implement building system

src/lib/sim/BeltSystem.cpp

@@ -135,6 +135,31 @@ std::optional<Item> BeltSystem::tryTakeItem(Port port)
     return std::nullopt;
 }
 
+std::optional<ItemType> BeltSystem::peekItem(Port port) const
+{
+    const std::map<std::pair<int, int>, BeltTile>::const_iterator it =
+        m_belts.find(key(port.tile));
+    if (it == m_belts.end())
+    {
+        return std::nullopt;
+    }
+    if (it->second.direction != port.direction)
+    {
+        return std::nullopt;
+    }
+
+    const BeltTile& bt = it->second;
+    if (bt.front)
+    {
+        return bt.front->item.type;
+    }
+    if (bt.back)
+    {
+        return bt.back->item.type;
+    }
+    return std::nullopt;
+}
+
 // ---------------------------------------------------------------------------
 // Maintenance
 // ---------------------------------------------------------------------------