CXX11/TensorSymmetry: add symmetry support for Tensor class

Add a symCoeff() method to the Tensor class template that allows the
user of the class to set multiple elements of a tensor at once if they
are connected by a symmetry operation with respect to the tensor's
indices (symmetry/antisymmetry/hermiticity/antihermiticity under
echange of two indices and combination thereof for different pairs of
indices).

A compile-time resolution of the required symmetry groups via meta
templates is also implemented. For small enough groups this is used to
unroll the loop that goes through all the elements of the Tensor that
are connected by this group. For larger groups or groups where the
symmetries are defined at run time, a standard run-time implementation
of the same algorithm is provided.

For example, the following code completely initializes all elements of
the totally antisymmetric tensor in three dimensions ('epsilon
tensor'):

SGroup<3, AntiSymmetry<0,1>, AntiSymmetry<1,2>> sym;
Eigen::Tensor<double, 3> epsilon(3,3,3);
epsilon.setZero();
epsilon.symCoeff(sym, 0, 1, 2) =  1;

1 files changed, 15 insertions, 0 deletions

diff --git a/unsupported/Eigen/CXX11/src/Tensor/Tensor.h b/unsupported/Eigen/CXX11/src/Tensor/Tensor.h
index c40905af4..ff3d6513e 100644
--- a/unsupported/Eigen/CXX11/src/Tensor/Tensor.h
+++ b/unsupported/Eigen/CXX11/src/Tensor/Tensor.h
@@ -92,6 +92,9 @@ struct tensor_index_linearization_helper<Index, NumIndices, 0, RowMajor>
     return std_array_get<RowMajor ? 0 : NumIndices - 1>(indices);
   }
 };
+
+/* Forward-declaration required for the symmetry support. */
+template<typename Tensor_, typename Symmetry_, int Flags = 0> class tensor_symmetry_value_setter;
 } // end namespace internal
 
 template<typename Scalar_, std::size_t NumIndices_, int Options_>
@@ -283,6 +286,18 @@ class Tensor
       #endif
     }
 
+    template<typename Symmetry_, typename... IndexTypes>
+    internal::tensor_symmetry_value_setter<Self, Symmetry_> symCoeff(const Symmetry_& symmetry, Index firstIndex, IndexTypes... otherIndices)
+    {
+      return symCoeff(symmetry, std::array<Index, NumIndices>{{firstIndex, otherIndices...}});
+    }
+
+    template<typename Symmetry_, typename... IndexTypes>
+    internal::tensor_symmetry_value_setter<Self, Symmetry_> symCoeff(const Symmetry_& symmetry, std::array<Index, NumIndices> const& indices)
+    {
+      return internal::tensor_symmetry_value_setter<Self, Symmetry_>(*this, symmetry, indices);
+    }
+
   protected:
     bool checkIndexRange(const std::array<Index, NumIndices>& indices) const
     {