, after_paragraph_line)
            }
            _ => {
                previous_list = None;
                write_block(block, options)?
            }
        };
        output.push_str(&written);
        wrote_block = true;
    }
    // Trailing blank lines each end the line before them; with no block,
    // the first of them starts the output.
    let trailing = if wrote_block {
        blank_lines
    } else {
        blank_lines.saturating_sub(1)
    };
    output.extend(core::iter::repeat_n('\n', trailing));
    Ok(output)
}

/// Writes `block` as canonical Markdown, and a leaf that keeps its text as
/// that text: `to_markdown_with` has cleared every kept text its leaf's
/// inlines no longer read as.
fn write_block(block: &Block, options: &SerializeOptions) -> Result<String, SerializeError> {
    match block {
        Block::BlankLine(_) => Ok(String::new()),
        Block::Paragraph(node) => match &node.source {
            Some(text) => Ok(text.clone()),
            None => write_inlines(&node.children, Context::BLOCK),
        },
        Block::Heading(node) => write_heading(node),
        Block::ThematicBreak(node) => Ok(write_thematic_break(node, false, false)),
        Block::BlockQuote(node) => {
            let inner = write_blocks(&node.children, options, Join::Gap, false, None)?;
            // No content, or one blank line, is one `>` line.
            Ok(if inner.is_empty() {
                ">".into()
            } else {
                quote_lines(&inner)
            })
        }
        Block::Alert(node) => write_alert(node, options),
        Block::List(node) => write_list(node, options, list_marker(node, options, None)),
        Block::CodeBlock(node) => Ok(write_code_block(node, options)),
        // An HTML block's lines are joined with `\n`, so a value ending in one
        // ends with an empty line that belongs to the block (an unclosed
        // comment, say); it is written as it is.
        Block::HtmlBlock(node) => Ok(node.value.clone()),
        Block::HtmlContainer(node) => write_html_container(node, options),
        Block::Definition(node) => Ok(write_definition(node)),
        Block::FootnoteDefinition(node) => {
            let inner = write_blocks(&node.children, options, Join::Gap, false, None)?;
            // Content opening with a space or a tab (indented code, say)
            // starts on the line after the label, as in a list item, where
            // the label's line would take that whitespace as its separator.
            Ok(if inner.starts_with([' ', '\t']) {
                format!(
                    "[^{}]:\n{}",
                    node.label,
                    prefix_container_lines(&inner, "    ")
                )
            } else {
                format!("[^{}]: {}", node.label, indent_continuation(&inner))
            })
        }
        Block::Table(node) => write_table(node),
        Block::MathBlock(node) => {
            let fence = block_math_fence(&node.value);
            Ok(fenced_body(&fence, &node.value, &fence))
        }
        Block::Frontmatter(node) => {
            let fence = match node.kind {
                FrontmatterKind::Yaml => "---",
                FrontmatterKind::Toml => "+++",
            };
            // The value holds the lines between the fences joined with `\n`,
            // so a final `\n` is an empty last line.
            Ok(format!("{fence}\n{}\n{fence}", node.value))
        }
        Block::LeafDirective(node) => Ok(format!(
            "::{}{}{}",
            node.name,
            write_directive_label(&node.label, node.label_source.as_deref())?,
            write_attributes(&node.attributes)
        )),
        Block::ContainerDirective(node) => {
            let mut inner = write_blocks(&node.children, options, Join::Gap, false, None)?;
            let fence = directive_fence(&inner);
            // Content ends with a line ending before the closing fence; an
            // empty directive takes no blank line, which would loosen a list
            // holding it.
            if !node.children.is_empty() {
                inner.push('\n');
            }
            Ok(format!(
                "{fence}{}{}{}\n{inner}{fence}",
                node.name,
                write_directive_label(&node.label, node.label_source.as_deref())?,
                write_attributes(&node.attributes)
            ))
        }
    }
}

/// A thematic break. A dash break is written spaced where a contiguous `---`
/// would read otherwise: at the document start, where it opens frontmatter,
/// and on the line after a paragraph's, where it underlines a setext heading.
fn write_thematic_break(
    node: &ThematicBreak,
    at_document_start: bool,
    after_paragraph_line: bool,
) -> String {
    match node.marker {
        ThematicBreakMarker::Dash if at_document_start || after_paragraph_line => "- - -".into(),
        ThematicBreakMarker::Dash => "---".into(),
        ThematicBreakMarker::Asterisk => "***".into(),
        ThematicBreakMarker::Underscore => "___".into(),
    }
}

/// Whether `node` is written as a setext heading: a setext underline can only
/// express depth 1 (`=`) or 2 (`-`), and it underlines content.
pub(crate) fn writes_setext(node: &Heading) -> bool {
    node.kind == HeadingKind::Setext && matches!(node.depth, 1 | 2) && !node.children.is_empty()
}

fn write_heading(node: &Heading) -> Result<String, SerializeError> {
    let setext = writes_setext(node);
    let content = match one_line(node.source.as_deref(), !setext) {
        Some(text) => text.into(),
        None => write_inlines(&node.children, Context::HEADING)?,
    };
    if setext {
        let marker = if node.depth == 1 { "=" } else { "-" };
        return Ok(format!(
            "{content}\n{}",
            marker.repeat(content.len().max(3))
        ));
    }
    let hashes = "#".repeat(usize::from(node.depth));
    Ok(if content.is_empty() {
        hashes
    } else if reads_as_closing_sequence(&content) {
        // Content ending in what would read as the closing sequence keeps it
        // by a closing sequence of its own.
        format!("{hashes} {content} #")
    } else {
        format!("{hashes} {content}")
    })
}

