//! Project-local agent prompt layer.
//!
//! Reads `.huskies/AGENT.md` from the project root and appends its content to
//! the baked-in agent prompt at spawn time.  This lets projects record
//! non-obvious facts (directory conventions, known traps, etc.) that every
//! agent should know without modifying the shared agent configuration.
//!
//! Behaviour contract:
//! - If the file is missing or empty the caller receives `None`; agents spawn
//!   normally with no warnings or errors.
//! - If the file exists and is non-empty, the content is returned and an
//!   INFO-level log line is emitted with the file path and byte count.
//! - The file is read fresh on every agent spawn — no caching.

use std::path::Path;

/// Attempt to load the project-local agent prompt from `.huskies/AGENT.md`.
///
/// Returns `Some(content)` when the file exists and is non-empty, or `None`
/// when the file is absent or empty.  Never returns an error; any I/O problem
/// is silently treated as "no local prompt".
pub fn read_project_local_prompt(project_root: &Path) -> Option<String> {
    let path = project_root.join(".huskies/AGENT.md");
    let content = std::fs::read_to_string(&path).ok()?;
    let trimmed = content.trim();
    if trimmed.is_empty() {
        return None;
    }
    crate::slog!(
        "[agents] project-local prompt loaded: {} ({} bytes)",
        path.display(),
        trimmed.len()
    );
    Some(trimmed.to_string())
}

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

    #[test]
    fn returns_none_when_file_absent() {
        let tmp = tempfile::tempdir().unwrap();
        let result = read_project_local_prompt(tmp.path());
        assert!(result.is_none(), "missing file must return None");
    }

    #[test]
    fn returns_none_when_file_empty() {
        let tmp = tempfile::tempdir().unwrap();
        let huskies_dir = tmp.path().join(".huskies");
        std::fs::create_dir_all(&huskies_dir).unwrap();
        std::fs::write(huskies_dir.join("AGENT.md"), "").unwrap();
        let result = read_project_local_prompt(tmp.path());
        assert!(result.is_none(), "empty file must return None");
    }

    #[test]
    fn returns_none_when_file_whitespace_only() {
        let tmp = tempfile::tempdir().unwrap();
        let huskies_dir = tmp.path().join(".huskies");
        std::fs::create_dir_all(&huskies_dir).unwrap();
        std::fs::write(huskies_dir.join("AGENT.md"), "   \n\n  ").unwrap();
        let result = read_project_local_prompt(tmp.path());
        assert!(result.is_none(), "whitespace-only file must return None");
    }

    #[test]
    fn returns_content_when_file_non_empty() {
        let tmp = tempfile::tempdir().unwrap();
        let huskies_dir = tmp.path().join(".huskies");
        std::fs::create_dir_all(&huskies_dir).unwrap();
        let marker = "DISTINCTIVE_MARKER_XYZ42";
        std::fs::write(huskies_dir.join("AGENT.md"), format!("# Hints\n{marker}\n")).unwrap();
        let result = read_project_local_prompt(tmp.path());
        assert!(result.is_some(), "non-empty file must return Some");
        let content = result.unwrap();
        assert!(
            content.contains(marker),
            "returned content must include the marker: {content}"
        );
    }

    #[test]
    fn appended_to_prompt_integration() {
        // Simulates the start.rs usage: marker appears in the constructed
        // system prompt when the file is present, absent when it is not.
        let tmp_with = tempfile::tempdir().unwrap();
        let huskies_dir = tmp_with.path().join(".huskies");
        std::fs::create_dir_all(&huskies_dir).unwrap();
        let marker = "INTEGRATION_MARKER_601";
        std::fs::write(huskies_dir.join("AGENT.md"), marker).unwrap();

        let base_prompt = "You are a coder agent.".to_string();
        let local = read_project_local_prompt(tmp_with.path());
        let effective = match local {
            Some(ref extra) => format!("{base_prompt}\n\n{extra}"),
            None => base_prompt.clone(),
        };
        assert!(
            effective.contains(marker),
            "marker must appear in effective prompt when file present: {effective}"
        );

        // Without the file
        let tmp_without = tempfile::tempdir().unwrap();
        let local2 = read_project_local_prompt(tmp_without.path());
        assert!(local2.is_none(), "no marker when file absent");
        let effective2 = match local2 {
            Some(ref extra) => format!("{base_prompt}\n\n{extra}"),
            None => base_prompt.clone(),
        };
        assert!(
            !effective2.contains(marker),
            "marker must NOT appear in effective prompt when file absent: {effective2}"
        );
    }
}