AK: Add generic reverse iterator for containers

Author: https://github.com/guerinoni
Commit: https://github.com/SerenityOS/serenity/commit/74650b4e32
Pull-request: https://github.com/SerenityOS/serenity/pull/12741
Reviewed-by: https://github.com/IdanHo
Reviewed-by: https://github.com/awesomekling
Reviewed-by: https://github.com/linusg

AK/ReverseIterator.h

@@ -0,0 +1,90 @@
+/*
+ * Copyright (c) 2022, the SerenityOS developers.
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <AK/Forward.h>
+
+namespace AK {
+
+template<typename Container, typename ValueType>
+class SimpleReverseIterator {
+public:
+    friend Container;
+
+    constexpr bool is_end() const { return m_index == SimpleReverseIterator::rend(m_container).m_index; }
+    constexpr int index() const { return m_index; }
+
+    constexpr bool operator==(SimpleReverseIterator other) const { return m_index == other.m_index; }
+    constexpr bool operator!=(SimpleReverseIterator other) const { return m_index != other.m_index; }
+    constexpr bool operator<(SimpleReverseIterator other) const { return m_index < other.m_index; }
+    constexpr bool operator>(SimpleReverseIterator other) const { return m_index > other.m_index; }
+    constexpr bool operator<=(SimpleReverseIterator other) const { return m_index <= other.m_index; }
+    constexpr bool operator>=(SimpleReverseIterator other) const { return m_index >= other.m_index; }
+
+    constexpr SimpleReverseIterator operator+(int delta) const { return SimpleReverseIterator { m_container, m_index - delta }; }
+    constexpr SimpleReverseIterator operator-(int delta) const { return SimpleReverseIterator { m_container, m_index + delta }; }
+
+    constexpr SimpleReverseIterator operator++()
+    {
+        --m_index;
+        return *this;
+    }
+    constexpr SimpleReverseIterator operator++(int)
+    {
+        --m_index;
+        return SimpleReverseIterator { m_container, m_index + 1 };
+    }
+    constexpr SimpleReverseIterator operator--()
+    {
+        ++m_index;
+        return *this;
+    }
+    constexpr SimpleReverseIterator operator--(int)
+    {
+        ++m_index;
+        return SimpleReverseIterator { m_container, m_index - 1 };
+    }
+
+    ALWAYS_INLINE constexpr ValueType const& operator*() const { return m_container[m_index]; }
+    ALWAYS_INLINE constexpr ValueType& operator*() { return m_container[m_index]; }
+
+    ALWAYS_INLINE constexpr ValueType const* operator->() const { return &m_container[m_index]; }
+    ALWAYS_INLINE constexpr ValueType* operator->() { return &m_container[m_index]; }
+
+    SimpleReverseIterator& operator=(const SimpleReverseIterator& other)
+    {
+        m_index = other.m_index;
+        return *this;
+    }
+    SimpleReverseIterator(SimpleReverseIterator const& obj) = default;
+
+private:
+    static constexpr SimpleReverseIterator rbegin(Container& container)
+    {
+        using RawContainerType = RemoveCV<Container>;
+        if constexpr (IsSame<StringView, RawContainerType> || IsSame<String, RawContainerType>)
+            return { container, static_cast<int>(container.length()) - 1 };
+        else
+            return { container, static_cast<int>(container.size()) - 1 };
+    }
+
+    static constexpr SimpleReverseIterator rend(Container& container)
+    {
+        return { container, -1 };
+    }
+
+    constexpr SimpleReverseIterator(Container& container, int index)
+        : m_container(container)
+        , m_index(index)
+    {
+    }
+
+    Container& m_container;
+    int m_index { 0 };
+};
+
+}