/// Whether written ATX content ends in a `#` run that is all of it or follows
/// a space or a tab, which a reader strips as the closing sequence.
fn reads_as_closing_sequence(content: &str) -> bool {
    let before = content.trim_end_matches('#');
    before.len() < content.len() && (before.is_empty() || before.ends_with([' ', '\t']))
}

fn write_definition(node: &Definition) -> String {
    let destination = write_destination(&node.destination, node.destination_kind);
    let mut output = format!("[{}]: {}", node.label, destination);
    if let Some(title) = &node.title {
        output.push(' ');
        output.push_str(&write_title(title));
    }
    output
}

fn write_html_container(
    node: &HtmlContainer,
    options: &SerializeOptions,
) -> Result<String, SerializeError> {
    match &node.content {
        HtmlContainerContent::Blocks(children) => {
            let inner = write_blocks(children, options, Join::Gap, false, None)?;
            // Content takes a blank line before the closing tag; blank lines
            // alone stand between the tags.
            let blank_lines = children
                .iter()
                .all(|block| matches!(block, Block::BlankLine(_)));
            Ok(if blank_lines {
                format!(
                    "{}\n{}{}",
                    node.opening.raw,
                    "\n".repeat(children.len()),
                    node.closing.raw
                )
            } else {
                format!("{}\n{}\n\n{}", node.opening.raw, inner, node.closing.raw)
            })
        }
        HtmlContainerContent::Inlines(children) => Ok(format!(
            "{}{}{}",
            node.opening.raw,
            write_inlines(children, Context::BLOCK)?,
            node.closing.raw
        )),
    }
}

fn write_alert(node: &Alert, options: &SerializeOptions) -> Result<String, SerializeError> {
    let mut output = String::from("> [!");
    output.push_str(alert_kind_name(node.kind));
    output.push(']');
    if let Some(title) = &node.title {
        if !title.is_empty() {
            output.push(' ');
            // A line ending would end the marker's line.
            output.push_str(&title.replace(['\n', '\r'], " "));
        }
    }
    let inner = write_blocks(&node.children, options, Join::Gap, false, None)?;
    if !inner.is_empty() {
        output.push('\n');
        output.push_str(&quote_lines(&inner));
    } else if !node.children.is_empty() {
        // One blank line.
        output.push_str("\n>");
    }
    Ok(output)
}

/// `inner` with every line in the quote: an empty last line, which a value
/// ending in a line ending holds, takes `>` too.
fn quote_lines(inner: &str) -> String {
    let mut output = prefix_container_lines(inner, "> ");
    if inner.ends_with(['\n', '\r']) {
        output.push('>');
    }
    output
}

fn alert_kind_name(kind: AlertKind) -> &'static str {
    match kind {
        AlertKind::Note => "NOTE",
        AlertKind::Tip => "TIP",
        AlertKind::Important => "IMPORTANT",
        AlertKind::Warning => "WARNING",
        AlertKind::Caution => "CAUTION",
    }
}

/// The marker char a list is written with: the one the options replace it
/// with, or else the one the AST records. A marker that the list written right
/// before it in the same container, `previous`, also uses yields to the next
/// one in the order `-`, `*`, `+`, or `.`, `)`; validation keeps two adjacent
/// recorded markers apart, so only a replaced marker can meet this.
fn list_marker(list: &List, options: &SerializeOptions, previous: Option<char>) -> char {
    let marker = if list.ordered {
        options
            .ordered_delimiter
            .map_or_else(|| written_marker(list), ordered_delimiter_char)
    } else {
        options
            .bullet
            .map_or_else(|| written_marker(list), bullet_char)
    };
    if previous == Some(marker) {
        let order: &[char] = if list.ordered {
            &['.', ')']
        } else {
            &['-', '*', '+']
        };
        let at = order.iter().position(|char| *char == marker).unwrap_or(0);
        return order[(at + 1) % order.len()];
    }
    marker
}

/// The marker char the AST records for a list: its bullet, or its ordered
/// delimiter. A delimiter of the other list kind is written `-` or `.`.
/// Serialization and validation's adjacent-list rule both read it.
pub(crate) fn written_marker(list: &List) -> char {
    match (list.ordered, list.delimiter) {
        (false, ListDelimiter::Asterisk) => '*',
        (false, ListDelimiter::Plus) => '+',
        (false, _) => '-',
        (true, ListDelimiter::Paren) => ')',
        (true, _) => '.',
    }
}

fn bullet_char(bullet: BulletMarker) -> char {
    match bullet {
        BulletMarker::Dash => '-',
        BulletMarker::Asterisk => '*',
        BulletMarker::Plus => '+',
    }
}

fn ordered_delimiter_char(delimiter: OrderedDelimiter) -> char {
    match delimiter {
        OrderedDelimiter::Period => '.',
        OrderedDelimiter::Paren => ')',
    }
}

fn write_list(
    node: &List,
    options: &SerializeOptions,
    marker_char: char,
) -> Result<String, SerializeError> {
    let mut output = String::new();
    // The number the previous item was written with.
    let mut previous: Option<u64> = None;
    for (index, item) in node.children.iter().enumerate() {
        if index > 0 {
            output.push_str(if node.tight { "\n" } else { "\n\n" });
        }
        output.push_str(&write_item(
            node,
            item,
            options,
            marker_char,
            &mut previous,
        )?);
    }
    Ok(output)
}

