Ring buffer: Add index operator and tests (#99)

Adds a new [] operator to access data at a specified index.

trellis/containers/ring_buffer.hpp

@@ -51,6 +51,8 @@ class RingBuffer {
     }
   }
 
+  const T& operator[](size_t index) const { return data_[(begin_ + index) % kArraySize]; }
+
   class ConstIterator {
    public:
     using iterator_category = std::random_access_iterator_tag;

trellis/containers/test/ring_buffer_test.cpp

@@ -60,4 +60,22 @@ TEST(RingBuffer, PopFront) {
   ASSERT_THAT(ring, IsEmpty());
 }
 
+TEST(RingBuffer, IndexOperator) {
+  auto ring = RingBuffer<int, 5>{};
+  ring.push_back(1);
+  ring.push_back(2);
+  ring.push_back(3);
+
+  // Test non-const access
+  EXPECT_EQ(ring[0], 1);
+  EXPECT_EQ(ring[1], 2);
+  EXPECT_EQ(ring[2], 3);
+
+  // Test const access
+  const auto& const_ring = ring;
+  EXPECT_EQ(const_ring[0], 1);
+  EXPECT_EQ(const_ring[1], 2);
+  EXPECT_EQ(const_ring[2], 3);
+}
+
 }  // namespace trellis::containers