Highlight tunnel connections in green when selected

src/lib/core/TunnelCompletion.cpp

@@ -113,3 +113,43 @@ TunnelCompletion resolveTunnelCompletion(const TunnelLookup& lookup, QPoint hove
     }
     return TunnelCompletion{BuildingType::TunnelEntry, exitPartner};
 }
+
+std::optional<QPoint> findTunnelPartner(const TunnelLookup& lookup, QPoint tile,
+                                        BuildingType type, Rotation rotation,
+                                        int maxDistance)
+{
+    if (type == BuildingType::TunnelEntry)
+    {
+        // An entry pairs with the first same-direction tunnel ahead, if it is an exit.
+        const std::optional<QPoint> ahead =
+            firstTunnelFacing(lookup, tile, rotation, rotation, maxDistance);
+        if (ahead.has_value())
+        {
+            const std::optional<TunnelTileInfo> info = lookup(*ahead);
+            if (info.has_value() && info->type == BuildingType::TunnelExit)
+            {
+                return ahead;
+            }
+        }
+        return std::nullopt;
+    }
+
+    if (type == BuildingType::TunnelExit)
+    {
+        // An exit is claimed by the first same-direction tunnel behind it, if it is an
+        // entry (its forward search reaches this exit as its first same-direction tunnel).
+        const std::optional<QPoint> behind =
+            firstTunnelFacing(lookup, tile, oppositeRotation(rotation), rotation, maxDistance);
+        if (behind.has_value())
+        {
+            const std::optional<TunnelTileInfo> info = lookup(*behind);
+            if (info.has_value() && info->type == BuildingType::TunnelEntry)
+            {
+                return behind;
+            }
+        }
+        return std::nullopt;
+    }
+
+    return std::nullopt;
+}