mirror of
https://github.com/LadybirdBrowser/ladybird.git
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388 lines
12 KiB
C++
388 lines
12 KiB
C++
/*
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* Copyright (c) 2021, Ali Mohammad Pur <mpfard@serenityos.org>
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*
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* SPDX-License-Identifier: BSD-2-Clause
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*/
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#pragma once
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#include <AK/Array.h>
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#include <AK/BitCast.h>
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#include <AK/StdLibExtras.h>
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namespace AK::Detail {
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template<typename T, typename IndexType, IndexType InitialIndex, typename... InTypes>
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struct VariantIndexOf {
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static_assert(DependentFalse<T, IndexType, InTypes...>, "Invalid VariantIndex instantiated");
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};
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template<typename T, typename IndexType, IndexType InitialIndex, typename InType, typename... RestOfInTypes>
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struct VariantIndexOf<T, IndexType, InitialIndex, InType, RestOfInTypes...> {
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consteval IndexType operator()()
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{
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if constexpr (IsSame<T, InType>)
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return InitialIndex;
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else
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return VariantIndexOf<T, IndexType, InitialIndex + 1, RestOfInTypes...> {}();
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}
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};
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template<typename T, typename IndexType, IndexType InitialIndex>
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struct VariantIndexOf<T, IndexType, InitialIndex> {
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consteval IndexType operator()() { return InitialIndex; }
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};
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template<typename T, typename IndexType, typename... Ts>
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consteval IndexType index_of()
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{
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return VariantIndexOf<T, IndexType, 0, Ts...> {}();
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}
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template<typename IndexType, IndexType InitialIndex, typename... Ts>
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struct Variant;
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template<typename IndexType, IndexType InitialIndex, typename F, typename... Ts>
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struct Variant<IndexType, InitialIndex, F, Ts...> {
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static constexpr auto current_index = VariantIndexOf<F, IndexType, InitialIndex, F, Ts...> {}();
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static void delete_(IndexType id, void* data)
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{
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if (id == current_index)
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bit_cast<F*>(data)->~F();
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else
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Variant<IndexType, InitialIndex + 1, Ts...>::delete_(id, data);
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}
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static void move_(IndexType old_id, void* old_data, void* new_data)
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{
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if (old_id == current_index)
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new (new_data) F(move(*bit_cast<F*>(old_data)));
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else
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Variant<IndexType, InitialIndex + 1, Ts...>::move_(old_id, old_data, new_data);
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}
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static void copy_(IndexType old_id, const void* old_data, void* new_data)
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{
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if (old_id == current_index)
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new (new_data) F(*bit_cast<F*>(old_data));
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else
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Variant<IndexType, InitialIndex + 1, Ts...>::copy_(old_id, old_data, new_data);
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}
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template<typename Visitor>
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static void visit_(IndexType id, void* data, Visitor&& visitor)
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{
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if (id == current_index)
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visitor(*bit_cast<F*>(data));
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else
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Variant<IndexType, InitialIndex + 1, Ts...>::visit_(id, data, forward<Visitor>(visitor));
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}
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template<typename Visitor>
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static void visit_(IndexType id, const void* data, Visitor&& visitor)
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{
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if (id == current_index)
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visitor(*bit_cast<const F*>(data));
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else
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Variant<IndexType, InitialIndex + 1, Ts...>::visit_(id, data, forward<Visitor>(visitor));
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}
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};
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template<typename IndexType, IndexType InitialIndex>
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struct Variant<IndexType, InitialIndex> {
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static void delete_(IndexType, void*) { }
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static void move_(IndexType, void*, void*) { }
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static void copy_(IndexType, const void*, void*) { }
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template<typename Visitor>
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static void visit_(IndexType, void*, Visitor&&) { }
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template<typename Visitor>
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static void visit_(IndexType, const void*, Visitor&&) { }
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};
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struct VariantNoClearTag {
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explicit VariantNoClearTag() = default;
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};
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struct VariantConstructTag {
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explicit VariantConstructTag() = default;
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};
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template<typename T, typename Base>
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struct VariantConstructors {
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VariantConstructors(T&& t)
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{
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internal_cast().template set<T>(forward<T>(t), VariantNoClearTag {});
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}
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VariantConstructors() { }
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Base& operator=(const T& value)
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{
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Base variant { value };
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internal_cast() = move(variant);
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return internal_cast();
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}
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Base& operator=(T&& value)
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{
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Base variant { move(value) };
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internal_cast() = move(variant);
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return internal_cast();
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}
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private:
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[[nodiscard]] Base& internal_cast()
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{
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// Warning: Internal type shenanigans - VariantsConstrutors<T, Base> <- Base
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// Not the other way around, so be _really_ careful not to cause issues.
