//! Thin read-only client over `~/.claude/registry.sqlite`.
//!
//! Fetches rule-fragment content by logical name (`rule::section`).
//! The registry stores the real filesystem path; this module reads that path.
//!
//! Constructor Pattern: one responsibility — lookup + read fragment body.
//! No writes. No schema migration. Opens DB read-only.

use rusqlite::{Connection, OpenFlags};
use std::path::{Path, PathBuf};

/// Open the registry at `db_path` in read-only mode.
pub fn open_read_only(db_path: &Path) -> Result<Connection, String> {
    Connection::open_with_flags(db_path, OpenFlags::SQLITE_OPEN_READ_ONLY)
        .map_err(|e| format!("open registry {}: {e}", db_path.display()))
}

/// Default path: `$KEI_REGISTRY_DB` (if set) or `~/.claude/registry.sqlite`.
pub fn default_db_path() -> PathBuf {
    if let Some(v) = std::env::var_os("KEI_REGISTRY_DB") {
        return PathBuf::from(v);
    }
    let home = std::env::var_os("HOME").unwrap_or_default();
    PathBuf::from(home).join(".claude/registry.sqlite")
}

/// Look up a rule fragment by `name` (e.g. `"karpathy-behavioral::1-think-before-coding"`).
///
/// Returns:
/// - `Ok(Some(body))` — fragment found and file readable.
/// - `Ok(None)` — name not in registry, or registry path does not exist on disk.
///   Caller should warn-and-skip.
/// - `Err(msg)` — DB query failure (not a missing-path issue). Propagate.
pub fn find_rule(conn: &Connection, name: &str) -> Result<Option<String>, String> {
    let path = match query_path(conn, name)? {
        Some(p) => p,
        None => return Ok(None),
    };
    read_fragment_body(name, &path)
}

/// Query the `path` column for the active row with `name` and `block_type='rule'`.
fn query_path(conn: &Connection, name: &str) -> Result<Option<String>, String> {
    let mut stmt = conn
        .prepare(
            "SELECT path FROM blocks \
             WHERE name = ?1 AND block_type = 'rule' AND superseded_by IS NULL \
             LIMIT 1",
        )
        .map_err(|e| format!("prepare query for {name}: {e}"))?;
    let row: Option<String> = stmt
        .query_row(rusqlite::params![name], |r| r.get(0))
        .optional()
        .map_err(|e| format!("query registry for {name}: {e}"))?;
    Ok(row)
}

/// Read the fragment body from `path`. Returns `Ok(None)` when the file is absent.
fn read_fragment_body(name: &str, path: &str) -> Result<Option<String>, String> {
    match std::fs::read_to_string(path) {
        Ok(body) => Ok(Some(body)),
        Err(e) if e.kind() == std::io::ErrorKind::NotFound => {
            eprintln!(
                "warn [assembler]: registry fragment for '{name}' has path '{path}' but file is missing — skipping. \
                 Run `kei-decompose decompose-rules --rebuild-fragments` to restore."
            );
            Ok(None)
        }
        Err(e) => Err(format!("read fragment for {name} at {path}: {e}")),
    }
}

trait OptionalExt<T>: Sized {
    fn optional(self) -> rusqlite::Result<Option<T>>;
}

impl<T> OptionalExt<T> for rusqlite::Result<T> {
    fn optional(self) -> rusqlite::Result<Option<T>> {
        match self {
            Ok(v) => Ok(Some(v)),
            Err(rusqlite::Error::QueryReturnedNoRows) => Ok(None),
            Err(e) => Err(e),
        }
    }
}

/// Check if `name` is a registered path-atom.
///
/// Convention: a path-atom is an atom whose source file is
/// `_blocks/path-<name>.md` and whose YAML frontmatter declares
/// `kind: path`. The DB stores only the file path (not body), so this
/// function uses the filename convention as a fast first check, then
/// reads the file and parses the frontmatter to confirm `kind: path`.
///
/// Returns:
/// - `Ok(true)` — atom registered under `name`, file exists, frontmatter
///   declares `kind: path`. Caller may emit an opaque resolved reference.
/// - `Ok(false)` — atom not found, or found but not a path-atom. Caller
///   should pass the original reference through unchanged (with optional
///   warn-and-skip in caller).
/// - `Err(msg)` — DB query failure. Propagate.
pub fn is_path_atom(conn: &Connection, name: &str) -> Result<bool, String> {
    let mut stmt = conn
        .prepare(
            "SELECT path FROM blocks \
             WHERE name = ?1 AND block_type = 'atom' AND superseded_by IS NULL \
             LIMIT 1",
        )
        .map_err(|e| format!("prepare path-atom query for {name}: {e}"))?;
    let path: Option<String> = stmt
        .query_row(rusqlite::params![name], |r| r.get(0))
        .optional()
        .map_err(|e| format!("query path-atom {name}: {e}"))?;
    let Some(p) = path else { return Ok(false) };
    // Filename convention check: `_blocks/path-<name>.md`. Cheap O(1) string
    // contains, avoids the file read on the common non-path-atom case.
    let expected_suffix = format!("/_blocks/path-{name}.md");
    if !p.ends_with(&expected_suffix) {
        return Ok(false);
    }
    // Read frontmatter to confirm `kind: path`. Defensive — convention is
    // not authoritative on its own; explicit declaration is.
    let body = match std::fs::read_to_string(&p) {
        Ok(b) => b,
        Err(_) => return Ok(false),
    };
    Ok(frontmatter_has_kind_path(&body))
}

/// Return true if `body` starts with a YAML frontmatter block (`---\n...---\n`)
/// containing a line whose key is `kind` and value is `path`. Tolerates
/// `---\r\n`, surrounding whitespace, and YAML quoting.
fn frontmatter_has_kind_path(body: &str) -> bool {
    let stripped = match body
        .strip_prefix("---\n")
        .or_else(|| body.strip_prefix("---\r\n"))
    {
        Some(s) => s,
        None => return false,
    };
    let end = match stripped
        .find("\n---\n")
        .or_else(|| stripped.find("\r\n---\r\n"))
    {
        Some(i) => i,
        None => return false,
    };
    let frontmatter = &stripped[..end];
    for line in frontmatter.lines() {
        let line = line.trim();
        if let Some(rest) = line.strip_prefix("kind:") {
            let val = rest.trim().trim_matches(&['\'', '"'][..]);
            return val == "path";
        }
    }
    false
}

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

    #[test]
    fn detects_kind_path_in_frontmatter() {
        let body = "---\ntype: atom\nkind: path\nname: foo\n---\n\n# body\n";
        assert!(frontmatter_has_kind_path(body));
    }

    #[test]
    fn rejects_kind_other() {
        let body = "---\ntype: atom\nkind: other\n---\n";
        assert!(!frontmatter_has_kind_path(body));
    }

    #[test]
    fn rejects_no_frontmatter() {
        let body = "# just markdown\n";
        assert!(!frontmatter_has_kind_path(body));
    }

    #[test]
    fn tolerates_quoted_value() {
        let body = "---\nkind: \"path\"\n---\n";
        assert!(frontmatter_has_kind_path(body));
    }

    #[test]
    fn rejects_kind_path_substring() {
        // `kind: pathological` must NOT match `kind: path`.
        let body = "---\nkind: pathological\n---\n";
        assert!(!frontmatter_has_kind_path(body));
    }
}