feat(geo): Portal gets fill with trivial method

The `fill` method accepts a volume and creates a trivial portal link on
the link that is not filled yet in the portal (along or opposite).

Core/include/Acts/Geometry/Portal.hpp

@@ -208,6 +208,10 @@ class Portal {
   /// @return True if the portal is valid
   bool isValid() const;
 
+  /// Create and attach a trivial portal link to the empty slot of this portal
+  /// @param volume The target volume to connect to
+  void fill(TrackingVolume& volume);
+
   /// Access the portal surface that is shared between the two links
   /// @return The portal surface
   const RegularSurface& surface() const;

Core/src/Geometry/Portal.cpp

@@ -303,4 +303,21 @@ bool Portal::isSameSurface(const GeometryContext& gctx, const Surface& a,
   return true;
 };
 
+void Portal::fill(TrackingVolume& volume) {
+  if (m_alongNormal != nullptr && m_oppositeNormal != nullptr) {
+    throw std::logic_error{"Portal is already filled"};
+  }
+
+  if (m_surface == nullptr) {
+    throw std::logic_error{"Portal has no existing link set, can't fill"};
+  }
+
+  if (m_alongNormal == nullptr) {
+    m_alongNormal = std::make_unique<TrivialPortalLink>(m_surface, volume);
+  } else {
+    assert(m_oppositeNormal == nullptr);
+    m_oppositeNormal = std::make_unique<TrivialPortalLink>(m_surface, volume);
+  }
+}
+
 }  // namespace Acts

Tests/UnitTests/Core/Geometry/PortalTests.cpp

@@ -567,6 +567,34 @@ BOOST_AUTO_TEST_CASE(InvalidConstruction) {
                     std::invalid_argument);
 }
 
+BOOST_AUTO_TEST_CASE(PortalFill) {
+  auto vol1 = makeDummyVolume();
+  auto vol2 = makeDummyVolume();
+
+  auto cyl1 = Surface::makeShared<CylinderSurface>(Transform3::Identity(),
+                                                   50_mm, 100_mm);
+
+  Portal portal1{gctx, {.oppositeNormal = {cyl1, *vol1}}};
+  Portal portal2{gctx, {.alongNormal = {cyl1, *vol2}}};
+
+  // Fuse these to make portal 1 and 2 empty
+  Portal::fuse(gctx, portal1, portal2, *logger);
+
+  BOOST_CHECK_THROW(portal1.fill(*vol2), std::logic_error);
+
+  portal1 = Portal{gctx, {.oppositeNormal = {cyl1, *vol1}}};
+  portal2 = Portal{gctx, {.alongNormal = {cyl1, *vol2}}};
+
+  BOOST_CHECK_EQUAL(portal1.getLink(Direction::AlongNormal), nullptr);
+  BOOST_CHECK_NE(portal1.getLink(Direction::OppositeNormal), nullptr);
+
+  portal1.fill(*vol2);
+  BOOST_CHECK_NE(portal1.getLink(Direction::AlongNormal), nullptr);
+  BOOST_CHECK_NE(portal1.getLink(Direction::OppositeNormal), nullptr);
+
+  BOOST_CHECK_THROW(portal1.fill(*vol2), std::logic_error);
+}
+
 BOOST_AUTO_TEST_SUITE_END()  // Portals
 
 BOOST_AUTO_TEST_SUITE_END()  // Geometry