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196 lines
5.7 KiB
C++
196 lines
5.7 KiB
C++
//===--- Pack.h - Helpers for wording with class template packs -*- C++ -*-===//
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//
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// This source file is part of the Swift.org open source project
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//
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// Copyright (c) 2014 - 2025 Apple Inc. and the Swift project authors
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// Licensed under Apache License v2.0 with Runtime Library Exception
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//
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// See https://swift.org/LICENSE.txt for license information
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// See https://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
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//
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//===----------------------------------------------------------------------===//
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#ifndef SWIFT_BASIC_PACKS_H
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#define SWIFT_BASIC_PACKS_H
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namespace swift {
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namespace packs {
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/// A pack of types.
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template <class...>
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struct Pack;
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/// A trait indicating whether the given type is a specialization
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/// of Pack<...>.
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template <class>
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struct IsPack {
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static constexpr bool value = false;
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};
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template <class... Components>
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struct IsPack<Pack<Components...>> {
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static constexpr bool value = true;
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};
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/// Transform a variadic list of types using the given transform, a class
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/// template which is expected to define a `result` type providing the
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/// result of the transform.
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///
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/// template <class Arg, class Component>
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/// class Transform {
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/// using result = ...;
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/// };
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///
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/// In principle, this would be cleaner as a member template, but that
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/// works out poorly because in practice the member template must be
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/// partially specialized in order to destructure it.
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template <template <class, class> class Transform,
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class TransformArg, class... Values>
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struct PackMapComponents {
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using result = Pack<typename Transform<TransformArg, Values>::result...>;
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};
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/// Transform the components of a pack using the given transform, a class
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/// template which is expected to define a `result` type providing the
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/// result of the transform.
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///
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/// template <class Arg, class Component>
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/// class Transform {
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/// using result = ...;
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/// };
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template <template <class, class> class Transform, class TransformArg,
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class P>
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struct PackMap;
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template <template <class, class> class Transform, class TransformArg,
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class... Components>
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struct PackMap<Transform, TransformArg, Pack<Components...>>
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: PackMapComponents<Transform, TransformArg, Components...> {};
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/// Concatenate the components of a variadic list of packs.
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template <class... Packs>
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struct PackConcat;
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template <>
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struct PackConcat<> {
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using result = Pack<>;
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};
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template <class First>
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struct PackConcat<First> {
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static_assert(IsPack<First>::value, "argument is not a pack");
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using result = First;
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};
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template <class... FirstComponents, class... SecondComponents>
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struct PackConcat<Pack<FirstComponents...>, Pack<SecondComponents...>> {
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using result = Pack<FirstComponents..., SecondComponents...>;
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};
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template <class Head, class... Tail>
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struct PackConcat<Head, Tail...>
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: PackConcat<Head, typename PackConcat<Tail...>::result> {};
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/// Flatten a pack of packs by concatenating their components.
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template <class>
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struct PackFlatten;
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template <class... PackComponents>
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struct PackFlatten<Pack<PackComponents...>>
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: PackConcat<PackComponents...> {};
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/// Apply the given pack-producing transform to each component of a pack,
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/// then concatenate the results.
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///
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/// For example, if we start with Pack<A, B, C>, and the transform turns:
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/// A => Pack<X, Y>
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/// B => Pack<>
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/// C => Pack<Z>
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/// then the result will be Pack<X, Y, Z>.
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template <template <class, class> class Transform, class TransformArg, class P>
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struct PackFlatMap
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: PackFlatten<typename PackMap<Transform, TransformArg, P>::result> {};
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/// Reverse a variadic list of types.
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template <class... PackComponents>
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struct PackReverseComponents;
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template <>
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struct PackReverseComponents<> {
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using result = Pack<>;
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};
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template <class Head>
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struct PackReverseComponents<Head> {
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using result = Pack<Head>;
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};
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template <class Head, class... Tail>
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struct PackReverseComponents<Head, Tail...>
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: PackConcat<typename PackReverseComponents<Tail...>::result,
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Pack<Head>> {};
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/// Reverse a pack.
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template <class>
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struct PackReverse;
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template <class... PackComponents>
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struct PackReverse<Pack<PackComponents...>>
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: PackReverseComponents<PackComponents...> {};
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/// Determine whether the given pack is transitively ordered according to
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/// the given predicate:
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///
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/// template <class First, class Second>
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/// struct IsOrdered {
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/// static constexpr bool value;
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/// };
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template <template <class, class> class IsOrdered,
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class... Components>
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struct PackComponentsAreOrdered;
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template <template <class, class> class IsOrdered>
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struct PackComponentsAreOrdered<IsOrdered> {
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static constexpr bool value = true;
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};
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template <template <class, class> class IsOrdered,
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class A>
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struct PackComponentsAreOrdered<IsOrdered, A> {
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static constexpr bool value = true;
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};
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template <template <class, class> class IsOrdered,
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class A, class B>
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struct PackComponentsAreOrdered<IsOrdered, A, B> : IsOrdered<A, B> {};
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template <template <class, class> class IsOrdered,
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class A, class B, class... C>
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struct PackComponentsAreOrdered<IsOrdered, A, B, C...> {
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static constexpr bool value =
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IsOrdered<A, B>::value &&
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PackComponentsAreOrdered<IsOrdered, B, C...>::value;
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};
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/// Determine whether the given pack is well-ordered according to the given
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/// predicate:
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///
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/// template <class First, class Second>
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/// struct IsOrdered {
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/// static constexpr bool value;
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/// };
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template <template <class, class> class IsOrdered,
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class P>
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struct PackIsOrdered;
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template <template <class, class> class IsOrdered,
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class... Components>
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struct PackIsOrdered<IsOrdered, Pack<Components...>>
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: PackComponentsAreOrdered<IsOrdered, Components...> {};
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} // end namespace packs
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} // end namespace swift
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#endif
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