//! # Rullst Task Scheduler (`rullst::scheduler`)
//!
//! Declarative Cron-like job scheduler that runs alongside your HTTP server.
//!
//! ## Quick Start
//! ```rust,ignore
//! use rullst::scheduler::Scheduler;
//!
//! let scheduler = Scheduler::new()
//!     .task("0 0 * * *", || async { cleanup_old_sessions().await })
//!     .task("*/5 * * * *", || async { check_health().await });
//!
//! Server::new(router)
//!     .schedule(scheduler)
//!     .run(3000)
//!     .await?;
//! ```

use std::future::Future;
use std::pin::Pin;
use std::sync::Arc;

// ─── Scheduled Task ─────────────────────────────────────────────────────────

/// A single scheduled task with a cron expression and an async handler.
#[allow(clippy::type_complexity)]
pub struct ScheduledTask {
    /// Human-readable label for logging (derived from cron expression).
    label: String,
    /// The parsed cron schedule.
    schedule: cron::Schedule,
    /// The async handler to execute when the schedule fires.
    handler: Arc<Box<dyn Fn() -> Pin<Box<dyn Future<Output = ()> + Send>> + Send + Sync>>,
}

// ─── Scheduler ──────────────────────────────────────────────────────────────

/// A declarative task scheduler that runs recurring async jobs on cron schedules.
///
/// Tasks are registered with standard 5-field cron expressions and async closures.
/// The scheduler runs as background Tokio tasks alongside your HTTP server.
///
/// # Cron Expression Format
/// ```text
/// ┌─────────── minute (0–59)
/// │ ┌───────── hour (0–23)
/// │ │ ┌─────── day of month (1–31)
/// │ │ │ ┌───── month (1–12)
/// │ │ │ │ ┌─── day of week (0–6, Sun=0)
/// │ │ │ │ │
/// * * * * *
/// ```
///
/// # Example
/// ```rust,ignore
/// Scheduler::new()
///     .task("0 0 * * *", || async { nightly_cleanup().await })
///     .task("*/10 * * * *", || async { health_check().await })
/// ```
pub struct Scheduler {
    tasks: Vec<ScheduledTask>,
}

impl Scheduler {
    /// Create a new empty scheduler.
    pub fn new() -> Self {
        Self { tasks: Vec::new() }
    }

    /// Register a recurring task with a cron expression.
    ///
    /// The cron expression uses the standard 5-field format extended to 7 fields
    /// internally (seconds and year are auto-filled).
    ///
    /// # Panics
    /// Panics if the cron expression is invalid.
    ///
    /// # Example
    /// ```rust,ignore
    /// scheduler.task("30 2 * * 1", || async {
    ///     println!("Running at 2:30 AM every Monday");
    /// });
    /// ```
    pub fn task<F, Fut>(mut self, cron_expr: &str, handler: F) -> Self
    where
        F: Fn() -> Fut + Send + Sync + 'static,
        Fut: Future<Output = ()> + Send + 'static,
    {
        // The `cron` crate requires 7 fields (sec min hr dom mon dow year).
        // We convert a standard 5-field expression by prepending "0" (seconds)
        // and appending "*" (year).
        let full_expr = format!("0 {} *", cron_expr);

        let schedule: cron::Schedule = full_expr
            .parse()
            .unwrap_or_else(|e| panic!("Invalid cron expression '{}': {}", cron_expr, e));

        let label = cron_expr.to_string();
        let boxed: Box<dyn Fn() -> Pin<Box<dyn Future<Output = ()> + Send>> + Send + Sync> =
            Box::new(move || Box::pin(handler()));

        self.tasks.push(ScheduledTask {
            label,
            schedule,
            handler: Arc::new(boxed),
        });

        self
    }

    /// Start all scheduled tasks as background Tokio tasks.
    ///
    /// Each task runs in its own spawned task, sleeping until the next
    /// cron-scheduled time, then executing the handler.
    ///
    /// This method consumes the scheduler and should be called once during
    /// server startup (typically via `Server::schedule()`).
    pub fn start(self) {
        for task in self.tasks {
            let schedule = task.schedule;
            let handler = task.handler;
            let label = task.label;

            tokio::spawn(async move {
                println!("📅 Scheduler: task '{}' registered.", label);
                loop {
                    let now = chrono::Utc::now();
                    if let Some(next) = schedule.upcoming(chrono::Utc).next() {
                        let duration = (next - now)
                            .to_std()
                            .unwrap_or(std::time::Duration::from_secs(60));
                        tokio::time::sleep(duration).await;

                        let handler_clone = Arc::clone(&handler);
                        tokio::spawn(async move {
                            handler_clone().await;
                        });
                    } else {
                        // No more upcoming executions — this shouldn't happen with standard cron
                        break;
                    }
                }
            });
        }
    }
}

impl Default for Scheduler {
    fn default() -> Self {
        Self::new()
    }
}

// ─── Tests ──────────────────────────────────────────────────────────────────

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

    #[test]
    fn test_scheduler_creation() {
        let scheduler = Scheduler::new();
        assert_eq!(scheduler.tasks.len(), 0);
    }

    #[test]
    fn test_scheduler_task_registration() {
        let scheduler = Scheduler::new()
            .task("* * * * *", || async {})
            .task("0 0 * * *", || async {});
        assert_eq!(scheduler.tasks.len(), 2);
    }

    #[test]
    fn test_scheduler_label_preserved() {
        let scheduler = Scheduler::new().task("30 2 * * 1", || async {});
        assert_eq!(scheduler.tasks[0].label, "30 2 * * 1");
    }

    #[test]
    #[should_panic(expected = "Invalid cron expression")]
    fn test_scheduler_invalid_cron() {
        let _scheduler = Scheduler::new().task("invalid cron", || async {});
    }

    #[tokio::test]
    async fn test_scheduler_cron_parses_correctly() {
        // Verify that a 5-field expression parses to a valid 7-field schedule
        let scheduler = Scheduler::new().task("*/5 * * * *", || async {});
        let next = scheduler.tasks[0].schedule.upcoming(chrono::Utc).next();
        assert!(next.is_some(), "Scheduler should have upcoming executions");
    }
}
