FixedSizeQueue: Bugfixes and improvements

- Fixed a bug where pushing items over queue's size left it in a corrupted state
- For non-trivial types, have clear() and pop() run destructors
- Added emplace(args...)
- Added empty()

FixedSizeQueue has semantics of a circular buffer,
so pushing items continuously is expected to keep overwriting oldest elements gracefully.

Tests have been updated to verify correctness of a previously bugged behaviour
and to verify correctness of destructing non-trivial types

Source/Core/Common/FixedSizeQueue.h

@@ -6,6 +6,7 @@
 
 #include <array>
 #include <cstddef>
+#include <type_traits>
 #include <utility>
 
 // STL-look-a-like interface, but name is mixed case to distinguish it clearly from the
@@ -19,6 +20,9 @@ class FixedSizeQueue
 public:
   void clear()
   {
+    if constexpr (!std::is_trivial_v<T>)
+      storage = {};
+
     head = 0;
     tail = 0;
     count = 0;
@@ -26,31 +30,47 @@ public:
 
   void push(T t)
   {
-    storage[tail] = std::move(t);
-    tail++;
-    if (tail == N)
-      tail = 0;
+    if (count == N)
+      head = (head + 1) % N;
+    else
       count++;
+
+    storage[tail] = std::move(t);
+    tail = (tail + 1) % N;
+  }
+
+  template <class... Args>
+  void emplace(Args&&... args)
+  {
+    if (count == N)
+      head = (head + 1) % N;
+    else
+      count++;
+
+    storage[tail] = T(std::forward<Args>(args)...);
+    tail = (tail + 1) % N;
   }
 
   void pop()
   {
-    head++;
-    if (head == N)
-      head = 0;
+    if constexpr (!std::is_trivial_v<T>)
+      storage[head] = {};
+
+    head = (head + 1) % N;
     count--;
   }
 
   T pop_front()
   {
-    T& temp = storage[head];
+    T temp = std::move(front());
     pop();
-    return std::move(temp);
+    return temp;
   }
 
-  T& front() { return storage[head]; }
-  const T& front() const { return storage[head]; }
-  size_t size() const { return count; }
+  T& front() noexcept { return storage[head]; }
+  const T& front() const noexcept { return storage[head]; }
+  size_t size() const noexcept { return count; }
+  bool empty() const noexcept { return size() == 0; }
 
 private:
   std::array<T, N> storage;

Source/UnitTests/Common/FixedSizeQueueTest.cpp

@@ -31,3 +31,85 @@ TEST(FixedSizeQueue, Simple)
 
   EXPECT_EQ(0u, q.size());
 }
+
+TEST(FixedSizeQueue, RingBuffer)
+{
+  // Testing if queue works when used as a ring buffer
+  FixedSizeQueue<int, 5> q;
+
+  EXPECT_EQ(0u, q.size());
+
+  q.push(0);
+  q.push(1);
+  q.push(2);
+  q.push(3);
+  q.push(4);
+  q.push(5);
+
+  EXPECT_EQ(5u, q.size());
+  EXPECT_EQ(1, q.pop_front());
+
+  EXPECT_EQ(4u, q.size());
+}
+
+// Local classes cannot have static fields,
+// therefore this has to be declared in global scope.
+class NonTrivialTypeTestData
+{
+public:
+  static inline int num_objects = 0;
+  static inline int total_constructed = 0;
+  static inline int default_constructed = 0;
+  static inline int total_destructed = 0;
+
+  NonTrivialTypeTestData()
+  {
+    num_objects++;
+    total_constructed++;
+    default_constructed++;
+  }
+
+  NonTrivialTypeTestData(int /*val*/)
+  {
+    num_objects++;
+    total_constructed++;
+  }
+
+  ~NonTrivialTypeTestData()
+  {
+    num_objects--;
+    total_destructed++;
+  }
+};
+
+TEST(FixedSizeQueue, NonTrivialTypes)
+{
+  // Testing if construction/destruction of non-trivial types happens as expected
+  FixedSizeQueue<NonTrivialTypeTestData, 2> q;
+
+  EXPECT_EQ(0u, q.size());
+
+  EXPECT_EQ(2, NonTrivialTypeTestData::num_objects);
+  EXPECT_EQ(2, NonTrivialTypeTestData::total_constructed);
+  EXPECT_EQ(2, NonTrivialTypeTestData::default_constructed);
+  EXPECT_EQ(0, NonTrivialTypeTestData::total_destructed);
+
+  q.emplace(4);
+  q.emplace(6);
+  q.emplace(8);
+
+  EXPECT_EQ(2, NonTrivialTypeTestData::num_objects);
+  EXPECT_EQ(2 + 3, NonTrivialTypeTestData::total_constructed);
+  EXPECT_EQ(2, NonTrivialTypeTestData::default_constructed);
+  EXPECT_EQ(3, NonTrivialTypeTestData::total_destructed);
+  EXPECT_EQ(2u, q.size());
+
+  q.pop();
+  q.pop();
+
+  EXPECT_EQ(2, NonTrivialTypeTestData::num_objects);
+  EXPECT_EQ(2 + 3 + 2, NonTrivialTypeTestData::total_constructed);
+  EXPECT_EQ(2 + 2, NonTrivialTypeTestData::default_constructed);
+  EXPECT_EQ(3 + 2, NonTrivialTypeTestData::total_destructed);
+  EXPECT_EQ(0u, q.size());
+}