/// The number an ordered item is written with: its own, or else one more than
/// the previous item's, `previous`, which it then becomes.
fn item_number(item: &ListItem, previous: &mut Option<u64>) -> u64 {
    // An item without a number counts on from the previous one, and stops at
    // the largest number a marker can hold.
    let number = item.number.unwrap_or_else(|| {
        previous.map_or(1, |previous| {
            previous
                .saturating_add(1)
                .min(crate::parse::MAX_ORDERED_NUMBER)
        })
    });
    *previous = Some(number);
    number
}

/// One item of `list`, written with `marker_char`; `previous` is the number
/// the item before it was written with.
fn write_item(
    list: &List,
    item: &ListItem,
    options: &SerializeOptions,
    marker_char: char,
    previous: &mut Option<u64>,
) -> Result<String, SerializeError> {
    let marker = if list.ordered {
        format!("{}{marker_char} ", item_number(item, previous))
    } else {
        format!("{marker_char} ")
    };
    let inner = write_item_blocks(&item.children, options, list.tight, item.checked)?;
    let break_after_bullet = breaks_after_bullet(list, item, marker_char);
    let opens_with_whitespace = opens_with_padding(&inner);
    Ok(if break_after_bullet || opens_with_whitespace {
        format!(
            "{}\n{}",
            marker.trim_end(),
            prefix_container_lines(&inner, &" ".repeat(marker.len()))
        )
    } else {
        format!("{marker}{}", indent_after_first_line(&inner, marker.len()))
    })
}

fn thematic_break_char(marker: ThematicBreakMarker) -> char {
    match marker {
        ThematicBreakMarker::Dash => '-',
        ThematicBreakMarker::Asterisk => '*',
        ThematicBreakMarker::Underscore => '_',
    }
}

/// An item's blocks; a task item's, `task` holding whether it is checked,
/// open with its checkbox: at the start of the first paragraph after the
/// definitions the item starts with, or else of the item.
/// A thematic break of the bullet's own char right after the bullet would
/// read as one longer break, so it starts on the next line.
fn breaks_after_bullet(list: &List, item: &ListItem, marker_char: char) -> bool {
    !list.ordered
        && item.checked.is_none()
        && matches!(
            item.children.first(),
            Some(Block::ThematicBreak(ThematicBreak { marker, .. }))
                if thematic_break_char(*marker) == marker_char
        )
}

/// Spaces or a tab opening an item's written content would read as padding
/// after the marker, so that content starts on the next line. Four spaces or
/// more open indented code, which the marker line holds after one space: an
/// item that starts empty cannot interrupt a paragraph.
fn opens_with_padding(inner: &str) -> bool {
    !inner.starts_with("    ") && inner.starts_with([' ', '\t'])
}

/// Whether `block`, written with default options on the line after a
/// paragraph's, starts there instead of continuing the paragraph: the
/// parser's rule (`interrupts_paragraph`) for the block as the serializer
/// writes it, a setext heading and a list whose first item starts below its
/// bullet continuing the paragraph.
pub(crate) fn written_after_paragraph_interrupts(block: &Block) -> bool {
    match block {
        Block::Heading(heading) => !writes_setext(heading),
        Block::List(list) => {
            crate::parse::interrupts_paragraph(block) && !first_item_starts_below_its_bullet(list)
        }
        _ => crate::parse::interrupts_paragraph(block),
    }
}

/// Whether, with default options, the serializer starts `list`'s first item
/// on a line after its bullet, so that the list, written after a paragraph's
/// line, cannot interrupt the paragraph. Only a leaf block can open with
/// whitespace, so only a leading leaf block is written to tell.
pub(crate) fn first_item_starts_below_its_bullet(list: &List) -> bool {
    let Some(item) = list.children.first() else {
        return false;
    };
    let options = SerializeOptions::default();
    if breaks_after_bullet(list, item, list_marker(list, &options, None)) {
        return true;
    }
    if item.checked.is_some() {
        return false;
    }
    match item.children.first() {
        Some(
            block @ (Block::Paragraph(_)
            | Block::Heading(_)
            | Block::HtmlBlock(_)
            | Block::CodeBlock(_)),
        ) => write_block(block, &options).is_ok_and(|written| opens_with_padding(&written)),
        _ => false,
    }
}

fn write_item_blocks(
    blocks: &[Block],
    options: &SerializeOptions,
    tight: bool,
    task: Option<bool>,
) -> Result<String, SerializeError> {
    let join = if tight { Join::Line } else { Join::Gap };
    let Some(checked) = task else {
        return write_blocks(blocks, options, join, false, None);
    };
    let checkbox = if checked { "[x]" } else { "[ ]" };
    let definitions = blocks
        .iter()
        .take_while(|block| matches!(block, Block::Definition(_)))
        .count();
    if matches!(blocks.get(definitions), Some(Block::Paragraph(_))) {
        let before = write_blocks(&blocks[..definitions], options, join, false, None)?;
        let rest = write_blocks(&blocks[definitions..], options, join, false, None)?;
        let mut output = before;
        if !output.is_empty() {
            match join {
                Join::Gap => push_block_gap(&mut output),
                Join::Line => output.push('\n'),
            }
        }
        output.push_str(checkbox);
        output.push(' ');
        output.push_str(&rest);
        return Ok(output);
    }
    let written = write_blocks(blocks, options, join, false, None)?;
    Ok(if written.is_empty() {
        checkbox.into()
    } else {
        format!("{checkbox} {written}")
    })
}

