add scale() method in DiscreteFactor

gtsam/discrete/DiscreteFactor.cpp

@@ -71,4 +71,12 @@ AlgebraicDecisionTree<Key> DiscreteFactor::errorTree() const {
   return AlgebraicDecisionTree<Key>(dkeys, errors);
 }
 
+/* ************************************************************************ */
+DiscreteFactor::shared_ptr DiscreteFactor::scale() const {
+  // Max over all the potentials by pretending all keys are frontal:
+  shared_ptr denominator = this->max(this->size());
+  // Normalize the product factor to prevent underflow.
+  return this->operator/(denominator);
+}
+
 }  // namespace gtsam

gtsam/discrete/DiscreteFactor.h

@@ -158,6 +158,14 @@ class GTSAM_EXPORT DiscreteFactor : public Factor {
   /// Create new factor by maximizing over all values with the same separator.
   virtual DiscreteFactor::shared_ptr max(const Ordering& keys) const = 0;
 
+  /**
+   * @brief Scale the factor values by the maximum
+   * to prevent underflow/overflow.
+   * 
+   * @return DiscreteFactor::shared_ptr 
+   */
+  DiscreteFactor::shared_ptr scale() const;
+
   /**
    * Get the number of non-zero values contained in this factor.
    * It could be much smaller than `prod_{key}(cardinality(key))`.