mars_schema/

frontmatter.rs

use indexmap::{IndexMap, IndexSet};
use serde_yaml::{Mapping, Value};

/// Parsed markdown frontmatter and body.
#[derive(Debug, Clone)]
pub struct Frontmatter {
    yaml: Mapping,
    body: String,
    has_frontmatter: bool,
    has_bom: bool,
}

/// Structured agent skill references.
///
/// A flat `skills: [a, b]` list is represented as all `load` entries for
/// backward compatibility. A structured `skills: { load: [...], available: [...] }`
/// keeps the two launch-bundle channels separate.
#[derive(Debug, Clone, Default, PartialEq, Eq, serde::Serialize)]
pub struct SkillsSpec {
    pub load: Vec<String>,
    pub available: Vec<String>,
}

impl SkillsSpec {
    pub fn all(&self) -> Vec<String> {
        self.load
            .iter()
            .chain(self.available.iter())
            .cloned()
            .collect()
    }
}

/// The most frontmatter bytes a file may carry. Parsing time grows with the square of the
/// nesting depth the text can hold, so an unbounded block is a way to stall a server that reads
/// untrusted files; real frontmatter is a few hundred bytes.
pub const MAX_FRONTMATTER_BYTES: usize = 64 * 1024;

/// The deepest run of `[` and `{` the frontmatter may open, counted over the raw text. The
/// YAML scanners take time quadratic in that depth, so a size cap alone still lets 64 KiB of `[`
/// stall a parse for seconds; real frontmatter nests a few levels.
pub const MAX_FLOW_DEPTH: usize = 64;

/// Errors from frontmatter parsing.
#[derive(Debug, thiserror::Error)]
pub enum FrontmatterError {
    #[error("frontmatter is {0} bytes; the limit is {MAX_FRONTMATTER_BYTES}")]
    TooLarge(usize),

    #[error("frontmatter nests `[` or `{{` more than {MAX_FLOW_DEPTH} deep")]
    TooDeep,

    #[error("malformed YAML frontmatter: {0}")]
    MalformedYaml(#[from] serde_yaml::Error),

    #[error("frontmatter is not closed with an exact `---` line")]
    Unclosed,

    #[error("YAML anchors and aliases are not supported")]
    Anchor,

    #[error("YAML document-end markers are not supported")]
    DocumentEnd,

    #[error(
        "YAML syntax accepted by the semantic parser is unsupported by the structural scanner: {0}"
    )]
    UnsupportedSyntax(String),

    #[error("frontmatter is not a YAML mapping")]
    NotAMapping,
}

/// Parse markdown content into frontmatter and body.
pub fn parse(content: &str) -> Result<Frontmatter, FrontmatterError> {
    Frontmatter::parse(content)
}

impl Frontmatter {
    /// Parse a markdown document into frontmatter + body.
    pub fn parse(content: &str) -> Result<Self, FrontmatterError> {
        let (frontmatter, mut structural_errors) = Self::parse_preserving_structure(content)?;
        if structural_errors.is_empty() {
            Ok(frontmatter)
        } else {
            Err(structural_errors.remove(0))
        }
    }

    /// Parse semantic content while returning every structural refusal found.
    ///
    /// Schema readers use this to avoid losing an anchor diagnostic when the
    /// structural parser later encounters syntax it cannot scan.
    pub(crate) fn parse_preserving_structure(
        content: &str,
    ) -> Result<(Self, Vec<FrontmatterError>), FrontmatterError> {
        let Some(regions) = split_regions(content)? else {
            return Ok((
                Self {
                    yaml: Mapping::new(),
                    body: content.to_string(),
                    has_frontmatter: false,
                    has_bom: false,
                },
                Vec::new(),
            ));
        };

        let yaml_text = regions.yaml;
        if yaml_text.trim().is_empty() {
            return Ok((
                Self {
                    yaml: Mapping::new(),
                    body: regions.body.to_string(),
                    has_frontmatter: true,
                    has_bom: regions.has_bom,
                },
                Vec::new(),
            ));
        }

        let value: Value = serde_yaml::from_str(yaml_text)?;
        let structural_errors = validate_yaml_structure(yaml_text);
        let yaml = match value {
            Value::Mapping(mapping) => mapping,
            Value::Null => Mapping::new(),
            _ => return Err(FrontmatterError::NotAMapping),
        };

