zed/crates/gpui/src/queue.rs

use std::{
    collections::VecDeque,
    fmt,
    iter::FusedIterator,
    sync::{Arc, atomic::AtomicUsize},
};

use rand::{Rng, SeedableRng, rngs::SmallRng};

use crate::Priority;

struct PriorityQueues<T> {
    high_priority: VecDeque<T>,
    medium_priority: VecDeque<T>,
    low_priority: VecDeque<T>,
}

impl<T> PriorityQueues<T> {
    fn is_empty(&self) -> bool {
        self.high_priority.is_empty()
            && self.medium_priority.is_empty()
            && self.low_priority.is_empty()
    }
}

struct PriorityQueueState<T> {
    queues: parking_lot::Mutex<PriorityQueues<T>>,
    condvar: parking_lot::Condvar,
    receiver_count: AtomicUsize,
    sender_count: AtomicUsize,
}

impl<T> PriorityQueueState<T> {
    fn send(&self, priority: Priority, item: T) -> Result<(), SendError<T>> {
        if self
            .receiver_count
            .load(std::sync::atomic::Ordering::Relaxed)
            == 0
        {
            return Err(SendError(item));
        }

        let mut queues = self.queues.lock();
        match priority {
            Priority::Realtime(_) => unreachable!(),
            Priority::High => queues.high_priority.push_back(item),
            Priority::Medium => queues.medium_priority.push_back(item),
            Priority::Low => queues.low_priority.push_back(item),
        };
        self.condvar.notify_one();
        Ok(())
    }

    fn recv<'a>(&'a self) -> Result<parking_lot::MutexGuard<'a, PriorityQueues<T>>, RecvError> {
        let mut queues = self.queues.lock();

        let sender_count = self.sender_count.load(std::sync::atomic::Ordering::Relaxed);
        if queues.is_empty() && sender_count == 0 {
            return Err(crate::queue::RecvError);
        }

        while queues.is_empty() {
            self.condvar.wait(&mut queues);
        }

        Ok(queues)
    }

    fn try_recv<'a>(
        &'a self,
    ) -> Result<Option<parking_lot::MutexGuard<'a, PriorityQueues<T>>>, RecvError> {
        let mut queues = self.queues.lock();

        let sender_count = self.sender_count.load(std::sync::atomic::Ordering::Relaxed);
        if queues.is_empty() && sender_count == 0 {
            return Err(crate::queue::RecvError);
        }

        if queues.is_empty() {
            Ok(None)
        } else {
            Ok(Some(queues))
        }
    }
}

pub(crate) struct PriorityQueueSender<T> {
    state: Arc<PriorityQueueState<T>>,
}

impl<T> PriorityQueueSender<T> {
    fn new(state: Arc<PriorityQueueState<T>>) -> Self {
        Self { state }
    }

    pub(crate) fn send(&self, priority: Priority, item: T) -> Result<(), SendError<T>> {
        self.state.send(priority, item)?;
        Ok(())
    }
}

impl<T> Drop for PriorityQueueSender<T> {
    fn drop(&mut self) {
        self.state
            .sender_count
            .fetch_sub(1, std::sync::atomic::Ordering::AcqRel);
    }
}

pub(crate) struct PriorityQueueReceiver<T> {
    state: Arc<PriorityQueueState<T>>,
    rand: SmallRng,
    disconnected: bool,
}

impl<T> Clone for PriorityQueueReceiver<T> {
    fn clone(&self) -> Self {
        self.state
            .receiver_count
            .fetch_add(1, std::sync::atomic::Ordering::AcqRel);
        Self {
            state: Arc::clone(&self.state),
            rand: SmallRng::seed_from_u64(0),
            disconnected: self.disconnected,
        }
    }
}

pub(crate) struct SendError<T>(T);

impl<T: fmt::Debug> fmt::Debug for SendError<T> {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        f.debug_tuple("SendError").field(&self.0).finish()
    }
}

#[derive(Debug)]
pub(crate) struct RecvError;

#[allow(dead_code)]
impl<T> PriorityQueueReceiver<T> {
    pub(crate) fn new() -> (PriorityQueueSender<T>, Self) {
        let state = PriorityQueueState {
            queues: parking_lot::Mutex::new(PriorityQueues {
                high_priority: VecDeque::new(),
                medium_priority: VecDeque::new(),
                low_priority: VecDeque::new(),
            }),
            condvar: parking_lot::Condvar::new(),
            receiver_count: AtomicUsize::new(1),
            sender_count: AtomicUsize::new(1),
        };
        let state = Arc::new(state);

        let sender = PriorityQueueSender::new(Arc::clone(&state));

        let receiver = PriorityQueueReceiver {
            state,
            rand: SmallRng::seed_from_u64(0),
            disconnected: false,
        };

