Fix DecisionTreeFactor division

gtsam/discrete/DecisionTreeFactor.h

@@ -154,7 +154,15 @@ namespace gtsam {
 
     static double safe_div(const double& a, const double& b);
 
-    /// divide by factor f (safely)
+    /**
+     * @brief Divide by factor f (safely).
+     * Division of a factor \f$f(x, y)\f$ by another factor \f$g(y, z)\f$
+     * results in a function which involves all keys
+     * \f$(\frac{f}{g})(x, y, z) = f(x, y) / g(y, z)\f$
+     * 
+     * @param f The DecisinTreeFactor to divide by.
+     * @return DecisionTreeFactor 
+     */
     DecisionTreeFactor operator/(const DecisionTreeFactor& f) const {
       return apply(f, safe_div);
     }

gtsam/discrete/tests/testDecisionTreeFactor.cpp

@@ -30,6 +30,12 @@
 using namespace std;
 using namespace gtsam;
 
+/** Convert Signature into CPT */
+DecisionTreeFactor create(const Signature& signature) {
+  DecisionTreeFactor p(signature.discreteKeys(), signature.cpt());
+  return p;
+}
+
 /* ************************************************************************* */
 TEST(DecisionTreeFactor, ConstructorsMatch) {
   // Declare two keys
@@ -105,6 +111,27 @@ TEST(DecisionTreeFactor, multiplication) {
   CHECK(assert_equal(expected2, actual));
 }
 
+/* ************************************************************************* */
+TEST(DecisionTreeFactor, Divide) {
+  DiscreteKey A(0, 2), S(1, 2);
+  DecisionTreeFactor pA = create(A % "99/1"), pS = create(S % "50/50");
+  DecisionTreeFactor joint = pA * pS;
+
+  DecisionTreeFactor s = joint / pA;
+
+  // Factors are not equal due to difference in keys
+  EXPECT(assert_inequal(pS, s));
+
+  // The underlying data should be the same
+  using ADT = AlgebraicDecisionTree<Key>;
+  EXPECT(assert_equal(ADT(pS), ADT(s)));
+
+  KeySet keys(joint.keys());
+  keys.insert(pA.keys().begin(), pA.keys().end());
+  EXPECT(assert_inequal(KeySet(pS.keys()), keys));
+
+}
+
 /* ************************************************************************* */
 TEST(DecisionTreeFactor, sum_max) {
   DiscreteKey v0(0, 3), v1(1, 2);
@@ -217,12 +244,6 @@ void maybeSaveDotFile(const DecisionTreeFactor& f, const string& filename) {
 #endif
 }
 
-/** Convert Signature into CPT */
-DecisionTreeFactor create(const Signature& signature) {
-  DecisionTreeFactor p(signature.discreteKeys(), signature.cpt());
-  return p;
-}
-
 /* ************************************************************************* */
 // test Asia Joint
 TEST(DecisionTreeFactor, joint) {