fn write_code_block(node: &CodeBlock, options: &SerializeOptions) -> String {
    match node.kind {
        CodeBlockKind::Indented => {
            // Each value line ends with a line ending; the block gap or the
            // final newline writes a last `\n`, so only `\r` and `\r\n` stay.
            let body = trim_trailing_newline(&node.value);
            let mut output = prefix_lines(body, "    ");
            let ending = &node.value[body.len()..];
            if matches!(ending, "\r" | "\r\n") {
                output.push_str(ending);
            }
            output
        }
        CodeBlockKind::Fenced { marker, length } => {
            let marker = code_block_fence_marker(node, marker, options);
            let (fence, indent) = code_block_fence(&node.value, marker, length.max(3));
            let mut opener = fence.clone();
            if let Some(info) = &node.info {
                opener.push(' ');
                opener.push_str(&escape_code_info(info));
            }
            let body = fenced_body(&opener, &node.value, &fence);
            if indent == 0 {
                body
            } else {
                prefix_lines(&body, &" ".repeat(indent))
            }
        }
    }
}

/// A fenced block: `opener`, then `value` (whose lines each keep their line
/// ending, the last one optionally), then `closer`. An empty value writes no
/// line between the fences.
fn fenced_body(opener: &str, value: &str, closer: &str) -> String {
    let mut output = String::with_capacity(opener.len() + value.len() + closer.len() + 2);
    output.push_str(opener);
    output.push('\n');
    output.push_str(value);
    if !value.is_empty() && !ends_with_line_ending(value) {
        output.push('\n');
    }
    output.push_str(closer);
    output
}

/// The fence char a code block is written with: tildes when its info string
/// holds a backtick, which a backtick fence's info string cannot, or else the
/// char the AST records or the options replace it with.
fn code_block_fence_marker(
    node: &CodeBlock,
    marker: FenceMarker,
    options: &SerializeOptions,
) -> FenceMarker {
    if node.info.as_deref().is_some_and(|info| info.contains('`')) {
        return FenceMarker::Tilde;
    }
    options.fence_marker.unwrap_or(marker)
}

fn escape_code_info(input: &str) -> String {
    let mut output = String::new();
    // The parser trims the info string, so whitespace at either end is
    // written as a character reference.
    let inner_start = input.len() - input.trim_start_matches([' ', '\t']).len();
    let inner_end = input.trim_end_matches([' ', '\t']).len().max(inner_start);
    for (offset, char) in input.char_indices() {
        match char {
            ' ' | '\t' if offset < inner_start || offset >= inner_end => {
                output.push_str(&char_reference(char));
            }
            '\t' => output.push(char),
            char if char.is_control() => output.push_str(&char_reference(char)),
            '\\' | '&' => {
                output.push('\\');
                output.push(char);
            }
            _ => output.push(char),
        }
    }
    output
}

/// A hexadecimal character reference for `char`.
fn char_reference(char: char) -> String {
    format!("&#x{:X};", char as u32)
}

fn write_table(node: &Table) -> Result<String, SerializeError> {
    let mut output = String::new();
    for (row_index, row) in node.rows.iter().enumerate() {
        if row_index > 0 {
            output.push('\n');
        }
        output.push_str(&write_row(row)?);
        if row_index == 0 {
            output.push('\n');
            output.push_str(&write_delimiter_row(&node.alignments));
        }
    }
    Ok(output)
}

fn write_delimiter_row(alignments: &[TableAlignment]) -> String {
    let cells = alignments
        .iter()
        .map(|alignment| match alignment {
            TableAlignment::None => "---",
            TableAlignment::Left => ":---",
            TableAlignment::Center => ":---:",
            TableAlignment::Right => "---:",
        })
        .collect::<Vec<_>>();
    format!("| {} |", cells.join(" | "))
}

fn write_row(row: &TableRow) -> Result<String, SerializeError> {
    let cells = row
        .cells
        .iter()
        .map(|cell| {
            Ok(encode_cell(&match one_line(cell.source.as_deref(), true) {
                Some(text) => text.into(),
                None => write_inlines(&cell.children, Context::BLOCK)?,
            }))
        })
        .collect::<Result<Vec<_>, SerializeError>>()?;
    Ok(format!("| {} |", cells.join(" | ")))
}

/// A cell's inline content, written as anywhere else, encoded as cell
/// source. The parser splits a row at each `|` after no backslash or an even
/// run of them, and reads an odd run before a `|` with one backslash less, so
/// a `\` added before each `|` of the first kind reads back as written. A `|`
/// that the content already puts after an odd run, an escaped `|` in text,
/// reads with one backslash less, still as an escaped `|`; validation rejects
/// the cell values whose meaning that would change.
fn encode_cell(written: &str) -> String {
    let mut output = String::with_capacity(written.len());
    let mut backslashes = 0usize;
    for char in written.chars() {
        if char == '|' && backslashes % 2 == 0 {
            output.push('\\');
        }
        backslashes = if char == '\\' { backslashes + 1 } else { 0 };
        output.push(char);
    }
    output
}

