Snap belt-drag end tile to a building's input edge

src/lib/sim/BuildingSystem.cpp

@@ -267,16 +267,23 @@ void BuildingSystem::initSalvageBayBuffer(Building& b) const
 }
 
 std::vector<Port> BuildingSystem::computeInputPorts(const Building& b) const
+{
+    return computeInputPorts(b.bodyCells, b.outputPorts);
+}
+
+std::vector<Port> BuildingSystem::computeInputPorts(
+    const std::vector<QPoint>& bodyCells,
+    const std::vector<Port>& outputPorts) const
 {
     // Build lookup sets for quick membership checks.
     std::set<std::pair<int, int>> bodySet;
-    for (const QPoint& cell : b.bodyCells)
+    for (const QPoint& cell : bodyCells)
     {
         bodySet.insert({cell.x(), cell.y()});
     }
 
     std::set<std::pair<int, int>> outputPortTiles;
-    for (const Port& port : b.outputPorts)
+    for (const Port& port : outputPorts)
     {
         outputPortTiles.insert({port.tile.x(), port.tile.y()});
     }
@@ -294,7 +301,7 @@ std::vector<Port> BuildingSystem::computeInputPorts(const Building& b) const
     std::set<std::pair<int, int>> seen;
     std::vector<Port> inputPorts;
 
-    for (const QPoint& cell : b.bodyCells)
+    for (const QPoint& cell : bodyCells)
     {
         for (int i = 0; i < 4; ++i)
         {
@@ -317,6 +324,30 @@ std::vector<Port> BuildingSystem::computeInputPorts(const Building& b) const
     return inputPorts;
 }
 
+std::vector<Port> BuildingSystem::getInputPorts(BuildingId id) const
+{
+    if (const Building* building = findBuilding(id))
+    {
+        return building->inputPorts;
+    }
+    if (const ConstructionSite* site = findSite(id))
+    {
+        // A site stores no ports; derive its output ports from the mask (absolute)
+        // and run the same input-edge scan (REQ-BLD-BELT-DRAG, REQ-MAT-INPUT-PORTS).
+        const BuildingDef* def = findBuildingDef(site->type);
+        if (def == nullptr) { return {}; }
+        const ParsedSurfaceMask mask = parseSurfaceMask(def->surfaceMask, site->rotation);
+        std::vector<Port> outputPortsAbsolute;
+        outputPortsAbsolute.reserve(mask.outputPorts.size());
+        for (const Port& port : mask.outputPorts)
+        {
+            outputPortsAbsolute.push_back(Port{ site->anchor + port.tile, port.direction });
+        }
+        return computeInputPorts(site->bodyCells, outputPortsAbsolute);
+    }
+    return {};
+}
+
 std::vector<Item> BuildingSystem::rollReprocessingOutput(const RecipeDef& recipe)
 {
     std::vector<const RecipeOutput*> eligible;

src/lib/sim/BuildingSystem.h

@@ -171,6 +171,12 @@ public:
     // Find nearest operational building of the given type; nullptr if none.
     const Building* findNearestBuilding(QVector2D worldPos, BuildingType type) const;
 
+    // Input-capable adjacent tiles for a building or construction site
+    // (REQ-BLD-BELT-DRAG, REQ-MAT-INPUT-PORTS): each returned Port.tile is the
+    // outside adjacent tile and Port.direction is the belt facing that points into
+    // the target. Output-port edges are excluded. Empty for an unknown id.
+    std::vector<Port> getInputPorts(BuildingId id) const;
+
     // Register / unregister tile occupancy for ECS station entities.
     void registerTileOccupancy(const std::vector<QPoint>& cells, BuildingId ownerPlaceholder);
     void unregisterTileOccupancy(const std::vector<QPoint>& cells);
@@ -246,6 +252,9 @@ private:
     void                initShipyardBuffers(Building& b) const;
     void                initSalvageBayBuffer(Building& b) const;
     std::vector<Port>   computeInputPorts(const Building& b) const;
+    // Core input-edge scan shared by operational buildings and construction sites.
+    std::vector<Port>   computeInputPorts(const std::vector<QPoint>& bodyCells,
+                                          const std::vector<Port>& outputPorts) const;
     std::vector<Item>   rollReprocessingOutput(const RecipeDef& recipe);
     bool                bodyCellsWithinWorldBounds(
                             const std::vector<QPoint>& bodyCells,