generalize inverse function

.clang-format

@@ -0,0 +1,7 @@
+---
+Language: Cpp
+BasedOnStyle: Google
+ColumnLimit: 100
+DerivePointerAlignment: false
+PointerAlignment: Left
+...

README.md

@@ -34,13 +34,14 @@ Refer to [`transforms_graph.h`](include/transforms_graph/transforms_graph.h) for
 ## Required types
 The `TransformsGraph` requires two template parameters:
 1. `Transform` type. This is the type that stores the transforms themselves (i.e., poses). This type should have the following defined:
-    - A default constructor that sets the transform to the identity transform
-    - A valid Multiplication operator `*` operator (i.e., `T1 = T2 * T2`)
-    - A valid `Transform inverse() const` method (i.e., `T1.inverse() * T1` should return an identity transform)
-    - An output stream operator `<<` (i.e., `std::cout << T`)
+    - A default constructor that sets the transform to the identity transform.
+    - A valid Multiplication operator `*` operator (i.e., `T1 = T2 * T2`).
+    - A valid `Transform inverse() const` method (i.e., `T1.inverse() * T1` should return an identity transform). Alternatively, a function `Transform inverse(Transform)` can be injected/passed to the `TransformsGraph` upon construction.
+    - An output stream operator `<<` (i.e., `std::cout << T`).
 2. `Frame` type (default set to `char`). This is the type that keeps track of the frames. The type should have the following defined:
-    - Greater-than comparison operator `>` (i.e., `frame_i > frame_j`)
-    - An output stream operator `<<` (i.e., `std::cout << T`)
+    - Greater-than comparison operator `>` (i.e., `frame_i > frame_j`).
+    - An output stream operator `<<` (i.e., `std::cout << T`).
+3. `Inv` function object (defaults to `Transform::inverse`). The function object is passed in the constructor.
 
 The classes from [Sophus](https://github.com/strasdat/Sophus) (e.g., `Sophus::SE2d`) and [Eigen](https://eigen.tuxfamily.org/dox/group__TutorialGeometry.html) (e.g., `Eigen::Affine2d`) already satisfy the `Transform` requirements, except for the output stream operator `<<` requirement.
 

examples/eigen_pose_example.cpp

@@ -29,7 +29,7 @@ class Pose {
 
   Pose inverse() const {
     Pose p;
-    p.pose_ = std::move(pose_.inverse());
+    p.pose_ = pose_.inverse();
     return p;
   }
   Eigen::Affine2d Affine() const { return pose_; }

examples/minimal_graph_example.cpp

@@ -20,7 +20,6 @@ class Displacement {
   Displacement() { d_ = 0; }
   Displacement(double d) : d_(d) {}
   double x() const { return d_; }
-  Displacement inverse() const { return Displacement(-d_); }
 
  private:
   double d_;
@@ -43,7 +42,8 @@ int main(int argc, char* argv[]) {
   using Transform = Displacement;
 
   // Construct a graph that consists of two unconnected subgraphs
-  tg::TransformsGraph<Transform, Frame> transforms;
+  auto displacement_inverse = [](const Transform& t) -> Transform { return -t.x(); };
+  tg::TransformsGraph<Transform, Frame> transforms(100, displacement_inverse);
   transforms.AddTransform('a', 'b', 1);
   transforms.AddTransform('a', 'c', 2);
   transforms.AddTransform('b', 'd', 3);

include/transforms_graph/graph_search.h

@@ -27,7 +27,6 @@ namespace tg {
  */
 template <typename Node, typename Graph = std::unordered_map<Node, std::unordered_set<Node>>>
 std::vector<Node> DFS(const Graph& graph, Node start, Node end) {
-  std::vector<Node> path;
   std::unordered_set<Node> visited;
   std::unordered_map<Node, Node> parent;
   std::stack<Node> stack;
@@ -43,6 +42,8 @@ std::vector<Node> DFS(const Graph& graph, Node start, Node end) {
       found_solution = true;
       break;
     }
+
+    // Mark as visited
     if (visited.count(current)) continue;
     visited.insert(current);
 
@@ -55,7 +56,8 @@ std::vector<Node> DFS(const Graph& graph, Node start, Node end) {
 
   if (!found_solution) return {};
 
