Add `front` and `back` methods to RingBuffer

Test: Corresponding added RingBuffer tests pass
(`atest libinput_tests`)

Bug: 268245099
Change-Id: If330940e67e70d809748b2aaa6156bed835c6c9d

include/input/RingBuffer.h

@@ -103,6 +103,11 @@ public:
     iterator end() { return {*this, mSize}; }
     const_iterator end() const { return {*this, mSize}; }
 
+    reference front() { return mBuffer[mBegin]; }
+    const_reference front() const { return mBuffer[mBegin]; }
+    reference back() { return mBuffer[bufferIndex(mSize - 1)]; }
+    const_reference back() const { return mBuffer[bufferIndex(mSize - 1)]; }
+
     reference operator[](size_type i) { return mBuffer[bufferIndex(i)]; }
     const_reference operator[](size_type i) const { return mBuffer[bufferIndex(i)]; }
 

libs/input/tests/RingBuffer_test.cpp

@@ -118,6 +118,21 @@ TEST(RingBufferTest, Assignment) {
     EXPECT_EQ(0u, d.capacity());
 }
 
+TEST(RingBufferTest, FrontBackAccess) {
+    RingBuffer<int> buffer(/*capacity=*/2);
+    buffer.pushBack(1);
+    EXPECT_EQ(1, buffer.front());
+    EXPECT_EQ(1, buffer.back());
+
+    buffer.pushFront(0);
+    EXPECT_EQ(0, buffer.front());
+    EXPECT_EQ(1, buffer.back());
+
+    buffer.pushFront(-1);
+    EXPECT_EQ(-1, buffer.front());
+    EXPECT_EQ(0, buffer.back());
+}
+
 TEST(RingBufferTest, Subscripting) {
     RingBuffer<int> buffer(/*capacity=*/2);
     buffer.pushBack(1);