Alternative Timer implementation using simply list-based priority heap.

sdk/lib/io/timer_impl.dart

Index: sdk/lib/io/timer_impl.dart
diff --git a/sdk/lib/io/timer_impl.dart b/sdk/lib/io/timer_impl.dart
index 883d09e42488bbefb6614636d2d12a9e363f82c4..ecbf17f451ee6f6d292e1e46f66846415370f823 100644
--- a/sdk/lib/io/timer_impl.dart
+++ b/sdk/lib/io/timer_impl.dart
@@ -4,19 +4,135 @@
 
 part of dart.io;
 
-class _Timer extends LinkedListEntry<_Timer> implements Timer {
+// Timer heap implemented as a array-based binary heap[0].
+// This allows for O(1) `first`, O(log(n)) `remove`/`removeFirst` and O(log(n))
+// `add`.
+//
+// To ensure the timers are ordered by insertion time, the _Timer class has a
+// `_id` field set when added to the heap.
+//
+// [0] http://en.wikipedia.org/wiki/Binary_heap
+class _TimerHeap {
+  List<_Timer> _list;
+  int _used = 0;
+
+  _TimerHeap([int initSize = 7])
+      : _list = new List<_Timer>(initSize);
+
+  bool get isEmpty => _used == 0;
+  bool get isNotEmpty => _used > 0;
+
+  _Timer get first => _list[0];
+
+  bool isFirst(_Timer timer) => timer._indexOrNext == 0;
+
+  void add(_Timer timer) {
+    if (_used == _list.length) {
+      _resize();
+    }
+    timer._indexOrNext = _used++;
+    _list[timer._indexOrNext] = timer;
+    _bubbleUp(timer);
+  }
+
+  _Timer removeFirst() {
+    var f = first;
+    remove(f);
+    return f;
+  }
+
+  void remove(_Timer timer) {
+    _used--;
+    timer._id = -1;
+    if (isEmpty) {
+      _list[0] = null;
+      timer._indexOrNext = null;
+      return;
+    }
+    var last = _list[_used];

[Lasse Reichstein Nielsen, 2013/12/13 14:07:20]

Consider wrapping from here to after the bubble in 
 if (!identical(last, timer)) {
or include this line and instead test:
 if (_used != timer._indexOrNext) { 

The code works, it just seems a little roundabout to move an element to its own
location (which means the reader must convince themself that it works -
alternatively, put a comment saying that we know about the case and the code
works).

[Anders Johnsen, 2013/12/16 12:36:15]

Done.

+    last._indexOrNext = timer._indexOrNext;
+    _list[last._indexOrNext] = last;
+    if (last._compareTo(timer) < 0) {
+      _bubbleUp(last);
+    } else {
+      _bubbleDown(last);
+    }
+    _list[_used] = null;
+    timer._indexOrNext = null;
+  }
+
+  void _resize() {
+    var newList = new List(_list.length * 2 + 1);
+    newList.setRange(0, _used, _list);
+    _list = newList;
+  }
+
+  void _bubbleUp(_Timer timer) {
+    while (!isFirst(timer)) {
+      Timer parent = _parent(timer);
+      if (timer._compareTo(parent) < 0) {
+        _swap(timer, parent);
+      } else {
+        break;
+      }
+    }
+  }
+
+  void _bubbleDown(_Timer timer) {
+    while (true) {
+      int leftIndex = _leftChildIndex(timer._indexOrNext);
+      int rightIndex = _rightChildIndex(timer._indexOrNext);
+      _Timer newest = timer;
+      if (leftIndex < _used && _list[leftIndex]._compareTo(newest) < 0) {
+        newest = _list[leftIndex];
+      }
+      if (rightIndex < _used && _list[rightIndex]._compareTo(newest) < 0) {
+        newest = _list[rightIndex];
+      }
+      if (newest == timer) {

[Lasse Reichstein Nielsen, 2013/12/13 14:07:20]

consider using identical.

[Anders Johnsen, 2013/12/16 12:36:15]

Done.

