unparser_shared/

markdown.rs

//! CommonMark syntax facts every Markdown writer in the family needs.
//!
//! Each library renders its own document model, and how it styles a run is its own business.
//! *Where* an emphasis delimiter may stand, and how text the document supplies — a picture's
//! description, a link's target — has to be written so that it stays inside the construct it
//! belongs to, are not: they are properties of CommonMark, and a writer that gets them wrong
//! prints its syntax as text. This module answers those questions once so the writers cannot
//! drift on them.
//!
//! `std`-only, like the rest of the crate root.

use std::ops::Range;

/// The part of a styled run that emphasis delimiters (`*`, `**`, `~~`) can wrap, given the
/// characters the run lands between, or `None` when no part can.
///
/// `before` is the last character already written, `after` the first one the next piece will
/// write (`None` at either end of the inline content). The text outside the returned range is
/// written as-is, outside the delimiters.
///
/// CommonMark reads a delimiter as emphasis only when it *flanks* the text (§6.2): it may not
/// touch whitespace on its inside, and when its inside is punctuation its outside must be
/// whitespace, punctuation, or the end of the line. So:
///
/// - leading and trailing whitespace always stays outside — `**word **` is not emphasis;
/// - leading or trailing punctuation stays outside too, with any whitespace behind it, when
///   the run touches a letter or digit on that side — an italic `", s"` after `32` written as
///   `32*, s*` shows its asterisks, written as `32, *s*` it does not;
/// - a run that is nothing but whitespace, or nothing but punctuation between words, has
///   nothing to wrap.
///
/// Punctuation that does not touch a word stays inside (`*i.e.,*`, `*(h)*`), so a run is not
/// reshaped unless it has to be.
///
/// `text` may already be escaped: a backslash escape (`\*`) is one unit and is never split
/// across a delimiter. Only escapes of ASCII punctuation are units, as in CommonMark.
///
/// ```
/// use unparser_shared::markdown::emphasis_span;
///
/// let wrap = |text: &str, before, after| {
///     emphasis_span(text, before, after).map_or(text.to_string(), |r| {
///         format!("{}*{}*{}", &text[..r.start], &text[r.clone()], &text[r.end..])
///     })
/// };
/// assert_eq!(wrap(", s", Some('2'), Some(' ')), ", *s*");
/// assert_eq!(wrap("word ", None, Some('x')), "*word* ");
/// assert_eq!(wrap(",", Some('8'), Some('a')), ",");
/// assert_eq!(wrap("(h)", Some(' '), None), "*(h)*");
/// ```
pub fn emphasis_span(
    text: &str,
    before: Option<char>,
    after: Option<char>,
) -> Option<Range<usize>> {
    let mut start = text.len() - text.trim_start().len();
    let mut end = text.trim_end().len();
    if start >= end {
        return None;
    }
    let touches_a_word = |c: Option<char>| c.is_some_and(|c| !is_flanking_neutral(c));
    if start == 0 && touches_a_word(before) {
        start = units(text, start, end)
            .find(|&(i, j)| !unit_is_neutral(&text[i..j]))
            .map_or(end, |(i, _)| i);
    }
    if end == text.len() && touches_a_word(after) {
        end = units(text, start, end)
            .filter(|&(i, j)| !unit_is_neutral(&text[i..j]))
            .last()
            .map_or(start, |(_, j)| j);
    }
    (start < end).then_some(start..end)
}

