LibMarkdown: Add blockquote support to LineIterator

This patch adds contexts to line iterator for nesting list items and
blockquotes. It also incidentally makes the api for LineIterator
simpler, and will make it easier to add other containers in the future.

Userland/Libraries/LibMarkdown/LineIterator.cpp

@@ -4,38 +4,62 @@
  * SPDX-License-Identifier: BSD-2-Clause
  */
 
+#include <AK/Format.h>
 #include <LibMarkdown/LineIterator.h>
 
 namespace Markdown {
 
-bool LineIterator::is_indented(StringView const& line) const
+void LineIterator::reset_ignore_prefix()
 {
-    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;
+    for (auto& context : m_context_stack) {
+        context.ignore_prefix = false;
     }
+}
 
-    return true;
+Optional<StringView> LineIterator::match_context(StringView const& line) const
+{
+    bool is_ws = line.is_whitespace();
+    size_t offset = 0;
+    for (auto& context : m_context_stack) {
+        switch (context.type) {
+        case Context::Type::ListItem:
+            if (is_ws)
+                break;
+
+            if (offset + context.indent > line.length())
+                return {};
+
+            if (!context.ignore_prefix && !line.substring_view(offset, context.indent).is_whitespace())
+                return {};
+
+            offset += context.indent;
+
+            break;
+        case Context::Type::BlockQuote:
+            for (; offset < line.length() && line[offset] == ' '; ++offset) { }
+            if (offset >= line.length() || line[offset] != '>') {
+                return {};
+            }
+            ++offset;
+            break;
+        }
+
+        if (offset > line.length())
+            return {};
+    }
+    return line.substring_view(offset);
 }
 
 bool LineIterator::is_end() const
 {
-    return m_iterator.is_end() || (!m_ignore_prefix_mode && !is_indented(*m_iterator));
+    return m_iterator.is_end() || !match_context(*m_iterator).has_value();
 }
 
 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);
+    auto line = match_context(*m_iterator);
+    VERIFY(line.has_value());
+    return line.value();
 }
 
 }