handle HybridConditional and explicitly set Gaussian Factor Graphs to empty

gtsam/hybrid/HybridGaussianFactorGraph.cpp

@@ -96,8 +96,15 @@ GaussianMixtureFactor::Sum sumFrontals(
       }
 
     } else if (f->isContinuous()) {
-      deferredFactors.push_back(
-          boost::dynamic_pointer_cast<HybridGaussianFactor>(f)->inner());
+      // Check if f is HybridConditional or HybridGaussianFactor.
+      if (auto hc = boost::dynamic_pointer_cast<HybridConditional>(f)) {
+        auto conditional =
+            boost::dynamic_pointer_cast<GaussianConditional>(hc->inner());
+        deferredFactors.push_back(conditional);
+      } else if (auto gf = boost::dynamic_pointer_cast<HybridGaussianFactor>(f)
+                               ->inner()) {
+        deferredFactors.push_back(gf);
+      }
 
     } else if (f->isDiscrete()) {
       // Don't do anything for discrete-only factors
@@ -184,6 +191,19 @@ hybridElimination(const HybridGaussianFactorGraph &factors,
   // sum out frontals, this is the factor on the separator
   GaussianMixtureFactor::Sum sum = sumFrontals(factors);
 
+  // If a tree leaf contains nullptr,
+  // convert that leaf to an empty GaussianFactorGraph.
+  // Needed since the DecisionTree will otherwise create
+  // a GFG with a single (null) factor.
+  auto emptyGaussian = [](const GaussianFactorGraph &gfg) {
+    bool hasNull =
+        std::any_of(gfg.begin(), gfg.end(),
+                    [](const GaussianFactor::shared_ptr &ptr) { return !ptr; });
+
+    return hasNull ? GaussianFactorGraph() : gfg;
+  };
+  sum = GaussianMixtureFactor::Sum(sum, emptyGaussian);
+
   using EliminationPair = GaussianFactorGraph::EliminationResult;
 
   KeyVector keysOfEliminated;  // Not the ordering
@@ -195,7 +215,10 @@ hybridElimination(const HybridGaussianFactorGraph &factors,
     if (graph.empty()) {
       return {nullptr, nullptr};
     }
-    auto result = EliminatePreferCholesky(graph, frontalKeys);
+    std::pair<boost::shared_ptr<GaussianConditional>,
+              boost::shared_ptr<GaussianFactor>>
+        result = EliminatePreferCholesky(graph, frontalKeys);
+
     if (keysOfEliminated.empty()) {
       keysOfEliminated =
           result.first->keys();  // Initialize the keysOfEliminated to be the
@@ -235,7 +258,7 @@ hybridElimination(const HybridGaussianFactorGraph &factors,
             boost::make_shared<HybridDiscreteFactor>(discreteFactor)};
 
   } else {
-    // Create a resulting DCGaussianMixture on the separator.
+    // Create a resulting GaussianMixtureFactor on the separator.
     auto factor = boost::make_shared<GaussianMixtureFactor>(
         KeyVector(continuousSeparator.begin(), continuousSeparator.end()),
         discreteSeparator, separatorFactors);