        (sender, receiver)
    }

    /// Tries to pop one element from the priority queue without blocking.
    ///
    /// This will early return if there are no elements in the queue.
    ///
    /// This method is best suited if you only intend to pop one element, for better performance
    /// on large queues see [`Self::try_iter`]
    ///
    /// # Errors
    ///
    /// If the sender was dropped
    pub(crate) fn try_pop(&mut self) -> Result<Option<T>, RecvError> {
        self.pop_inner(false)
    }

    /// Pops an element from the priority queue blocking if necessary.
    ///
    /// This method is best suited if you only intend to pop one element, for better performance
    /// on large queues see [`Self::iter``]
    ///
    /// # Errors
    ///
    /// If the sender was dropped
    pub(crate) fn pop(&mut self) -> Result<T, RecvError> {
        self.pop_inner(true).map(|e| e.unwrap())
    }

    /// Returns an iterator over the elements of the queue
    /// this iterator will end when all elements have been consumed and will not wait for new ones.
    pub(crate) fn try_iter(self) -> TryIter<T> {
        TryIter {
            receiver: self,
            ended: false,
        }
    }

    /// Returns an iterator over the elements of the queue
    /// this iterator will wait for new elements if the queue is empty.
    pub(crate) fn iter(self) -> Iter<T> {
        Iter(self)
    }

    #[inline(always)]
    // algorithm is the loaded die from biased coin from
    // https://www.keithschwarz.com/darts-dice-coins/
    fn pop_inner(&mut self, block: bool) -> Result<Option<T>, RecvError> {
        use Priority as P;

        let mut queues = if !block {
            let Some(queues) = self.state.try_recv()? else {
                return Ok(None);
            };
            queues
        } else {
            self.state.recv()?
        };

        let high = P::High.probability() * !queues.high_priority.is_empty() as u32;
        let medium = P::Medium.probability() * !queues.medium_priority.is_empty() as u32;
        let low = P::Low.probability() * !queues.low_priority.is_empty() as u32;
        let mut mass = high + medium + low; //%

        if !queues.high_priority.is_empty() {
            let flip = self.rand.random_ratio(P::High.probability(), mass);
            if flip {
                return Ok(queues.high_priority.pop_front());
            }
            mass -= P::High.probability();
        }

        if !queues.medium_priority.is_empty() {
            let flip = self.rand.random_ratio(P::Medium.probability(), mass);
            if flip {
                return Ok(queues.medium_priority.pop_front());
            }
            mass -= P::Medium.probability();
        }

        if !queues.low_priority.is_empty() {
            let flip = self.rand.random_ratio(P::Low.probability(), mass);
            if flip {
                return Ok(queues.low_priority.pop_front());
            }
        }

        Ok(None)
    }
}

impl<T> Drop for PriorityQueueReceiver<T> {
    fn drop(&mut self) {
        self.state
            .receiver_count
            .fetch_sub(1, std::sync::atomic::Ordering::AcqRel);
    }
}

/// If None is returned the sender disconnected
pub(crate) struct Iter<T>(PriorityQueueReceiver<T>);
impl<T> Iterator for Iter<T> {
    type Item = T;

    fn next(&mut self) -> Option<Self::Item> {
        self.0.pop().ok()
    }
}
impl<T> FusedIterator for Iter<T> {}

/// If None is returned there are no more elements in the queue
pub(crate) struct TryIter<T> {
    receiver: PriorityQueueReceiver<T>,
    ended: bool,
}
impl<T> Iterator for TryIter<T> {
    type Item = Result<T, RecvError>;

    fn next(&mut self) -> Option<Self::Item> {
        if self.ended {
            return None;
        }

        let res = self.receiver.try_pop();
        self.ended = res.is_err();

        res.transpose()
    }
}
impl<T> FusedIterator for TryIter<T> {}

#[cfg(test)]
mod tests {
    use collections::HashSet;

    use super::*;

    #[test]
    fn all_tasks_get_yielded() {
        let (tx, mut rx) = PriorityQueueReceiver::new();
        tx.send(Priority::Medium, 20).unwrap();
        tx.send(Priority::High, 30).unwrap();
        tx.send(Priority::Low, 10).unwrap();
        tx.send(Priority::Medium, 21).unwrap();
        tx.send(Priority::High, 31).unwrap();

        drop(tx);

        assert_eq!(
            rx.iter().collect::<HashSet<_>>(),
            [30, 31, 20, 21, 10].into_iter().collect::<HashSet<_>>()
        )
    }

    #[test]
    fn new_high_prio_task_get_scheduled_quickly() {
        let (tx, mut rx) = PriorityQueueReceiver::new();
        for _ in 0..100 {
            tx.send(Priority::Low, 1).unwrap();
        }

        assert_eq!(rx.pop().unwrap(), 1);
        tx.send(Priority::High, 3).unwrap();
        assert_eq!(rx.pop().unwrap(), 3);
        assert_eq!(rx.pop().unwrap(), 1);
    }
}