-  // Found the end
+  // Get path from start -> end
+  std::vector<Node> path;
   Node node = end;
   while (node != start) {
     path.push_back(node);

include/transforms_graph/transforms_graph.h

@@ -22,8 +22,11 @@ namespace tg {
  * @brief Transform graph class
  * @tparam Transform Transform/pose type. Should have `*`, `<<`, and `inverse()` defined
  * @tparam Frame Frame type (e.g. char, int, etc.). Should have `<<` defined.
+ * @tparam Inv Function object, where the instance takes an argument of type Transform and returns
+ * an inverse Transform.
  */
-template <typename Transform, typename Frame = char>
+template <typename Transform, typename Frame = char,
+          typename Inv = std::function<Transform(Transform)>>
 class TransformsGraph {
  public:
   /** Id/key used to store transforms in the Transforms map */
@@ -32,6 +35,9 @@ class TransformsGraph {
   /** Raw transforms */
   using Transforms = std::unordered_map<TransformId, Transform>;
 
+  /** Inverse function object */
+  using TransformInverse = Inv;
+
   /** Adjacency matrix of adjacent frames */
   using AdjacentFrames = std::unordered_map<Frame, std::unordered_set<Frame>>;
 
@@ -45,8 +51,13 @@ class TransformsGraph {
    * @details The maximum number of frames cannot be changed after construction
    *
    * @param[in] max_frames Maximum number of frames allowed in the graph
+   * @param[in] transform_inverse Function object to invert a transform. Should take a
+   *            transform as an argument and return its inverse.
    */
-  TransformsGraph(int max_frames = 100) : max_frames_(max_frames) {}
+  TransformsGraph(int max_frames = 100,
+                  TransformInverse transform_inverse = std::bind(&Transform::inverse,
+                                                                 std::placeholders::_1))
+      : max_frames_(max_frames), transform_inverse_(transform_inverse) {}
 
   /**
    * @brief Get maximum number allowed in the transform graph
@@ -130,7 +141,7 @@ class TransformsGraph {
     const auto transform_id = ComputeTransformId(parent, child);
     auto transform = raw_transforms_.at(transform_id);
 
-    return ShouldInvertFrames(parent, child) ? transform.inverse() : transform;
+    return ShouldInvertFrames(parent, child) ? transform_inverse_(transform) : transform;
   }
 
   /**
@@ -160,7 +171,7 @@ class TransformsGraph {
       const auto transform_id = ComputeTransformId(prev, frame);
       auto T_prev_curr = raw_transforms_.at(transform_id);
       if (ShouldInvertFrames(prev, frame)) {
-        T_prev_curr = T_prev_curr.inverse();
+        T_prev_curr = transform_inverse_(T_prev_curr);
       }
 
       T_parent_child = T_parent_child * T_prev_curr;
@@ -354,7 +365,8 @@ class TransformsGraph {
       throw std::runtime_error("Transform does not exist in the graph");
     }
     const auto transform_id = ComputeTransformId(parent, child);
-    raw_transforms_[transform_id] = ShouldInvertFrames(parent, child) ? pose.inverse() : pose;
+    raw_transforms_[transform_id] =
+        ShouldInvertFrames(parent, child) ? transform_inverse_(pose) : pose;
   }
 
   /**
@@ -457,7 +469,8 @@ class TransformsGraph {
     if (!HasFrame(child)) AddFrame(child);
 
     const auto transform_id = ComputeTransformId(parent, child);
-    raw_transforms_[transform_id] = ShouldInvertFrames(parent, child) ? pose.inverse() : pose;
+    raw_transforms_[transform_id] =
+        ShouldInvertFrames(parent, child) ? transform_inverse_(pose) : pose;
   }
 
   /**
@@ -508,6 +521,9 @@ class TransformsGraph {
   /** Maximum number of frames expected to be in the graph */
   int max_frames_ = 100;
 
+  /** Function object to invert transform */
+  TransformInverse transform_inverse_;
+
   /** Acyclic graph where the vertices are the frames and the edges are transforms between the two
    * frames */
   AdjacentFrames adjacent_frames_;