ilist.h

#pragma once
#include "gaia/config/config.h"
 
#include <cstddef>
#include <cstdint>
#include <type_traits>
 
#include "gaia/cnt/bitset.h"
#include "gaia/cnt/darray.h"
#include "gaia/core/utility.h"
#include "gaia/mem/mem_alloc.h"
#include "gaia/mem/raw_data_holder.h"
 
namespace gaia {
  namespace cnt {
    template <typename TListItem>
    struct ilist_item_traits;
 
    namespace detail {
      template <typename T, typename = void>
      struct ilist_has_idx_member: std::false_type {};
      template <typename T>
      struct ilist_has_idx_member<T, std::void_t<decltype(std::declval<T&>().idx)>>: std::true_type {};
 
      template <typename T, typename = void>
      struct ilist_has_gen_member: std::false_type {};
      template <typename T>
      struct ilist_has_gen_member<T, std::void_t<decltype(std::declval<T&>().gen)>>: std::true_type {};
 
      template <typename T, typename = void>
      struct ilist_has_data_gen_member: std::false_type {};
      template <typename T>
      struct ilist_has_data_gen_member<T, std::void_t<decltype(std::declval<T&>().data.gen)>>: std::true_type {};
 
      template <typename T, typename = void>
      struct ilist_has_id_mask: std::false_type {};
      template <typename T>
      struct ilist_has_id_mask<T, std::void_t<decltype(T::IdMask)>>: std::true_type {};
 
      template <typename T, typename = void>
      struct ilist_has_handle_id: std::false_type {};
      template <typename T>
      struct ilist_has_handle_id<T, std::void_t<decltype(std::declval<const T&>().id())>>: std::true_type {};
 
      template <typename T, typename = void>
      struct ilist_has_handle_gen: std::false_type {};
      template <typename T>
      struct ilist_has_handle_gen<T, std::void_t<decltype(std::declval<const T&>().gen())>>: std::true_type {};
 
      template <typename T, typename = void>
      struct ilist_has_create: std::false_type {};
      template <typename T>
      struct ilist_has_create<
          T, std::void_t<decltype(T::create(std::declval<uint32_t>(), std::declval<uint32_t>(), (void*)nullptr))>>:
          std::true_type {};
 
      template <typename T, typename THandle, typename = void>
      struct ilist_has_handle: std::false_type {};
      template <typename T, typename THandle>
      struct ilist_has_handle<T, THandle, std::void_t<decltype(T::handle(std::declval<const T&>()))>>:
          std::bool_constant<std::is_convertible_v<decltype(T::handle(std::declval<const T&>())), THandle>> {};
 
      template <typename T, typename = void>
      struct ilist_traits_has_idx: std::false_type {};
      template <typename T>
      struct ilist_traits_has_idx<T, std::void_t<decltype(ilist_item_traits<T>::idx(std::declval<const T&>()))>>:
          std::true_type {};
 
      template <typename T, typename = void>
      struct ilist_traits_has_set_idx: std::false_type {};
      template <typename T>
      struct ilist_traits_has_set_idx<
          T, std::void_t<decltype(ilist_item_traits<T>::set_idx(std::declval<T&>(), std::declval<uint32_t>()))>>:
          std::true_type {};
 
      template <typename T, typename = void>
      struct ilist_traits_has_gen: std::false_type {};
      template <typename T>
      struct ilist_traits_has_gen<T, std::void_t<decltype(ilist_item_traits<T>::gen(std::declval<const T&>()))>>:
          std::true_type {};
 
      template <typename T, typename = void>
      struct ilist_traits_has_set_gen: std::false_type {};
      template <typename T>
      struct ilist_traits_has_set_gen<
          T, std::void_t<decltype(ilist_item_traits<T>::set_gen(std::declval<T&>(), std::declval<uint32_t>()))>>:
          std::true_type {};
    } // namespace detail
 
    template <typename TListItem>
    struct ilist_item_traits {
      static_assert(
          detail::ilist_has_idx_member<TListItem>::value,
          "ilist item type must expose idx or specialize ilist_item_traits");
      static_assert(
          detail::ilist_has_gen_member<TListItem>::value || detail::ilist_has_data_gen_member<TListItem>::value,
          "ilist item type must expose gen/data.gen or specialize ilist_item_traits");
 
      GAIA_NODISCARD static uint32_t idx(const TListItem& item) noexcept {
        return (uint32_t)item.idx;
      }
 
      static void set_idx(TListItem& item, uint32_t value) noexcept {
        item.idx = value;
      }
 
