draconisplusplus/include/rfl/parsing/FieldVariantParser.hpp

#ifndef RFL_PARSING_FIELD_VARIANT_PARSER_HPP_
#define RFL_PARSING_FIELD_VARIANT_PARSER_HPP_

#include <map>
#include <stdexcept>
#include <string>
#include <type_traits>
#include <variant>

#include "../Result.hpp"
#include "../always_false.hpp"
#include "FieldVariantReader.hpp"
#include "Parser_base.hpp"
#include "schema/Type.hpp"

namespace rfl {
  namespace parsing {

    /// To be used when all options of the variants are rfl::Field. Essentially,
    /// this is an externally tagged union.
    template <class R, class W, class ProcessorsType, class... FieldTypes>
      requires AreReaderAndWriter<R, W, std::variant<FieldTypes...>>
    struct FieldVariantParser {
      using FieldVariantType = std::variant<FieldTypes...>;
      using ResultType       = Result<FieldVariantType>;

     public:
      using InputObjectType = typename R::InputObjectType;
      using InputVarType    = typename R::InputVarType;

      using OutputObjectType = typename W::OutputObjectType;
      using OutputVarType    = typename W::OutputVarType;

      static ResultType read(const R& _r, const InputVarType& _var) noexcept {
        static_assert(
          internal::no_duplicate_field_names<std::tuple<FieldTypes...>>(),
          "Externally tagged variants cannot have duplicate field "
          "names."
        );

        const auto to_result = [&](const auto _obj) -> ResultType {
          auto       field_variant = std::optional<Result<FieldVariantType>>();
          const auto reader =
            FieldVariantReader<R, W, ProcessorsType, FieldTypes...>(&_r, &field_variant);
          auto err = _r.read_object(reader, _obj);
          if (err) {
            return *err;
          }
          if (!field_variant) {
            return Error(
              "Could not parse: Expected the object to have "
              "exactly one field, but found more than one."
            );
          }
          return std::move(*field_variant);
        };

        return _r.to_object(_var).and_then(to_result);
      }

      template <class P>
      static void
      write(const W& _w, const std::variant<FieldTypes...>& _v, const P& _parent) noexcept {
        static_assert(
          internal::no_duplicate_field_names<std::tuple<FieldTypes...>>(),
          "Externally tagged variants cannot have duplicate field "
          "names."
        );

        const auto handle = [&](const auto& _field) {
          const auto named_tuple = make_named_tuple(internal::to_ptr_field(_field));
          using NamedTupleType   = std::remove_cvref_t<decltype(named_tuple)>;
          Parser<R, W, NamedTupleType, ProcessorsType>::write(_w, named_tuple, _parent);
        };

        std::visit(handle, _v);
      }

      static schema::Type to_schema(
        std::map<std::string, schema::Type>* _definitions,
        std::vector<schema::Type>            _types = {}
      ) {
        using VariantType = std::variant<NamedTuple<FieldTypes>...>;
        return Parser<R, W, VariantType, ProcessorsType>::to_schema(_definitions);
      }
    };
  } // namespace parsing
} // namespace rfl

#endif