use std::{
    collections::HashMap,
    sync::{
        atomic::{AtomicBool, AtomicPtr, Ordering},
        Arc, Mutex,
    },
    thread::{self, JoinHandle},
};

use eyre::{eyre, Result};

use {
    super::SchedulingRule,
    chrono::{DateTime, Local},
};

// TODO: add logging (always as debug)

type Action = Box<dyn FnMut() + Send + Sync + 'static>;

/// 🧉 » a scheduled task
///
/// this structs represents a task that has been scheduled in the scheduler.
///
/// this is returned by the `Scheduler::schedule` method, and can be used to check and control the
/// status of the task.
pub struct ScheduledTask {
    #[allow(dead_code)]
    name: String,
    action: Action,
    rules: Arc<Vec<SchedulingRule>>,
    is_running: Arc<AtomicBool>,
    is_stopped: Arc<AtomicBool>,
    is_removed: Arc<AtomicBool>,
    last_run: Arc<AtomicPtr<DateTime<Local>>>,
}

impl ScheduledTask {
    fn run(&mut self) {
        let action = self.action.as_mut();
        action();
    }

    fn make_handler(&self) -> TaskHandler {
        TaskHandler {
            name: self.name.clone(),
            rules: self.rules.clone(),
            is_running: self.is_running.clone(),
            is_stopped: self.is_stopped.clone(),
            is_removed: self.is_removed.clone(),
            last_run: self.last_run.clone(),
        }
    }

    fn is_active(&self) -> bool {
        !self.is_stopped.load(Ordering::Relaxed) && !self.is_removed.load(Ordering::Relaxed)
    }

    fn is_removed(&self) -> bool {
        self.is_removed.load(Ordering::Relaxed)
    }
}

/// 🧉 » a task scheduler.
///
/// this struct is responsible for scheduling tasks to be executed at specific times, depending on
/// the rules provided for each task.
///
/// Each task can have n rules, and the task will be executed when any of the rules is met.
pub struct Scheduler {
    tasks: HashMap<String, Arc<Mutex<ScheduledTask>>>,
}

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

impl Scheduler {
    /// 🧉 » create a new scheduler
    pub fn new() -> Self {
        Self {
            tasks: HashMap::new(),
        }
    }

    /// 🧉 » schedule a task
    ///
    /// schedules a task to be executed at times determined by the provided rules.
    pub fn schedule<F>(&mut self, name: &str, action: F, rules: SchedulingRule) -> TaskHandler
    where
        F: FnMut() + Send + Sync + 'static,
    {
        self.schedule_many_rules(name, action, vec![rules])
    }

    /// 🧉 » schedule a task
    ///
    /// schedules a task to be executed at times determined by the provided rules.
    pub fn schedule_many_rules<F>(
        &mut self,
        name: &str,
        action: F,
        rules: Vec<SchedulingRule>,
    ) -> TaskHandler
    where
        F: FnMut() + Send + Sync + 'static,
    {
        let task = Arc::new(Mutex::new(ScheduledTask {
            name: name.to_string(),
            action: Box::new(action),
            rules: Arc::new(rules),
            is_running: Arc::new(AtomicBool::new(false)),
            is_stopped: Arc::new(AtomicBool::new(false)),
            is_removed: Arc::new(AtomicBool::new(false)),
            last_run: Arc::new(AtomicPtr::new(std::ptr::null_mut())),
        }));

        self.tasks.insert(name.to_string(), task.clone());

        // launch the task in its own thread
        spawn_task(task.clone());

        let handler: TaskHandler = {
            let task = task.lock().unwrap();
            task.make_handler()
        };

        handler
    }

    /// 🧉 » stop a task
    pub fn stop(&mut self, handler: &TaskHandler) -> Result<()> {
        // get the task from the tasks map
        let task = self.tasks.get(handler.name());

        if let Some(task) = task {
            if let Ok(task) = task.lock() {
                task.is_stopped.store(true, Ordering::Relaxed);
                Ok(())
            } else {
                Err(eyre!("error stopping task {}", handler.name()))
            }
        } else {
            Err(eyre!("task {} was not found", handler.name()))
        }
    }

    /// 🧉 » resume a task
    pub fn resume(&mut self, handler: &TaskHandler) -> Result<()> {
        // get the task from the tasks map
        let task = self.tasks.get(handler.name());

        if let Some(task) = task {
            if let Ok(task) = task.lock() {
                task.is_stopped.store(false, Ordering::Relaxed);
                Ok(())
            } else {
                Err(eyre!("error resuming task {}", handler.name()))
            }
        } else {
            Err(eyre!("task {} was not found", handler.name()))
        }
    }

    /// 🧉 » remove a task
    pub fn remove(&mut self, handler: &TaskHandler) -> Result<()> {
        // get the task from the tasks map
        let task = self.tasks.remove(handler.name());

        if let Some(task) = task {
            if let Ok(task) = &mut task.lock() {
                task.is_removed.store(true, Ordering::Relaxed);

