Animate items emerging from building output ports

src/lib/sim/Building.h

@@ -12,6 +12,7 @@
 #include "BuildingId.h"
 
 #include "entt/entity/entity.hpp"
+#include "BeltSlot.h"
 #include "Item.h"
 #include "ItemType.h"
 #include "Port.h"
@@ -75,6 +76,26 @@ struct Building
     OutputBuffer outputBuffer;
     std::optional<Production> production;
 
+    // Items currently emerging from each output port on its virtual output belt
+    // (REQ-MAT-OUTPUT-EMERGE); one lane per output port, parallel to outputPorts.
+    // Each lane holds slots at progress [0.5, 1.0], front (highest progress) first.
+    // An emerging item still counts as residing in the output buffer until it hands
+    // off onto a real belt at progress 1.0.
+    std::vector<std::vector<BeltItemSlot>> emergingItems;
+
+    // Total items held on the output side: buffered plus still-emerging. The
+    // output-buffer capacity rule (REQ-MAT-OUTPUT-BUFFER) counts emerging items,
+    // since they have not yet left the building.
+    int outputItemCount() const
+    {
+        int count = static_cast<int>(outputBuffer.items.size());
+        for (const std::vector<BeltItemSlot>& lane : emergingItems)
+        {
+            count += static_cast<int>(lane.size());
+        }
+        return count;
+    }
+
     // Pre-computed from surface mask at placement; in absolute world coordinates.
     std::vector<QPoint> bodyCells;
     std::vector<Port>   outputPorts;