/// A picture as an inline image: `![alt](destination)`.
///
/// The alt text is data from the document — whatever a person typed into a description box:
/// several lines, blank lines, brackets. Written verbatim, a blank line ends the paragraph and
/// the image reads back as literal text plus a stray paragraph, and an unbalanced `]` ends the
/// link text early. So the alt text is flattened to one line — every run of whitespace becomes
/// one space, the ends trimmed — and the characters that are syntax inside link text are
/// escaped: `\`, `[`, `]`, `` ` `` (a code span would swallow the closing bracket), `<` (raw
/// HTML or an autolink), and an `&` that would start a character reference (`&amp;` would read
/// back as `&`). Inside a table cell `|` is escaped too, since it would end the cell. `*` and
/// `_` are left alone: unmatched they are literal, matched they only style the text, and
/// neither can end the image.
///
/// The destination is written by [`link_destination`].
///
/// ```
/// use unparser_shared::markdown::image;
///
/// assert_eq!(
///     image("A view\n\nof the harbour", "img/1.png", false),
///     "![A view of the harbour](img/1.png)"
/// );
/// assert_eq!(image("a [draft]", "my dir/a.png", false), r"![a \[draft\]](<my dir/a.png>)");
/// assert_eq!(image("x | y", "a.png", true), r"![x \| y](a.png)");
/// ```
pub fn image(alt: &str, destination: &str, in_table_cell: bool) -> String {
    let mut out = String::with_capacity(alt.len() + destination.len() + 5);
    out.push_str("![");
    for (n, word) in alt.split_whitespace().enumerate() {
        if n > 0 {
            out.push(' ');
        }
        for (i, c) in word.char_indices() {
            match c {
                '\\' | '[' | ']' | '`' | '<' => {
                    out.push('\\');
                    out.push(c);
                }
                '&' if starts_character_reference(&word[i + 1..]) => out.push_str("\\&"),
                '|' if in_table_cell => out.push_str("\\|"),
                _ => out.push(c),
            }
        }
    }
    out.push_str("](");
    out.push_str(&link_destination(destination, in_table_cell));
    out.push(')');
    out
}

/// A link or image destination, written so that CommonMark reads back exactly `url` (§6.3).
///
/// The bare form is used when it can carry the URL: no spaces, no ASCII control characters,
/// no `<` or `>`, and parentheses that balance — an unbalanced `)` would end the destination
/// early, a `(` would leave it open. Anything else is written in the pointy-bracket form
/// `<...>`, which takes spaces and any parentheses. A line ending cannot stand in either form;
/// it is percent-encoded (`%0A`, `%0D`), the one place the URL is changed rather than
/// escaped. In both forms a backslash that would otherwise escape the character after it — an
/// ASCII punctuation character, or the end of the destination — is doubled; other backslashes
/// (`C:\dir`) are literal and stay as they are. An `&` that would start a character reference
/// is escaped, so `?a=1&amp;b` stays as written. Inside a table cell `|` is escaped, since a
/// GFM table splits its cells before it reads links. An empty URL is `<>`.
///
/// A link that is not in a table cell passes `in_table_cell: false`.
///
/// ```
/// use unparser_shared::markdown::link_destination;
///
/// assert_eq!(link_destination("https://example.com/a(b)", false), "https://example.com/a(b)");
/// assert_eq!(link_destination("my folder/file.png", false), "<my folder/file.png>");
/// assert_eq!(link_destination("notes).txt", false), "<notes).txt>");
/// assert_eq!(link_destination("a<b>c", false), r"<a\<b\>c>");
/// ```
pub fn link_destination(url: &str, in_table_cell: bool) -> String {
    let bare = !url.is_empty()
        && !url
            .chars()
            .any(|c| c == ' ' || c == '<' || c == '>' || c.is_ascii_control())
        && parentheses_balance(url);
    let mut out = String::with_capacity(url.len() + 2);
    if !bare {
        out.push('<');
    }
    let mut chars = url.char_indices().peekable();
    while let Some((i, c)) = chars.next() {
        match c {
            // Judged on what is written next: a line ending goes out as `%`.
            '\\' if chars
                .peek()
                .is_none_or(|&(_, n)| n.is_ascii_punctuation() || n == '\n' || n == '\r') =>
            {
                out.push_str("\\\\")
            }
            '&' if starts_character_reference(&url[i + 1..]) => out.push_str("\\&"),
            '<' | '>' => {
                out.push('\\');
                out.push(c);
            }
            '|' if in_table_cell => out.push_str("\\|"),
            '\n' => out.push_str("%0A"),
            '\r' => out.push_str("%0D"),
            _ => out.push(c),
        }
    }
    if !bare {
        out.push('>');
    }
    out
}

/// Whether the text after an `&` makes it a character reference (§6.2) — `name;`, `#123;` or
/// `#x1F;` — which CommonMark would replace by the character it names.
fn starts_character_reference(rest: &str) -> bool {
    let Some(end) = rest.find(';') else {
        return false;
    };
    let body = &rest[..end];
    if let Some(number) = body.strip_prefix('#') {
        return match number.strip_prefix(['x', 'X']) {
            Some(hex) => (1..=6).contains(&hex.len()) && hex.chars().all(|c| c.is_ascii_hexdigit()),
            None => (1..=7).contains(&number.len()) && number.chars().all(|c| c.is_ascii_digit()),
        };
    }
    body.starts_with(|c: char| c.is_ascii_alphabetic())
        && (2..=32).contains(&body.len())
        && body.chars().all(|c| c.is_ascii_alphanumeric())
}

