Straight-up depth-first fold method

# Conflicts:
#	gtsam/discrete/tests/testDecisionTree.cpp

gtsam/discrete/DecisionTree-inl.h

@@ -630,6 +630,48 @@ namespace gtsam {
     return LY::compose(functions.begin(), functions.end(), newLabel);
   }
 
+  /*********************************************************************************/
+  template <typename L, typename Y, typename X>
+  struct Fold {
+    std::function<X(const Y&, X)> f;
+
+    /// Construct from folding function
+    Fold(std::function<X(const Y&, X)> f) : f(f) {}
+
+    using NodePtr = typename DecisionTree<L, Y>::NodePtr;
+    using Choice = typename DecisionTree<L, Y>::Choice;
+    using Leaf = typename DecisionTree<L, Y>::Leaf;
+
+    /**
+     * @brief Do a depth-first fold on the tree rooted at node.
+     *
+     * @param node root of a (sub-) tree, or a leaf.
+     * @param x0 Initial accumulator value.
+     * @return X Final accumulator value.
+     */
+    X fold(const NodePtr& node, X x0) const {
+      if (auto leaf = boost::dynamic_pointer_cast<const Leaf>(node)) {
+        return f(leaf->constant(), x0);
+      } else if (auto choice =
+                     boost::dynamic_pointer_cast<const Choice>(node)) {
+        for (auto&& branch : choice->branches()) x0 = fold(branch, x0);
+        return x0;
+      } else {
+        throw std::invalid_argument("Fold: Invalid NodePtr");
+      }
+    }
+
+    // alias for fold:
+    X operator()(const NodePtr& node, X x0) const { return fold(node, x0); }
+  };
+
+  template <typename L, typename Y>
+  template <typename Func, typename X>
+  X DecisionTree<L, Y>::fold(Func f, X x0) const {
+    Fold<L, Y, X> fold(f);
+    return fold(root_, x0);
+  }
+
   /*********************************************************************************/
   template <typename L, typename Y>
   bool DecisionTree<L, Y>::equals(const DecisionTree& other,

gtsam/discrete/DecisionTree.h

@@ -229,6 +229,22 @@ namespace gtsam {
     /** evaluate */
     const Y& operator()(const Assignment<L>& x) const;
 
+    /**
+     * @brief Fold a binary function over the tree, returning accumulator.
+     *
+     * @tparam X type for accumulator.
+     * @param f binary function: Y * X -> X returning an updated accumulator.
+     * @param x0 initial value for accumulator.
+     * @return X final value for accumulator.
+     * 
+     * @note X is always passed by value.
+     */
+    template <typename Func, typename X>
+    X fold(Func f, X x0) const;
+
+    /** Retrieve all labels. */
+    std::vector<L> labels() const { return std::vector<L>(); }
+
     /** apply Unary operation "op" to f */
     DecisionTree apply(const Unary& op) const;
 

gtsam/discrete/tests/testDecisionTree.cpp

@@ -123,8 +123,7 @@ struct Ring {
 
 /* ******************************************************************************** */
 // test DT
-TEST(DT, example)
+TEST(DecisionTree, example) {
-{
   // Create labels
   string A("A"), B("B"), C("C");
 
@@ -236,8 +235,7 @@ std::function<bool(const int&)> bool_of_int = [](const int& y) {
 };
 typedef DecisionTree<string, bool> StringBoolTree;
 
-TEST(DT, ConvertValuesOnly)
+TEST(DecisionTree, ConvertValuesOnly) {
-{
   // Create labels
   string A("A"), B("B");
 
@@ -260,8 +258,7 @@ enum Label {
 };
 typedef DecisionTree<Label, bool> LabelBoolTree;
 
-TEST(DT, ConvertBoth)
+TEST(DecisionTree, ConvertBoth) {
-{
   // Create labels
   string A("A"), B("B");
 
@@ -288,8 +285,7 @@ TEST(DT, ConvertBoth)
 
 /* ******************************************************************************** */
 // test Compose expansion
-TEST(DT, Compose)
+TEST(DecisionTree, Compose) {
-{
   // Create labels
   string A("A"), B("B"), C("C");
 
@@ -314,6 +310,49 @@ TEST(DT, Compose)
   DOT(f5);
 }
 
+/* ******************************************************************************** */
+// Check we can create a decision tree of containers.
+TEST(DecisionTree, Containers) {
+  using Container = std::vector<double>;
+  using StringContainerTree = DecisionTree<string, Container>;
+
+  // Check default constructor
+  StringContainerTree tree;
+
+  // Create small two-level tree
+  string A("A"), B("B"), C("C");
+  DT stringIntTree(B, DT(A, 0, 1), DT(A, 2, 3));
+
+  // Check conversion
+  std::function<Container(const int& i)> container_of_int = [](const int& i) {
+    Container c;
+    c.emplace_back(i);
+    return c;
+  };
+  StringContainerTree converted(stringIntTree, container_of_int);
+}
+
+/* ******************************************************************************** */
+// Test fold.
+TEST(DecisionTree, fold) {
+  // Create small two-level tree
+  string A("A"), B("B"), C("C");
+  DT tree(B, DT(A, 0, 1), DT(A, 2, 3));
+  auto add = [](const int& y, double x) { return y + x; };
+  double sum = tree.fold(add, 0.0);
+  EXPECT_DOUBLES_EQUAL(6.0, sum, 1e-9);
+}
+
+/* ******************************************************************************** */
+// Test retrieving all labels.
+TEST_DISABLED(DecisionTree, labels) {
+  // Create small two-level tree
+  string A("A"), B("B"), C("C");
+  DT tree(B, DT(A, 0, 1), DT(A, 2, 3));
+  auto labels = tree.labels();
+  EXPECT_LONGS_EQUAL(2, labels.size());
+}
+
 /* ************************************************************************* */
 int main() {
   TestResult tr;