drop.hpp

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// Copyright 2026, Toyota Motor Corporation
//
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
#ifndef INCLUDE_GUARD_ARENE_BASE_ARENE_BASE_TYPE_LIST_DROP_HPP_
#define INCLUDE_GUARD_ARENE_BASE_ARENE_BASE_TYPE_LIST_DROP_HPP_
 
#include "arene/base/constraints/constraints.hpp"
#include "arene/base/stdlib_choice/cstddef.hpp"
#include "arene/base/stdlib_choice/declval.hpp"
#include "arene/base/stdlib_choice/enable_if.hpp"
#include "arene/base/stdlib_choice/integer_sequence.hpp"
#include "arene/base/type_list/at.hpp"
 
namespace arene {
namespace base {
namespace type_lists {
 
namespace drop_detail {
 
// parasoft-begin-suppress CERT_C-EXP37-a "False positive: The rule does not mention naming all parameters"
/// @brief helper removing the first @c Count types
/// @tparam Count number of types to remove
/// @tparam Is indices
/// @tparam List type list type
/// @tparam Ts types
/// @return @c List after removing the first @c Count types from @c Ts...
template <std::size_t Count, std::size_t... Is, template <class...> class List, class... Ts>
auto drop_impl(std::index_sequence<Is...>, List<Ts...>) -> List<at_t<List<Ts...>, Count + Is>...>;
// parasoft-end-suppress CERT_C-EXP37-a
 
/// @brief implementation helper for drop
/// @tparam List type list
/// @tparam Count number of types to drop
///
/// Undefined primary template.
template <class List, std::size_t Count, class = constraints<>>
class drop;
 
/// @brief implementation helper for drop
/// @tparam List type list type
/// @tparam Ts types
/// @tparam Count number of types to drop
///
/// Specialization for @c Count less than the size of @c List<Ts...>
template <template <class...> class List, class... Ts, std::size_t Count>
class drop<List<Ts...>, Count, constraints<std::enable_if_t<(Count < sizeof...(Ts))>>> {
 public:
  /// @brief resulting type list
  using type =
      decltype(drop_impl<Count>(std::make_index_sequence<sizeof...(Ts) - Count>{}, std::declval<List<Ts...>>()));
};
 
/// @brief implementation helper for drop
/// @tparam List type list type
/// @tparam Ts types
/// @tparam Count number of types to drop
///
/// Specialization for @c Count greater than or equal to the size of @c List<Ts...>
template <template <class...> class List, class... Ts, std::size_t Count>
class drop<List<Ts...>, Count, constraints<std::enable_if_t<(Count >= sizeof...(Ts))>>> {
 public:
  /// @brief resulting type list
  using type = List<>;
};
 
}  // namespace drop_detail
 
/// @brief drops types from the front of a type list
/// @tparam List type-list-like
/// @tparam Count number of types to drop
///
/// For any type-list-like @c List, provides the member typedef @c type which is
/// the same as @c List but with the first @c Count types omitted. If @c Count
/// exceeds the number of types in @c List, @c type specifies an "empty"
/// type-list.
///
/// Usage:
/// @snippet docs/examples/type_list_examples.cpp drop_example
///
///
template <class List, std::size_t Count>
using drop = drop_detail::drop<List, Count>;
 
/// @brief drops types from the front of a type list
/// @tparam List type-list-like
/// @tparam Count number of types to drop
/// @copydoc drop
template <class List, std::size_t Count>
using drop_t = typename drop<List, Count>::type;
 
}  // namespace type_lists
}  // namespace base
}  // namespace arene
 
#endif  // INCLUDE_GUARD_ARENE_BASE_ARENE_BASE_TYPE_LIST_DROP_HPP_