libcamera: value_node: Support looking up descendant node by path

Looking up a descendant node based on a path is a common operation. Add
a helper function to do so, to avoid loops in the callers.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Barnabás Pőcze <barnabas.pocze@ideasonboard.com>
Reviewed-by: Isaac Scott <isaac.scott@ideasonboard.com>

include/libcamera/internal/value_node.h

@@ -237,6 +237,7 @@ public:
 	bool contains(std::string_view key) const;
 	ValueNode *at(std::string_view key);
 	const ValueNode &operator[](std::string_view key) const;
+	const ValueNode &operator[](std::initializer_list<std::string_view> path) const;
 
 	ValueNode *add(std::unique_ptr<ValueNode> &&child);
 	ValueNode *add(std::string key, std::unique_ptr<ValueNode> &&child);

src/libcamera/value_node.cpp

@@ -490,6 +490,29 @@ const ValueNode &ValueNode::operator[](std::string_view key) const
 	return *iter->second;
 }
 
+/**
+ * \brief Retrieve a descendant node by path
+ * \param[in] path The path
+ *
+ * This function retrieves a descendant of a ValueNode by following a \a path.
+ * The path is a list of keys that index nested dictionary nodes. If any node
+ * along the path is not a Dictionary node, an empty node is returned.
+ *
+ * \return The ValueNode corresponding to the \a path
+ */
+const ValueNode &ValueNode::operator[](std::initializer_list<std::string_view> path) const
+{
+	const ValueNode *node = this;
+
+	for (const auto &part : path) {
+		node = &(*node)[part];
+		if (!*node)
+			return empty;
+	}
+
+	return *node;
+}
+
 /**
  * \brief Add a child node to a list
  * \param[in] child The child node