/// Where inline content is written, as the value encodings of its nodes read
/// it.
#[derive(Clone, Copy, Debug, Eq, PartialEq)]
struct Context {
    /// In a heading, which a line ending would end.
    heading: bool,
}

impl Context {
    const BLOCK: Self = Self { heading: false };
    const HEADING: Self = Self { heading: true };
}

fn write_inlines(inlines: &[Inline], context: Context) -> Result<String, SerializeError> {
    let mut output = InlineOut::default();
    for inline in inlines {
        write_inline(inline, context, &mut output)?;
    }
    Ok(output.text)
}

/// The Markdown written so far for the inline content of one block, cell, or
/// label, which the parser reads in one inline pass, and the backtick runs in
/// it that a code span's fence must not close.
#[derive(Default)]
struct InlineOut {
    text: String,
    /// `open_runs[n]`: a backtick run of length `n` that opens no code span
    /// was written, outside every code span: a later fence of that length
    /// would close it.
    open_runs: Vec<bool>,
    /// How far `text` has been read for `open_runs`.
    scanned: usize,
}

impl InlineOut {
    /// Reads the text written since the last code span for backtick runs. A
    /// backslash escapes the first backtick of a run, which then opens with
    /// the rest of the run.
    fn scan_open_runs(&mut self) {
        let mut escaped = false;
        let mut run = 0;
        let mut run_escaped = false;
        for byte in self.text.as_bytes()[self.scanned..]
            .iter()
            .copied()
            .chain(core::iter::once(b' '))
        {
            if byte == b'`' {
                if run == 0 {
                    run_escaped = escaped;
                }
                run += 1;
                escaped = false;
                continue;
            }
            let opening = run - usize::from(run_escaped && run > 0);
            if opening > 0 {
                if self.open_runs.len() <= opening {
                    self.open_runs.resize(opening + 1, false);
                }
                self.open_runs[opening] = true;
            }
            run = 0;
            escaped = byte == b'\\' && !escaped;
        }
        self.scanned = self.text.len();
    }

    fn is_open_run(&self, length: usize) -> bool {
        self.open_runs.get(length).copied().unwrap_or(false)
    }

    /// Writes, with `write`, content that the parser reads whole from its
    /// first char, so no backtick run in it opens a code span.
    fn push_opaque(&mut self, write: impl FnOnce(&mut String)) {
        self.scan_open_runs();
        write(&mut self.text);
        self.scanned = self.text.len();
    }
}

impl core::ops::Deref for InlineOut {
    type Target = String;

    fn deref(&self) -> &String {
        &self.text
    }
}

impl core::ops::DerefMut for InlineOut {
    fn deref_mut(&mut self) -> &mut String {
        &mut self.text
    }
}

fn write_inline(
    inline: &Inline,
    context: Context,
    out: &mut InlineOut,
) -> Result<(), SerializeError> {
    match inline {
        Inline::Text(node) => out.push_str(&node.value),
        Inline::Escape(node) => {
            out.push('\\');
            out.push(node.value);
        }
        Inline::CharacterReference(node) => out.push_str(&node.reference),
        Inline::Emphasis(node) => {
            let delimiter = emphasis_delimiter(node.delimiter);
            write_span(out, delimiter, &node.children, delimiter, context)?;
        }
        Inline::Strong(node) => {
            let delimiter = emphasis_delimiter(node.delimiter).repeat(2);
            write_span(out, &delimiter, &node.children, &delimiter, context)?;
        }
        Inline::Delete(node) => write_span(out, "~~", &node.children, "~~", context)?,
        Inline::Mark(node) => write_span(out, "==", &node.children, "==", context)?,
        Inline::InlineFootnote(node) => write_span(out, "^[", &node.children, "]", context)?,
        Inline::Shortcode(node) => {
            out.push(':');
            out.push_str(&node.name);
            out.push(':');
        }
        Inline::Tag(node) => {
            out.push('#');
            out.push_str(&node.name);
        }
        Inline::Code(node) => write_code_span(node, out),
        Inline::Link(node) => {
            write_span(out, "[", &node.children, "](", context)?;
            out.push_opaque(|out| {
                write_resource(
                    out,
                    &node.destination,
                    node.destination_kind,
                    node.title.as_ref(),
                );
            });
        }
        Inline::Autolink(node) => out.push_opaque(|out| match node.form {
            AutolinkForm::Literal => out.push_str(&node.text),
            AutolinkForm::Angle => {
                out.push('<');
                out.push_str(&node.text);
                out.push('>');
            }
        }),
        Inline::Image(node) => {
            write_span(out, "![", &node.alt, "](", context)?;
            out.push_opaque(|out| {
                write_resource(
                    out,
                    &node.destination,
                    node.destination_kind,
                    node.title.as_ref(),
                );
            });
        }
        Inline::LinkReference(node) => {
            write_reference(out, "[", &node.children, node.kind, &node.label, context)?;
        }
        Inline::ImageReference(node) => {
            write_reference(out, "![", &node.alt, node.kind, &node.label, context)?;
        }
        Inline::Html(node) => out.push_opaque(|out| out.push_str(&node.value)),
        Inline::SoftBreak(_) if context.heading => out.push(' '),
        Inline::SoftBreak(_) => out.push('\n'),
        Inline::LineBreak(node) => match node.kind {
            LineBreakKind::Backslash => out.push_str("\\\n"),
            LineBreakKind::Spaces => out.push_str("  \n"),
        },
        Inline::Math(node) => {
            let math = write_inline_math(node);
            out.push_opaque(|out| out.push_str(&math));
        }
        Inline::FootnoteReference(node) => {
            out.push_str("[^");
            out.push_str(&node.label);
            out.push(']');
        }
        Inline::WikiLink(node) => out.push_opaque(|out| {
            if node.embed {
                out.push('!');
            }
            out.push_str("[[");
            out.push_str(&node.target);
            if node.target != node.label {
                out.push('|');
                out.push_str(&node.label);
            }
            out.push_str("]]");
        }),
        Inline::TextDirective(node) => {
            // The label is read in an inline pass of its own.
            let label = if node.label.is_empty() {
                String::new()
            } else {
                format!("[{}]", write_inlines(&node.label, context)?)
            };
            out.push_opaque(|out| {
                out.push(':');
                out.push_str(&node.name);
                out.push_str(&label);
                out.push_str(&write_attributes(&node.attributes));
            });
        }
    }
    Ok(())
}

