From 8e264f4289f70440c7382e90453a4c0c1e89c831 Mon Sep 17 00:00:00 2001
From: dellaert <dellaert@cc.gatech.edu>
Date: Thu, 9 Oct 2014 14:38:16 +0200
Subject: [PATCH] Attempt at defining Trace recursively

---
 gtsam_unstable/nonlinear/Expression-inl.h | 50 ++++++++++++++---------
 1 file changed, 31 insertions(+), 19 deletions(-)

diff --git a/gtsam_unstable/nonlinear/Expression-inl.h b/gtsam_unstable/nonlinear/Expression-inl.h
index c97a903eb..b7038fc4c 100644
--- a/gtsam_unstable/nonlinear/Expression-inl.h
+++ b/gtsam_unstable/nonlinear/Expression-inl.h
@@ -378,6 +378,36 @@ public:
 
 };
 
+//-----------------------------------------------------------------------------
+
+#include <boost/mpl/list.hpp>
+
+/// Recursive Trace Class
+template<class T, class LIST>
+struct MakeTrace: public JacobianTrace<T> {
+  typedef boost::mpl::front<LIST> A1;
+  static const size_t dimA = A1::dimension;
+  typedef Eigen::Matrix<double, T::dimension, A1::dimension> JacobianTA;
+  typedef Eigen::Matrix<double, 2, T::dimension> Jacobian2T;
+
+  typename JacobianTrace<A1>::Pointer trace1;
+  JacobianTA dTdA1;
+
+  /// Start the reverse AD process
+  virtual void startReverseAD(JacobianMap& jacobians) const {
+    Select<T::dimension, A1>::reverseAD(trace1, dTdA1, jacobians);
+  }
+  /// Given df/dT, multiply in dT/dA and continue reverse AD process
+  virtual void reverseAD(const Matrix& dFdT, JacobianMap& jacobians) const {
+    trace1.reverseAD(dFdT * dTdA1, jacobians);
+  }
+  /// Version specialized to 2-dimensional output
+  virtual void reverseAD2(const Jacobian2T& dFdT,
+      JacobianMap& jacobians) const {
+    trace1.reverseAD2(dFdT * dTdA1, jacobians);
+  }
+};
+
 //-----------------------------------------------------------------------------
 /// Unary Function Expression
 template<class T, class A1>
@@ -423,25 +453,7 @@ public:
   }
 
   /// Trace structure for reverse AD
-  struct Trace: public JacobianTrace<T> {
-    typename JacobianTrace<A1>::Pointer trace1;
-    JacobianTA dTdA1;
-
-    /// Start the reverse AD process
-    virtual void startReverseAD(JacobianMap& jacobians) const {
-      Select<T::dimension, A1>::reverseAD(trace1, dTdA1, jacobians);
-    }
-    /// Given df/dT, multiply in dT/dA and continue reverse AD process
-    virtual void reverseAD(const Matrix& dFdT, JacobianMap& jacobians) const {
-      trace1.reverseAD(dFdT * dTdA1, jacobians);
-    }
-    /// Version specialized to 2-dimensional output
-    typedef Eigen::Matrix<double, 2, T::dimension> Jacobian2T;
-    virtual void reverseAD2(const Jacobian2T& dFdT,
-        JacobianMap& jacobians) const {
-      trace1.reverseAD2(dFdT * dTdA1, jacobians);
-    }
-  };
+  typedef MakeTrace<T, boost::mpl::list1<A1> > Trace;
 
   /// Construct an execution trace for reverse AD
   virtual T traceExecution(const Values& values,