Return an Iterator from expired method

Return an `Expired` struct that implements Iterator from `expired()`
instead of a Vec to avoid an allocation.

src/lib.rs

@@ -46,6 +46,42 @@ pub enum TimerType {
     Recurring
 }
 
+/// An Iterator over expired timers
+pub struct Expired<'a, T> where T: 'a {
+    now: Instant,
+    heap: &'a mut TimerHeap<T>
+}
+
+impl<'a, T> Iterator for Expired<'a, T> where T: Eq + Clone + Hash {
+    type Item = T;
+
+    fn next(&mut self) -> Option<T> {
+        while let Some(mut popped) = self.heap.timers.pop() {
+            if popped.expires_at <= self.now {
+                if self.heap.active.get(&popped.key) != Some(&popped.counter) {
+                    // Drop an old deleted timer
+                    continue;
+                }
+                if popped.recurring {
+                    let key = popped.key.clone();
+                    // We use the expired_at time so we don't keep skewing later and later
+                    // by adding the duration to the current time.
+                    popped.expires_at += popped.duration;
+                    self.heap.timers.push(popped);
+                    return Some(key);
+                } else {
+                    let _ = self.heap.active.remove(&popped.key);
+                    return Some(popped.key);
+                }
+            } else {
+                self.heap.timers.push(popped);
+                return None;
+            }
+        }
+        None
+    }
+}
+
 /// Store timers in a binary heap. Keep them sorted by which timer is going to expire first.
 pub struct TimerHeap<T> {
     timers: BinaryHeap<TimerEntry<T>>,
@@ -146,35 +182,16 @@ impl<T: Eq + Clone + Hash> TimerHeap<T>  {
     /// Return all expired keys
     ///
     /// Any recurring timers will be re-added to the heap in the correct spot
-    pub fn expired(&mut self) -> Vec<T> {
+    pub fn expired(&mut self) -> Expired<T> {
         self._expired(Instant::now())
     }
 
     /// A deterministically testable version of `expired()`
-    fn _expired(&mut self, now: Instant) -> Vec<T> {
-        let mut expired = Vec::new();
-        while let Some(mut popped) = self.timers.pop() {
-            if popped.expires_at <= now {
-                if self.active.get(&popped.key) != Some(&popped.counter) {
-                    // Drop an old deleted timer
-                    continue;
-                }
-                if popped.recurring {
-                    expired.push(popped.key.clone());
-                    // We use the expired_at time so we don't keep skewing later and later
-                    // by adding the duration to the current time.
-                    popped.expires_at += popped.duration;
-                    self.timers.push(popped);
-                } else {
-                    let _ = self.active.remove(&popped.key);
-                    expired.push(popped.key)
-                }
-            } else {
-                self.timers.push(popped);
-                return expired;
-            }
+    fn _expired(&mut self, now: Instant) -> Expired<T> {
+        Expired {
+            now: now,
+            heap: self
         }
-        expired
     }
 }
 
@@ -258,12 +275,12 @@ mod tests {
         let now = Instant::now();
         let duration = Duration::from_millis(500);
         heap._insert(1u64, duration, TimerType::Oneshot, now).unwrap();
-        assert_eq!(heap._expired(now), vec![]);
-        let v = heap._expired(now + duration);
+        assert_eq!(heap._expired(now).count(), 0);
+        let count = heap._expired(now + duration).count();
         assert_eq!(heap.active.len(), 0);
-        assert_eq!(v.len(), 1);
+        assert_eq!(count, 1);
         assert_eq!(heap.len(), 0);
-        assert_eq!(heap._expired(now + duration), vec![]);
+        assert_eq!(heap._expired(now + duration).next(), None);
     }
 
     #[test]
@@ -272,15 +289,15 @@ mod tests {
         let now = Instant::now();
         let duration = Duration::from_millis(500);
         heap._insert(1u64, duration, TimerType::Recurring, now).unwrap();
-        assert_eq!(heap._expired(now), vec![]);
-        let v = heap._expired(now + duration);
-        assert_eq!(v.len(), 1);
+        assert_eq!(heap._expired(now).count(), 0);
+        let count = heap._expired(now + duration).count();
+        assert_eq!(count, 1);
         assert_eq!(heap.len(), 1);
-        assert_eq!(heap._expired(now + duration + Duration::from_millis(1)), vec![]);
-        let v = heap._expired(now + duration + duration);
-        assert_eq!(v.len(), 1);
+        assert_eq!(heap._expired(now + duration + Duration::from_millis(1)).count(), 0);
+        let count = heap._expired(now + duration + duration).count();
+        assert_eq!(count, 1);
         assert_eq!(heap.len(), 1);
-        assert_eq!(heap._expired(now + duration + duration), vec![]);
+        assert_eq!(heap._expired(now + duration + duration).count(), 0);
     }
 
     #[test]
@@ -298,7 +315,7 @@ mod tests {
         let duration = Duration::from_millis(500);
         heap._insert(1u64, duration, TimerType::Recurring, now).unwrap();
         assert_eq!(heap.remove(1u64), true);
-        assert_eq!(heap._expired(now + duration), vec![]);
+        assert_eq!(heap._expired(now + duration).count(), 0);
         assert_eq!(heap.len(), 0);
     }
 
@@ -310,8 +327,7 @@ mod tests {
         heap._insert(1u64, duration, TimerType::Oneshot, now).unwrap();
         assert_eq!(heap.remove(1u64), true);
         heap._insert(1u64, duration, TimerType::Oneshot, now + duration).unwrap();
-        let v = heap._expired(now + duration + duration);
-        assert_eq!(v.len(), 1);
+        assert_eq!(heap._expired(now + duration + duration).count(), 1);
         assert_eq!(heap.active.len(), 0);
         assert_eq!(heap.len(), 0);
     }