draconisplusplus/include/rfl/internal/remove_fields.hpp

#ifndef RFL_INTERNAL_REMOVEFIELDS_HPP_
#define RFL_INTERNAL_REMOVEFIELDS_HPP_

#include <algorithm>
#include <tuple>
#include <type_traits>

#include "../NamedTuple.hpp"
#include "../define_named_tuple.hpp"
#include "StringLiteral.hpp"

namespace rfl {
  namespace internal {

    /// Recursively builds a new NamedTuple type from the FieldTypes, leaving
    /// out the field signified by _name.
    template <class _OldNamedTupleType, StringLiteral _name, class _NewNamedTupleType, int _i>
    struct remove_single_field;

    /// Special case - _i == 0
    template <class _OldNamedTupleType, StringLiteral _name, class _NewNamedTupleType>
    struct remove_single_field<_OldNamedTupleType, _name, _NewNamedTupleType, 0> {
      using type = _NewNamedTupleType;
    };

    /// General case.
    template <class _OldNamedTupleType, StringLiteral _name, class _NewNamedTupleType, int _i>
    struct remove_single_field {
      using OldNamedTupleType = std::remove_cvref_t<_OldNamedTupleType>;

      constexpr static int num_fields = std::tuple_size_v<typename OldNamedTupleType::Fields>;

      using FieldType = std::remove_cvref_t<
        typename std::tuple_element<num_fields - _i, typename OldNamedTupleType::Fields>::type>;

      using NewNamedTupleType = std::conditional_t<
        _name == FieldType::name_,
        _NewNamedTupleType,
        define_named_tuple_t<_NewNamedTupleType, FieldType>>;

      using type =
        typename remove_single_field<OldNamedTupleType, _name, NewNamedTupleType, _i - 1>::type;
    };

    /// Recursively removes all of the fields signified by _head and _tail from
    /// the NamedTupleType.
    template <class _NamedTupleType, StringLiteral _head, StringLiteral... _tail>
    struct remove_fields;

    /// Special case - only head is left.
    template <class _NamedTupleType, StringLiteral _head>
    struct remove_fields<_NamedTupleType, _head> {
      using NamedTupleType = std::remove_cvref_t<_NamedTupleType>;

      constexpr static int num_fields = std::tuple_size_v<typename NamedTupleType::Fields>;

      using type =
        typename remove_single_field<NamedTupleType, _head, NamedTuple<>, num_fields>::type;
    };

    /// General case.
    template <class _NamedTupleType, StringLiteral _head, StringLiteral... _tail>
    struct remove_fields {
      using NamedTupleType = std::remove_cvref_t<_NamedTupleType>;

      constexpr static int num_fields = std::tuple_size_v<typename NamedTupleType::Fields>;

      using NewNamedTupleType =
        typename remove_single_field<NamedTupleType, _head, NamedTuple<>, num_fields>::type;

      using type = typename remove_fields<NewNamedTupleType, _tail...>::type;
    };

  } // namespace internal
} // namespace rfl

#endif // RFL_REMOVEFIELDS_HPP_