Fix circular_queue pop_front(count) debug assert and empty()

fatal/container/circular_queue.h

@@ -259,7 +259,9 @@ class circular_queue {
     FATAL_ASSUME_LT(offset_, queue_.size());
 
     FATAL_ASSUME_LE(chunk, count);
-    FATAL_ASSUME_EQ(offset_ == 0, count == chunk);
+    // when there are more elements to destroy beyond the first chunk, the
+    // first chunk must have wrapped around the buffer (resetting offset_)
+    FATAL_ASSUME_IF(FATAL_GT(count, chunk), FATAL_IS_TRUE(offset_ == 0));
     for (auto const end = count - chunk; offset_ < end; ++offset_) {
       queue_[offset_].value.~value_type();
     }
@@ -475,13 +477,7 @@ class circular_queue {
 
   fatal::fast_pass<size_type> size() const noexcept { return size_; }
 
-  bool empty() const noexcept {
-    static_assert(
-      noexcept(queue_.empty()),
-      "underlying container must provide a noexcept empty()"
-    );
-    return queue_.empty();
-  }
+  bool empty() const noexcept { return size_ == 0; }
 
   using const_iterator = random_access_iterator<circular_queue, true>;
   using iterator = random_access_iterator<circular_queue, false>;

fatal/container/test/circular_queue_test.cpp

@@ -306,6 +306,69 @@ FATAL_TEST(circular_queue, shift_to_back_by) {
   }
 }
 
+FATAL_TEST(circular_queue, pop_front_count) {
+  circular_queue<int> q;
+  for (int i = 0; i < 10; ++i) {
+    q.push_back(i);
+  }
+  CHECK_CONTENTS(0, 1, 2, 3, 4, 5, 6, 7, 8, 9);
+
+  // popping a prefix that does not wrap around the buffer
+  q.pop_front(3);
+  CHECK_CONTENTS(3, 4, 5, 6, 7, 8, 9);
+
+  // popping zero elements is a no-op
+  q.pop_front(0);
+  CHECK_CONTENTS(3, 4, 5, 6, 7, 8, 9);
+
+  // popping the remainder drains the queue
+  q.pop_front(7);
+  CHECK_CONTENTS();
+
+  FATAL_EXPECT_TRUE(q.empty());
+}
+
+FATAL_TEST(circular_queue, pop_front_count_wrap) {
+  circular_queue<int> q;
+  for (int i = 0; i < 10; ++i) {
+    q.push_back(i);
+  }
+
+  // drain the queue so the elements wrap around the buffer on the next pushes
+  q.pop_front(10);
+  for (int i = 0; i < 8; ++i) {
+    q.push_back(i);
+  }
+  CHECK_CONTENTS(0, 1, 2, 3, 4, 5, 6, 7);
+
+  // the first chunk does not reach the end of the buffer
+  q.pop_front(5);
+  CHECK_CONTENTS(5, 6, 7);
+
+  // the first chunk reaches the end of the buffer and the remainder wraps
+  q.pop_front(3);
+  CHECK_CONTENTS();
+
+  FATAL_EXPECT_TRUE(q.empty());
+}
+
+FATAL_TEST(circular_queue, empty_after_drain) {
+  circular_queue<int> q;
+  FATAL_EXPECT_TRUE(q.empty());
+
+  for (int i = 0; i < 10; ++i) {
+    q.push_back(i);
+    FATAL_EXPECT_FALSE(q.empty());
+  }
+
+  while (!q.empty()) {
+    q.pop_front();
+  }
+
+  FATAL_EXPECT_EQ(0, q.size());
+  FATAL_EXPECT_TRUE(q.empty());
+}
+
 # undef CHECK_CONTENTS
 
 template <typename Data, typename Factory>