/// Whether every `)` closes an earlier `(` and every `(` is closed. A backslash in the URL
/// does not shield a parenthesis: [`link_destination`] writes it as a literal backslash.
fn parentheses_balance(url: &str) -> bool {
    let mut depth = 0usize;
    for c in url.chars() {
        match c {
            '(' => depth += 1,
            ')' => match depth.checked_sub(1) {
                Some(d) => depth = d,
                None => return false,
            },
            _ => {}
        }
    }
    depth == 0
}

/// Whitespace or punctuation — what a delimiter may touch on its outside when its inside is
/// punctuation. CommonMark's punctuation is the Unicode P and S categories; a character that
/// is neither alphanumeric, whitespace nor a control is one of them, save for combining
/// marks, which do not stand at a run's edge.
fn is_flanking_neutral(c: char) -> bool {
    c.is_whitespace() || !(c.is_alphanumeric() || c.is_control())
}

/// A unit is neutral when the character it writes is: an escape writes punctuation.
fn unit_is_neutral(unit: &str) -> bool {
    unit.chars().last().is_some_and(is_flanking_neutral)
}

/// The byte ranges of `text[start..end]`'s units: one character, or a backslash and the ASCII
/// punctuation it escapes.
fn units(text: &str, start: usize, end: usize) -> impl Iterator<Item = (usize, usize)> + '_ {
    let mut chars = text[start..end].char_indices().peekable();
    std::iter::from_fn(move || {
        let (i, c) = chars.next()?;
        let mut j = i + c.len_utf8();
        if c == '\\' {
            if let Some(&(k, escaped)) = chars.peek() {
                if escaped.is_ascii_punctuation() {
                    j = k + escaped.len_utf8();
                    chars.next();
                }
            }
        }
        Some((start + i, start + j))
    })
}

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

    /// `text` with `*` around its emphasis span.
    fn wrap(text: &str, before: Option<char>, after: Option<char>) -> String {
        emphasis_span(text, before, after).map_or(text.to_string(), |r| {
            format!(
                "{}*{}*{}",
                &text[..r.start],
                &text[r.clone()],
                &text[r.end..]
            )
        })
    }

    #[test]
    fn whitespace_stays_outside() {
        assert_eq!(wrap("  word  ", Some('a'), Some('b')), "  *word*  ");
        assert_eq!(emphasis_span("   ", None, None), None);
        assert_eq!(emphasis_span("", None, None), None);
    }

    #[test]
    fn leading_punctuation_against_a_word_moves_out_with_its_space() {
        assert_eq!(wrap(", s", Some('2'), Some(' ')), ", *s*");
    }

    #[test]
    fn trailing_punctuation_against_a_word_moves_out() {
        assert_eq!(wrap("word.", Some(' '), Some('x')), "*word*.");
        assert_eq!(wrap("word, ", Some(' '), Some('x')), "*word,* ");
    }

    #[test]
    fn punctuation_between_words_alone_has_nothing_to_wrap() {
        assert_eq!(emphasis_span(",", Some('8'), Some('a')), None);
        assert_eq!(emphasis_span(", ", Some('8'), None), None);
    }

    #[test]
    fn punctuation_against_space_punctuation_or_an_edge_stays_inside() {
        assert_eq!(wrap("i.e.,", Some(' '), Some(' ')), "*i.e.,*");
        assert_eq!(wrap("(h)", None, None), "*(h)*");
        assert_eq!(wrap("(h)", Some('('), Some(')')), "*(h)*");
        assert_eq!(
            wrap("Continued pretraining.", None, Some(' ')),
            "*Continued pretraining.*"
        );
    }

    #[test]
    fn a_word_on_either_side_moves_both_ends() {
        assert_eq!(wrap("(h)", Some('Q'), Some('x')), "(*h*)");
    }

    #[test]
    fn an_escape_is_never_split() {
        assert_eq!(wrap(r"\*x\_", Some('a'), Some('b')), r"\**x*\_");
    }

    #[test]
    fn a_backslash_before_a_letter_is_not_an_escape() {
        assert_eq!(wrap(r"\a", Some('b'), None), r"\*a*");
    }

    #[test]
    fn non_ascii_punctuation_and_letters_count() {
        assert_eq!(wrap("“word”", Some('x'), Some('y')), "“*word*”");
        assert_eq!(wrap("단어", Some('가'), Some('나')), "*단어*");
    }
}