      GAIA_NODISCARD static uint32_t gen(const TListItem& item) noexcept {
        if constexpr (detail::ilist_has_gen_member<TListItem>::value)
          return (uint32_t)item.gen;
        else
          return (uint32_t)item.data.gen;
      }
 
      static void set_gen(TListItem& item, uint32_t value) noexcept {
        if constexpr (detail::ilist_has_gen_member<TListItem>::value)
          item.gen = value;
        else
          item.data.gen = value;
      }
    };
 
    struct ilist_item {
      struct ItemData {
        uint32_t gen;
      };
 
      uint32_t idx;
      ItemData data;
 
      ilist_item() = default;
      ilist_item(uint32_t index, uint32_t generation): idx(index) {
        data.gen = generation;
      }
 
      ilist_item(const ilist_item& other) {
        idx = other.idx;
        data.gen = other.data.gen;
      }
      ilist_item& operator=(const ilist_item& other) {
        GAIA_ASSERT(core::addressof(other) != this);
        idx = other.idx;
        data.gen = other.data.gen;
        return *this;
      }
 
      ilist_item(ilist_item&& other) {
        idx = other.idx;
        data.gen = other.data.gen;
 
        other.idx = (uint32_t)-1;
        other.data.gen = (uint32_t)-1;
      }
      ilist_item& operator=(ilist_item&& other) {
        GAIA_ASSERT(core::addressof(other) != this);
        idx = other.idx;
        data.gen = other.data.gen;
 
        other.idx = (uint32_t)-1;
        other.data.gen = (uint32_t)-1;
        return *this;
      }
    };
 
    template <typename TListItem>
    struct darray_ilist_storage: public cnt::darray<TListItem> {
      void add_item(TListItem&& container) {
        this->push_back(GAIA_MOV(container));
      }
 
      void del_item([[maybe_unused]] TListItem& container) {}
    };
 
    template <typename TListItem, typename TItemHandle, typename TInternalStorage = darray_ilist_storage<TListItem>>
    struct ilist {
      using internal_storage = TInternalStorage;
 
      using value_type = TListItem;
      using reference = TListItem&;
      using const_reference = const TListItem&;
      using pointer = TListItem*;
      using const_pointer = const TListItem*;
      using difference_type = typename internal_storage::difference_type;
      using size_type = typename internal_storage::size_type;
 
      // TODO: replace this iterator with a real list iterator
      using iterator = typename internal_storage::iterator;
      using const_iterator = typename internal_storage::const_iterator;
      using iterator_category = typename internal_storage::iterator_category;
 
      static_assert(detail::ilist_traits_has_idx<TListItem>::value, "ilist_item_traits<T> must expose idx(const T&)");
      static_assert(
          detail::ilist_traits_has_set_idx<TListItem>::value, "ilist_item_traits<T> must expose set_idx(T&, uint32_t)");
      static_assert(detail::ilist_traits_has_gen<TListItem>::value, "ilist_item_traits<T> must expose gen(const T&)");
      static_assert(
          detail::ilist_traits_has_set_gen<TListItem>::value, "ilist_item_traits<T> must expose set_gen(T&, uint32_t)");
      static_assert(
          detail::ilist_has_create<TListItem>::value,
          "ilist item type must expose static create(index, generation, ctx)");
      static_assert(
          detail::ilist_has_handle<TListItem, TItemHandle>::value,
          "ilist item type must expose static handle(const item) returning the handle type");
      static_assert(detail::ilist_has_id_mask<TItemHandle>::value, "ilist handle type must expose IdMask");
      static_assert(detail::ilist_has_handle_id<TItemHandle>::value, "ilist handle type must expose id()");
      static_assert(detail::ilist_has_handle_gen<TItemHandle>::value, "ilist handle type must expose gen()");
      internal_storage m_items;
      size_type m_nextFreeIdx = (size_type)-1;
      size_type m_freeItems = 0;
 
      GAIA_NODISCARD pointer data() noexcept {
        return reinterpret_cast<pointer>(m_items.data());
      }
 
      GAIA_NODISCARD const_pointer data() const noexcept {
        return reinterpret_cast<const_pointer>(m_items.data());
      }
 
      GAIA_NODISCARD reference operator[](size_type index) {
        return m_items[index];
      }
      GAIA_NODISCARD const_reference operator[](size_type index) const {
        return m_items[index];
      }
 
      void clear() {
        m_items.clear();
        m_nextFreeIdx = (size_type)-1;
        m_freeItems = 0;
      }
 
