Revert "kill toDecisionTreeFactor to force rethink"

This reverts commit 306a3bae52.

gtsam/discrete/DecisionTreeFactor.h

@@ -169,6 +169,9 @@ namespace gtsam {
       }
     }
 
+    /// Convert into a decision tree
+    DecisionTreeFactor toDecisionTreeFactor() const override { return *this; }
+
     /// Create new factor by summing all values with the same separator values
     DiscreteFactor::shared_ptr sum(size_t nrFrontals) const override {
       return combine(nrFrontals, ADT::Ring::add);

gtsam/discrete/DiscreteFactor.h

@@ -113,6 +113,8 @@ class GTSAM_EXPORT DiscreteFactor : public Factor {
   virtual DiscreteFactor::shared_ptr operator*(
       const DiscreteFactor::shared_ptr&) const = 0;
 
+  virtual DecisionTreeFactor toDecisionTreeFactor() const = 0;
+
   /// Create new factor by summing all values with the same separator values
   virtual DiscreteFactor::shared_ptr sum(size_t nrFrontals) const = 0;
 

gtsam/discrete/TableFactor.cpp

@@ -180,6 +180,20 @@ DiscreteFactor::shared_ptr TableFactor::operator*(
   }
 }
 
+/* ************************************************************************ */
+DecisionTreeFactor TableFactor::toDecisionTreeFactor() const {
+  DiscreteKeys dkeys = discreteKeys();
+  std::vector<double> table;
+  for (auto i = 0; i < sparse_table_.size(); i++) {
+    table.push_back(sparse_table_.coeff(i));
+  }
+  gttic_(toDecisionTreeFactor_Constructor);
+  // NOTE(Varun): This constructor is really expensive!!
+  DecisionTreeFactor f(dkeys, table);
+  gttoc_(toDecisionTreeFactor_Constructor);
+  return f;
+}
+
 /* ************************************************************************ */
 TableFactor TableFactor::choose(const DiscreteValues parent_assign,
                                 DiscreteKeys parent_keys) const {

gtsam/discrete/TableFactor.h

@@ -207,6 +207,9 @@ class GTSAM_EXPORT TableFactor : public DiscreteFactor {
     }
   }
 
+  /// Convert into a decisiontree
+  DecisionTreeFactor toDecisionTreeFactor() const override;
+
   /// Create a TableFactor that is a subset of this TableFactor
   TableFactor choose(const DiscreteValues assignments,
                      DiscreteKeys parent_keys) const;