libcamera: yaml_parser: Add iterator API

Allow using range-based for loops over YamlObject instances by
implementing iterators. New YamlObject::DictAdapter and
YamlObject::ListAdapter adapter classes are introduced to provide
different iterators depending on the object type.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Jacopo Mondi <jacopo@jmondi.org>
Reviewed-by: Han-Lin Chen <hanlinchen@chromium.org>

include/libcamera/internal/yaml_parser.h

@@ -7,6 +7,7 @@
 
 #pragma once
 
+#include <iterator>
 #include <map>
 #include <string>
 #include <vector>
@@ -22,7 +23,116 @@ class YamlParserContext;
 
 class YamlObject
 {
+private:
+	using DictContainer = std::map<std::string, std::unique_ptr<YamlObject>>;
+	using ListContainer = std::vector<std::unique_ptr<YamlObject>>;
+
 public:
+#ifndef __DOXYGEN__
+	template<typename Container, typename Derived>
+	class Iterator
+	{
+	public:
+		using difference_type = std::ptrdiff_t;
+		using iterator_category = std::forward_iterator_tag;
+
+		Iterator(typename Container::const_iterator it)
+			: it_(it)
+		{
+		}
+
+		Derived &operator++()
+		{
+			++it_;
+			return *static_cast<Derived *>(this);
+		}
+
+		Derived operator++(int)
+		{
+			Derived it = *static_cast<Derived *>(this);
+			it_++;
+			return it;
+		}
+
+		friend bool operator==(const Iterator &a, const Iterator &b)
+		{
+			return a.it_ == b.it_;
+		}
+
+		friend bool operator!=(const Iterator &a, const Iterator &b)
+		{
+			return a.it_ != b.it_;
+		}
+
+	protected:
+		typename Container::const_iterator it_;
+	};
+
+	template<typename Container, typename Iterator>
+	class Adapter
+	{
+	public:
+		Adapter(const Container &container)
+			: container_(container)
+		{
+		}
+
+		Iterator begin() const
+		{
+			return Iterator{ container_.begin() };
+		}
+
+		Iterator end() const
+		{
+			return Iterator{ container_.end() };
+		}
+
+	protected:
+		const Container &container_;
+	};
+
+	class ListIterator : public Iterator<ListContainer, ListIterator>
+	{
+	public:
+		using value_type = const YamlObject &;
+		using pointer = const YamlObject *;
+		using reference = value_type;
+
+		value_type operator*() const
+		{
+			return *it_->get();
+		}
+
+		pointer operator->() const
+		{
+			return it_->get();
+		}
+	};
+
+	class DictIterator : public Iterator<DictContainer, DictIterator>
+	{
+	public:
+		using value_type = std::pair<const std::string &, const YamlObject &>;
+		using pointer = value_type *;
+		using reference = value_type &;
+
+		value_type operator*() const
+		{
+			return { it_->first, *it_->second.get() };
+		}
+	};
+
+	class DictAdapter : public Adapter<DictContainer, DictIterator>
+	{
+	public:
+		using key_type = std::string;
+	};
+
+	class ListAdapter : public Adapter<ListContainer, ListIterator>
+	{
+	};
+#endif /* __DOXYGEN__ */
+
 	YamlObject();
 	~YamlObject();
 
@@ -55,6 +165,9 @@ public:
 #endif
 	T get(const T &defaultValue, bool *ok = nullptr) const;
 
+	DictAdapter asDict() const { return DictAdapter{ dictionary_ }; }
+	ListAdapter asList() const { return ListAdapter{ list_ }; }
+
 	const YamlObject &operator[](std::size_t index) const;
 
 	bool contains(const std::string &key) const;
@@ -75,8 +188,8 @@ private:
 	Type type_;
 
 	std::string value_;
-	std::vector<std::unique_ptr<YamlObject>> list_;
-	std::map<const std::string, std::unique_ptr<YamlObject>> dictionary_;
+	ListContainer list_;
+	DictContainer dictionary_;
 };
 
 class YamlParser final