      GAIA_NODISCARD size_type get_next_free_item() const noexcept {
        return m_nextFreeIdx;
      }
 
      GAIA_NODISCARD size_type get_free_items() const noexcept {
        return m_freeItems;
      }
 
      GAIA_NODISCARD size_type item_count() const noexcept {
        return size() - m_freeItems;
      }
 
      GAIA_NODISCARD size_type size() const noexcept {
        return (size_type)m_items.size();
      }
 
      GAIA_NODISCARD bool empty() const noexcept {
        return size() == 0;
      }
 
      GAIA_NODISCARD size_type capacity() const noexcept {
        return (size_type)m_items.capacity();
      }
 
      GAIA_NODISCARD iterator begin() noexcept {
        return m_items.begin();
      }
 
      GAIA_NODISCARD const_iterator begin() const noexcept {
        return m_items.begin();
      }
 
      GAIA_NODISCARD const_iterator cbegin() const noexcept {
        return m_items.begin();
      }
 
      GAIA_NODISCARD iterator end() noexcept {
        return m_items.end();
      }
 
      GAIA_NODISCARD const_iterator end() const noexcept {
        return m_items.end();
      }
 
      GAIA_NODISCARD const_iterator cend() const noexcept {
        return m_items.end();
      }
 
      void reserve(size_type cap) {
        m_items.reserve(cap);
      }
 
      GAIA_NODISCARD TItemHandle alloc(void* ctx) {
        if GAIA_UNLIKELY (m_freeItems == 0U) {
          // We don't want to go out of range for new item
          const auto itemCnt = (size_type)m_items.size();
          GAIA_ASSERT(itemCnt < TItemHandle::IdMask && "Trying to allocate too many items!");
 
          GAIA_GCC_WARNING_PUSH()
          GAIA_CLANG_WARNING_PUSH()
          GAIA_GCC_WARNING_DISABLE("-Wstringop-overflow");
          GAIA_GCC_WARNING_DISABLE("-Wmissing-field-initializers");
          GAIA_CLANG_WARNING_DISABLE("-Wmissing-field-initializers");
          m_items.add_item(TListItem::create(itemCnt, 0U, ctx));
          return TListItem::handle(m_items.back());
          GAIA_GCC_WARNING_POP()
          GAIA_CLANG_WARNING_POP()
        }
 
        // Make sure the list is not broken
        GAIA_ASSERT(m_nextFreeIdx < (size_type)m_items.size() && "Item recycle list broken!");
 
        --m_freeItems;
        const auto index = m_nextFreeIdx;
        auto& j = m_items[m_nextFreeIdx];
        m_nextFreeIdx = ilist_item_traits<TListItem>::idx(j);
        j = TListItem::create(index, ilist_item_traits<TListItem>::gen(j), ctx);
        return TListItem::handle(j);
      }
 
      GAIA_NODISCARD TItemHandle alloc() {
        if GAIA_UNLIKELY (m_freeItems == 0U) {
          // We don't want to go out of range for new item
          const auto itemCnt = (size_type)m_items.size();
          GAIA_ASSERT(itemCnt < TItemHandle::IdMask && "Trying to allocate too many items!");
 
          GAIA_GCC_WARNING_PUSH()
          GAIA_CLANG_WARNING_PUSH()
          GAIA_GCC_WARNING_DISABLE("-Wstringop-overflow");
          GAIA_GCC_WARNING_DISABLE("-Wmissing-field-initializers");
          GAIA_CLANG_WARNING_DISABLE("-Wmissing-field-initializers");
          m_items.add_item(TListItem(itemCnt, 0U));
          return {itemCnt, 0U};
          GAIA_GCC_WARNING_POP()
          GAIA_CLANG_WARNING_POP()
        }
 
        // Make sure the list is not broken
        GAIA_ASSERT(m_nextFreeIdx < (size_type)m_items.size() && "Item recycle list broken!");
 
        --m_freeItems;
        const auto index = m_nextFreeIdx;
        auto& j = m_items[m_nextFreeIdx];
        m_nextFreeIdx = ilist_item_traits<TListItem>::idx(j);
        return {index, ilist_item_traits<TListItem>::gen(m_items[index])};
      }
 
      TListItem& free(TItemHandle handle) {
        auto& item = m_items[handle.id()];
        m_items.del_item(item);
 
