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+/* Copyright 2019 Google LLC. All Rights Reserved.
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+==============================================================================*/
+
+#ifndef TENSORFLOW_LITE_EXPERIMENTAL_RUY_RUY_SIDE_PAIR_H_
+#define TENSORFLOW_LITE_EXPERIMENTAL_RUY_RUY_SIDE_PAIR_H_
+
+#include "ruy/check_macros.h"
+
+namespace ruy {
+
+// Enumeration of the sides, i.e. the operands 'slots', in a matrix
+// multiplication. The numerical values of these enumeration constants matter
+// because these will be used as indices into the array underlying a SidePair.
+enum class Side {
+  // Left-hand side
+  kLhs = 0,
+  // Right-hand side
+  kRhs = 1
+};
+
+// SidePair is a pair container where the two elements are indexed by a Side
+// enum.
+template <typename T>
+class SidePair final {
+ public:
+  SidePair() {}
+  SidePair(const T& a, const T& b) : elem_{a, b} {}
+  const T& operator[](Side side) const {
+    const int index = static_cast<int>(side);
+    // Technically this check is vacuous, since other values would be
+    // out-of-range for enum Side.
+    RUY_DCHECK(index == 0 || index == 1);
+    return elem_[index];
+  }
+
+  T& operator[](Side side) {
+    const int index = static_cast<int>(side);
+    // Technically this check is vacuous, since other values would be
+    // out-of-range for enum Side.
+    RUY_DCHECK(index == 0 || index == 1);
+    return elem_[index];
+  }
+
+ private:
+  static_assert(static_cast<int>(Side::kLhs) == 0, "");
+  static_assert(static_cast<int>(Side::kRhs) == 1, "");
+  T elem_[2];
+};
+
+}  // namespace ruy
+
+#endif  // TENSORFLOW_LITE_EXPERIMENTAL_RUY_RUY_SIDE_PAIR_H_