From 811aadbe000a01a47bc53cc228bc269fb0653be5 Mon Sep 17 00:00:00 2001
From: Rasmus Munk Larsen <rmlarsen@google.com>
Date: Thu, 2 Jun 2016 12:41:28 -0700
Subject: Add syntactic sugar to Eigen tensors to allow more natural syntax.
 Specifically, this enables expressions involving:

scalar + tensor
scalar * tensor
scalar / tensor
scalar - tensor
---
 unsupported/Eigen/CXX11/src/Tensor/TensorBase.h | 30 ++++++++++++++++++++++
 unsupported/test/cxx11_tensor_sugar.cpp         | 34 ++++++++++++++++++++-----
 2 files changed, 57 insertions(+), 7 deletions(-)

diff --git a/unsupported/Eigen/CXX11/src/Tensor/TensorBase.h b/unsupported/Eigen/CXX11/src/Tensor/TensorBase.h
index 1eaa8d4fc..12f8a1499 100644
--- a/unsupported/Eigen/CXX11/src/Tensor/TensorBase.h
+++ b/unsupported/Eigen/CXX11/src/Tensor/TensorBase.h
@@ -215,6 +215,13 @@ class TensorBase<Derived, ReadOnlyAccessors>
       return unaryExpr(internal::scalar_add_op<Scalar>(rhs));
     }
 
+    EIGEN_DEVICE_FUNC
+    EIGEN_STRONG_INLINE friend
+    const TensorCwiseUnaryOp<internal::scalar_add_op<Scalar>, const Derived>
+    operator+ (Scalar lhs, const Derived& rhs) {
+      return rhs + lhs;
+    }
+
     EIGEN_DEVICE_FUNC
     EIGEN_STRONG_INLINE const TensorCwiseUnaryOp<internal::scalar_sub_op<Scalar>, const Derived>
     operator- (Scalar rhs) const {
@@ -222,18 +229,41 @@ class TensorBase<Derived, ReadOnlyAccessors>
       return unaryExpr(internal::scalar_sub_op<Scalar>(rhs));
     }
 
+    EIGEN_DEVICE_FUNC
+    EIGEN_STRONG_INLINE friend
+    const TensorCwiseUnaryOp<internal::scalar_add_op<Scalar>,
+                             const TensorCwiseUnaryOp<internal::scalar_opposite_op<Scalar>, const Derived> >
+    operator- (Scalar lhs, const Derived& rhs) {
+      return -rhs + lhs;
+    }
+
     EIGEN_DEVICE_FUNC
     EIGEN_STRONG_INLINE const TensorCwiseUnaryOp<internal::scalar_multiple_op<Scalar>, const Derived>
     operator* (Scalar rhs) const {
       return unaryExpr(internal::scalar_multiple_op<Scalar>(rhs));
     }
 
+    EIGEN_DEVICE_FUNC
+    EIGEN_STRONG_INLINE friend
+    const TensorCwiseUnaryOp<internal::scalar_multiple_op<Scalar>, const Derived>
+    operator* (Scalar lhs, const Derived& rhs) {
+      return rhs * lhs;
+    }
+
     EIGEN_DEVICE_FUNC
     EIGEN_STRONG_INLINE const TensorCwiseUnaryOp<internal::scalar_quotient1_op<Scalar>, const Derived>
     operator/ (Scalar rhs) const {
       return unaryExpr(internal::scalar_quotient1_op<Scalar>(rhs));
     }
 
+    EIGEN_DEVICE_FUNC
+    EIGEN_STRONG_INLINE friend
+    const TensorCwiseUnaryOp<internal::scalar_multiple_op<Scalar>,
+                             const TensorCwiseUnaryOp<internal::scalar_inverse_op<Scalar>, const Derived> >
+    operator/ (Scalar lhs, const Derived& rhs) {
+      return rhs.inverse() * lhs;
+    }
+
     EIGEN_DEVICE_FUNC
     EIGEN_STRONG_INLINE const TensorCwiseUnaryOp<internal::scalar_mod_op<Scalar>, const Derived>
     operator% (Scalar rhs) const {
diff --git a/unsupported/test/cxx11_tensor_sugar.cpp b/unsupported/test/cxx11_tensor_sugar.cpp
index a03f75cfe..2f56eb495 100644
--- a/unsupported/test/cxx11_tensor_sugar.cpp
+++ b/unsupported/test/cxx11_tensor_sugar.cpp
@@ -33,7 +33,7 @@ static void test_comparison_sugar() {
 }
 
 
-static void test_scalar_sugar() {
+static void test_scalar_sugar_add_mul() {
   Tensor<float, 3> A(6, 7, 5);
   Tensor<float, 3> B(6, 7, 5);
   A.setRandom();
@@ -41,21 +41,41 @@ static void test_scalar_sugar() {
 
   const float alpha = 0.43f;
   const float beta = 0.21f;
+  const float gamma = 0.14f;
 
-  Tensor<float, 3> R = A * A.constant(alpha) + B * B.constant(beta);
-  Tensor<float, 3> S = A * alpha + B * beta;
-
-  // TODO: add enough syntactic sugar to support this
-  // Tensor<float, 3> T = alpha * A + beta * B;
+  Tensor<float, 3> R = A.constant(gamma) + A * A.constant(alpha) + B * B.constant(beta);
+  Tensor<float, 3> S = A * alpha + B * beta + gamma;
+  Tensor<float, 3> T = gamma + alpha * A + beta * B;
 
   for (int i = 0; i < 6*7*5; ++i) {
     VERIFY_IS_APPROX(R(i), S(i));
+    VERIFY_IS_APPROX(R(i), T(i));
   }
 }
 
+static void test_scalar_sugar_sub_div() {
+  Tensor<float, 3> A(6, 7, 5);
+  Tensor<float, 3> B(6, 7, 5);
+  A.setRandom();
+  B.setRandom();
+
+  const float alpha = 0.43f;
+  const float beta = 0.21f;
+  const float gamma = 0.14f;
+  const float delta = 0.32f;
+
+  Tensor<float, 3> R = A.constant(gamma) - A / A.constant(alpha)
+      - B.constant(beta) / B - A.constant(delta);
+  Tensor<float, 3> S = gamma - A / alpha - beta / B - delta;
+
+  for (int i = 0; i < 6*7*5; ++i) {
+    VERIFY_IS_APPROX(R(i), S(i));
+  }
+}
 
 void test_cxx11_tensor_sugar()
 {
   CALL_SUBTEST(test_comparison_sugar());
-  CALL_SUBTEST(test_scalar_sugar());
+  CALL_SUBTEST(test_scalar_sugar_add_mul());
+  CALL_SUBTEST(test_scalar_sugar_sub_div());
 }
-- 
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