improve dead mode removal by checking for empty discrete joints and adding the marginals for future factors

gtsam/hybrid/HybridBayesNet.cpp

@@ -58,15 +58,12 @@ HybridBayesNet HybridBayesNet::prune(size_t maxNrLeaves,
     joint = joint * (*conditional);
   }
 
-  // Create the result starting with the pruned joint.
+  // Initialize the resulting HybridBayesNet.
   HybridBayesNet result;
-  result.emplace_shared<DiscreteConditional>(joint);
-  // Prune the joint. NOTE: imperative and, again, possibly quite expensive.
-  result.back()->asDiscrete()->prune(maxNrLeaves);
 
-  // Get pruned discrete probabilities so
-  // we can prune HybridGaussianConditionals.
-  DiscreteConditional pruned = *result.back()->asDiscrete();
+  // Prune the joint. NOTE: imperative and, again, possibly quite expensive.
+  DiscreteConditional pruned = joint;
+  joint.prune(maxNrLeaves);
 
   DiscreteValues deadModesValues;
   if (removeDeadModes) {
@@ -88,8 +85,26 @@ HybridBayesNet HybridBayesNet::prune(size_t maxNrLeaves,
     }
 
     // Remove the modes (imperative)
-    result.back()->asDiscrete()->removeDiscreteModes(deadModesValues);
-    pruned = *result.back()->asDiscrete();
+    pruned.removeDiscreteModes(deadModesValues);
+
+    /*
+      If the pruned discrete conditional has any keys left,
+      we add it to the HybridBayesNet.
+      If not, it means it is an orphan so we don't add this pruned joint,
+      and instead add only the marginals below.
+    */
+    if (pruned.keys().size() > 0) {
+      result.emplace_shared<DiscreteConditional>(pruned);
+    }
+
+    // Add the marginals for future factors
+    for (auto &&[key, _] : deadModesValues) {
+      result.push_back(
+          std::dynamic_pointer_cast<DiscreteConditional>(marginals(key)));
+    }
+
+  } else {
+    result.emplace_shared<DiscreteConditional>(pruned);
   }
 
   /* To prune, we visitWith every leaf in the HybridGaussianConditional.
@@ -186,6 +201,7 @@ DiscreteValues HybridBayesNet::mpe() const {
       }
     }
   }
+
   return discrete_fg.optimize();
 }