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//! Timer queue operations.
use core::cell::Cell;
use core::cmp::min;
use embassy_executor::raw::TaskRef;
/// A timer queue, with items integrated into tasks.
pub struct TimerQueue {
head: Cell<Option<TaskRef>>,
}
impl TimerQueue {
/// Creates a new timer queue.
pub const fn new() -> Self {
Self { head: Cell::new(None) }
}
/// Schedules a task to run at a specific time.
///
/// If this function returns `true`, the called should find the next expiration time and set
/// a new alarm for that time.
pub fn schedule_wake(&mut self, at: u64, p: TaskRef) -> bool {
let item = p.timer_queue_item();
if item.next.get().is_none() {
// If not in the queue, add it and update.
let prev = self.head.replace(Some(p));
item.next.set(if prev.is_none() {
Some(unsafe { TaskRef::dangling() })
} else {
prev
});
item.expires_at.set(at);
true
} else if at <= item.expires_at.get() {
// If expiration is sooner than previously set, update.
item.expires_at.set(at);
true
} else {
// Task does not need to be updated.
false
}
}
/// Dequeues expired timers and returns the next alarm time.
///
/// The provided callback will be called for each expired task. Tasks that never expire
/// will be removed, but the callback will not be called.
pub fn next_expiration(&mut self, now: u64) -> u64 {
let mut next_expiration = u64::MAX;
self.retain(|p| {
let item = p.timer_queue_item();
let expires = item.expires_at.get();
if expires <= now {
// Timer expired, process task.
embassy_executor::raw::wake_task(p);
false
} else {
// Timer didn't yet expire, or never expires.
next_expiration = min(next_expiration, expires);
expires != u64::MAX
}
});
next_expiration
}
fn retain(&self, mut f: impl FnMut(TaskRef) -> bool) {
let mut prev = &self.head;
while let Some(p) = prev.get() {
if unsafe { p == TaskRef::dangling() } {
// prev was the last item, stop
break;
}
let item = p.timer_queue_item();
if f(p) {
// Skip to next
prev = &item.next;
} else {
// Remove it
prev.set(item.next.get());
item.next.set(None);
unsafe { p.timer_dequeue() };
}
}
}
}
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