AK: Add enqueue_begin() for the CircularDeque class (#1320)

Also add tests for CircularDeque.

AK/CircularDeque.h

@@ -35,6 +35,24 @@ namespace AK {
 template<typename T, size_t Capacity>
 class CircularDeque : public CircularQueue<T, Capacity> {
 public:
+    void enqueue_begin(T&& value)
+    {
+        const auto new_head = (this->m_head - 1 + Capacity) % Capacity;
+        auto& slot = this->elements()[new_head];
+        if (this->m_size == Capacity)
+            slot.~T();
+        else
+            ++this->m_size;
+
+        new (&slot) T(move(value));
+        this->m_head = new_head;
+    }
+
+    void enqueue_begin(const T& value)
+    {
+        enqueue_begin(T(value));
+    }
+
     T dequeue_end()
     {
         ASSERT(!this->is_empty());

AK/Tests/TestCircularDeque.cpp

@@ -0,0 +1,84 @@
+/*
+ * Copyright (c) 2018-2020, Fei Wu <f.eiwu@yahoo.com>
+ * All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions are met:
+ *
+ * 1. Redistributions of source code must retain the above copyright notice, this
+ *    list of conditions and the following disclaimer.
+ *
+ * 2. Redistributions in binary form must reproduce the above copyright notice,
+ *    this list of conditions and the following disclaimer in the documentation
+ *    and/or other materials provided with the distribution.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
+ * AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+ * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
+ * DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE
+ * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
+ * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
+ * SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
+ * CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
+ * OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+ * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+ */
+
+#include <AK/CircularDeque.h>
+#include <AK/StdLibExtras.h>
+#include <AK/String.h>
+#include <AK/TestSuite.h>
+
+TEST_CASE(enqueue_begin)
+{
+    CircularDeque<int, 3> ints;
+
+    ints.enqueue_begin(0);
+    EXPECT_EQ(ints.size(), 1);
+    EXPECT_EQ(ints.first(), 0);
+
+    ints.enqueue_begin(1);
+    EXPECT_EQ(ints.size(), 2);
+    EXPECT_EQ(ints.first(), 1);
+    EXPECT_EQ(ints.last(), 0);
+
+    ints.enqueue_begin(2);
+    EXPECT_EQ(ints.size(), 3);
+    EXPECT_EQ(ints.first(), 2);
+    EXPECT_EQ(ints.last(), 0);
+
+    ints.enqueue_begin(3);
+    EXPECT_EQ(ints.size(), 3);
+    EXPECT_EQ(ints.first(), 3);
+    EXPECT_EQ(ints.at(1), 2);
+    EXPECT_EQ(ints.last(), 1);
+}
+
+TEST_CASE(enqueue_begin_being_moved_from)
+{
+    CircularDeque<String, 2> strings;
+
+    String str{"test"};
+    strings.enqueue_begin(move(str));
+    EXPECT(str.is_null());
+}
+
+TEST_CASE(deque_end)
+{
+    CircularDeque<int, 3> ints;
+    ints.enqueue(0);
+    ints.enqueue(1);
+    ints.enqueue(2);
+    EXPECT_EQ(ints.size(), 3);
+
+    EXPECT_EQ(ints.dequeue_end(), 2);
+    EXPECT_EQ(ints.size(), 2);
+
+    EXPECT_EQ(ints.dequeue_end(), 1);
+    EXPECT_EQ(ints.size(), 1);
+
+    EXPECT_EQ(ints.dequeue_end(), 0);
+    EXPECT(ints.is_empty());
+}
+
+TEST_MAIN(CircularDeque)