//! Creates a worker thread and exposes a sender to communidate with
use async_trait::async_trait;
use chrono::Utc;
use parking_lot::RwLock;
use std::{
    ops::Deref,
    sync::{atomic::AtomicBool, Arc},
    time::Duration,
};
use tokio::{
    sync::mpsc::{channel, error::TrySendError, Sender},
    time::sleep,
};

/// Time to awake the main thread to check if the parent struct is still in memory if it was dropped
/// already. If it was dropped the main thread has to be stopped.
const CHECK_WORKER_IN_SCOPE_MS: u64 = 50;
/// The maximum size for buffering messages
const WORKER_BUFFER_SIZE: usize = 1_000;
/// The maximum time to be idle waiting for requests to be processed. After this is reached the main
/// loop is stopped.
const MAXIMUM_IDLE_TIME_SEC: i64 = 60;

/// Worker trait
///
/// The worker trait has the definition of the code the worker has to perform in a different thread
#[async_trait]
pub trait Worker: Send + Sync {
    type Payload: Send + Sync + Clone;

    /// Method to be executed with a given task
    async fn handler(&self, payload: Self::Payload);

    /// Whether or not to process the request
    fn process_request(&self) -> bool {
        true
    }
}

/// Worker manager
///
/// The worker manager manages the instances of the Worker trait, which is executed asynchronously
/// in a separate thread from the send() context.
///
/// The logic of having one or more instances of the Worker trait is abstracted in this structure.
#[derive(Debug)]
pub struct WorkerManager<W: Worker> {
    sender: RwLock<Option<Sender<W::Payload>>>,
    is_running: Arc<AtomicBool>,
    worker: Arc<W>,
}

impl<W: Worker> Drop for WorkerManager<W> {
    fn drop(&mut self) {
        self.is_running
            .store(false, std::sync::atomic::Ordering::Release);
    }
}

impl<W: Worker> Deref for WorkerManager<W> {
    type Target = Arc<W>;

    fn deref(&self) -> &Self::Target {
        &self.worker
    }
}

impl<W: Worker + 'static> WorkerManager<W> {
    /// Creates a new WorkerManager given a struct that implements the Worker trait
    pub fn new(worker: W) -> Self {
        Self {
            sender: RwLock::new(None),
            is_running: Arc::new(true.into()),
            worker: Arc::new(worker),
        }
    }

    fn start_background_worker(&self) -> Sender<W::Payload> {
        let (sender, mut receiver) = channel(WORKER_BUFFER_SIZE);
        let worker_for_thread = self.worker.clone();
        let worker_in_scope = self.is_running.clone();
        tokio::spawn(async move {
            let mut last_time = Utc::now();
            loop {
                tokio::select! {
                    Some(message) = receiver.recv() => {
                        worker_for_thread.handler(message).await;
                        last_time = Utc::now();
                    }
                    _ = sleep(Duration::from_millis(CHECK_WORKER_IN_SCOPE_MS))  => {}
                }

                if !worker_in_scope.load(std::sync::atomic::Ordering::Acquire)
                    || (last_time - Utc::now()).num_seconds() > MAXIMUM_IDLE_TIME_SEC
                {
                    break;
                }
            }
        });
        sender
    }

    /// Sends a message to be processed in another thread
    pub fn process(&self, message: W::Payload) {
        if self.worker.process_request() {
            let sender = self.sender.read();
            match sender
                .as_ref()
                .map(|sender| sender.try_send(message.clone()))
            {
                None | Some(Err(TrySendError::Closed(_))) => {
                    drop(sender);

                    let mut sender = self.sender.write();

                    if let Some(sender) = sender.as_ref() {
                        // Check if another faster thread did not set sender to Some already
                        let _ = sender.try_send(message.clone());
                    } else {
                        // Either there is no running worker thread and it is closed already, and
                        // this thread is fastest and therefore will start the background worker and
                        // will set the sender to Some
                        let new_worker = self.start_background_worker();
                        let _ = new_worker.try_send(message);
                        *sender = Some(new_worker);
                    }
                }
                _ => {}
            }
        }
    }
}