feat: add features to tflite::Span (#2664)

To tflite::Span, add constructors for arrays and std::arrays, equality
and inequality operators, and tests thereof.

BUG=see description

tensorflow/lite/micro/BUILD

diff --git a/tensorflow/lite/micro/BUILD b/tensorflow/lite/micro/BUILD
index 97d6897..1753465 100644
--- a/tensorflow/lite/micro/BUILD
+++ b/tensorflow/lite/micro/BUILD
@@ -569,6 +569,18 @@
 )
 
 cc_test(
+    name = "span_test",
+    size = "small",
+    srcs = [
+        "span_test.cc",
+    ],
+    deps = [
+        ":span",
+        "//tensorflow/lite/micro/testing:micro_test",
+    ],
+)
+
+cc_test(
     name = "testing_helpers_test",
     srcs = [
         "testing_helpers_test.cc",

tensorflow/lite/micro/span.h

diff --git a/tensorflow/lite/micro/span.h b/tensorflow/lite/micro/span.h
index eeb4c0b..9399f1d 100644
--- a/tensorflow/lite/micro/span.h
+++ b/tensorflow/lite/micro/span.h
@@ -16,6 +16,7 @@
 #ifndef TENSORFLOW_LITE_MICRO_SPAN_H_
 #define TENSORFLOW_LITE_MICRO_SPAN_H_
 
+#include <array>
 #include <cstddef>
 
 namespace tflite {
@@ -26,6 +27,13 @@
  public:
   constexpr Span(T* data, size_t size) noexcept : data_(data), size_(size) {}
 
+  template <size_t N>
+  constexpr Span(T (&data)[N]) noexcept : data_(data), size_(N) {}
+
+  template <size_t N>
+  constexpr Span(std::array<T, N>& array) noexcept
+      : data_(array.data()), size_(N) {}
+
   constexpr T& operator[](size_t idx) const noexcept { return *(data_ + idx); }
 
   constexpr T* data() const noexcept { return data_; }
@@ -36,6 +44,26 @@
   size_t size_;
 };
 
+template <typename A, typename B>
+bool operator==(const Span<A>& a, const Span<B>& b) {
+  if (a.size() != b.size()) {
+    return false;
+  }
+
+  for (size_t i = 0; i < a.size(); ++i) {
+    if (a[i] != b[i]) {
+      return false;
+    }
+  }
+
+  return true;
+}
+
+template <typename A, typename B>
+bool operator!=(const Span<A>& a, const Span<B>& b) {
+  return !(a == b);
+}
+
 }  // end namespace tflite
 
 #endif  // TENSORFLOW_LITE_MICRO_SPAN_H_

tensorflow/lite/micro/span_test.cc

diff --git a/tensorflow/lite/micro/span_test.cc b/tensorflow/lite/micro/span_test.cc
new file mode 100644
index 0000000..ef906c6
--- /dev/null
+++ b/tensorflow/lite/micro/span_test.cc
@@ -0,0 +1,59 @@
+// Copyright 2024 The TensorFlow Authors. 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.
+
+#include "tensorflow/lite/micro/span.h"
+
+#include <array>
+
+#include "tensorflow/lite/micro/testing/micro_test.h"
+
+TF_LITE_MICRO_TESTS_BEGIN
+
+TF_LITE_MICRO_TEST(TestArrayInitialization) {
+  int a[]{1, 2, 3};
+  tflite::Span<int> s{a};
+  TF_LITE_MICRO_EXPECT(s.data() == a);
+  TF_LITE_MICRO_EXPECT(s.size() == sizeof(a) / sizeof(int));
+}
+
+TF_LITE_MICRO_TEST(TestStdArrayInitialization) {
+  std::array<char, 20> a;
+  tflite::Span<char> s{a};
+  TF_LITE_MICRO_EXPECT(s.data() == a.data());
+  TF_LITE_MICRO_EXPECT(s.size() == a.size());
+}
+
+TF_LITE_MICRO_TEST(TestEquality) {
+  constexpr int a[]{1, 2, 3};
+  constexpr int b[]{1, 2, 3};
+  constexpr int c[]{3, 2, 1};
+  tflite::Span<const int> s_a{a};
+  tflite::Span<const int> s_b{b};
+  tflite::Span<const int> s_c{c};
+  TF_LITE_MICRO_EXPECT_TRUE(s_a == s_b);
+  TF_LITE_MICRO_EXPECT_FALSE(s_a == s_c);
+}
+
+TF_LITE_MICRO_TEST(TestInequality) {
+  constexpr int a[]{1, 2, 3};
+  constexpr int b[]{1, 2, 3};
+  constexpr int c[]{3, 2, 1};
+  tflite::Span<const int> s_a{a};
+  tflite::Span<const int> s_b{b};
+  tflite::Span<const int> s_c{c};
+  TF_LITE_MICRO_EXPECT_FALSE(s_a != s_b);
+  TF_LITE_MICRO_EXPECT_TRUE(s_a != s_c);
+}
+
+TF_LITE_MICRO_TESTS_END