fn emphasis_delimiter(delimiter: EmphasisDelimiter) -> &'static str {
    match delimiter {
        EmphasisDelimiter::Asterisk => "*",
        EmphasisDelimiter::Underscore => "_",
    }
}

/// Writes `open`, `children`, and `close`.
fn write_span(
    out: &mut InlineOut,
    open: &str,
    children: &[Inline],
    close: &str,
    context: Context,
) -> Result<(), SerializeError> {
    out.push_str(open);
    for child in children {
        write_inline(child, context, out)?;
    }
    out.push_str(close);
    Ok(())
}

/// A link's or image's `destination "title")`, after its `](`.
fn write_resource(
    out: &mut String,
    destination: &str,
    kind: LinkDestinationKind,
    title: Option<&Title>,
) {
    out.push_str(&write_destination(destination, kind));
    if let Some(title) = title {
        out.push(' ');
        out.push_str(&write_title(title));
    }
    out.push(')');
}

/// What follows a reference's text: nothing, `[]`, or `[label]`.
/// A reference link or image. A full reference writes its text and then its
/// label; a shortcut or collapsed one writes its label as written, which is
/// both its text and its key, so the text it reads back as matches the
/// definition it names.
fn write_reference(
    out: &mut InlineOut,
    open: &str,
    children: &[Inline],
    kind: ReferenceKind,
    label: &str,
    context: Context,
) -> Result<(), SerializeError> {
    match kind {
        ReferenceKind::Full => write_span(out, open, children, "]", context)?,
        ReferenceKind::Shortcut | ReferenceKind::Collapsed => {
            out.push_str(open);
            out.push_str(label);
            out.push(']');
        }
    }
    out.push_opaque(|out| write_reference_kind(out, kind, label));
    Ok(())
}

fn write_reference_kind(out: &mut String, kind: ReferenceKind, label: &str) {
    match kind {
        ReferenceKind::Shortcut => {}
        ReferenceKind::Collapsed => out.push_str("[]"),
        ReferenceKind::Full => {
            out.push('[');
            // A label is matched as written.
            out.push_str(label);
            out.push(']');
        }
    }
}

/// Writes a code span from its value: fenced by the shortest backtick run
/// that neither the value holds nor a run written before it in the same
/// inline content opens, and padded with a space at each end when the value
/// would otherwise lose an end to the fence or to the space stripping.
fn write_code_span(node: &CodeInline, out: &mut InlineOut) {
    let value = &node.value;
    out.scan_open_runs();
    let held = backtick_runs(value);
    // An escaped backtick written just before joins the opening fence into
    // one longer run, which an earlier open run of that length would take as
    // its close.
    let before = out.len() - out.trim_end_matches('`').len();
    let length = (1..)
        .find(|length| {
            !held.get(*length).copied().unwrap_or(false)
                && !out.is_open_run(*length)
                && (before == 0 || !out.is_open_run(before + *length))
        })
        .unwrap_or(1);
    let fence = "`".repeat(length);
    out.push_str(&fence);
    if code_span_needs_padding(value) {
        out.push(' ');
        out.push_str(value);
        out.push(' ');
    } else {
        out.push_str(value);
    }
    out.push_str(&fence);
    // The span's own runs are matched; what follows is read afresh.
    out.scanned = out.text.len();
}

/// `runs[n]`: whether `value` holds a maximal run of `n` backticks.
fn backtick_runs(value: &str) -> Vec<bool> {
    // A value of `len` bytes holds runs of at most `len` backticks.
    let mut runs = alloc::vec![false; value.len() + 1];
    let mut run = 0;
    for byte in value.bytes() {
        if byte == b'`' {
            run += 1;
        } else {
            runs[run] = true;
            run = 0;
        }
    }
    runs[run] = true;
    runs
}

fn code_span_needs_padding(input: &str) -> bool {
    input.starts_with('`')
        || input.ends_with('`')
        || (input.starts_with(' ') && input.ends_with(' ') && input.chars().any(|char| char != ' '))
}

pub(crate) fn write_inline_math(node: &MathInline) -> String {
    match node.kind {
        MathInlineKind::Code => format!("$`{}`$", node.value),
        // Dollar math is written verbatim behind its exact-length fence.
        MathInlineKind::Dollar { dollars } => {
            let fence = "$".repeat(usize::from(dollars));
            format!("{fence}{}{fence}", node.value)
        }
    }
}

