[libcamera-devel] [RFC PATCH v2 06/14] libcamera: yaml_parser: Preserve order of items in dictionary

[Laurent Pinchart]

YAML dictionaries are ordered, but the std::map container used to store
dictionary items in YamlObject doesn't preserve that order as maps are
ordered by key, not by insertion order.

As the order of items in the YAML file may be important, preserve it by
storing items in list_ unconditionally. Turn the list_ vector from
storing YamlObject unique pointers to storing key-value pairs, with the
key being absent when the object is a list, not a dictionary.

The YamlObject implementation is updated to preserve the existing API,
with the only difference being that YamlObject::memberNames() now
returns member names in the same order as in the YAML file.

Signed-off-by: Laurent Pinchart <laurent.pinchart at ideasonboard.com>
---
 include/libcamera/internal/yaml_parser.h | 13 +++++++--
 src/libcamera/yaml_parser.cpp            | 37 +++++++++++++++---------
 2 files changed, 35 insertions(+), 15 deletions(-)

diff --git a/include/libcamera/internal/yaml_parser.h b/include/libcamera/internal/yaml_parser.h
index be5f0914703f..bd1d06d5c488 100644
--- a/include/libcamera/internal/yaml_parser.h
+++ b/include/libcamera/internal/yaml_parser.h
@@ -72,11 +72,20 @@ private:
 		Value,
 	};
 
+	struct Value {
+		Value(std::string &&k, std::unique_ptr<YamlObject> &&v)
+			: key(std::move(k)), value(std::move(v))
+		{
+		}
+		std::string key;
+		std::unique_ptr<YamlObject> value;
+	};
+
 	Type type_;
 
 	std::string value_;
-	std::vector<std::unique_ptr<YamlObject>> list_;
-	std::map<const std::string, std::unique_ptr<YamlObject>> dictionary_;
+	std::vector<Value> list_;
+	std::map<std::string, YamlObject *> dictionary_;
 };
 
 class YamlParser final
diff --git a/src/libcamera/yaml_parser.cpp b/src/libcamera/yaml_parser.cpp
index 85f6694f5fde..83813e2a8552 100644
--- a/src/libcamera/yaml_parser.cpp
+++ b/src/libcamera/yaml_parser.cpp
@@ -90,7 +90,6 @@ std::size_t YamlObject::size() const
 {
 	switch (type_) {
 	case Type::Dictionary:
-		return dictionary_.size();
 	case Type::List:
 		return list_.size();
 	default:
@@ -245,11 +244,11 @@ Size YamlObject::get(const Size &defaultValue, bool *ok) const
 	 * that ok == nullptr.
 	 */
 	bool valid;
-	uint32_t width = list_[0]->get<uint32_t>(0, &valid);
+	uint32_t width = list_[0].value->get<uint32_t>(0, &valid);
 	if (!valid)
 		return defaultValue;
 
-	uint32_t height = list_[1]->get<uint32_t>(0, &valid);
+	uint32_t height = list_[1].value->get<uint32_t>(0, &valid);
 	if (!valid)
 		return defaultValue;
 
@@ -274,7 +273,7 @@ const YamlObject &YamlObject::operator[](std::size_t index) const
 	if (type_ != Type::List || index >= size())
 		return empty;
 
-	return *list_[index];
+	return *list_[index].value;
 }
 
 /**
@@ -290,7 +289,7 @@ const YamlObject &YamlObject::operator[](std::size_t index) const
  */
 bool YamlObject::contains(const std::string &key) const
 {
-	if (dictionary_.find(key) == dictionary_.end())
+	if (dictionary_.find(std::ref(key)) == dictionary_.end())
 		return false;
 
 	return true;
@@ -312,7 +311,7 @@ bool YamlObject::contains(const std::string &key) const
 std::vector<std::string> YamlObject::memberNames() const
 {
 	std::vector<std::string> memberNames;
-	for (auto &[key, _] : dictionary_)
+	for (auto &[key, _] : list_)
 		memberNames.push_back(key);
 
 	return memberNames;
@@ -584,16 +583,16 @@ int YamlParserContext::parseNextYamlObject(YamlObject &yamlObject, EventPtr even
 		yamlObject.type_ = YamlObject::Type::List;
 		auto &list = yamlObject.list_;
 		auto handler = [this, &list](EventPtr evt) {
-			list.emplace_back(new YamlObject());
-			return parseNextYamlObject(*list.back(), std::move(evt));
+			list.emplace_back(std::string{}, std::make_unique<YamlObject>());
+			return parseNextYamlObject(*list.back().value, std::move(evt));
 		};
 		return parseDictionaryOrList(YamlObject::Type::List, handler);
 	}
 
 	case YAML_MAPPING_START_EVENT: {
 		yamlObject.type_ = YamlObject::Type::Dictionary;
-		auto &dictionary = yamlObject.dictionary_;
-		auto handler = [this, &dictionary](EventPtr evtKey) {
+		auto &list = yamlObject.list_;
+		auto handler = [this, &list](EventPtr evtKey) {
 			/* Parse key */
 			if (evtKey->type != YAML_SCALAR_EVENT) {
 				LOG(YamlParser, Error) << "Expect key at line: "
@@ -611,10 +610,19 @@ int YamlParserContext::parseNextYamlObject(YamlObject &yamlObject, EventPtr even
 			if (!evtValue)
 				return -EINVAL;
 
-			auto elem = dictionary.emplace(key, std::make_unique<YamlObject>());
-			return parseNextYamlObject(*elem.first->second.get(), std::move(evtValue));
+			auto &elem = list.emplace_back(std::move(key),
+						       std::make_unique<YamlObject>());
+			return parseNextYamlObject(*elem.value, std::move(evtValue));
 		};
-		return parseDictionaryOrList(YamlObject::Type::Dictionary, handler);
+		int ret = parseDictionaryOrList(YamlObject::Type::Dictionary, handler);
+		if (ret)
+			return ret;
+
+		auto &dictionary = yamlObject.dictionary_;
+		for (const auto &elem : list)
+			dictionary.emplace(elem.key, elem.value.get());
+
+		return 0;
 	}
 
 	default:
@@ -670,6 +678,9 @@ int YamlParserContext::parseNextYamlObject(YamlObject &yamlObject, EventPtr even
  * The YamlParser::parse() function takes an open FILE, parses its contents, and
  * returns a pointer to a YamlObject corresponding to the root node of the YAML
  * document.
+ *
+ * The parser preserves the order of items in the YAML file, for both lists and
+ * dictionaries.
  */
 
 /**
-- 
Regards,

Laurent Pinchart