AK: Add `CircularBuffer`

The class is very similar to `CircularDuplexStream` in its behavior.
Main differences are that `CircularBuffer`:
 - does not inherit from `AK::Stream`
 - uses `ErrorOr` for its API
 - is heap allocated (and OOM-Safe)

 This patch also add some tests.

1 files changed, 67 insertions, 0 deletions

diff --git a/AK/CircularBuffer.h b/AK/CircularBuffer.h
new file mode 100644
index 0000000000..0f121986f4
--- /dev/null
+++ b/AK/CircularBuffer.h
@@ -0,0 +1,67 @@
+/*
+ * Copyright (c) 2022, Lucas Chollet <lucas.chollet@free.fr>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <AK/ByteBuffer.h>
+#include <AK/Error.h>
+#include <AK/Noncopyable.h>
+
+namespace AK {
+
+class CircularBuffer {
+    AK_MAKE_NONCOPYABLE(CircularBuffer);
+
+public:
+    static ErrorOr<CircularBuffer> create_empty(size_t size);
+    static ErrorOr<CircularBuffer> create_initialized(ByteBuffer);
+
+    CircularBuffer(CircularBuffer&& other)
+    {
+        operator=(move(other));
+    }
+
+    CircularBuffer& operator=(CircularBuffer&& other)
+    {
+        if (&other == this)
+            return *this;
+
+        swap(m_buffer, other.m_buffer);
+        swap(m_reading_head, other.m_reading_head);
+        swap(m_used_space, other.m_used_space);
+
+        return *this;
+    }
+
+    ~CircularBuffer() = default;
+
+    size_t write(ReadonlyBytes bytes);
+    Bytes read(Bytes bytes);
+    ErrorOr<void> discard(size_t discarded_bytes);
+
+    [[nodiscard]] size_t empty_space() const;
+    [[nodiscard]] size_t used_space() const;
+    [[nodiscard]] size_t capacity() const;
+
+    Optional<size_t> offset_of(StringView needle, Optional<size_t> until = {}) const;
+
+    void clear();
+
+private:
+    CircularBuffer(ByteBuffer);
+
+    [[nodiscard]] bool is_wrapping_around() const;
+
+    [[nodiscard]] Bytes next_write_span();
+    [[nodiscard]] ReadonlyBytes next_read_span() const;
+
+    ByteBuffer m_buffer {};
+
+    size_t m_reading_head {};
+    size_t m_used_space {};
+};
+
+}