        Ok((
            Self {
                yaml,
                body: regions.body.to_string(),
                has_frontmatter: true,
                has_bom: regions.has_bom,
            },
            structural_errors,
        ))
    }

    /// Read all referenced skills as a flat list (`load` followed by `available`).
    pub fn skills(&self) -> Vec<String> {
        self.skills_structured().all()
    }

    /// Read `skills` in structured load/available form.
    pub fn skills_structured(&self) -> SkillsSpec {
        match self.get("skills") {
            Some(Value::Mapping(mapping)) => SkillsSpec {
                load: mapping
                    .get(yaml_key("load"))
                    .map(yaml_str_list)
                    .unwrap_or_default(),
                available: mapping
                    .get(yaml_key("available"))
                    .map(yaml_str_list)
                    .unwrap_or_default(),
            },
            Some(value) => SkillsSpec {
                load: yaml_str_list(value),
                available: Vec::new(),
            },
            None => SkillsSpec::default(),
        }
    }

    /// Read the `name` field if present.
    pub fn name(&self) -> Option<&str> {
        self.get("name").and_then(Value::as_str)
    }

    /// Read any YAML field by key.
    pub fn get(&self, key: &str) -> Option<&Value> {
        self.yaml.get(yaml_key(key))
    }

    /// Mutably read a YAML field by key.
    pub fn get_mut(&mut self, key: &str) -> Option<&mut Value> {
        self.yaml.get_mut(yaml_key(key))
    }

    /// Markdown body after frontmatter.
    pub fn body(&self) -> &str {
        &self.body
    }

    /// Replace the Markdown body while preserving the parsed frontmatter.
    pub fn set_body(&mut self, body: String) {
        self.body = body;
    }

    /// Whether this document contains frontmatter delimiters.
    pub fn has_frontmatter(&self) -> bool {
        self.has_frontmatter
    }

    /// All frontmatter keys as strings.
    pub fn keys(&self) -> Vec<String> {
        self.yaml
            .keys()
            .filter_map(|k| k.as_str().map(str::to_owned))
            .collect()
    }

    /// Parsed top-level YAML, for schema transforms that must verify an edited source.
    pub(crate) fn mapping(&self) -> &Mapping {
        &self.yaml
    }

    /// Insert or replace a top-level frontmatter field.
    pub fn insert(&mut self, key: &str, value: Value) {
        self.has_frontmatter = true;
        self.yaml.insert(yaml_key(key), value);
    }

    /// Remove a top-level frontmatter field.
    pub fn remove(&mut self, key: &str) -> Option<Value> {
        self.yaml.shift_remove(yaml_key(key))
    }

    /// Serialize back to full markdown.
    pub fn render(&self) -> String {
        if !self.has_frontmatter && self.yaml.is_empty() {
            return self.body.clone();
        }
        let mut out = self.render_frontmatter();
        out.push_str(&self.body);
        out
    }

    /// Serialize only the frontmatter block, delimiters included.
    ///
    /// A caller that changes the frontmatter and must keep the authored body byte for byte
    /// (`mars migrate`'s full rewrite) renders this and appends the parsed body itself.
    pub(crate) fn render_frontmatter(&self) -> String {
        if !self.has_frontmatter && self.yaml.is_empty() {
            return String::new();
        }

        let mut out = if self.has_bom {
            String::from("\u{feff}---\n")
        } else {
            String::from("---\n")
        };
        if !self.yaml.is_empty() {
            let mut yaml = serde_yaml::to_string(&self.yaml)
                .expect("serializing frontmatter mapping should succeed");
            if let Some(stripped) = yaml.strip_prefix("---\n") {
                yaml = stripped.to_string();
            }
            out.push_str(&yaml);
            if !yaml.ends_with('\n') {
                out.push('\n');
            }
        }
        out.push_str("---\n");
        out
    }
}