/// A directive's `[label]`, written as the text it keeps if any, or nothing
/// for an empty label.
fn write_directive_label(label: &[Inline], source: Option<&str>) -> Result<String, SerializeError> {
    if let Some(text) = one_line(source, true) {
        return Ok(if text.is_empty() {
            String::new()
        } else {
            format!("[{text}]")
        });
    }
    if label.is_empty() {
        Ok(String::new())
    } else {
        Ok(format!("[{}]", write_inlines(label, Context::BLOCK)?))
    }
}

/// The text a leaf keeps, where it can be written in place of its inlines:
/// on one line when `single` holds, as in a cell, a label, or an ATX heading,
/// which a line ending would end.
fn one_line(source: Option<&str>, single: bool) -> Option<&str> {
    source.filter(|text| !single || !text.contains(['\n', '\r']))
}

fn write_attributes(attributes: &[DirectiveAttribute]) -> String {
    if attributes.is_empty() {
        return String::new();
    }
    let mut output = String::from("{");
    for (index, attribute) in attributes.iter().enumerate() {
        if index > 0 {
            output.push(' ');
        }
        match (&*attribute.name, &attribute.value) {
            ("id", Some(value)) if is_directive_shorthand_value(value) => {
                output.push('#');
                output.push_str(value);
            }
            ("class", Some(value)) if is_directive_shorthand_value(value) => {
                output.push('.');
                output.push_str(value);
            }
            (_, Some(value)) => {
                output.push_str(&attribute.name);
                output.push_str("=\"");
                output.push_str(&escape_title(value, LinkTitleKind::DoubleQuote));
                output.push('"');
            }
            (_, None) => output.push_str(&attribute.name),
        }
    }
    output.push('}');
    output
}

fn is_directive_shorthand_value(input: &str) -> bool {
    !input.is_empty()
        && input
            .chars()
            .all(|char| char.is_ascii_alphanumeric() || matches!(char, '_' | '-'))
}

fn write_destination(input: &str, kind: LinkDestinationKind) -> String {
    match kind {
        LinkDestinationKind::Omitted if input.is_empty() => String::new(),
        LinkDestinationKind::Angle => {
            let mut output = String::from("<");
            for char in input.chars() {
                match char {
                    char if char.is_control() => output.push_str(&char_reference(char)),
                    '\\' | '<' | '>' => {
                        output.push('\\');
                        output.push(char);
                    }
                    _ => output.push(char),
                }
            }
            output.push('>');
            output
        }
        LinkDestinationKind::Bare | LinkDestinationKind::Omitted if input.is_empty() => "<>".into(),
        LinkDestinationKind::Bare | LinkDestinationKind::Omitted => {
            let mut output = String::new();
            for char in input.chars() {
                match char {
                    // A space would end the destination, and `\ ` is no escape.
                    char if char.is_control() || char == ' ' => {
                        output.push_str(&char_reference(char));
                    }
                    '(' | ')' | '\\' | '<' | '>' | '&' => {
                        output.push('\\');
                        output.push(char);
                    }
                    _ => output.push(char),
                }
            }
            output
        }
    }
}

fn write_title(title: &Title) -> String {
    let (open, close) = match title.kind {
        LinkTitleKind::DoubleQuote => ('"', '"'),
        LinkTitleKind::SingleQuote => ('\'', '\''),
        LinkTitleKind::Paren => ('(', ')'),
    };
    let mut output = String::new();
    output.push(open);
    output.push_str(&escape_title(&title.value, title.kind));
    output.push(close);
    output
}

fn escape_title(input: &str, kind: LinkTitleKind) -> String {
    let mut output = String::new();
    for char in input.chars() {
        match char {
            char if char.is_control() => output.push_str(&char_reference(char)),
            '\\' | '&' => {
                output.push('\\');
                output.push(char);
            }
            '"' if kind == LinkTitleKind::DoubleQuote => output.push_str("\\\""),
            '\'' if kind == LinkTitleKind::SingleQuote => output.push_str("\\'"),
            '(' | ')' if kind == LinkTitleKind::Paren => {
                output.push('\\');
                output.push(char);
            }
            _ => output.push(char),
        }
    }
    output
}

/// Prefixes every line of `input` with a container's `prefix`, keeping each
/// line's ending; an empty line takes the prefix without its trailing
/// whitespace, so that no blank line the writer writes ends in whitespace.
fn prefix_container_lines(input: &str, prefix: &str) -> String {
    prefix_lines_with(input, prefix, prefix.trim_end_matches([' ', '\t']))
}

/// Prefixes every line of `input` with `prefix`, keeping each line's ending.
fn prefix_lines(input: &str, prefix: &str) -> String {
    prefix_lines_with(input, prefix, prefix)
}

/// Prefixes every line of `input` with `prefix`, or an empty one with
/// `empty_prefix`, keeping each line's ending.
fn prefix_lines_with(input: &str, prefix: &str, empty_prefix: &str) -> String {
    if input.is_empty() {
        return String::new();
    }
    let bytes = input.as_bytes();
    let mut output = String::new();
    let mut line_start = 0;
    let mut cursor = 0;
    while cursor < input.len() {
        let eol_end = match bytes[cursor] {
            b'\n' => Some(cursor + 1),
            b'\r' if bytes.get(cursor + 1) == Some(&b'\n') => Some(cursor + 2),
            b'\r' => Some(cursor + 1),
            _ => None,
        };
        if let Some(end) = eol_end {
            output.push_str(if cursor == line_start {
                empty_prefix
            } else {
                prefix
            });
            output.push_str(&input[line_start..end]);
            cursor = end;
            line_start = cursor;
        } else {
            cursor += 1;
        }
    }
    if line_start < input.len() {
        output.push_str(prefix);
        output.push_str(&input[line_start..]);
    }
    output
}

