World|Sector: Properly resolve cyclic sector cluster linking

If two sector clusters determine they should both link to the other
then by virtue of the clustering this inherently means that one of
the two contains the other. To resolve this situation we'll simply
compare the bounding boxes to determine which.

doomsday/client/src/world/sector.cpp

@@ -18,6 +18,7 @@
  * 02110-1301 USA</small>
  */
 
+#include <QRect>
 #include <QSet>
 #include <QtAlgorithms>
 
@@ -43,6 +44,11 @@
 
 using namespace de;
 
+static QRectF qrectFromAABox(AABoxd const &aaBox)
+{
+    return QRectF(QPointF(aaBox.minX, aaBox.maxY), QPointF(aaBox.maxX, aaBox.minY));
+}
+
 void Sector::Cluster::remapVisPlanes()
 {
     // By default both planes are mapped to the parent sector.
@@ -61,7 +67,7 @@ void Sector::Cluster::remapVisPlanes()
             {
                 if(hedge->mapElement()->as<LineSideSegment>().line().isSelfReferencing())
                 {
-                    if(!exteriorCluster && hedge->twin().hasFace())
+                    if(hedge->twin().hasFace())
                     {
                         BspLeaf &otherLeaf = hedge->twin().face().mapElement()->as<BspLeaf>();
                         if(otherLeaf.hasCluster())
@@ -89,11 +95,25 @@ void Sector::Cluster::remapVisPlanes()
     {
         // Ensure we don't produce a cyclic dependency...
         Sector *finalSector = &exteriorCluster->visPlane(Floor).sector();
-        if(finalSector != &sector())
+        if(finalSector == &sector())
         {
-            _mappedVisFloor   = exteriorCluster;
-            _mappedVisCeiling = exteriorCluster;
+            // Must share a boundary edge.
+            QRectF boundingRect = qrectFromAABox(aaBox());
+            if(boundingRect.contains(qrectFromAABox(exteriorCluster->aaBox())))
+            {
+                // The contained cluster will link to this.
+                return;
+            }
+            else
+            {
+                // Reverse the linkage (this cluster is containted).
+                exteriorCluster->_mappedVisFloor =
+                    exteriorCluster->_mappedVisCeiling = exteriorCluster;
+            }
         }
+
+        _mappedVisFloor   = exteriorCluster;
+        _mappedVisCeiling = exteriorCluster;
     }
 }