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//! Timer wheel for scheduling periodic and one-shot
//! callbacks without threads or async.
//!
//! The event loop calls `tick()` each iteration to
//! fire expired timers.
use std::collections::{BTreeMap, HashMap};
use std::time::{Duration, Instant};
/// Opaque timer identifier.
#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
pub struct TimerId(u64);
/// A simple timer wheel driven by the event loop.
///
/// Timers are stored in a BTreeMap keyed by deadline,
/// which gives O(log n) insert/cancel and efficient
/// scanning of expired timers.
pub struct TimerWheel {
deadlines: BTreeMap<Instant, Vec<TimerId>>,
intervals: HashMap<TimerId, Duration>,
next_id: u64,
}
impl TimerWheel {
pub fn new() -> Self {
Self {
deadlines: BTreeMap::new(),
intervals: HashMap::new(),
next_id: 0,
}
}
/// Schedule a one-shot timer that fires after `delay`.
pub fn schedule(&mut self, delay: Duration) -> TimerId {
let id = self.alloc_id();
let deadline = Instant::now() + delay;
self.deadlines.entry(deadline).or_default().push(id);
id
}
/// Schedule a repeating timer that fires every
/// `interval`, starting after one interval.
pub fn schedule_repeating(&mut self, interval: Duration) -> TimerId {
let id = self.alloc_id();
let deadline = Instant::now() + interval;
self.deadlines.entry(deadline).or_default().push(id);
self.intervals.insert(id, interval);
id
}
/// Cancel a pending timer.
///
/// Returns `true` if the timer was found and removed.
pub fn cancel(&mut self, id: TimerId) -> bool {
self.intervals.remove(&id);
let mut found = false;
// Remove from deadline map
let mut empty_keys = Vec::new();
for (key, ids) in self.deadlines.iter_mut() {
if let Some(pos) = ids.iter().position(|i| *i == id) {
ids.swap_remove(pos);
found = true;
if ids.is_empty() {
empty_keys.push(*key);
}
break;
}
}
for k in empty_keys {
self.deadlines.remove(&k);
}
found
}
/// Fire all expired timers, returning their IDs.
///
/// Repeating timers are automatically rescheduled.
/// Call this once per event loop iteration.
pub fn tick(&mut self) -> Vec<TimerId> {
let now = Instant::now();
let mut fired = Vec::new();
// Collect all deadlines <= now
let expired: Vec<Instant> =
self.deadlines.range(..=now).map(|(k, _)| *k).collect();
for deadline in expired {
if let Some(ids) = self.deadlines.remove(&deadline) {
for id in ids {
fired.push(id);
// Reschedule if repeating
if let Some(&interval) = self.intervals.get(&id) {
let next = now + interval;
self.deadlines.entry(next).or_default().push(id);
}
}
}
}
fired
}
/// Duration until the next timer fires, or `None`
/// if no timers are scheduled.
///
/// Useful for determining the poll timeout.
pub fn next_deadline(&self) -> Option<Duration> {
self.deadlines.keys().next().map(|deadline| {
let now = Instant::now();
if *deadline <= now {
Duration::ZERO
} else {
*deadline - now
}
})
}
/// Number of pending timers.
pub fn pending_count(&self) -> usize {
self.deadlines.values().map(|v| v.len()).sum()
}
/// Check if there are no pending timers.
pub fn is_empty(&self) -> bool {
self.deadlines.is_empty()
}
fn alloc_id(&mut self) -> TimerId {
let id = TimerId(self.next_id);
self.next_id += 1;
id
}
}
impl Default for TimerWheel {
fn default() -> Self {
Self::new()
}
}
#[cfg(test)]
mod tests {
use super::*;
use std::thread;
#[test]
fn schedule_and_tick() {
let mut tw = TimerWheel::new();
let id = tw.schedule(Duration::from_millis(10));
// Not yet expired
assert!(tw.tick().is_empty());
assert_eq!(tw.pending_count(), 1);
// Wait and tick
thread::sleep(Duration::from_millis(15));
let fired = tw.tick();
assert_eq!(fired, vec![id]);
assert!(tw.is_empty());
}
#[test]
fn cancel_timer() {
let mut tw = TimerWheel::new();
let id = tw.schedule(Duration::from_millis(100));
assert!(tw.cancel(id));
assert!(tw.is_empty());
// Cancel non-existent returns false
assert!(!tw.cancel(TimerId(999)));
}
#[test]
fn repeating_timer() {
let mut tw = TimerWheel::new();
let id = tw.schedule_repeating(Duration::from_millis(10));
thread::sleep(Duration::from_millis(15));
let fired = tw.tick();
assert_eq!(fired, vec![id]);
// Should be rescheduled, not empty
assert_eq!(tw.pending_count(), 1);
// Cancel the repeating timer
tw.cancel(id);
assert!(tw.is_empty());
}
#[test]
fn next_deadline_empty() {
let tw = TimerWheel::new();
assert!(tw.next_deadline().is_none());
}
#[test]
fn next_deadline_returns_duration() {
let mut tw = TimerWheel::new();
tw.schedule(Duration::from_secs(10));
let d = tw.next_deadline().unwrap();
assert!(d <= Duration::from_secs(10));
assert!(d > Duration::from_secs(9));
}
#[test]
fn multiple_timers_fire_in_order() {
let mut tw = TimerWheel::new();
let a = tw.schedule(Duration::from_millis(5));
let b = tw.schedule(Duration::from_millis(10));
thread::sleep(Duration::from_millis(15));
let fired = tw.tick();
assert!(fired.contains(&a));
assert!(fired.contains(&b));
assert!(tw.is_empty());
}
}
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