draconisplusplus/include/rfl/parsing/FieldVariantReader.hpp

#ifndef RFL_PARSING_FIELDVARIANTREADER_HPP_
#define RFL_PARSING_FIELDVARIANTREADER_HPP_

#include <array>
#include <optional>
#include <string_view>
#include <type_traits>
#include <variant>
#include <vector>

#include "../Result.hpp"
#include "../internal/is_array.hpp"

namespace rfl::parsing {

  template <class R, class W, class ProcessorsType, class... FieldTypes>
  class FieldVariantReader {
   private:
    using InputVarType     = typename R::InputVarType;
    using FieldVariantType = std::variant<FieldTypes...>;
    using ResultType       = Result<FieldVariantType>;

   public:
    FieldVariantReader(const R* _r, std::optional<Result<FieldVariantType>>* _field_variant)
      : r_(_r), field_variant_(_field_variant) {}

    ~FieldVariantReader() = default;

    template <size_t _i = 0>
    void read(const std::string_view& _disc_value, const InputVarType& _var) const noexcept {
      if constexpr (_i < sizeof...(FieldTypes)) {
        if constexpr (_i == 0) {
          if (*field_variant_) {
            *field_variant_ = Error(
              "Could not parse: Expected the object to have "
              "exactly one field, but found more than one."
            );
            return;
          }
        }

        using FieldType =
          std::remove_cvref_t<typename std::tuple_element<_i, std::tuple<FieldTypes...>>::type>;

        using ValueType = std::remove_cvref_t<typename FieldType::Type>;

        constexpr auto key = FieldType::name_.string_view();

        if (key == _disc_value) {
          const auto to_variant = [](ValueType&& _val) {
            return std::variant<FieldTypes...>(FieldType(std::move(_val)));
          };
          const auto embellish_error = [&](const Error& _e) {
            return Error(
              "Could not parse std::variant with field '" + std::string(_disc_value) +
              "': " + _e.what()
            );
          };
          *field_variant_ = Parser<R, W, ValueType, ProcessorsType>::read(*r_, _var)
                              .transform(to_variant)
                              .or_else(embellish_error);
          return;
        } else {
          read<_i + 1>(_disc_value, _var);
        }
      } else {
        *field_variant_ = Error(
          "Could not parse std::variant, could not match field named "
          "'" +
          std::string(_disc_value) + "'."
        );
      }
    }

   private:
    /// The underlying reader.
    const R* r_;

    /// The underlying field variant.
    std::optional<Result<FieldVariantType>>* field_variant_;
  };

} // namespace rfl::parsing

#endif