// Copyright 2020, The Tari Project
//
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use std::{future::Future, sync::Arc};

use tokio::{
    sync::{OwnedSemaphorePermit, Semaphore},
    task::JoinHandle,
};

/// Error emitted from [`try_spawn`](self::BoundedExecutor::try_spawn) when there are no tasks available
#[derive(Debug)]
pub struct TrySpawnError;

/// A task executor bounded by a semaphore.
///
/// Use the asynchronous spawn method to spawn a task. If a given number of tasks are already spawned and have not
/// completed, the spawn function will block (asynchronously) until a previously spawned task completes.
pub struct BoundedExecutor {
    // inner: runtime::Handle,
    semaphore: Arc<Semaphore>,
    max_available: usize,
}

impl BoundedExecutor {
    pub fn new(num_permits: usize) -> Self {
        Self {
            semaphore: Arc::new(Semaphore::new(num_permits)),
            max_available: num_permits,
        }
    }

    pub fn allow_maximum() -> Self {
        Self::new(Self::max_theoretical_tasks())
    }

    pub const fn max_theoretical_tasks() -> usize {
        // Maximum from here: https://github.com/tokio-rs/tokio/blob/ce9eabfdd12a14efb74f5e6d507f2acbe7a814c9/tokio/src/sync/batch_semaphore.rs#L101
        // NOTE: usize::MAX >> 3 does not work. The reason is not clear, however 1152921504606846975 tasks seems
        // sufficient
        usize::MAX >> 4
    }

    pub fn can_spawn(&self) -> bool {
        self.num_available() > 0
    }

    /// Returns the remaining number of tasks that can be spawned on this executor without waiting.
    pub fn num_available(&self) -> usize {
        self.semaphore.available_permits()
    }

    /// Returns the maximum number of concurrent tasks that can be spawned on this executor without waiting.
    pub fn max_available(&self) -> usize {
        self.max_available
    }

    pub fn try_spawn<F>(&self, future: F) -> Result<JoinHandle<F::Output>, TrySpawnError>
    where
        F: Future + Send + 'static,
        F::Output: Send + 'static,
    {
        let permit = self.semaphore.clone().try_acquire_owned().map_err(|_| TrySpawnError)?;
        let handle = self.do_spawn(permit, future);
        Ok(handle)
    }

    /// Spawn a future onto the Tokio runtime asynchronously blocking if there are too many
    /// spawned tasks.
    ///
    /// This spawns the given future onto the runtime's executor, usually a
    /// thread pool. The thread pool is then responsible for polling the future
    /// until it completes.
    ///
    /// If the number of pending tasks exceeds the num_permits value given to `BoundedExecutor::new`
    /// the future returned from spawn will block until a permit is released.
    ///
    /// See [module level][mod] documentation for more details.
    ///
    /// [mod]: index.html
    ///
    /// # Examples
    ///
    /// ```
    /// use tokio::runtime::Handle;
    /// use tari_comms::bounded_executor::BoundedExecutor;
    ///
    /// # fn dox() {
    /// // Create the runtime
    /// let executor = BoundedExecutor::new(1);
    ///
    /// // Spawn a future onto the runtime
    /// // NOTE: BoundedExecutor::spawn is an async function and therefore, must be polled/awaited for the task to be spawned
    /// let task1 = executor.spawn(async {
    ///     println!("now running on a worker thread");
    /// });
    /// // This will spawn after task1
    /// let task2 = executor.spawn(async {
    ///     println!("will always run after the first task");
    /// });
    ///
    /// Handle::current().block_on(task1);
    /// Handle::current().block_on(task2);
    /// # }
    /// ```
    ///
    /// # Panics
    ///
    /// This function panics if the spawn fails. Failure occurs if the executor
    /// is currently at capacity and is unable to spawn a new future.
    pub async fn spawn<F>(&self, future: F) -> JoinHandle<F::Output>
    where
        F: Future + Send + 'static,
        F::Output: Send + 'static,
    {
        // SAFETY: acquire_owned only fails if the semaphore is closed (i.e self.semaphore.close() is called) - this
        // never happens in this implementation
        let permit = self.semaphore.clone().acquire_owned().await.expect("semaphore closed");
        self.do_spawn(permit, future)
    }

    fn do_spawn<F>(&self, permit: OwnedSemaphorePermit, future: F) -> JoinHandle<F::Output>
    where
        F: Future + Send + 'static,
        F::Output: Send + 'static,
    {
        // let task = task::Builder::new().inner
        tokio::spawn(async move {
            // Task is finished, release the permit
            let ret = future.await;
            drop(permit);
            ret
        })
    }
}

#[cfg(test)]
mod test {
    use std::{
        sync::atomic::{AtomicBool, Ordering},
        time::Duration,
    };

    use tokio::time::sleep;

    use super::*;

    #[tokio::test]
    async fn spawn() {
        let flag = Arc::new(AtomicBool::new(false));
        let flag_cloned = flag.clone();
        let executor = BoundedExecutor::new(1);

        // Spawn 1
        let task1_fut = executor
            .spawn(async move {
                sleep(Duration::from_millis(1)).await;
                flag_cloned.store(true, Ordering::SeqCst);
            })
            .await;

        // Spawn 2
        let task2_fut = executor
            .spawn(async move {
                // This will panic if this task is spawned before task1 completes (e.g if num_permitted > 1)
                assert!(flag.load(Ordering::SeqCst));
            })
            .await;

        task2_fut.await.unwrap();
        task1_fut.await.unwrap();
    }
}