        // Update our implicit list
        if GAIA_UNLIKELY (m_freeItems == 0)
          ilist_item_traits<TListItem>::set_idx(item, TItemHandle::IdMask);
        else
          ilist_item_traits<TListItem>::set_idx(item, m_nextFreeIdx);
        ilist_item_traits<TListItem>::set_gen(item, ilist_item_traits<TListItem>::gen(item) + 1);
 
        m_nextFreeIdx = handle.id();
        ++m_freeItems;
 
        return item;
      }
 
      void validate() const {
        bool hasThingsToRemove = m_freeItems > 0;
        if (!hasThingsToRemove)
          return;
 
        // If there's something to remove there has to be at least one entity left
        GAIA_ASSERT(!m_items.empty());
 
        auto freeItems = m_freeItems;
        auto nextFreeItem = m_nextFreeIdx;
        while (freeItems > 0) {
          GAIA_ASSERT(nextFreeItem < m_items.size() && "Item recycle list broken!");
 
          nextFreeItem = ilist_item_traits<TListItem>::idx(m_items[nextFreeItem]);
          --freeItems;
        }
 
        // At this point the index of the last item in list should
        // point to -1 because that's the tail of our implicit list.
        GAIA_ASSERT(nextFreeItem == TItemHandle::IdMask);
      }
    };
 
    template <typename TItemHandle, typename = void>
    struct ilist_handle_traits {
      static TItemHandle make(uint32_t id, uint32_t gen, const TItemHandle& prev) {
        (void)prev;
        return TItemHandle(id, gen);
      }
    };
 
    template <typename TItemHandle>
    struct ilist_handle_traits<TItemHandle, std::void_t<decltype(std::declval<const TItemHandle&>().prio())>> {
      static TItemHandle make(uint32_t id, uint32_t gen, const TItemHandle& prev) {
        return TItemHandle(id, gen, prev.prio());
      }
    };
 
    template <typename TListItem, typename TItemHandle, uint32_t MaxPages = 0>
    struct paged_ilist;
 
    template <typename TPagedIList, bool IsConst>
    class paged_ilist_iterator {
      using owner_type = std::conditional_t<IsConst, const TPagedIList, TPagedIList>;
      using owner_pointer = owner_type*;
 
      owner_pointer m_pOwner = nullptr;
      typename TPagedIList::size_type m_index = 0;
 
      void skip_dead() {
        while (m_pOwner != nullptr && m_index < m_pOwner->size() && !m_pOwner->has(m_index))
          ++m_index;
      }
 
    public:
      using value_type = typename TPagedIList::value_type;
      using reference =
          std::conditional_t<IsConst, typename TPagedIList::const_reference, typename TPagedIList::reference>;
      using pointer = std::conditional_t<IsConst, typename TPagedIList::const_pointer, typename TPagedIList::pointer>;
      using difference_type = typename TPagedIList::difference_type;
      using iterator_category = core::forward_iterator_tag;
 
      paged_ilist_iterator() = default;
 
      paged_ilist_iterator(owner_pointer pOwner, typename TPagedIList::size_type index):
          m_pOwner(pOwner), m_index(index) {
        skip_dead();
      }
 
      GAIA_NODISCARD reference operator*() const {
        return (*m_pOwner)[m_index];
      }
 
      GAIA_NODISCARD pointer operator->() const {
        return &(*m_pOwner)[m_index];
      }
 
      paged_ilist_iterator& operator++() {
        ++m_index;
        skip_dead();
        return *this;
      }
 
      paged_ilist_iterator operator++(int) {
        auto tmp = *this;
        ++(*this);
        return tmp;
      }
 
      GAIA_NODISCARD bool operator==(const paged_ilist_iterator& other) const {
        return m_pOwner == other.m_pOwner && m_index == other.m_index;
      }
 
      GAIA_NODISCARD bool operator!=(const paged_ilist_iterator& other) const {
        return !(*this == other);
      }
    };
 
    template <typename TListItem, typename TItemHandle, uint32_t MaxPages>
    struct paged_ilist {
      using value_type = TListItem;
      using reference = TListItem&;
      using const_reference = const TListItem&;
      using pointer = TListItem*;
      using const_pointer = const TListItem*;
      using difference_type = std::ptrdiff_t;
      using size_type = uint32_t;
      using iterator_category = core::forward_iterator_tag;
 
