Initial community commit

Src/nu/LockFreeItem.h

@@ -0,0 +1,47 @@
+#pragma once
+#include <windows.h>
+
+/* This data structure holds one item, and returns you the old item when you replace it 
+   it's sort of a "stack of 1" */
+
+template <class value_t>
+class LockFreeItem
+{
+public:
+	typedef value_t *ptr_t;
+	LockFreeItem() { item = 0; }
+	
+	value_t *GetItem()
+	{
+		for(;;) // keep trying until we get this right
+		{
+			volatile ptr_t ret;
+			InterlockedExchangePointer(&ret, item);
+			if (InterlockedCompareExchangePointer((volatile PVOID *)&item, 0, ret) == ret)
+				return ret;
+		}
+	}
+
+	// returns the previous value
+	value_t *SetItem(value_t *new_item)
+	{
+		for(;;) // keep trying until we get this right
+		{
+			volatile ptr_t ret;
+			InterlockedExchangePointer(&ret, item);
+			if (InterlockedCompareExchangePointer((volatile PVOID *)&item, new_item, ret) == ret)
+				return ret;
+		}
+	}
+
+	// if there's already a value, returns what you passed in
+	value_t *SetItemIfZero(value_t *new_item)
+	{
+		if (InterlockedCompareExchangePointer((volatile PVOID *)&item, new_item, 0) == 0)
+				return 0;
+		else
+			return new_item;
+	}
+
+	volatile ptr_t item;
+};