ImplicitSurfaceNetwork/shared_module/container/variadic_.hpp

#pragma once

#include <type_traits>
#include <limits>
#include <array>
#include <cstddef>
#include <new>
#include <utility>
#include <memory>

#include <utils/no_unique_address_emulation.hpp>

namespace detail
{
template <std::size_t _Np, typename... _Types>
struct __nth__ {
};

template <typename _Tp0, typename... _Rest>
struct __nth__<0, _Tp0, _Rest...> {
    using type = _Tp0;
};

template <typename _Tp0, typename _Tp1, typename... _Rest>
struct __nth__<1, _Tp0, _Tp1, _Rest...> {
    using type = _Tp1;
};

template <typename _Tp0, typename _Tp1, typename _Tp2, typename... _Rest>
struct __nth__<2, _Tp0, _Tp1, _Tp2, _Rest...> {
    using type = _Tp2;
};

template <const std::size_t _Np, typename _Tp0, typename _Tp1, typename _Tp2, typename... _Rest>
struct __nth__<_Np, _Tp0, _Tp1, _Tp2, _Rest...> : __nth__<_Np - 3, _Rest...> {
};

template <const std::size_t _Idx, typename... _Types>
using _Select_nth_type = typename __nth__<_Idx, _Types...>::type;

template <unsigned long long _Val, typename... _Ints>
struct _Select_int_base;

template <unsigned long long _Val, typename _IntType, typename... _Ints>
struct _Select_int_base<_Val, _IntType, _Ints...> : std::conditional_t<(_Val <= std::numeric_limits<_IntType>::max()),
                                                                       std::integral_constant<_IntType, (_IntType)_Val>,
                                                                       _Select_int_base<_Val, _Ints...>> {
};

template <unsigned long long _Val>
struct _Select_int_base<_Val> {
};

template <unsigned long long _Val>
using _Select_int =
    typename _Select_int_base<_Val, unsigned char, unsigned short, unsigned int, unsigned long, unsigned long long>::type::
        value_type;
} // namespace detail

namespace impl::variadic
{
#pragma packed(push, 1)

template <typename _Index_type>
struct _Index_base {
    _Index_type __this_;
    _Index_type __next_;

    constexpr _Index_base(const _Index_type __i, const _Index_type __j) noexcept : __this_{__i}, __next_{__j} {}
};

#pragma packed(pop)

template <typename _Ty, typename _Index_type>
struct _Element : public _Index_base<_Index_type>,
                  public no_unique_address_emulation<_Ty> {
    template <typename... _Args>
    explicit constexpr _Element(const std::size_t __i, _Args&&... __ctors) //
        noexcept(std::is_nothrow_constructible_v<_Ty, _Args...>)
        : _Index_base<_Index_type>{static_cast<_Index_type>(__i), std::numeric_limits<_Index_type>::max()},
          no_unique_address_emulation<_Ty>(std::forward<_Args>(__ctors)...)
    {
    }
};

template <typename _Visitor>
struct __vtable__ {
    template <std::size_t _Idx, typename _Ty = _Select_nth_type<_Idx, _Types...>>
    static void _S_invoke(_Visitor&& __visitor, void* __ptr) noexcept(std::is_nothrow_invocable_v<_Visitor, _Ty&>)
    {
        auto* __box = std::launder(reinterpret_cast<_Box<_Ty>*>(__ptr));
        std::forward<_Visitor>(__visitor)(__box->__val_);
    }

    template <std::size_t... _Indices>
    static constexpr auto _S_make_vtab(std::index_sequence<_Indices...>) noexcept
    {
        return std::array<void (*)(_Visitor&&, void*), sizeof...(_Types)>{&_S_invoke<_Indices>...};
    }

    static constexpr auto __vtab_ = _S_make_vtab(std::index_sequence_for<_Types...>{});
};
} // namespace impl::variadic

template <const std::size_t __Sp_, typename... _Types>
struct variadic {
    using _Index_type = detail::_Select_int<sizeof...(_Types)>;

    struct _Idx_base {
        _Index_type __this_;
        _Index_type __next_;
    } __attribute__((__packed__));

    template <typename _Ty>
    struct _Box : public _Idx_base {
        [[__no_unique_address__]] _Ty __val_;

        template <typename... _Args>
        explicit constexpr _Box(const std::size_t __i,
                                _Args&&... __ctors) noexcept(std::is_nothrow_constructible_v<_Ty, _Args...>)
            : _Idx_base{static_cast<_Index_type>(__i), std::numeric_limits<_Index_type>::max()},
              __val_(std::forward<_Args>(__ctors)...)
        {
        }
    };

    static constexpr std::size_t __max_align_ = std::max({std::alignment_of_v<_Box<_Types>>...});