      static_assert(detail::ilist_traits_has_idx<TListItem>::value, "ilist_item_traits<T> must expose idx(const T&)");
      static_assert(
          detail::ilist_traits_has_set_idx<TListItem>::value, "ilist_item_traits<T> must expose set_idx(T&, uint32_t)");
      static_assert(detail::ilist_traits_has_gen<TListItem>::value, "ilist_item_traits<T> must expose gen(const T&)");
      static_assert(
          detail::ilist_traits_has_set_gen<TListItem>::value, "ilist_item_traits<T> must expose set_gen(T&, uint32_t)");
      static_assert(
          detail::ilist_has_create<TListItem>::value,
          "paged_ilist item type must expose static create(index, generation, ctx)");
      static_assert(
          detail::ilist_has_handle<TListItem, TItemHandle>::value,
          "paged_ilist item type must expose static handle(const item) returning the handle type");
      static_assert(detail::ilist_has_id_mask<TItemHandle>::value, "paged_ilist handle type must expose IdMask");
      static_assert(detail::ilist_has_handle_id<TItemHandle>::value, "paged_ilist handle type must expose id()");
      static_assert(detail::ilist_has_handle_gen<TItemHandle>::value, "paged_ilist handle type must expose gen()");
 
    private:
      static constexpr size_type target_payload_bytes() noexcept {
        return 16384;
      }
 
      static constexpr size_type min_page_capacity() noexcept {
        return 8;
      }
 
      static constexpr size_type max_page_capacity() noexcept {
        return 256;
      }
 
      static constexpr size_type calc_page_capacity() noexcept {
        const size_type payloadItemBytes = sizeof(TListItem) == 0 ? 1U : (size_type)sizeof(TListItem);
        const size_type desired = target_payload_bytes() / payloadItemBytes;
        if (desired < min_page_capacity())
          return min_page_capacity();
        if (desired > max_page_capacity())
          return max_page_capacity();
        return desired;
      }
 
      static constexpr size_type PageCapacity = calc_page_capacity();
      static constexpr bool FixedPageTable = MaxPages != 0;
      static constexpr size_type StaticPageCount = MaxPages == 0 ? 1U : MaxPages;
 
      struct page_type {
        using alive_mask_type = cnt::bitset<PageCapacity>;
        using storage_type = mem::raw_data_holder<TListItem, sizeof(TListItem) * PageCapacity>;
 
        alive_mask_type aliveMask;
        TItemHandle handles[PageCapacity]{};
        uint32_t nextFree[PageCapacity];
        uint32_t liveCount = 0;
        storage_type* pStorage = nullptr;
 
        page_type() {
          GAIA_FOR(PageCapacity)
          nextFree[i] = TItemHandle::IdMask;
        }
 
        ~page_type() {
          clear();
        }
 
        page_type(const page_type&) = delete;
        page_type& operator=(const page_type&) = delete;
 
        page_type(page_type&& other) noexcept:
            aliveMask(other.aliveMask), liveCount(other.liveCount), pStorage(other.pStorage) {
          GAIA_FOR(PageCapacity) {
            handles[i] = other.handles[i];
            nextFree[i] = other.nextFree[i];
          }
 
          other.aliveMask.reset();
          other.liveCount = 0;
          other.pStorage = nullptr;
        }
 
        page_type& operator=(page_type&& other) noexcept {
          GAIA_ASSERT(core::addressof(other) != this);
 
          clear();
          aliveMask = other.aliveMask;
          liveCount = other.liveCount;
          pStorage = other.pStorage;
          GAIA_FOR(PageCapacity) {
            handles[i] = other.handles[i];
            nextFree[i] = other.nextFree[i];
          }
 
          other.aliveMask.reset();
          other.liveCount = 0;
          other.pStorage = nullptr;
          return *this;
        }
 
        GAIA_NODISCARD pointer data() noexcept {
          return pStorage == nullptr ? nullptr : reinterpret_cast<pointer>((uint8_t*)*pStorage);
        }
 
        GAIA_NODISCARD const_pointer data() const noexcept {
          return pStorage == nullptr ? nullptr : reinterpret_cast<const_pointer>((const uint8_t*)*pStorage);
        }
 
        GAIA_NODISCARD pointer ptr(size_type slot) noexcept {
          GAIA_ASSERT(slot < PageCapacity);
          GAIA_ASSERT(pStorage != nullptr);
          return data() + slot;
        }
 
