WIP Add a steal_half method to sync::chase_lev

src/sync/chase_lev.rs

@@ -102,6 +102,19 @@ pub enum Steal<T> {
     Data(T),
 }
 
+/// When stealing half of the data, this is an enumeration of the possible
+/// outcomes.
+#[derive(PartialEq, Eq, Debug)]
+pub enum StealHalf<T> {
+    /// The deque was empty at the time of stealing.
+    Empty,
+    /// The stealer lost the race for stealing data, and a retry may return more
+    /// data.
+    Abort,
+    /// The stealer has successfully stolen some data.
+    Data(Vec<T>),
+}
+
 // An internal buffer used by the chase-lev deque. This structure is actually
 // implemented as a circular buffer, and is used as the intermediate storage of
 // the data in the deque.
@@ -137,6 +150,12 @@ impl<T> Stealer<T> {
     pub fn steal(&self) -> Steal<T> {
         self.deque.steal()
     }
+
+    /// Steals half of the work off the end of the queue (opposite of the
+    /// worker's end)
+    pub fn steal_half(&self) -> StealHalf<T> {
+        self.deque.steal_half()
+    }
 }
 
 impl<T> Clone for Stealer<T> {
@@ -254,6 +273,40 @@ impl<T> Deque<T> {
         }
     }
 
+    fn steal_half(&self) -> StealHalf<T> {
+        let guard = epoch::pin();
+
+        let t = self.top.load(Acquire);
+        fence(SeqCst);
+        let b = self.bottom.load(Acquire);
+
+        let size = b - t;
+        if size <= 0 {
+            return StealHalf::Empty;
+        }
+
+        let half = size + 1 / 2;
+
+        unsafe {
+            let a = self.array.load(Acquire, &guard).unwrap();
+
+            let mut data = Vec::with_capacity(half as usize);
+            for i in t..t + half {
+                data.push(a.get(i));
+            }
+
+            if self.top.compare_and_swap(t, t + half, SeqCst) == t {
+                StealHalf::Data(data)
+            } else {
+                while data.len() > 0 {
+                    mem::forget(data.pop());
+                }
+                StealHalf::Abort
+            }
+        }
+
+    }
+
     // potentially shrink the array. This can be called only from the worker.
     unsafe fn maybe_shrink(&self, b: isize, t: isize, guard: &epoch::Guard) {
         let a = self.array.load(SeqCst, guard).unwrap();