/// Rename skills in frontmatter using exact-match replacement.
fn rewrite_skills(fm: &mut Frontmatter, renames: &IndexMap<String, String>) -> IndexSet<String> {
    let mut renamed = IndexSet::new();
    let key = yaml_key("skills");
    if let Some(value) = fm.yaml.get_mut(&key) {
        rewrite_skill_value(value, renames, &mut renamed);
    }

    renamed
}

/// Rename subagents in frontmatter using exact-match replacement.
fn rewrite_subagents(fm: &mut Frontmatter, renames: &IndexMap<String, String>) -> IndexSet<String> {
    rewrite_string_list_field(fm, "subagents", renames)
}

/// Parse content, rewrite skills, and render updated content if changed.
pub fn rewrite_content_skills(
    content: &str,
    renames: &IndexMap<String, String>,
) -> Result<Option<String>, FrontmatterError> {
    let mut fm = Frontmatter::parse(content)?;
    let renamed = rewrite_skills(&mut fm, renames);
    if renamed.is_empty() {
        Ok(None)
    } else {
        Ok(Some(fm.render()))
    }
}

/// Parse content, rewrite subagents, and render updated content if changed.
pub fn rewrite_content_subagents(
    content: &str,
    renames: &IndexMap<String, String>,
) -> Result<Option<String>, FrontmatterError> {
    let mut fm = Frontmatter::parse(content)?;
    let renamed = rewrite_subagents(&mut fm, renames);
    if renamed.is_empty() {
        Ok(None)
    } else {
        Ok(Some(fm.render()))
    }
}

fn rewrite_string_list_field(
    fm: &mut Frontmatter,
    field_name: &str,
    renames: &IndexMap<String, String>,
) -> IndexSet<String> {
    let mut renamed = IndexSet::new();
    let key = yaml_key(field_name);
    if let Some(value) = fm.yaml.get_mut(&key) {
        rewrite_string_list_value(value, renames, &mut renamed);
    }

    renamed
}

fn yaml_str_list(val: &Value) -> Vec<String> {
    match val {
        Value::Sequence(seq) => seq
            .iter()
            .filter_map(Value::as_str)
            .map(str::to_owned)
            .collect(),
        Value::String(s) => vec![s.clone()],
        _ => vec![],
    }
}

fn rewrite_skill_value(
    value: &mut Value,
    renames: &IndexMap<String, String>,
    renamed: &mut IndexSet<String>,
) {
    match value {
        Value::Sequence(_) | Value::String(_) => rewrite_string_list_value(value, renames, renamed),
        Value::Mapping(mapping) => {
            for field in ["load", "available"] {
                if let Some(child) = mapping.get_mut(yaml_key(field)) {
                    rewrite_skill_value(child, renames, renamed);
                }
            }
        }
        _ => {}
    }
}

fn rewrite_string_list_value(
    value: &mut Value,
    renames: &IndexMap<String, String>,
    renamed: &mut IndexSet<String>,
) {
    match value {
        Value::Sequence(seq) => {
            for item in seq {
                let Some(name) = item.as_str() else {
                    continue;
                };
                if let Some(new_name) = renames.get(name) {
                    renamed.insert(name.to_string());
                    *item = Value::String(new_name.clone());
                }
            }
        }
        Value::String(name) => {
            if let Some(new_name) = renames.get(name.as_str()) {
                renamed.insert(name.clone());
                *name = new_name.clone();
            }
        }
        _ => {}
    }
}

fn exceeds_flow_depth(yaml: &str) -> bool {
    let mut depth = 0usize;
    for byte in yaml.bytes() {
        match byte {
            b'[' | b'{' => {
                depth += 1;
                if depth > MAX_FLOW_DEPTH {
                    return true;
                }
            }
            b']' | b'}' => depth = depth.saturating_sub(1),
            _ => {}
        }
    }
    false
}

/// A document's frontmatter region: the YAML between the fences, where it starts in the
/// document, and the Markdown after the closing fence.
struct Regions<'a> {
    has_bom: bool,
    /// Byte offset of `yaml`: past a byte-order mark and the opening fence line.
    yaml_start: usize,
    yaml: &'a str,
    body: &'a str,
}