        GAIA_NODISCARD const_pointer ptr(size_type slot) const noexcept {
          GAIA_ASSERT(slot < PageCapacity);
          GAIA_ASSERT(pStorage != nullptr);
          return data() + slot;
        }
 
        void ensure_storage() {
          if GAIA_LIKELY (pStorage != nullptr)
            return;
 
          constexpr auto StorageAlignment =
              alignof(storage_type) < sizeof(void*) ? sizeof(void*) : alignof(storage_type);
          pStorage = mem::AllocHelper::alloc_alig<storage_type>("PagedIListPage", StorageAlignment);
        }
 
        void maybe_release_storage() {
          if (liveCount != 0 || pStorage == nullptr)
            return;
 
          mem::AllocHelper::free_alig("PagedIListPage", pStorage);
          pStorage = nullptr;
        }
 
        void clear() {
          if (pStorage != nullptr) {
            for (auto slot: aliveMask)
              core::call_dtor(ptr(slot));
            mem::AllocHelper::free_alig("PagedIListPage", pStorage);
            pStorage = nullptr;
          }
 
          aliveMask.reset();
          liveCount = 0;
          GAIA_FOR(PageCapacity)
          nextFree[i] = TItemHandle::IdMask;
        }
 
        template <typename... Args>
        void construct(size_type slot, Args&&... args) {
          GAIA_ASSERT(slot < PageCapacity);
          ensure_storage();
          core::call_ctor(ptr(slot), GAIA_FWD(args)...);
          aliveMask.set(slot);
          ++liveCount;
        }
 
        void destroy(size_type slot) {
          GAIA_ASSERT(slot < PageCapacity);
          GAIA_ASSERT(aliveMask.test(slot));
          core::call_dtor(ptr(slot));
          aliveMask.reset(slot);
          GAIA_ASSERT(liveCount > 0);
          --liveCount;
          maybe_release_storage();
        }
      };
 
      cnt::darray<page_type*> m_pages;
      page_type* m_staticPages[StaticPageCount]{};
      size_type m_size = 0;
 
    public:
      size_type m_nextFreeIdx = (size_type)-1;
      size_type m_freeItems = 0;
 
    private:
      GAIA_NODISCARD static constexpr size_type page_index(size_type index) noexcept {
        return index / PageCapacity;
      }
 
      GAIA_NODISCARD static constexpr size_type slot_index(size_type index) noexcept {
        return index % PageCapacity;
      }
 
      GAIA_NODISCARD static constexpr size_type page_count_for_slots(size_type slotCnt) noexcept {
        return slotCnt == 0 ? 0U : (slotCnt + PageCapacity - 1) / PageCapacity;
      }
 
    public:
      GAIA_NODISCARD static constexpr size_type page_capacity() noexcept {
        return PageCapacity;
      }
 
      GAIA_NODISCARD static constexpr size_type page_count_for_capacity(size_type slotCnt) noexcept {
        return page_count_for_slots(slotCnt);
      }
 
    private:
      GAIA_NODISCARD size_type page_table_size() const noexcept {
        if constexpr (FixedPageTable)
          return MaxPages;
        else
          return (size_type)m_pages.size();
      }
 
      GAIA_NODISCARD page_type*& page_slot(size_type pageIdx) noexcept {
        if constexpr (FixedPageTable) {
          GAIA_ASSERT(pageIdx < MaxPages);
          return m_staticPages[pageIdx];
        } else {
          GAIA_ASSERT(pageIdx < (size_type)m_pages.size());
          return m_pages[pageIdx];
        }
      }
 
      GAIA_NODISCARD const page_type* page_slot(size_type pageIdx) const noexcept {
        if constexpr (FixedPageTable) {
          GAIA_ASSERT(pageIdx < MaxPages);
          return m_staticPages[pageIdx];
        } else {
          GAIA_ASSERT(pageIdx < (size_type)m_pages.size());
          return m_pages[pageIdx];
        }
      }
 
      void clear_pages() {
        const auto pageCnt = page_table_size();
        GAIA_FOR(pageCnt) {
          auto*& pPage = page_slot(i);
          if (pPage == nullptr)
            continue;
          delete pPage;
          pPage = nullptr;
        }
 
        if constexpr (!FixedPageTable)
          m_pages.clear();
      }
 
      void ensure_page_count(size_type slotCnt) {
        const auto pageCnt = page_count_for_slots(slotCnt);
        if constexpr (FixedPageTable) {
          GAIA_ASSERT(pageCnt <= MaxPages && "Trying to allocate too many paged_ilist pages!");
        } else {
          if (pageCnt > (size_type)m_pages.size())
            m_pages.resize(pageCnt, nullptr);
        }
      }
 
      GAIA_NODISCARD page_type& ensure_page(size_type index) {
        ensure_page_count(index + 1);
        auto*& pPage = page_slot(page_index(index));
        if (pPage == nullptr)
          pPage = new page_type();
        return *pPage;
      }
 
      GAIA_NODISCARD page_type* try_page(size_type index) noexcept {
        const auto pageIdx = page_index(index);
        if (pageIdx >= page_table_size())
          return nullptr;
        return page_slot(pageIdx);
      }
 
