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.cpp

@@ -0,0 +1,41 @@
+/*
+ * Copyright (c) 2021, Peter Elliott <pelliott@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include <LibMarkdown/LineIterator.h>
+
+namespace Markdown {
+
+bool LineIterator::is_indented(StringView const& line) const
+{
+    if (line.is_whitespace())
+        return true;
+
+    if (line.length() < m_indent)
+        return false;
+
+    for (size_t i = 0; i < m_indent; ++i) {
+        if (line[i] != ' ')
+            return false;
+    }
+
+    return true;
+}
+
+bool LineIterator::is_end() const
+{
+    return m_iterator.is_end() || (!m_ignore_prefix_mode && !is_indented(*m_iterator));
+}
+
+StringView LineIterator::operator*() const
+{
+    VERIFY(m_ignore_prefix_mode || is_indented(*m_iterator));
+    if (m_iterator->is_whitespace())
+        return *m_iterator;
+
+    return m_iterator->substring_view(m_indent);
+}
+
+}