+        // We are where we should be, break.
+        break;
+      }
+      _swap(newest, timer);
+    }
+  }
+
+  void _swap(_Timer first, _Timer second) {
+    int tmp = first._indexOrNext;
+    first._indexOrNext = second._indexOrNext;
+    second._indexOrNext = tmp;
+    _list[first._indexOrNext] = first;
+    _list[second._indexOrNext] = second;
+  }
+
+  Timer _parent(_Timer timer) => _list[_parentIndex(timer._indexOrNext)];
+  Timer _leftChild(_Timer timer) => _list[_leftChildIndex(timer._indexOrNext)];
+  Timer _rightChild(_Timer timer) =>
+      _list[_rightChildIndex(timer._indexOrNext)];
+
+  static int _parentIndex(int index) => (index - 1) ~/ 2;
+  static int _leftChildIndex(int index) => 2 * index + 1;
+  static int _rightChildIndex(int index) => 2 * index + 2;
+}
+
+class _Timer implements Timer {
   // Disables the timer.
   static const int _NO_TIMER = -1;
 
   // Timers are ordered by wakeup time.
-  static LinkedList<_Timer> _timers = new LinkedList<_Timer>();
+  static _TimerHeap _heap = new _TimerHeap();
+  static _Timer _firstZeroTimer;
+  static _Timer _lastZeroTimer;
+  static int _idCount = 0;
 
   static RawReceivePort _receivePort;
-  static bool _handling_callbacks = false;
+  static bool _handlingCallbacks = false;
 
   Function _callback;
   int _milliSeconds;
   int _wakeupTime = 0;
+  var _indexOrNext;
+  int _id = -1;
 
   static Timer _createTimer(void callback(Timer timer),
                             int milliSeconds,
@@ -32,8 +148,7 @@ class _Timer extends LinkedListEntry<_Timer> implements Timer {
           new DateTime.now().millisecondsSinceEpoch + 1 + milliSeconds;
     }
     timer._milliSeconds = repeating ? milliSeconds : -1;
-    timer._addTimerToList();
-    if (identical(timer, _timers.first)) {
+    if (timer._addTimerToHeap()) {
       // The new timer is the first in queue. Update event handler.
       timer._notifyEventHandler();
     }
@@ -50,10 +165,16 @@ class _Timer extends LinkedListEntry<_Timer> implements Timer {
 
   _Timer._internal() {}
 
+  bool get _isInHeap => _id >= 0;
+
   void _clear() {
     _callback = null;
-    _milliSeconds = 0;
-    _wakeupTime = 0;
+  }
+
+  int _compareTo(_Timer other) {
+    int c = _wakeupTime - other._wakeupTime;
+    if (c != 0) return c;
+    return _id - other._id;
   }
 
   bool get _repeating => _milliSeconds >= 0;
@@ -64,12 +185,11 @@ class _Timer extends LinkedListEntry<_Timer> implements Timer {
   // the given timer is the earliest timer the native timer is reset.
   void cancel() {
     _clear();
-    // Return if already canceled.
-    if (list == null) return;
-    assert(!_timers.isEmpty);
-    _Timer first = _timers.first;
-    unlink();
-    if (identical(first, this)) {
+    if (!_isInHeap) return;
+    assert(_wakeupTime != 0);
+    bool update = _firstZeroTimer == null && _heap.isFirst(this);
+    _heap.remove(this);
+    if (update) {
       _notifyEventHandler();
     }
   }
@@ -81,34 +201,32 @@ class _Timer extends LinkedListEntry<_Timer> implements Timer {
 
   // Adds a timer to the timer list. Timers with the same wakeup time are
   // enqueued in order and notified in FIFO order.
-  void _addTimerToList() {
-    _Timer entry = _timers.isEmpty ? null : _timers.first;
-    // If timer is last, add to end.
-    if (entry == null || _timers.last._wakeupTime <= _wakeupTime) {
-      _timers.add(this);
-      return;
-    }
-    // Otherwise scan through and find the right position.
-    while (entry != null) {
-      if (_wakeupTime < entry._wakeupTime) {
-        entry.insertBefore(this);
-        return;
+  bool _addTimerToHeap() {
+    if (_wakeupTime == 0) {
+      if (_firstZeroTimer == null) {
+        _lastZeroTimer = _firstZeroTimer = this;
+        return true;
+      } else {
+        _lastZeroTimer = _lastZeroTimer._indexOrNext = this;
+        return false;
       }
-      entry = entry.next;
+    } else {
+      _id = _idCount++;
+      _heap.add(this);
+      return _firstZeroTimer == null && _heap.isFirst(this);
     }
-    _timers.add(this);
   }
 
 
   void _notifyEventHandler() {
-    if (_handling_callbacks) {
+    if (_handlingCallbacks) {
       // While we are already handling callbacks we will not notify the event
       // handler. _handleTimeout will call _notifyEventHandler once all pending
       // timers are processed.
       return;
     }
 