      GAIA_NODISCARD const page_type* try_page(size_type index) const noexcept {
        const auto pageIdx = page_index(index);
        if (pageIdx >= page_table_size())
          return nullptr;
        return page_slot(pageIdx);
      }
 
      GAIA_NODISCARD reference slot_ref(size_type index) {
        auto* pPage = try_page(index);
        GAIA_ASSERT(pPage != nullptr);
        return *pPage->ptr(slot_index(index));
      }
 
      GAIA_NODISCARD const_reference slot_ref(size_type index) const {
        auto* pPage = try_page(index);
        GAIA_ASSERT(pPage != nullptr);
        return *pPage->ptr(slot_index(index));
      }
 
    public:
      using iterator = paged_ilist_iterator<paged_ilist, false>;
      using const_iterator = paged_ilist_iterator<paged_ilist, true>;
 
      ~paged_ilist() {
        clear_pages();
      }
 
      paged_ilist() = default;
      paged_ilist(const paged_ilist&) = delete;
      paged_ilist& operator=(const paged_ilist&) = delete;
      paged_ilist(paged_ilist&& other) noexcept:
          m_size(other.m_size), m_nextFreeIdx(other.m_nextFreeIdx), m_freeItems(other.m_freeItems) {
        if constexpr (FixedPageTable) {
          GAIA_FOR(MaxPages) {
            m_staticPages[i] = other.m_staticPages[i];
            other.m_staticPages[i] = nullptr;
          }
        } else {
          m_pages = GAIA_MOV(other.m_pages);
        }
 
        other.m_size = 0;
        other.m_nextFreeIdx = (size_type)-1;
        other.m_freeItems = 0;
      }
      paged_ilist& operator=(paged_ilist&& other) noexcept {
        GAIA_ASSERT(core::addressof(other) != this);
        clear_pages();
 
        if constexpr (FixedPageTable) {
          GAIA_FOR(MaxPages) {
            m_staticPages[i] = other.m_staticPages[i];
            other.m_staticPages[i] = nullptr;
          }
        } else {
          m_pages = GAIA_MOV(other.m_pages);
        }
 
        m_size = other.m_size;
        m_nextFreeIdx = other.m_nextFreeIdx;
        m_freeItems = other.m_freeItems;
 
        other.m_size = 0;
        other.m_nextFreeIdx = (size_type)-1;
        other.m_freeItems = 0;
        return *this;
      }
 
      GAIA_NODISCARD pointer data() noexcept {
        return nullptr;
      }
 
      GAIA_NODISCARD const_pointer data() const noexcept {
        return nullptr;
      }
 
      GAIA_NODISCARD bool has(size_type index) const noexcept {
        if (index >= m_size)
          return false;
 
        const auto* pPage = try_page(index);
        return pPage != nullptr && pPage->aliveMask.test(slot_index(index));
      }
 
      GAIA_NODISCARD bool has(TItemHandle handle) const noexcept {
        return has(handle.id()) && this->handle(handle.id()) == handle;
      }
 
      GAIA_NODISCARD TItemHandle handle(size_type index) const noexcept {
        GAIA_ASSERT(index < m_size);
        const auto* pPage = try_page(index);
        GAIA_ASSERT(pPage != nullptr);
        return pPage->handles[slot_index(index)];
      }
 
      GAIA_NODISCARD uint32_t generation(size_type index) const noexcept {
        return handle(index).gen();
      }
 
      GAIA_NODISCARD uint32_t next_free(size_type index) const noexcept {
        GAIA_ASSERT(index < m_size);
        const auto* pPage = try_page(index);
        GAIA_ASSERT(pPage != nullptr);
        return pPage->nextFree[slot_index(index)];
      }
 
      GAIA_NODISCARD reference operator[](size_type index) {
        GAIA_ASSERT(has(index));
        return slot_ref(index);
      }
 
      GAIA_NODISCARD const_reference operator[](size_type index) const {
        GAIA_ASSERT(has(index));
        return slot_ref(index);
      }
 
      void clear() {
        clear_pages();
        m_size = 0;
        m_nextFreeIdx = (size_type)-1;
        m_freeItems = 0;
      }
 
      GAIA_NODISCARD size_type get_next_free_item() const noexcept {
        return m_nextFreeIdx;
      }
 
      GAIA_NODISCARD size_type get_free_items() const noexcept {
        return m_freeItems;
      }
 
      GAIA_NODISCARD size_type item_count() const noexcept {
        return m_size - m_freeItems;
      }
 
