NeighborListCache: don't build neighbor list during initialization (#983)

Given n radios, neighbor list construction has O(n^2) complexity.
If, during initialization, when each radio is added, the neighbor list
is reconstructed, we end up having a simulation initialization with
O(n^3) complexity.
Sidenote: In one of our tests, without this patch, when running a WiFi
simulation of 5000 nodes, it took ~40 minutes to just initialize the
simulation.

Co-authored-by: Levente Mészáros <[email protected]>

Author: chenlijun99

src/inet/physicallayer/wireless/common/neighborcache/NeighborListNeighborCache.cc

@@ -96,10 +96,13 @@ void NeighborListNeighborCache::addRadio(const IRadio *radio)
     RadioEntry *newEntry = new RadioEntry(radio);
     radios.push_back(newEntry);
     radioToEntry[radio] = newEntry;
-    updateNeighborLists();
-    maxSpeed = radioMedium->getMediumLimitCache()->getMaxSpeed().get<mps>();
-    if (maxSpeed != 0 && !updateNeighborListsTimer->isScheduled() && initialized())
-        scheduleAfter(refillPeriod, updateNeighborListsTimer);
+    if (initialized()) {
+        updateNeighborLists();
+        maxSpeed = radioMedium->getMediumLimitCache()->getMaxSpeed().get<mps>();
+        if (maxSpeed != 0 && !updateNeighborListsTimer->isScheduled()) {
+            scheduleAfter(refillPeriod, updateNeighborListsTimer);
+        }
+    }
 }
 
 void NeighborListNeighborCache::removeRadio(const IRadio *radio)
@@ -144,4 +147,3 @@ NeighborListNeighborCache::~NeighborListNeighborCache()
 
 } // namespace physicallayer
 } // namespace inet
-