Add a maximum number of forward/backward time steps that can be queued

scwx-qt/source/scwx/qt/util/queue_counter.hpp

@@ -0,0 +1,64 @@
+#pragma once
+
+#include <memory>
+#include <atomic>
+
+namespace scwx::qt::util
+{
+
+class QueueCounter
+{
+public:
+   /**
+    * Counts the number of items in a queue, and prevents it from exceeding a
+    * count in a thread safe manor. This is lock free, assuming
+    * std::atomic<size_t> supports lock free fetch_add and fetch_sub.
+    */
+
+   /**
+    * Construct a QueueCounter with a given maximum count
+    *
+    * @param maxCount The maximum number of items in the queue
+    */
+   explicit QueueCounter(size_t maxCount);
+
+   ~QueueCounter();
+   QueueCounter(const QueueCounter&)            = delete;
+   QueueCounter(QueueCounter&&)                 = delete;
+   QueueCounter& operator=(const QueueCounter&) = delete;
+   QueueCounter& operator=(QueueCounter&&)      = delete;
+
+   /**
+    * Called before adding an item. If it returns true, it is ok to add. If it
+    * returns false, it should not be added
+    *
+    * @return true if it is ok to add, false if the queue is full
+    */
+   bool add();
+
+   /**
+    * Called when item is removed from the queue. Should only be called after a
+    * corresponding and successful call to add.
+    */
+   void remove();
+
+   /**
+    * Tells if this instance is lock free
+    *
+    * @return true if it is lock free, false otherwise
+    */
+   bool is_lock_free();
+
+   /**
+    * Tells if this class is always lock free. True if it is lock free, false
+    * otherwise
+    */
+   static constexpr bool is_always_lock_free =
+      std::atomic<size_t>::is_always_lock_free;
+
+private:
+   class Impl;
+   std::unique_ptr<Impl> p;
+};
+
+} // namespace scwx::qt::util