From 5e89573e2a1018afa1e0ef6394f5f33851c3bc6f Mon Sep 17 00:00:00 2001
From: Rasmus Munk Larsen <rmlarsen@google.com>
Date: Sat, 20 Nov 2021 00:22:46 +0000
Subject: [PATCH] Implement Eigen::array<...>::reverse_iterator if
 std::reverse_iterator exists.

---
 unsupported/Eigen/CXX11/Tensor                |  3 +-
 .../Eigen/CXX11/src/util/EmulateArray.h       | 36 ++++++++++++++-----
 2 files changed, 29 insertions(+), 10 deletions(-)

diff --git a/unsupported/Eigen/CXX11/Tensor b/unsupported/Eigen/CXX11/Tensor
index 5251ff9e4..c269289d9 100644
--- a/unsupported/Eigen/CXX11/Tensor
+++ b/unsupported/Eigen/CXX11/Tensor
@@ -38,9 +38,10 @@
 #include <cmath>
 #include <cstddef>
 #include <cstring>
+#include <iterator>
+#include <numeric>
 #include <random>
 #include <thread>
-#include <numeric>
 
 #if defined(EIGEN_USE_THREADS) || defined(EIGEN_USE_SYCL)
 #include "ThreadPool"
diff --git a/unsupported/Eigen/CXX11/src/util/EmulateArray.h b/unsupported/Eigen/CXX11/src/util/EmulateArray.h
index e51001070..f87cb811a 100644
--- a/unsupported/Eigen/CXX11/src/util/EmulateArray.h
+++ b/unsupported/Eigen/CXX11/src/util/EmulateArray.h
@@ -17,11 +17,38 @@
 
 namespace Eigen {
 template <typename T, size_t n> class array {
+
  public:
   typedef T value_type;
   typedef T* iterator;
   typedef const T* const_iterator;
 
+  EIGEN_DEVICE_FUNC
+  EIGEN_STRONG_INLINE iterator begin() { return values; }
+  EIGEN_DEVICE_FUNC
+  EIGEN_STRONG_INLINE const_iterator begin() const { return values; }
+
+  EIGEN_DEVICE_FUNC
+  EIGEN_STRONG_INLINE iterator end() { return values + n; }
+  EIGEN_DEVICE_FUNC
+  EIGEN_STRONG_INLINE const_iterator end() const { return values + n; }
+
+
+#if !defined(EIGEN_GPUCC)
+  typedef std::reverse_iterator<iterator> reverse_iterator;
+  typedef std::reverse_iterator<const_iterator> const_reverse_iterator;
+
+  EIGEN_DEVICE_FUNC
+  EIGEN_STRONG_INLINE reverse_iterator rbegin() { return reverse_iterator(end());}
+  EIGEN_DEVICE_FUNC
+  EIGEN_STRONG_INLINE const_reverse_iterator rbegin() const { return const_reverse_iterator(end()); }
+
+  EIGEN_DEVICE_FUNC
+  EIGEN_STRONG_INLINE reverse_iterator rend() { return reverse_iterator(begin()); }
+  EIGEN_DEVICE_FUNC
+  EIGEN_STRONG_INLINE const_reverse_iterator rend() const { return const_reverse_iterator(begin()); }
+#endif
+
   EIGEN_DEVICE_FUNC
   EIGEN_STRONG_INLINE T& operator[] (size_t index) { eigen_internal_assert(index < size()); return values[index]; }
   EIGEN_DEVICE_FUNC
@@ -42,15 +69,6 @@ template <typename T, size_t n> class array {
   EIGEN_DEVICE_FUNC
   EIGEN_STRONG_INLINE const T& back() const { return values[n-1]; }
 
-  EIGEN_DEVICE_FUNC
-  EIGEN_STRONG_INLINE iterator begin() { return values; }
-  EIGEN_DEVICE_FUNC
-  EIGEN_STRONG_INLINE const_iterator begin() const { return values; }
-
-  EIGEN_DEVICE_FUNC
-  EIGEN_STRONG_INLINE iterator end() { return values + n; }
-  EIGEN_DEVICE_FUNC
-  EIGEN_STRONG_INLINE const_iterator end() const { return values + n; }
 
   EIGEN_DEVICE_FUNC EIGEN_ALWAYS_INLINE
   static std::size_t size() { return n; }