[WIP] Implement grammar-aware YAML stringifying

src/core/yaml/CMakeLists.txt

@@ -1,11 +1,12 @@
 sourcemeta_library(NAMESPACE sourcemeta PROJECT core NAME yaml
-  PRIVATE_HEADERS error.h
-  SOURCES yaml.cc lexer.h parser.h stringify.h)
+  PRIVATE_HEADERS error.h roundtrip.h
+  SOURCES yaml.cc roundtrip.cc lexer.h parser.h stringify.h)
 
 if(SOURCEMETA_CORE_INSTALL)
   sourcemeta_library_install(NAMESPACE sourcemeta PROJECT core NAME yaml)
 endif()
 
 target_link_libraries(sourcemeta_core_yaml PUBLIC sourcemeta::core::json)
+target_link_libraries(sourcemeta_core_yaml PUBLIC sourcemeta::core::jsonpointer)
 target_link_libraries(sourcemeta_core_yaml PRIVATE sourcemeta::core::io)
 target_link_libraries(sourcemeta_core_yaml PRIVATE sourcemeta::core::unicode)

src/core/yaml/include/sourcemeta/core/yaml.h

@@ -9,6 +9,7 @@
 
 // NOLINTBEGIN(misc-include-cleaner)
 #include <sourcemeta/core/yaml_error.h>
+#include <sourcemeta/core/yaml_roundtrip.h>
 // NOLINTEND(misc-include-cleaner)
 
 #include <filesystem> // std::filesystem
@@ -110,6 +111,46 @@ SOURCEMETA_CORE_YAML_EXPORT
 auto read_yaml_or_json(const std::filesystem::path &path,
                        const JSON::ParseCallback &callback = nullptr) -> JSON;
 
+/// @ingroup yaml
+///
+/// Create a JSON document from a YAML string, collecting round-trip metadata
+/// to reproduce the original formatting. For example:
+///
+/// ```cpp
+/// #include <sourcemeta/core/json.h>
+/// #include <sourcemeta/core/yaml.h>
+///
+/// sourcemeta::core::YAMLRoundTrip roundtrip;
+/// const std::string input{"hello: world"};
+/// const sourcemeta::core::JSON document =
+///   sourcemeta::core::parse_yaml(input, roundtrip);
+/// ```
+SOURCEMETA_CORE_YAML_EXPORT
+auto parse_yaml(const JSON::String &input, YAMLRoundTrip &roundtrip,
+                const JSON::ParseCallback &callback = nullptr) -> JSON;
+
+/// @ingroup yaml
+///
+/// Stringify a JSON document as YAML, using round-trip metadata collected
+/// during parsing to preserve the original formatting. For example:
+///
+/// ```cpp
+/// #include <sourcemeta/core/json.h>
+/// #include <sourcemeta/core/yaml.h>
+///
+/// #include <iostream>
+///
+/// sourcemeta::core::YAMLRoundTrip roundtrip;
+/// const std::string input{"hello: world"};
+/// const sourcemeta::core::JSON document =
+///   sourcemeta::core::parse_yaml(input, roundtrip);
+/// sourcemeta::core::stringify_yaml(document, std::cout, roundtrip);
+/// ```
+SOURCEMETA_CORE_YAML_EXPORT
+auto stringify_yaml(const JSON &document,
+                    std::basic_ostream<JSON::Char, JSON::CharTraits> &stream,
+                    const YAMLRoundTrip &roundtrip) -> void;
+
 /// @ingroup yaml
 ///
 /// Stringify a JSON document as YAML. For example:

src/core/yaml/include/sourcemeta/core/yaml_roundtrip.h

@@ -0,0 +1,68 @@
+#ifndef SOURCEMETA_CORE_YAML_ROUNDTRIP_H_
+#define SOURCEMETA_CORE_YAML_ROUNDTRIP_H_
+
+#ifndef SOURCEMETA_CORE_YAML_EXPORT
+#include <sourcemeta/core/yaml_export.h>
+#endif
+
+#include <sourcemeta/core/json.h>
+#include <sourcemeta/core/jsonpointer.h>
+
+#include <cstdint>  // std::uint8_t, std::size_t
+#include <map>      // std::map
+#include <optional> // std::optional
+#include <string>   // std::string
+#include <vector>   // std::vector
+
+namespace sourcemeta::core {
+
+/// @ingroup yaml
+enum class YAMLScalarStyle : std::uint8_t {
+  Plain,
+  SingleQuoted,
+  DoubleQuoted,
+  Literal,
+  Folded
+};
+
+/// @ingroup yaml
+enum class YAMLCollectionStyle : std::uint8_t { Block, Flow };
+
+/// @ingroup yaml
+enum class YAMLChomping : std::uint8_t { Clip, Strip, Keep };
+
+/// @ingroup yaml
+struct SOURCEMETA_CORE_YAML_EXPORT YAMLNodeStyle {
+  std::optional<YAMLScalarStyle> scalar;
+  std::optional<YAMLCollectionStyle> collection;
+  std::optional<YAMLChomping> chomping;
+  std::optional<std::string> block_content;
+  std::optional<std::string> anchor;
+  std::vector<std::string> comments_before;
+  std::optional<std::string> comment_inline;
+  std::optional<std::string> comment_on_indicator;
+};
+
+/// @ingroup yaml
+/// Holds per-node metadata collected during YAML parsing to reproduce the
+/// original formatting
+class SOURCEMETA_CORE_YAML_EXPORT YAMLRoundTrip {
+public:
+  auto clear() -> void;
+  std::map<Pointer, YAMLNodeStyle> styles;
+  std::map<Pointer, std::string> aliases;
+  std::map<Pointer, YAMLScalarStyle> key_styles;
+  bool explicit_document_start{false};
+  bool explicit_document_end{false};
+  std::optional<std::string> document_start_comment;
+  std::optional<std::string> document_end_comment;
+  std::vector<std::string> leading_comments;
+  std::vector<std::string> post_start_comments;
+  std::vector<std::string> pre_end_comments;
+  std::vector<std::string> trailing_comments;
+  std::size_t indent_width{2};
+};
+
+} // namespace sourcemeta::core
+
+#endif