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return *reinterpret_cast<Base*>(this);
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}
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};
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// Type list deduplication
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// Since this is a big template mess, each template is commented with how and why it works.
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struct ParameterPackTag {
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};
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// Pack<Ts...> is just a way to pass around the type parameter pack Ts
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template<typename... Ts>
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struct ParameterPack : ParameterPackTag {
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};
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// Blank<T> is a unique replacement for T, if T is a duplicate type.
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template<typename T>
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struct Blank {
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};
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template<typename A, typename P>
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inline constexpr bool IsTypeInPack = false;
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// IsTypeInPack<T, Pack<Ts...>> will just return whether 'T' exists in 'Ts'.
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template<typename T, typename... Ts>
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inline constexpr bool IsTypeInPack<T, ParameterPack<Ts...>> = (IsSame<T, Ts> || ...);
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// Replaces T with Blank<T> if it exists in Qs.
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template<typename T, typename... Qs>
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using BlankIfDuplicate = Conditional<(IsTypeInPack<T, Qs> || ...), Blank<T>, T>;
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template<unsigned I, typename...>
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struct InheritFromUniqueEntries;
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// InheritFromUniqueEntries will inherit from both Qs and Ts, but only scan entries going *forwards*
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// that is to say, if it's scanning from index I in Qs, it won't scan for duplicates for entries before I
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// as that has already been checked before.
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// This makes sure that the search is linear in time (like the 'merge' step of merge sort).
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template<unsigned I, typename... Ts, unsigned... Js, typename... Qs>
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struct InheritFromUniqueEntries<I, ParameterPack<Ts...>, IndexSequence<Js...>, Qs...>
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: public BlankIfDuplicate<Ts, Conditional<Js <= I, ParameterPack<>, Qs>...>... {
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using BlankIfDuplicate<Ts, Conditional<Js <= I, ParameterPack<>, Qs>...>::BlankIfDuplicate...;
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};
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template<typename...>
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struct InheritFromPacks;
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// InheritFromPacks will attempt to 'merge' the pack 'Ps' with *itself*, but skip the duplicate entries
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// (via InheritFromUniqueEntries).
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template<unsigned... Is, typename... Ps>
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struct InheritFromPacks<IndexSequence<Is...>, Ps...>
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: public InheritFromUniqueEntries<Is, Ps, IndexSequence<Is...>, Ps...>... {
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using InheritFromUniqueEntries<Is, Ps, IndexSequence<Is...>, Ps...>::InheritFromUniqueEntries...;
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};
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// Just a nice wrapper around InheritFromPacks, which will wrap any parameter packs in ParameterPack (unless it alread is one).
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template<typename... Ps>
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using MergeAndDeduplicatePacks = InheritFromPacks<MakeIndexSequence<sizeof...(Ps)>, Conditional<IsBaseOf<ParameterPackTag, Ps>, Ps, ParameterPack<Ps>>...>;
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}
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namespace AK {
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struct Empty {
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};
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template<typename... Ts>
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struct Variant
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: public Detail::MergeAndDeduplicatePacks<Detail::VariantConstructors<Ts, Variant<Ts...>>...> {
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private:
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using IndexType = Conditional<sizeof...(Ts) < 255, u8, size_t>; // Note: size+1 reserved for internal value checks
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static constexpr IndexType invalid_index = sizeof...(Ts);
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template<typename T>
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static constexpr IndexType index_of() { return Detail::index_of<T, IndexType, Ts...>(); }
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public:
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template<typename... NewTs>
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friend struct Variant;
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Variant(const Variant& old)
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: Detail::MergeAndDeduplicatePacks<Detail::VariantConstructors<Ts, Variant<Ts...>>...>()
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, m_index(old.m_index)
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{
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Helper::copy_(old.m_index, old.m_data, m_data);
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}
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// Note: A moved-from variant emulates the state of the object it contains
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// so if a variant containing an int is moved from, it will still contain that int
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// and if a variant with a nontrivial move ctor is moved from, it may or may not be valid
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// but it will still contain the "moved-from" state of the object it previously contained.
