Create LockFreeQueue.chpl

example/LockFreeQueue.chpl

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+/**
+ * Created by Garvit Dewan - 
+ * https://github.com/dgarvit/epoch-based-manager/blob/master/src/LockFreeQueue.chpl
+ *   
+ * Lock-Free Queue that uses ABA feature of LocalAtomicObject
+ * Based on Michael Scott Queue
+ */
+module LockFreeQueue {
+
+  use LocalAtomics;
+
+  class node {
+    type eltType;
+    var val : eltType;
+    var next : LocalAtomicObject(unmanaged node(eltType));
+
+    proc init(val : ?eltType) {
+      this.eltType = eltType;
+      this.val = val;
+    }
+
+    proc init(type eltType) {
+      this.eltType = eltType;
+    }
+  }
+
+  class LockFreeQueue {
+    type objType;
+    var _head : LocalAtomicObject(unmanaged node(objType));
+    var _tail : LocalAtomicObject(unmanaged node(objType));
+
+    proc init(type objType) {
+      this.objType = objType;
+      this.complete();
+      var _node = new unmanaged node(objType);
+      _head.write(_node);
+      _tail.write(_node);
+    }
+
+    proc enqueue(newObj : objType) {
+      var n = new unmanaged node(newObj);
+      while (true) {
+        var curr_tail = _tail.readABA();
+        var next = curr_tail.next.readABA();
+        if (next.getObject() == nil) {
+          if (curr_tail.next.compareExchangeABA(next, n)) {
+            _tail.compareExchangeABA(curr_tail, n);
+            break;
+          }
+        }
+        else {
+          _tail.compareExchangeABA(curr_tail, next.getObject());
+        }
+      }
+    }
+
+    proc dequeue() : objType {
+      while (true) {
+        var curr_head = _head.readABA();
+        var curr_tail = _tail.readABA();
+        var next = curr_head.next.readABA();
+        if (curr_head.getObject() == curr_tail.getObject()) {
+          if (next.getObject() == nil) then
+            return nil;
+          _tail.compareExchangeABA(curr_tail, next.getObject());
+        }
+        else {
+          if (_head.compareExchangeABA(curr_head, next.getObject())) then
+            return next.getObject().val;
+        }
+      }
+      return nil;
+    }
+
+    iter these() : objType {
+      var ptr = _head.read().next.read();
+      while (ptr != nil) {
+        yield ptr.val;
+        ptr = ptr.next.read();
+      }
+    }
+
+    proc peek() : objType {
+      var actual_head = _head.read().next.read();
+      if (actual_head != nil) then
+        return actual_head.val;
+      return nil;
+    }
+
+    proc deinit() {
+      var ptr = _head.read();
+      while (ptr != nil) {
+        _head = ptr.next;
+        delete ptr.val;
+        delete ptr;
+        ptr = _head.read();
+      }
+    }
+  }
+}