From cc3c0fcfeca05dc55881b048e3537728eee29bbe Mon Sep 17 00:00:00 2001
From: Sungtae An <stan84@gatech.edu>
Date: Sun, 5 Oct 2014 17:38:09 -0400
Subject: [PATCH] add trace structure for reverse AD in TernaryExpression

---
 gtsam_unstable/nonlinear/Expression-inl.h | 20 ++++++++++++++++++++
 1 file changed, 20 insertions(+)

diff --git a/gtsam_unstable/nonlinear/Expression-inl.h b/gtsam_unstable/nonlinear/Expression-inl.h
index 7d375e737..2b02e6991 100644
--- a/gtsam_unstable/nonlinear/Expression-inl.h
+++ b/gtsam_unstable/nonlinear/Expression-inl.h
@@ -533,6 +533,26 @@ public:
     return Augmented<T>(t, H1, argument1.jacobians(), H2, argument2.jacobians(), H3, argument3.jacobians());
   }
 
+  /// Trace structure for reverse AD
+  struct Trace: public JacobianTrace<T> {
+    boost::shared_ptr<JacobianTrace<A1> > trace1;
+    boost::shared_ptr<JacobianTrace<A2> > trace2;
+    boost::shared_ptr<JacobianTrace<A3> > trace3;
+    Matrix H1, H2, H3;
+    /// Start the reverse AD process
+    virtual void reverseAD(JacobianMap& jacobians) const {
+      trace1->reverseAD(H1, jacobians);
+      trace2->reverseAD(H2, jacobians);
+      trace3->reverseAD(H3, jacobians);
+    }
+    /// Given df/dT, multiply in dT/dA and continue reverse AD process
+    virtual void reverseAD(const Matrix& H, JacobianMap& jacobians) const {
+      trace1->reverseAD(H * H1, jacobians);
+      trace2->reverseAD(H * H2, jacobians);
+      trace3->reverseAD(H * H3, jacobians);
+    }
+  };
+
 };
 //-----------------------------------------------------------------------------
 }