/// The frontmatter region of `content`, and `None` when it has no frontmatter.
fn split_regions(content: &str) -> Result<Option<Regions<'_>>, FrontmatterError> {
    let (has_bom, document) = content
        .strip_prefix('\u{feff}')
        .map_or((false, content), |document| (true, document));
    let (first_line, after_first_line) = split_first_line(document);
    if !is_delimiter_line(first_line) {
        return Ok(None);
    }

    let mut yaml_end = None;
    let mut offset = 0usize;
    for line in after_first_line.split_inclusive('\n') {
        if is_delimiter_line(line) {
            yaml_end = Some((offset, line.len()));
            break;
        }
        offset += line.len();
    }

    let Some((yaml_len, closing_len)) = yaml_end else {
        return Err(FrontmatterError::Unclosed);
    };

    if yaml_len > MAX_FRONTMATTER_BYTES {
        return Err(FrontmatterError::TooLarge(yaml_len));
    }
    let yaml = &after_first_line[..yaml_len];
    if exceeds_flow_depth(yaml) {
        return Err(FrontmatterError::TooDeep);
    }
    Ok(Some(Regions {
        has_bom,
        yaml_start: content.len() - after_first_line.len(),
        yaml,
        body: &after_first_line[yaml_len + closing_len..],
    }))
}

/// The YAML text between a document's frontmatter fences, and where it starts in the document.
pub(crate) fn yaml_text(content: &str) -> Result<Option<(&str, usize)>, FrontmatterError> {
    Ok(split_regions(content)?.map(|regions| (regions.yaml, regions.yaml_start)))
}

fn split_first_line(content: &str) -> (&str, &str) {
    match content.split_once('\n') {
        Some((first, rest)) => (first, rest),
        None => (content, ""),
    }
}

fn is_delimiter_line(line: &str) -> bool {
    line.strip_suffix('\n')
        .unwrap_or(line)
        .strip_suffix('\r')
        .unwrap_or(line.strip_suffix('\n').unwrap_or(line))
        == "---"
}

fn validate_yaml_structure(text: &str) -> Vec<FrontmatterError> {
    use yaml_rust2::parser::{Event, MarkedEventReceiver, Parser};
    use yaml_rust2::scanner::Marker;

    #[derive(Default)]
    struct StructuralEvents {
        anchor_or_alias: bool,
        document_end: bool,
        source: String,
        char_to_byte: Vec<usize>,
    }
    impl MarkedEventReceiver for StructuralEvents {
        fn on_event(&mut self, event: Event, marker: Marker) {
            match event {
                Event::Alias(_) | Event::Scalar(_, _, 1.., _) => self.anchor_or_alias = true,
                Event::SequenceStart(1.., _) | Event::MappingStart(1.., _) => {
                    self.anchor_or_alias = true;
                }
                Event::DocumentEnd => {
                    let byte = self.char_to_byte.get(marker.index()).copied();
                    if byte.is_some_and(|byte| self.source[byte..].starts_with("...")) {
                        self.document_end = true;
                    }
                }
                _ => {}
            }
        }
    }

    let mut char_to_byte: Vec<usize> = text.char_indices().map(|(byte, _)| byte).collect();
    char_to_byte.push(text.len());
    let mut events = StructuralEvents {
        source: text.to_owned(),
        char_to_byte,
        ..StructuralEvents::default()
    };
    // serde_yaml already accepted the document. The event parser is used only
    // to distinguish YAML structure from identical text inside scalar values.
    let parse_error = Parser::new_from_str(text).load(&mut events, true).err();
    let mut errors = Vec::new();
    if events.anchor_or_alias {
        errors.push(FrontmatterError::Anchor);
    }
    if events.document_end {
        errors.push(FrontmatterError::DocumentEnd);
    }
    if let Some(error) = parse_error {
        errors.push(FrontmatterError::UnsupportedSyntax(error.to_string()));
    }
    errors
}

fn yaml_key(key: &str) -> Value {
    Value::String(key.to_string())
}

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

    #[test]
    fn parse_and_render_roundtrip() {
        let input = "---\nname: coder\nskills:\n- plan\n- review\n---\n# Body\ntext";
        let fm = Frontmatter::parse(input).unwrap();
        assert_eq!(fm.name(), Some("coder"));
        assert_eq!(fm.skills(), vec!["plan", "review"]);
        assert_eq!(fm.body(), "# Body\ntext");
        assert!(fm.has_frontmatter());

        let rendered = fm.render();
        let reparsed = Frontmatter::parse(&rendered).unwrap();
        assert_eq!(reparsed.name(), Some("coder"));
        assert_eq!(reparsed.skills(), vec!["plan", "review"]);
        assert_eq!(reparsed.body(), "# Body\ntext");
    }

