draconisplusplus/include/rfl/OneOf.hpp

#ifndef RFL_ONEOF_HPP_
#define RFL_ONEOF_HPP_

#include <string>
#include <utility>
#include <vector>

#include "Result.hpp"
#include "parsing/schema/ValidationType.hpp"

namespace rfl {

  /// Requires that all of the contraints C and Cs be true.
  template <class C, class... Cs>
  struct OneOf {
    template <class T>
    static rfl::Result<T> validate(const T& _value) noexcept {
      return validate_impl<T, C, Cs...>(_value, {});
    }

    template <class T>
    static parsing::schema::ValidationType to_schema() {
      using ValidationType = parsing::schema::ValidationType;
      const auto types     = std::vector<ValidationType>(
          {C::template to_schema<T>(), Cs::template to_schema<T>()...}
      );
      return ValidationType {ValidationType::OneOf {.types_ = types}};
    }

   private:
    static Error make_error_message(const std::vector<Error>& _errors) {
      std::string msg = "Expected exactly 1 out of " +
                        std::to_string(sizeof...(Cs) + 1) +
                        " validations to pass, but " +
                        std::to_string(sizeof...(Cs) + 1 - _errors.size()) +
                        " of them did. The following errors were generated: ";
      for (size_t i = 0; i < _errors.size(); ++i) {
        msg += "\n" + std::to_string(i + 1) + ") " + _errors.at(i).what();
      }
      return Error(msg);
    }

    template <class T, class Head, class... Tail>
    static rfl::Result<T>
    validate_impl(const T& _value, std::vector<Error> _errors) {
      const auto push_back = [&](Error&& _err) -> rfl::Result<T> {
        _errors.emplace_back(std::forward<Error>(_err));
        return _err;
      };

      const auto next_validation = [&](rfl::Result<T>&& _r) -> rfl::Result<T> {
        _r.or_else(push_back);

        if constexpr (sizeof...(Tail) == 0) {
          if (_errors.size() == sizeof...(Cs)) { return _value; }
          return make_error_message(_errors);
        } else {
          return validate_impl<T, Tail...>(
              _value, std::forward<std::vector<Error>>(_errors)
          );
        }
      };

      return Head::validate(_value)
          .and_then(next_validation)
          .or_else(next_validation);
    }
  };

} // namespace rfl

#endif