SpMcRingBuff: Add getNextIndexForProducer/abortProduction

These two methods form the core of the SpMcRingbuffer's wrap-around
behaviour.

include/user/spMcRingBuffer.h

@@ -40,7 +40,8 @@ public:
 	:
 	nBuffers(frameAssemblyDesc_ ? frameAssemblyDesc_->slots.size() : 0),
 	frameAssemblyDesc(frameAssemblyDesc_),
-	slots(nBuffers)  // Default-construct all frames
+	slots(nBuffers),  // Default-construct all frames
+	producerNextUsableIndex(0)
 	{
 		if (!frameAssemblyDesc)
 		{
@@ -100,6 +101,42 @@ public:
 		return slots[slotIndex].lock;
 	}
 
+	/**
+	 * @brief Get the next index to produce into, atomically incrementing it
+	 *
+	 * Uses sequence lock to perform an emulated fetch_add with modulo nBuffers
+	 * applied, ensuring the returned index is always < nBuffers.
+	 *
+	 * @return The index to produce into (always < nBuffers)
+	 */
+	size_t getIndexToProduceInto()
+	{
+		producerNextUsableIndexLock.writeAcquire();
+		size_t currentIndex = producerNextUsableIndex;
+		size_t nextIndex = (currentIndex + 1) % nBuffers;
+		producerNextUsableIndex = nextIndex;
+		producerNextUsableIndexLock.writeRelease();
+		return currentIndex;
+	}
+
+	/**
+	 * @brief Abort production by setting the producer index to a specific value
+	 *
+	 * @param index The index to set (must be < nBuffers)
+	 * @throws std::out_of_range if index >= nBuffers
+	 */
+	void abortProduction(size_t index)
+	{
+		if (index >= nBuffers)
+		{
+			throw std::out_of_range(std::string(__func__)
+				+ ": SpMcRingBuffer: index must be < nBuffers");
+		}
+		producerNextUsableIndexLock.writeAcquire();
+		producerNextUsableIndex = index;
+		producerNextUsableIndexLock.writeRelease();
+	}
+
 public:
 	// Layout/invariants
 	size_t nBuffers;
@@ -109,6 +146,8 @@ private:
 	std::shared_ptr<FrameAssemblyDesc> frameAssemblyDesc;
 	// Frames vector: each frame contains a sequence lock and SlotDesc
 	std::vector<StimulusFrame> slots;
+	SequenceLock producerNextUsableIndexLock;
+	size_t producerNextUsableIndex;
 };
 
 } // namespace stim_buff