    template <typename _Visitor>
    struct __vtable__ {
        template <std::size_t _Idx, typename _Ty = _Select_nth_type<_Idx, _Types...>>
        static void _S_invoke(_Visitor&& __visitor, void* __ptr) noexcept(std::is_nothrow_invocable_v<_Visitor, _Ty&>)
        {
            auto* __box = std::launder(reinterpret_cast<_Box<_Ty>*>(__ptr));
            std::forward<_Visitor>(__visitor)(__box->__val_);
        }

        template <std::size_t... _Indices>
        static constexpr auto _S_make_vtab(std::index_sequence<_Indices...>) noexcept
        {
            return std::array<void (*)(_Visitor&&, void*), sizeof...(_Types)>{&_S_invoke<_Indices>...};
        }

        static constexpr auto __vtab_ = _S_make_vtab(std::index_sequence_for<_Types...>{});
    };

    alignas(__max_align_) std::byte __data_[__Sp_];
    void*        __p_     = &__data_[0];
    std::size_t  __sz_    = __Sp_;
    std::size_t  __count_ = 0;
    _Index_type* __next_  = nullptr;

    template <typename _Ty>
    [[__nodiscard__]] //
    _Ty* _M_implicit_aligned_alloc(const std::size_t __align = alignof(_Ty)) noexcept
    {
        if (std::align(__align, sizeof(_Ty), __p_, __sz_)) [[__likely__]] {
            _Ty* __result  = std::launder(reinterpret_cast<_Ty*>(__p_));
            __p_           = static_cast<std::byte*>(__p_) + sizeof(_Ty);
            __sz_         -= sizeof(_Ty);
            return __result;
        }
        return nullptr;
    }

    template <std::size_t _Idx, typename... _Args, typename _Result = _Select_nth_type<_Idx, _Types...>>
    _Result& emplace_back(_Args&&... __ctors)
    {
        if (__next_) { *__next_ = static_cast<_Index_type>(_Idx); }
        if (auto* __ptr = _M_implicit_aligned_alloc<_Box<_Result>>()) [[__likely__]] {
            ::new (__ptr) _Box<_Result>{_Idx, std::forward<_Args>(__ctors)...};
            __next_ = &__ptr->__next_;
            ++__count_;
            return __ptr->__val_;
        }
        throw std::bad_alloc{};
    }

    template <typename _Visitor>
    void loop(_Visitor&& __visitor)
    {
        if (__count_ == 0) [[__unlikely__]] { return; }

        static constexpr auto& __table_ = __vtable__<_Visitor&&>::__vtab_;

        void*             __current = &__data_[0];
        const void* const __end     = __p_;

        while (__current < __end) [[__likely__]] {
            auto*      __base = std::launder(reinterpret_cast<_Idx_base*>(__current));
            const auto __i    = __base->__this_;
            const auto __j    = __base->__next_;
            if (__i < sizeof...(_Types)) [[__likely__]] { __table_[__i](std::forward<_Visitor>(__visitor), __current); }
            __current = _S_advance_ptr(__current, __i, __j);
        }
    }

private:
    template <std::size_t... _Indices>
    static void* _S_advance_ptr_impl(const void*       __ptr,
                                     const std::size_t __i,
                                     const std::size_t __j,
                                     std::index_sequence<_Indices...>) noexcept
    {
        static constexpr std::array<std::size_t, sizeof...(_Types)> __alignments_ = {std::alignment_of_v<_Box<_Types>>...};
        static constexpr std::array<std::size_t, sizeof...(_Types)> __sizes_      = {sizeof(_Box<_Types>)...};
        const auto __intptr = reinterpret_cast<std::uintptr_t>(__ptr) + __sizes_[__i];
        if (__j == std::numeric_limits<_Index_type>::max()) {
            // NOLINTNEXTLINE
            return reinterpret_cast<void*>(__intptr);
        }

        const auto __align  = __alignments_[__j];
        const auto __result = (__intptr - 1U + __align) & -__align;
        // NOLINTNEXTLINE
        return reinterpret_cast<void*>(__result);
    }

    static void* _S_advance_ptr(const void* __ptr, const std::size_t __i, const std::size_t __j)
    {
        return _S_advance_ptr_impl(__ptr, __i, __j, std::index_sequence_for<_Types...>{});
    }

public:
    [[__nodiscard__]] std::size_t size() const noexcept { return __count_; }

    [[__nodiscard__]] bool empty() const noexcept { return __count_ == 0; }

    [[__nodiscard__]] std::size_t capacity() const noexcept { return __Sp_; }

    [[__nodiscard__]] std::size_t available() const noexcept { return __sz_; }

    void clear() noexcept
    {
        loop([] [[__gnu__::__always_inline__]] (auto& __obj) noexcept {
            using _Type = std::decay_t<decltype(__obj)>;
            if constexpr (!std::is_trivially_destructible_v<_Type>) { __obj.~_Type(); }
        });

        __p_     = &__data_[0];
        __sz_    = __Sp_;
        __count_ = 0;
    }

    ~variadic() { clear(); }
};