//! Session metadata: id, title, workspace roots, and message/token counts,
//! persisted as `session.json` per session directory.
use chrono::Utc;
use serde::{Deserialize, Serialize};
use std::path::{Path, PathBuf};

/// Metadata for one conversation session (distinct from the message
/// history itself, which lives in `Conversation`/the msglog).
#[derive(Debug, Clone, Serialize, Deserialize)]
pub struct Session {
    pub id: String,
    pub created_at: i64,
    pub updated_at: i64,
    pub title: String,
    pub model: String,
    pub workspace_roots: Vec<PathBuf>,
    pub message_count: u32,
    pub token_count: u32,
    pub archived: bool,
    pub summary: Option<String>,
}

impl Session {
    /// Create a new session with the given id/title, defaulting the
    /// model, workspace root (current dir), and counters.
    pub fn new(id: String, title: String) -> Self {
        let now = Utc::now().timestamp_millis();
        Session {
            id,
            created_at: now,
            updated_at: now,
            title,
            model: "anthropic/claude-opus-4-8".to_string(),
            workspace_roots: vec![std::env::current_dir().unwrap_or_default()],
            message_count: 0,
            token_count: 0,
            archived: false,
            summary: None,
        }
    }

    /// Compute this session's directory under `<base_dir>/sessions/<id>`.
    pub fn session_dir(&self, base_dir: &Path) -> PathBuf {
        base_dir.join("sessions").join(&self.id)
    }

    /// Compute this session's `conversation.json` path.
    pub fn conversation_path(&self, base_dir: &Path) -> PathBuf {
        self.session_dir(base_dir).join("conversation.json")
    }

    /// Persist this session's metadata to `session.json`, atomically
    /// with fsync for crash safety.
    ///
    /// Flow: ensure the session directory exists → serialize to pretty
    /// JSON → write to `session.json.tmp` → fsync → rename over
    /// `session.json` → fsync parent directory.
    ///
    /// Why: write-then-rename avoids a torn/partial `session.json` if
    /// interrupted mid-write; fsync before rename ensures the data is
    /// on disk before the rename makes it visible.
    ///
    /// Return: `Ok(())` on success, or an `io::Error` from any step.
    pub fn save(&self, base_dir: &Path) -> std::io::Result<()> {
        let dir = self.session_dir(base_dir);
        std::fs::create_dir_all(&dir)?;
        let path = dir.join("session.json");
        let data = serde_json::to_string_pretty(self)?;
        let tmp = dir.join("session.json.tmp");
        std::fs::write(&tmp, data)?;
        let f = std::fs::File::open(&tmp)?;
        f.sync_all()?;
        std::fs::rename(&tmp, path)?;
        let _ = std::fs::File::open(&dir).and_then(|d| d.sync_all());
        Ok(())
    }

    /// Load a session's metadata by id from `<base_dir>/sessions/<id>/session.json`.
    ///
    /// Security: the session id is validated to prevent directory traversal
    /// (e.g. `../../etc/passwd`). Only alphanumeric, hyphens, underscores,
    /// and dots are allowed — no path separators.
    ///
    /// Return: the parsed `Session`, or an `io::Error` if the file is
    /// missing or malformed.
    pub fn load(id: &str, base_dir: &Path) -> std::io::Result<Self> {
        // Reject session ids that contain path separators or parent dir refs
        if id.contains('/') || id.contains('\\') || id.contains("..") {
            return Err(std::io::Error::new(
                std::io::ErrorKind::InvalidInput,
                format!("invalid session id '{id}': must not contain path separators"),
            ));
        }
        let path = base_dir.join("sessions").join(id).join("session.json");
        let data = std::fs::read_to_string(path)?;
        let session: Session = serde_json::from_str(&data)?;
        Ok(session)
    }

    /// List all loadable sessions under `<base_dir>/sessions/`.
    ///
    /// Flow: read the sessions directory → keep subdirectories → attempt
    /// `Session::load` for each by its directory name, discarding any
    /// that fail to load.
    ///
    /// Return: a `Vec<Session>`, empty if the directory can't be read or
    /// contains no valid sessions.
    pub fn list(base_dir: &Path) -> Vec<Self> {
        let sessions_dir = base_dir.join("sessions");
        let Ok(entries) = std::fs::read_dir(&sessions_dir) else {
            return Vec::new();
        };
        entries
            .filter_map(std::result::Result::ok)
            .filter(|e| e.path().is_dir())
            .filter_map(|e| {
                let id = e.file_name().to_string_lossy().to_string();
                Session::load(&id, base_dir).ok()
            })
            .collect()
    }
}