Kigi-CLI/crates/codegen/kigi-markdown/src/syntax.rs

//! Syntax highlighting support using syntect.

use std::io::Cursor;
use std::path::Path;

use syntect::{
    easy::HighlightLines,
    highlighting::{Theme as SyntectTheme, ThemeSet},
    parsing::{SyntaxReference, SyntaxSet},
};

/// Syntax highlighting configuration.
///
/// Loading the syntax set is expensive; create one instance and pass it to
/// the markdown renderer.
pub struct Syntect {
    pub theme: SyntectTheme,
    pub syntax_set: SyntaxSet,
}

impl Syntect {
    /// `theme_bytes` must be a TextMate `.tmTheme` file. The syntax set is
    /// two-face's extended one, covering the 250+ languages bat ships.
    ///
    /// # Example
    ///
    /// ```ignore
    /// let syntect = Syntect::new(include_bytes!("assets/tokyo-night.tmTheme"));
    /// ```
    pub fn new(theme_bytes: &[u8]) -> Self {
        let mut cursor = Cursor::new(theme_bytes);
        let theme = ThemeSet::load_from_reader(&mut cursor).expect("Failed to load theme");
        let syntax_set = two_face::syntax::extra_newlines();
        Self { theme, syntax_set }
    }

    /// Resolves on the extension alone; the rest of the file name is ignored.
    pub fn find_syntax_by_file_path(&self, file_path: &Path) -> Option<&SyntaxReference> {
        let ext = file_path.extension()?.to_str()?;
        self.syntax_set.find_syntax_by_extension(ext)
    }

    /// Find a syntax definition by language token (e.g., "rust", "python").
    pub fn find_syntax_by_token(&self, token: &str) -> Option<&SyntaxReference> {
        self.syntax_set.find_syntax_by_token(token)
    }

    pub fn highlight_lines_by_file_path(&self, file_path: &Path) -> Option<HighlightLines<'_>> {
        Some(HighlightLines::new(
            self.find_syntax_by_file_path(file_path)?,
            &self.theme,
        ))
    }

    pub fn highlight_lines_for_token(&self, token: &str) -> Option<HighlightLines<'_>> {
        Some(HighlightLines::new(
            self.find_syntax_by_token(token)?,
            &self.theme,
        ))
    }

    /// Highlighter for a fenced code block *info* string: a normal language token
    /// (e.g. `rust`, `python`), or a **line-range citation** of the form
    /// `lineStart:lineEnd:path/to/file.ext` where the syntax is resolved the same
    /// way as [`Syntect::highlight_lines_by_file_path`] (see
    /// [`Syntect::find_syntax_by_file_path`]).
    ///
    /// If the string matches the citation form but no syntax is found for the
    /// path, this falls back to [`Syntect::find_syntax_by_token`] with the full
    /// `fence_info` string, so plain ` ```lang` blocks keep working and odd
    /// citations degrade to a plain token lookup.
    pub fn highlight_lines_for_fence_info(&self, fence_info: &str) -> Option<HighlightLines<'_>> {
        Some(HighlightLines::new(
            self.find_syntax_for_fence_info(fence_info)?,
            &self.theme,
        ))
    }

    /// Resolve the [`SyntaxReference`] for a fenced code block *info* string,
    /// using the SAME rules as [`Syntect::highlight_lines_for_fence_info`]:
    /// a `lineStart:lineEnd:path` citation resolves by file path, otherwise
    /// (or if the path has no known syntax) it falls back to a language token.
    ///
    /// Exposed so the incremental open-code highlighter can build its own
    /// resumable `ParseState`/`HighlightState` against exactly the syntax the
    /// batch `HighlightLines` path would have used — keeping the two
    /// byte-identical.
    pub(crate) fn find_syntax_for_fence_info(&self, fence_info: &str) -> Option<&SyntaxReference> {
        if let Some((_, _, path)) = parse_line_citation_fence_info(fence_info)
            && let Some(s) = self.find_syntax_by_file_path(Path::new(path))
        {
            return Some(s);
        }
        self.find_syntax_by_token(fence_info)
    }
}

/// ```text
/// lineStart:lineEnd:path/to/file.ext
/// ```
///
/// The path is the segment after the **second** colon; it is then parsed with
/// [`Path::new`]. Paths with extra colons in the first two segments (e.g. some
/// Windows `C:...` forms) are not supported; use a repo-relative or
/// forward-slash form.
fn parse_line_citation_fence_info(info: &str) -> Option<(&str, &str, &str)> {
    let mut it = info.splitn(3, ':');
    let start = it.next()?;
    let end = it.next()?;
    let path = it.next()?;
    if start.is_empty() || !start.chars().all(|c| c.is_ascii_digit()) {
        return None;
    }
    if end.is_empty() || !end.chars().all(|c| c.is_ascii_digit()) {
        return None;
    }
    if path.is_empty() {
        return None;
    }
    Some((start, end, path))
}

/// Syntax highlight code, returning raw styled segments per line.
///
/// `fence_info` is the fenced code block *info* string (language tag or
/// `lineStart:lineEnd:path` citation form); see
/// [`Syntect::highlight_lines_for_fence_info`]. Lives here (not in `parse`)
/// so both the parser and the streaming highlighter caches depend one-way on
/// `syntax`.
pub(crate) fn syntax_highlight_raw(
    syntect: Option<&Syntect>,
    fence_info: &str,
    text: &str,
) -> Option<Vec<Vec<(syntect::highlighting::Style, String)>>> {
    use syntect::util::LinesWithEndings;

    let syn = syntect?;
    let mut hl = syn.highlight_lines_for_fence_info(fence_info)?;
    let mut lines = Vec::new();
    for line in LinesWithEndings::from(text) {
        let highlighted = hl.highlight_line(line, &syn.syntax_set).ok()?;
        lines.push(
            highlighted
                .into_iter()
                .map(|(s, t)| (s, t.to_string()))
                .collect(),
        );
    }
    Some(lines)
}

/// Shared Syntect instance for tests, using the crate's bundled tokyo-night
/// theme.
#[cfg(any(test, fuzzing))]
#[allow(dead_code)]
pub fn test_syntect() -> &'static Syntect {
    use std::sync::OnceLock;
    static TEST_SYNTECT: OnceLock<Syntect> = OnceLock::new();
    TEST_SYNTECT.get_or_init(|| Syntect::new(include_bytes!("../assets/tokyo-night.tmTheme")))
}

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

    #[test]
    fn line_citation_fence_parses_start_end_path() {
        assert_eq!(
            parse_line_citation_fence_info("37:65:crates/example/src/tools/read.rs"),
            Some(("37", "65", "crates/example/src/tools/read.rs"))
        );
    }

    #[test]
    fn line_citation_rejects_non_numeric_line() {
        assert_eq!(parse_line_citation_fence_info("37:ab:file.rs"), None);
    }

    #[test]
    fn line_citation_rejects_plain_lang_token() {
        assert_eq!(parse_line_citation_fence_info("rust"), None);
        assert_eq!(parse_line_citation_fence_info(""), None);
    }

    #[test]
    fn highlight_lines_for_fence_info_resolves_citation_path_to_rust() {
        let s = super::test_syntect();
        assert!(
            s.highlight_lines_for_fence_info("37:65:crates/codegen/kigi-markdown/src/parse.rs")
                .is_some()
        );
    }

    #[test]
    fn highlight_lines_for_fence_info_still_accepts_rust_token() {
        let s = super::test_syntect();
        assert!(s.highlight_lines_for_fence_info("rust").is_some());
    }
}