    #[test]
    fn parse_without_frontmatter_keeps_body() {
        let input = "# Markdown only\ntext";
        let fm = parse(input).unwrap();
        assert!(!fm.has_frontmatter());
        assert!(fm.skills().is_empty());
        assert_eq!(fm.body(), input);
        assert_eq!(fm.render(), input);
    }

    #[test]
    fn parse_empty_frontmatter_roundtrips_delimiters() {
        let input = "---\n---\nbody";
        let fm = Frontmatter::parse(input).unwrap();
        assert!(fm.has_frontmatter());
        assert!(fm.skills().is_empty());
        assert_eq!(fm.body(), "body");
        assert_eq!(fm.render(), input);
    }

    #[test]
    fn parse_malformed_yaml_errors() {
        let input = "---\ninvalid: [:\n---\nbody";
        assert!(matches!(
            Frontmatter::parse(input),
            Err(FrontmatterError::MalformedYaml(_))
        ));
    }

    #[test]
    fn parse_flow_style_skills() {
        let input = "---\nskills: [plan, review]\n---\nbody";
        let fm = Frontmatter::parse(input).unwrap();
        assert_eq!(fm.skills(), vec!["plan", "review"]);
        assert_eq!(fm.skills_structured().load, vec!["plan", "review"]);
        assert!(fm.skills_structured().available.is_empty());
    }

    #[test]
    fn parse_structured_skills() {
        let input = "---\nskills:\n  load: [principles]\n  available:\n    - planning\n    - spawn\n---\nbody";
        let fm = Frontmatter::parse(input).unwrap();
        assert_eq!(fm.skills(), vec!["principles", "planning", "spawn"]);
        assert_eq!(fm.skills_structured().load, vec!["principles"]);
        assert_eq!(fm.skills_structured().available, vec!["planning", "spawn"]);
    }

    #[test]
    fn rewrite_structured_skills_preserves_split() {
        let input = "---\nskills:\n  load: [plan]\n  available: [review]\n---\nbody\n";
        let renames = IndexMap::from([
            ("plan".to_string(), "plan__org".to_string()),
            ("review".to_string(), "review__org".to_string()),
        ]);

        let rewritten = rewrite_content_skills(input, &renames).unwrap().unwrap();
        let fm = Frontmatter::parse(&rewritten).unwrap();
        assert_eq!(fm.skills_structured().load, vec!["plan__org"]);
        assert_eq!(fm.skills_structured().available, vec!["review__org"]);
    }

    #[test]
    fn rewrite_does_not_corrupt_substrings() {
        let input = "---\nskills:\n- plan\n- planner\n- planning-extended\n---\nbody\n";
        let renames = IndexMap::from([(
            "plan".to_string(),
            "plan__meridian-flow_meridian-base".to_string(),
        )]);

        let rewritten = rewrite_content_skills(input, &renames).unwrap().unwrap();
        let fm = Frontmatter::parse(&rewritten).unwrap();
        assert_eq!(
            fm.skills(),
            vec![
                "plan__meridian-flow_meridian-base",
                "planner",
                "planning-extended"
            ]
        );
    }

    #[test]
    fn rewrite_subagents_uses_exact_matches() {
        let input = "---\nsubagents:\n- web-researcher\n- web\n---\nbody\n";
        let renames = IndexMap::from([(
            "web-researcher".to_string(),
            "web-researcher__source-a".to_string(),
        )]);

        let rewritten = rewrite_content_subagents(input, &renames).unwrap().unwrap();
        let fm = Frontmatter::parse(&rewritten).unwrap();
        assert_eq!(
            fm.get("subagents").map(yaml_str_list).unwrap(),
            vec!["web-researcher__source-a", "web"]
        );
    }
}