fn indent_after_first_line(input: &str, width: usize) -> String {
    indent_lines_of(input, width)
}

fn indent_continuation(input: &str) -> String {
    indent_lines_of(input, 4)
}

/// Indents every line of `input` after its first by `width` spaces, as a
/// container's continuation; an empty line stays empty. Each line keeps its
/// line ending, a final one included.
fn indent_lines_of(input: &str, width: usize) -> String {
    let mut output = String::with_capacity(input.len());
    let bytes = input.as_bytes();
    let mut line_start = 0;
    let mut index = 0;
    let push = |output: &mut String, line: &str, number: usize| {
        if number > 0 && !line.starts_with(['\n', '\r']) {
            output.extend(core::iter::repeat_n(' ', width));
        }
        output.push_str(line);
    };
    let mut number = 0;
    while index < bytes.len() {
        let end = match bytes[index] {
            b'\n' => Some(index + 1),
            b'\r' if bytes.get(index + 1) == Some(&b'\n') => Some(index + 2),
            b'\r' => Some(index + 1),
            _ => None,
        };
        match end {
            Some(end) => {
                push(&mut output, &input[line_start..end], number);
                number += 1;
                index = end;
                line_start = end;
            }
            None => index += 1,
        }
    }
    if line_start < input.len() {
        push(&mut output, &input[line_start..], number);
    }
    output
}

fn trim_trailing_newline(input: &str) -> &str {
    input.trim_end_matches('\n').trim_end_matches('\r')
}

fn ends_with_line_ending(input: &str) -> bool {
    input.ends_with('\n') || input.ends_with('\r')
}

/// The fence for a code block's `value` and the columns the block is indented
/// by, so that no line of `value` closes it. A closing fence is up to three
/// spaces, at least as many marker chars, and nothing else but spaces and
/// tabs. The fence keeps `min_len` chars when indenting the block (which the
/// value's lines lose again) moves every closing-like line past three spaces;
/// otherwise it grows past the longest of them.
fn code_block_fence(value: &str, marker: FenceMarker, min_len: usize) -> (String, usize) {
    let char = match marker {
        FenceMarker::Backtick => '`',
        FenceMarker::Tilde => '~',
    };
    let closing_like = |line: &str, length: usize| {
        let indent = line.len() - line.trim_start_matches(' ').len();
        let rest = &line[indent..];
        let run = rest.len() - rest.trim_start_matches(char).len();
        (indent <= 3 && run >= length && rest[run..].trim_matches([' ', '\t']).is_empty())
            .then_some((indent, run))
    };
    let lines = || value.split(['\n', '\r']);
    let least_indent = lines()
        .filter_map(|line| closing_like(line, min_len))
        .map(|(indent, _)| indent)
        .min();
    match least_indent {
        None => (char.to_string().repeat(min_len), 0),
        Some(indent) if indent > 0 => (char.to_string().repeat(min_len), 4 - indent),
        Some(_) => {
            let mut length = min_len;
            for line in lines() {
                if let Some((_, run)) = closing_like(line, length) {
                    length = run + 1;
                }
            }
            (char.to_string().repeat(length), 0)
        }
    }
}

fn block_math_fence(input: &str) -> String {
    let mut length = 2;
    for line in trim_trailing_newline(input).lines() {
        let trimmed = line.trim();
        if trimmed.len() >= 2 && trimmed.chars().all(|char| char == '$') {
            length = length.max(trimmed.len() + 1);
        }
    }
    "$".repeat(length)
}

fn directive_fence(inner: &str) -> String {
    let mut length = 3;
    for line in inner.lines() {
        if let Some(closing) = directive_closing_fence_len(line) {
            length = length.max(closing + 1);
        }
    }
    ":".repeat(length)
}

fn directive_closing_fence_len(line: &str) -> Option<usize> {
    let trimmed = trim_up_to_three_indent_columns(line)?;
    let length = trimmed
        .as_bytes()
        .iter()
        .take_while(|byte| **byte == b':')
        .count();
    if length >= 3 && trimmed[length..].trim().is_empty() {
        Some(length)
    } else {
        None
    }
}

fn trim_up_to_three_indent_columns(input: &str) -> Option<&str> {
    let mut columns = 0usize;
    let mut bytes = 0usize;
    for byte in input.as_bytes() {
        match *byte {
            b' ' => columns += 1,
            b'\t' => columns += 4 - (columns % 4),
            _ => break,
        }
        bytes += 1;
    }
    (columns <= 3).then_some(&input[bytes..])
}

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

    // serialization: Unchanged source is kept
    #[test]
    fn the_read_back_check_names_each_block_that_reads_differently() {
        let blocks = crate::parse("> x\n\n# y\n\nz").document.children;
        assert_eq!(misread(&blocks, "> x\n\n# y\n\nz", &[0, 2]), [0usize; 0]);
        // A block read differently in its place.
        assert_eq!(misread(&blocks, "> w\n\n# y\n\nz", &[0, 2]), [0]);
        assert_eq!(misread(&blocks, "> w\n\n# y\n\nz", &[2]), [0usize; 0]);
        // `z` read into the quote: the block count differs.
        assert_eq!(misread(&blocks, "> x\nz\n\n# y", &[0, 2]), [0, 2]);
    }
}