From 15084cf1ac1f58085cd0635676aa1d28efb268de Mon Sep 17 00:00:00 2001
From: Gael Guennebaud <g.gael@free.fr>
Date: Fri, 29 Jan 2016 22:09:45 +0100
Subject: [PATCH] bug #632: add support for "dense +/- sparse" operations. The
 current implementation is based on SparseView to make the dense subexpression
 compatible with the sparse one.

---
 Eigen/src/SparseCore/SparseCwiseBinaryOp.h | 30 ++++++++++++++++++++++
 test/sparse_basic.cpp                      |  5 ++++
 2 files changed, 35 insertions(+)

diff --git a/Eigen/src/SparseCore/SparseCwiseBinaryOp.h b/Eigen/src/SparseCore/SparseCwiseBinaryOp.h
index d9420ac63..06c6d0e4d 100644
--- a/Eigen/src/SparseCore/SparseCwiseBinaryOp.h
+++ b/Eigen/src/SparseCore/SparseCwiseBinaryOp.h
@@ -60,6 +60,8 @@ namespace internal {
 
   
 // Generic "sparse OP sparse"
+template<typename XprType> struct binary_sparse_evaluator;
+
 template<typename BinaryOp, typename Lhs, typename Rhs>
 struct binary_evaluator<CwiseBinaryOp<BinaryOp, Lhs, Rhs>, IteratorBased, IteratorBased>
   : evaluator_base<CwiseBinaryOp<BinaryOp, Lhs, Rhs> >
@@ -428,6 +430,34 @@ SparseMatrixBase<Derived>::cwiseProduct(const MatrixBase<OtherDerived> &other) c
   return typename CwiseProductDenseReturnType<OtherDerived>::Type(derived(), other.derived());
 }
 
+template<typename SparseDerived, typename DenseDerived>
+EIGEN_STRONG_INLINE const CwiseBinaryOp<internal::scalar_sum_op<typename DenseDerived::Scalar>, const SparseDerived, const SparseView<DenseDerived,true> >
+operator+(const SparseMatrixBase<SparseDerived> &a, const MatrixBase<DenseDerived> &b)
+{
+  return a.derived() + SparseView<DenseDerived,true>(b.derived());
+}
+
+template<typename DenseDerived, typename SparseDerived>
+EIGEN_STRONG_INLINE const CwiseBinaryOp<internal::scalar_sum_op<typename DenseDerived::Scalar>, const SparseView<DenseDerived,true>, const SparseDerived>
+operator+(const MatrixBase<DenseDerived> &a, const SparseMatrixBase<SparseDerived> &b)
+{
+  return SparseView<DenseDerived,true>(a.derived()) + b.derived();
+}
+
+template<typename SparseDerived, typename DenseDerived>
+EIGEN_STRONG_INLINE const CwiseBinaryOp<internal::scalar_difference_op<typename DenseDerived::Scalar>, const SparseDerived, const SparseView<DenseDerived,true> >
+operator-(const SparseMatrixBase<SparseDerived> &a, const MatrixBase<DenseDerived> &b)
+{
+  return a.derived() - SparseView<DenseDerived,true>(b.derived());
+}
+
+template<typename DenseDerived, typename SparseDerived>
+EIGEN_STRONG_INLINE const CwiseBinaryOp<internal::scalar_difference_op<typename DenseDerived::Scalar>, const SparseView<DenseDerived,true>, const SparseDerived>
+operator-(const MatrixBase<DenseDerived> &a, const SparseMatrixBase<SparseDerived> &b)
+{
+  return SparseView<DenseDerived,true>(a.derived()) - b.derived();
+}
+
 } // end namespace Eigen
 
 #endif // EIGEN_SPARSE_CWISE_BINARY_OP_H
diff --git a/test/sparse_basic.cpp b/test/sparse_basic.cpp
index d803e7dae..5a5650705 100644
--- a/test/sparse_basic.cpp
+++ b/test/sparse_basic.cpp
@@ -192,6 +192,11 @@ template<typename SparseMatrixType> void sparse_basic(const SparseMatrixType& re
     VERIFY_IS_APPROX(refM4.cwiseProduct(m3), refM4.cwiseProduct(refM3));
 //     VERIFY_IS_APPROX(m3.cwise()/refM4, refM3.cwise()/refM4);
 
+    VERIFY_IS_APPROX(refM4 + m3, refM4 + refM3);
+    VERIFY_IS_APPROX(m3 + refM4, refM3 + refM4);
+    VERIFY_IS_APPROX(refM4 - m3, refM4 - refM3);
+    VERIFY_IS_APPROX(m3 - refM4, refM3 - refM4);
+
     // test aliasing
     VERIFY_IS_APPROX((m1 = -m1), (refM1 = -refM1));
     VERIFY_IS_APPROX((m1 = m1.transpose()), (refM1 = refM1.transpose().eval()));