LibMarkdown: Add LineIterator

LineIterator wraps a vector's ConstIterator, to provide an iterator that
can work on indented container blocks (like lists and blockquotes).

Userland/Libraries/LibMarkdown/LineIterator.h

@@ -0,0 +1,75 @@
+/*
+ * Copyright (c) 2021, Peter Elliott <pelliott@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <AK/Iterator.h>
+#include <AK/StringView.h>
+#include <AK/Vector.h>
+
+namespace Markdown {
+
+template<typename T>
+class FakePtr {
+public:
+    FakePtr(T item)
+        : m_item(move(item))
+    {
+    }
+
+    T const* operator->() const { return &m_item; }
+    T* operator->() { return &m_item; }
+
+private:
+    T m_item;
+};
+
+class LineIterator {
+public:
+    LineIterator(Vector<StringView>::ConstIterator const& lines, size_t indent = 0)
+        : m_iterator(lines)
+        , m_indent(indent)
+    {
+    }
+
+    bool is_end() const;
+    StringView operator*() const;
+
+    LineIterator operator++()
+    {
+        reset_ignore_prefix();
+        ++m_iterator;
+        return *this;
+    }
+
+    LineIterator operator++(int)
+    {
+        LineIterator tmp = *this;
+        reset_ignore_prefix();
+        ++m_iterator;
+        return tmp;
+    }
+
+    LineIterator operator+(ptrdiff_t delta) const { return LineIterator { m_iterator + delta, m_indent }; }
+    LineIterator operator-(ptrdiff_t delta) const { return LineIterator { m_iterator - delta, m_indent }; }
+    ptrdiff_t operator-(LineIterator other) const { return m_iterator - other.m_iterator; }
+
+    FakePtr<StringView> operator->() const { return FakePtr<StringView>(operator*()); }
+
+    size_t indent() const { return m_indent; }
+    void set_indent(size_t indent) { m_indent = indent; }
+    void ignore_next_prefix() { m_ignore_prefix_mode = true; }
+
+private:
+    void reset_ignore_prefix() { m_ignore_prefix_mode = false; }
+    bool is_indented(StringView const& line) const;
+
+    Vector<StringView>::ConstIterator m_iterator;
+    size_t m_indent;
+    bool m_ignore_prefix_mode { false };
+};
+
+}