-    if (_timers.isEmpty) {
+    if (_firstZeroTimer == null && _heap.isEmpty) {
       // No pending timers: Close the receive port and let the event handler
       // know.
       if (_receivePort != null) {
@@ -123,61 +241,63 @@ class _Timer extends LinkedListEntry<_Timer> implements Timer {
       }
       _EventHandler._sendData(null,
                               _receivePort,
-                              _timers.first._wakeupTime);
+                              _firstZeroTimer != null ?
+                                  0 : _heap.first._wakeupTime);
     }
   }
 
-
-  // Creates a receive port and registers the timer handler on that
-  // receive port.
-  void _createTimerHandler() {
-
-    void _handleTimeout() {
-      int currentTime = new DateTime.now().millisecondsSinceEpoch;
-
-      // Collect all pending timers.
-      var pending_timers = new List();
-      while (!_timers.isEmpty) {
-        _Timer entry = _timers.first;
-        if (entry._wakeupTime <= currentTime) {
-          entry.unlink();
-          pending_timers.add(entry);
-        } else {
-          break;
-        }
+  void _handleTimeout(_) {
+    int currentTime = new DateTime.now().millisecondsSinceEpoch;
+    // Collect all pending timers.
+    var timer = _firstZeroTimer;
+    var nextTimer = _lastZeroTimer;
+    _firstZeroTimer = _lastZeroTimer = null;
+    while (_heap.isNotEmpty && _heap.first._wakeupTime <= currentTime) {
+      var next = _heap.removeFirst();
+      if (timer == null) {
+        nextTimer = timer = next;
+      } else {
+        nextTimer = nextTimer._indexOrNext = next;
       }
+    }
 
-      // Trigger all of the pending timers. New timers added as part of the
-      // callbacks will be enqueued now and notified in the next spin at the
-      // earliest.
-      _handling_callbacks = true;
-      try {
-        for (var timer in pending_timers) {
-          // One of the timers in the pending_timers list can cancel
-          // one of the later timers which will set the callback to
-          // null.
-          if (timer._callback != null) {
-            var callback = timer._callback;
-            if (!timer._repeating) {
-              //Mark timer as inactive.
-              timer._callback = null;
-            }
-            callback(timer);
-            // Re-insert repeating timer if not canceled.
-            if (timer._repeating && timer._callback != null) {
-              timer._advanceWakeupTime();
-              timer._addTimerToList();
-            }
+    // Trigger all of the pending timers. New timers added as part of the
+    // callbacks will be enqueued now and notified in the next spin at the
+    // earliest.
+    _handlingCallbacks = true;
+    try {
+      while (timer != null) {
+        var next = timer._indexOrNext;
+        timer._indexOrNext = null;
+        // One of the timers in the pending_timers list can cancel
+        // one of the later timers which will set the callback to
+        // null.
+        if (timer._callback != null) {
+          var callback = timer._callback;
+          if (!timer._repeating) {
+            // Mark timer as inactive.
+            timer._callback = null;
+          }
+          callback(timer);
+          // Re-insert repeating timer if not canceled.
+          if (timer._repeating && timer._callback != null) {
+            timer._advanceWakeupTime();
+            timer._addTimerToHeap();
           }
         }
-      } finally {
-        _handling_callbacks = false;
-        _notifyEventHandler();
+        timer = next;
       }
+    } finally {
+      _handlingCallbacks = false;
+      _notifyEventHandler();
     }
+  }
 
+  // Creates a receive port and registers the timer handler on that
+  // receive port.
+  void _createTimerHandler() {
     if(_receivePort == null) {
-      _receivePort = new RawReceivePort((_) { _handleTimeout(); });
+      _receivePort = new RawReceivePort(_handleTimeout);
     }
   }