use std::sync::Arc;
use std::time::Duration;

use chrono::Utc;
use serde::{Deserialize, Serialize};

use super::batch::Batch;
use super::error::Result;
#[cfg(test)]
use super::task_store::test::MockTaskStore as TaskStore;
use super::task_store::Pending;
#[cfg(not(test))]
use super::task_store::TaskStore;
use super::TaskPerformer;
use crate::tasks::task::TaskEvent;

/// The scheduler roles is to perform batches of tasks one at a time. It will monitor the TaskStore
/// for new tasks, put them in a batch, and process the batch as soon as possible.
///
/// When a batch is currently processing, the scheduler is just waiting.
pub struct Scheduler<P: TaskPerformer> {
    store: TaskStore,
    performer: Arc<P>,

    /// The interval at which the the `TaskStore` should be checked for new updates
    task_store_check_interval: Duration,
}

impl<P> Scheduler<P>
where
    P: TaskPerformer + Send + Sync + 'static,
    P::Error: Serialize + for<'de> Deserialize<'de> + Send + Sync + 'static,
{
    pub fn new(store: TaskStore, performer: Arc<P>, task_store_check_interval: Duration) -> Self {
        Self {
            store,
            performer,
            task_store_check_interval,
        }
    }

    pub async fn run(self) {
        loop {
            if let Err(e) = self.process_next_batch().await {
                log::error!("an error occured while processing an update batch: {}", e);
            }
        }
    }

    async fn process_next_batch(&self) -> Result<()> {
        match self.prepare_batch().await? {
            Some(mut batch) => {
                for task in &mut batch.tasks {
                    match task {
                        Pending::Task(task) => task.events.push(TaskEvent::Processing(Utc::now())),
                        Pending::Job(_) => (),
                    }
                }

                // the jobs are ignored
                batch.tasks = self.store.update_tasks(batch.tasks).await?;

                let performer = self.performer.clone();
                let batch_result = performer.process(batch).await;
                self.handle_batch_result(batch_result).await?;
            }
            None => {
                // No update found to create a batch we wait a bit before we retry.
                tokio::time::sleep(self.task_store_check_interval).await;
            }
        }

        Ok(())
    }

    /// Checks for pending tasks and groups them in a batch. If there are no pending update,
    /// return Ok(None)
    ///
    /// Until batching is properly implemented, the batches contain only one task.
    async fn prepare_batch(&self) -> Result<Option<Batch>> {
        match self.store.peek_pending_task().await {
            Some(Pending::Task(next_task_id)) => {
                let mut task = self.store.get_task(next_task_id, None).await?;

                task.events.push(TaskEvent::Batched {
                    timestamp: Utc::now(),
                    batch_id: 0,
                });

                let batch = Batch {
                    id: 0,
                    // index_uid: task.index_uid.clone(),
                    created_at: Utc::now(),
                    tasks: vec![Pending::Task(task)],
                };
                Ok(Some(batch))
            }
            Some(Pending::Job(job)) => Ok(Some(Batch {
                id: 0,
                created_at: Utc::now(),
                tasks: vec![Pending::Job(job)],
            })),
            None => Ok(None),
        }
    }

    /// Handles the result from a batch processing.
    ///
    /// When a task is processed, the result of the processing is pushed to its event list. The
    /// handle batch result make sure that the new state is save into its store.
    /// The tasks are then removed from the processing queue.
    async fn handle_batch_result(&self, mut batch: Batch) -> Result<()> {
        let tasks = self.store.update_tasks(batch.tasks).await?;
        batch.tasks = tasks;
        self.store.delete_pending(&batch.tasks[0]).await;
        self.performer.finish(&batch).await;
        Ok(())
    }
}

#[cfg(test)]
mod test {
    use nelson::Mocker;

    use crate::index_resolver::IndexUid;
    use crate::tasks::task::Task;
    use crate::tasks::task_store::TaskFilter;

    use super::super::task::{TaskContent, TaskEvent, TaskId, TaskResult};
    use super::super::MockTaskPerformer;
    use super::*;

    #[tokio::test]
    async fn test_prepare_batch_full() {
        let mocker = Mocker::default();

        mocker
            .when::<(TaskId, Option<TaskFilter>), Result<Option<Task>>>("get_task")
            .once()
            .then(|(id, _filter)| {
                let task = Task {
                    id,
                    index_uid: IndexUid::new("Test".to_string()).unwrap(),
                    content: TaskContent::IndexDeletion,
                    events: vec![TaskEvent::Created(Utc::now())],
                };
                Ok(Some(task))
            });

        mocker
            .when::<(), Option<Pending<TaskId>>>("peek_pending_task")
            .then(|()| Some(Pending::Task(1)));

        let store = TaskStore::mock(mocker);
        let performer = Arc::new(MockTaskPerformer::new());

        let scheduler = Scheduler {
            store,
            performer,
            task_store_check_interval: Duration::from_millis(1),
        };

        let batch = scheduler.prepare_batch().await.unwrap().unwrap();

        assert_eq!(batch.tasks.len(), 1);
        assert!(
            matches!(batch.tasks[0], Pending::Task(Task { id: 1, .. })),
            "{:?}",
            batch.tasks[0]
        );
    }

    #[tokio::test]
    async fn test_prepare_batch_empty() {
        let mocker = Mocker::default();
        mocker
            .when::<(), Option<Pending<TaskId>>>("peek_pending_task")
            .then(|()| None);

        let store = TaskStore::mock(mocker);
        let performer = Arc::new(MockTaskPerformer::new());

        let scheduler = Scheduler {
            store,
            performer,
            task_store_check_interval: Duration::from_millis(1),
        };

        assert!(scheduler.prepare_batch().await.unwrap().is_none());
    }

    #[tokio::test]
    async fn test_loop_run_normal() {
        let mocker = Mocker::default();
        let mut id = Some(1);
        mocker
            .when::<(), Option<Pending<TaskId>>>("peek_pending_task")
            .then(move |()| id.take().map(Pending::Task));
        mocker
            .when::<(TaskId, Option<TaskFilter>), Result<Task>>("get_task")
            .once()
            .then(|(id, _)| {
                let task = Task {
                    id,
                    index_uid: IndexUid::new("Test".to_string()).unwrap(),
                    content: TaskContent::IndexDeletion,
                    events: vec![TaskEvent::Created(Utc::now())],
                };
                Ok(task)
            });

        mocker
            .when::<Vec<Pending<Task>>, Result<Vec<Pending<Task>>>>("update_tasks")
            .times(2)
            .then(|tasks| {
                assert_eq!(tasks.len(), 1);
                Ok(tasks)
            });

        mocker.when::<(), ()>("delete_pending").once().then(|_| ());

        let store = TaskStore::mock(mocker);

        let mut performer = MockTaskPerformer::new();
        performer.expect_process().once().returning(|mut batch| {
            batch.tasks.iter_mut().for_each(|t| match t {
                Pending::Task(Task { ref mut events, .. }) => events.push(TaskEvent::Succeded {
                    result: TaskResult::Other,
                    timestamp: Utc::now(),
                }),
                _ => panic!("expected a task, found a job"),
            });

            batch
        });

        performer.expect_finish().once().returning(|_| ());

        let performer = Arc::new(performer);

        let scheduler = Scheduler {
            store,
            performer,
            task_store_check_interval: Duration::from_millis(1),
        };

        let handle = tokio::spawn(scheduler.run());

        if let Ok(r) = tokio::time::timeout(Duration::from_millis(100), handle).await {
            r.unwrap();
        }
    }
}