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Variant(Variant&& old)
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: Detail::MergeAndDeduplicatePacks<Detail::VariantConstructors<Ts, Variant<Ts...>>...>()
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, m_index(old.m_index)
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{
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Helper::move_(old.m_index, old.m_data, m_data);
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}
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~Variant()
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{
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Helper::delete_(m_index, m_data);
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}
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Variant& operator=(const Variant& other)
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{
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m_index = other.m_index;
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Helper::copy_(other.m_index, other.m_data, m_data);
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return *this;
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}
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Variant& operator=(Variant&& other)
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{
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m_index = other.m_index;
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Helper::move_(other.m_index, other.m_data, m_data);
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return *this;
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}
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using Detail::MergeAndDeduplicatePacks<Detail::VariantConstructors<Ts, Variant<Ts...>>...>::MergeAndDeduplicatePacks;
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template<typename T>
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void set(T&& t) requires(index_of<T>() != invalid_index)
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{
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using StrippedT = RemoveCV<RemoveReference<T>>;
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constexpr auto new_index = index_of<StrippedT>();
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Helper::delete_(m_index, m_data);
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new (m_data) StrippedT(forward<T>(t));
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m_index = new_index;
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}
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template<typename T>
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void set(T&& t, Detail::VariantNoClearTag)
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{
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using StrippedT = RemoveCV<RemoveReference<T>>;
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constexpr auto new_index = index_of<StrippedT>();
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new (m_data) StrippedT(forward<T>(t));
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m_index = new_index;
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}
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template<typename T>
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T* get_pointer()
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{
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if (index_of<T>() == m_index)
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return bit_cast<T*>(&m_data);
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return nullptr;
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}
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template<typename T>
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[[gnu::noinline]] T& get()
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{
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VERIFY(has<T>());
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return *bit_cast<T*>(&m_data);
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}
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template<typename T>
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[[gnu::noinline]] const T* get_pointer() const
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{
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if (index_of<T>() == m_index)
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return bit_cast<const T*>(&m_data);
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return nullptr;
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}
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template<typename T>
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[[gnu::noinline]] const T& get() const
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{
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VERIFY(has<T>());
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return *bit_cast<const T*>(&m_data);
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}
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template<typename T>
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[[nodiscard]] bool has() const
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{
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return index_of<T>() == m_index;
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}
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template<typename... Fs>
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void visit(Fs&&... functions)
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{
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Visitor<Fs...> visitor { forward<Fs>(functions)... };
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Helper::visit_(m_index, m_data, visitor);
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}
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template<typename... Fs>
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void visit(Fs&&... functions) const
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{
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Visitor<Fs...> visitor { forward<Fs>(functions)... };
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Helper::visit_(m_index, m_data, visitor);
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}
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template<typename... NewTs>
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Variant<NewTs...> downcast() &&
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{
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Variant<NewTs...> instance { Variant<NewTs...>::invalid_index, Detail::VariantConstructTag {} };
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visit([&](auto& value) {
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if constexpr (Variant<NewTs...>::template can_contain<RemoveCV<RemoveReference<decltype(value)>>>())
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instance.set(move(value), Detail::VariantNoClearTag {});
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});
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VERIFY(instance.m_index != instance.invalid_index);
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return instance;
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}
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template<typename... NewTs>
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Variant<NewTs...> downcast() &
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{
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Variant<NewTs...> instance { Variant<NewTs...>::invalid_index, Detail::VariantConstructTag {} };
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visit([&](const auto& value) {
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if constexpr (Variant<NewTs...>::template can_contain<RemoveCV<RemoveReference<decltype(value)>>>())
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instance.set(value, Detail::VariantNoClearTag {});
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});
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VERIFY(instance.m_index != instance.invalid_index);
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return instance;
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}
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template<typename T>
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static constexpr bool can_contain()
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{
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return index_of<T>() != invalid_index;
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}
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private:
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static constexpr auto data_size = integer_sequence_generate_array<size_t>(0, IntegerSequence<size_t, sizeof(Ts)...>()).max();
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static constexpr auto data_alignment = integer_sequence_generate_array<size_t>(0, IntegerSequence<size_t, alignof(Ts)...>()).max();
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using Helper = Detail::Variant<IndexType, 0, Ts...>;
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explicit Variant(IndexType index, Detail::VariantConstructTag)
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: Detail::MergeAndDeduplicatePacks<Detail::VariantConstructors<Ts, Variant<Ts...>>...>()
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, m_index(index)
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{
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}
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template<typename... Fs>
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struct Visitor : Fs... {
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Visitor(Fs&&... args)
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: Fs(args)...
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{
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}
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using Fs::operator()...;
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};
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alignas(data_alignment) u8 m_data[data_size];
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// Note: Make sure not to default-initialize!
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// VariantConstructors::VariantConstructors(T) will set this to the correct value
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// So default-constructing to anything will leave the first initialization with that value instead of the correct one.
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IndexType m_index;
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};
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}
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using AK::Empty;
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using AK::Variant;
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