                Ok(())
            } else {
                Err(eyre!("error removing task {}", handler.name()))
            }
        } else {
            handler.is_removed.store(true, Ordering::Relaxed);
            Err(eyre!("task {} was not found", handler.name()))
        }
    }
}

/// 🧉 » task handler
///
/// returned by the `Scheduler::schedule` method,
/// this struct can be used to check and control
/// the status of the task.
#[derive(Clone)]
pub struct TaskHandler {
    name: String,
    rules: Arc<Vec<SchedulingRule>>,
    is_running: Arc<AtomicBool>,
    is_stopped: Arc<AtomicBool>,
    is_removed: Arc<AtomicBool>,
    last_run: Arc<AtomicPtr<DateTime<Local>>>,
}

impl TaskHandler {
    /// 🧉 » last run date
    ///
    /// returns a `DateTime<Local>` representing the last time the task was run or None if the task
    /// has never been run before.
    pub fn get_last_run(&self) -> Option<DateTime<Local>> {
        let last_run = self.last_run.load(Ordering::Relaxed);
        if last_run.is_null() {
            None
        } else {
            Some(unsafe { *last_run })
        }
    }

    /// 🧉 » next run date
    ///
    /// returns a `DateTime<Local>` representing the next time the task is scheduled to run
    pub fn get_next_run(&self) -> Option<DateTime<Local>> {
        if self.is_active() {
            return get_next_run_time(&self.rules, None);
        }

        None
    }

    /// 🧉 » is running?
    ///
    /// returns a `bool` indicating if the task is currently running in this moment
    pub fn is_running(&self) -> bool {
        self.is_running.load(Ordering::Relaxed)
    }

    /// 🧉 » name
    ///
    /// returns the name of the task
    pub fn name(&self) -> &str {
        &self.name
    }

    /// 🧉 » is active?
    ///
    /// returns a `bool` indicating if the task is active
    pub fn is_active(&self) -> bool {
        !self.is_stopped.load(Ordering::Relaxed) && !self.is_removed.load(Ordering::Relaxed)
    }

    /// 🧉 » is stopped?
    ///
    /// returns a `bool` indicating if the task has been stopped
    ///
    /// a task being stopped means that it has been paused, but not removed from the scheduler.
    /// So, although it's not running, it's still in the scheduler and can be resumed.
    pub fn is_stopped(&self) -> bool {
        self.is_stopped.load(Ordering::Relaxed)
    }

    /// 🧉 » is removed?
    ///
    /// returns a `bool` indicating if the task has been removed
    ///
    /// once a task is removed, it's no longer in the scheduler and can't be resumed.
    pub fn is_removed(&self) -> bool {
        self.is_removed.load(Ordering::Relaxed)
    }
}

/// **main function to run the task in its own thread**
///
/// it creates a new thread and runs the task according to its scheduling rules.
fn spawn_task(task_mutex: Arc<Mutex<ScheduledTask>>) -> JoinHandle<()> {
    let thread = thread::spawn(move || {
        let (mut maybe_next_run, name) = {
            let task = task_mutex.lock().unwrap();
            let rules = &task.rules;

            (get_next_run_time(rules, None), task.name.clone())
        };

        while let Some(run_date) = maybe_next_run {
            let now = Local::now();
            if run_date > now {
                // if the next run is in the future, go to bed until then
                let sleep_until = run_date - now;
                println!(
                    "task {} will run in {} seconds",
                    name,
                    sleep_until.num_seconds()
                );
                std::thread::sleep(sleep_until.to_std().unwrap());
            } else {
                // if the next run is in the past, run the task immediately, probably missed the
                // run time for a few nanos
                println!("task will run in 0 seconds");
            }

            let mut task = task_mutex.lock().unwrap();

            if task.is_active() {
                let run_date_box = Box::new(run_date);
                let run_date_raw = Box::into_raw(run_date_box);

                task.last_run.store(run_date_raw, Ordering::Relaxed);
                task.is_running.store(true, Ordering::SeqCst);
                task.run();
                task.is_running.store(false, Ordering::SeqCst);
            }

            if !task.is_removed() {
                maybe_next_run = get_next_run_time(&task.rules, Some(run_date));
            } else {
                maybe_next_run = None;
            }
        }
    });

    thread
}

/// **get next run time**
///
/// this function takes a list of scheduling rules and a base time, and returns the next time the
/// task should run.
///
/// to determine the next run time, it iterates over the list of rules and calculates the next run
/// time for each of them, returning the earliest of them all.
fn get_next_run_time(
    rules: &Vec<SchedulingRule>,
    from: Option<DateTime<Local>>,
) -> Option<DateTime<Local>> {
    let mut next_run_so_far: Option<DateTime<Local>> = None;

    let base = if let Some(from) = from {
        from
    } else {
        Local::now()
    };

    for rule in rules {
        let rule_next_run = rule.next_from(base);

        if let Some(next_run) = rule_next_run {
            if let Some(d) = next_run_so_far {
                if next_run < d {
                    next_run_so_far = Some(next_run);
                }
            } else {
                next_run_so_far = Some(next_run);
            }
        }
    }

    next_run_so_far
}