      GAIA_NODISCARD size_type size() const noexcept {
        return m_size;
      }
 
      GAIA_NODISCARD bool empty() const noexcept {
        return m_size == 0;
      }
 
      GAIA_NODISCARD size_type capacity() const noexcept {
        if constexpr (FixedPageTable)
          return MaxPages * PageCapacity;
        else
          return (size_type)m_pages.capacity() * PageCapacity;
      }
 
      GAIA_NODISCARD iterator begin() noexcept {
        return iterator(this, 0);
      }
 
      GAIA_NODISCARD const_iterator begin() const noexcept {
        return const_iterator(this, 0);
      }
 
      GAIA_NODISCARD const_iterator cbegin() const noexcept {
        return const_iterator(this, 0);
      }
 
      GAIA_NODISCARD iterator end() noexcept {
        return iterator(this, m_size);
      }
 
      GAIA_NODISCARD const_iterator end() const noexcept {
        return const_iterator(this, m_size);
      }
 
      GAIA_NODISCARD const_iterator cend() const noexcept {
        return const_iterator(this, m_size);
      }
 
      void reserve(size_type cap) {
        const auto pageCnt = page_count_for_slots(cap);
        if constexpr (FixedPageTable) {
          GAIA_ASSERT(pageCnt <= MaxPages);
        } else {
          m_pages.reserve(pageCnt);
        }
      }
 
      void reserve_slot_table(size_type cap) {
        const auto pageCnt = page_count_for_slots(cap);
        if constexpr (FixedPageTable) {
          GAIA_ASSERT(pageCnt <= MaxPages);
        } else {
          ensure_page_count(cap);
        }
      }
 
      GAIA_NODISCARD reference live_unsafe(size_type index) {
        auto* pPage = try_page(index);
        GAIA_ASSERT(pPage != nullptr);
        const auto slot = slot_index(index);
        GAIA_ASSERT(pPage->aliveMask.test(slot));
        return *pPage->ptr(slot);
      }
 
      GAIA_NODISCARD const_reference live_unsafe(size_type index) const {
        const auto* pPage = try_page(index);
        GAIA_ASSERT(pPage != nullptr);
        const auto slot = slot_index(index);
        GAIA_ASSERT(pPage->aliveMask.test(slot));
        return *pPage->ptr(slot);
      }
 
      GAIA_NODISCARD reference payload_unsafe(size_type index) {
        auto* pPage = try_page(index);
        GAIA_ASSERT(pPage != nullptr);
        return *pPage->ptr(slot_index(index));
      }
 
      GAIA_NODISCARD const_reference payload_unsafe(size_type index) const {
        const auto* pPage = try_page(index);
        GAIA_ASSERT(pPage != nullptr);
        return *pPage->ptr(slot_index(index));
      }
 
      GAIA_NODISCARD pointer try_get(size_type index) noexcept {
        if (!has(index))
          return nullptr;
        return &slot_ref(index);
      }
 
      GAIA_NODISCARD const_pointer try_get(size_type index) const noexcept {
        if (!has(index))
          return nullptr;
        return &slot_ref(index);
      }
 
      void add_live(TListItem&& item) {
        const auto index = (size_type)ilist_item_traits<TListItem>::idx(item);
        auto& page = ensure_page(index);
        const auto slot = slot_index(index);
        const bool existed = index < m_size;
        const bool wasAlive = existed && page.aliveMask.test(slot);
        const bool wasFree = existed && !wasAlive;
 
        if (index >= m_size)
          m_size = index + 1;
        else if (wasAlive)
          page.destroy(slot);
        else if (wasFree && m_freeItems > 0)
          --m_freeItems;
 
        page.construct(slot, GAIA_MOV(item));
        page.handles[slot] = TListItem::handle(*page.ptr(slot));
        page.nextFree[slot] = TItemHandle::IdMask;
      }
 
      void add_free(TItemHandle handle, uint32_t nextFreeIdx) {
        const auto index = (size_type)handle.id();
        auto& page = ensure_page(index);
        const auto slot = slot_index(index);
        const bool existed = index < m_size;
        const bool wasAlive = existed && page.aliveMask.test(slot);
        const bool wasFree = existed && !wasAlive;
 
        if (index >= m_size)
          m_size = index + 1;
        else if (wasAlive)
          page.destroy(slot);
 
        page.handles[slot] = handle;
        page.nextFree[slot] = nextFreeIdx;
        if (!wasFree)
          ++m_freeItems;
        if (m_nextFreeIdx == (size_type)-1)
          m_nextFreeIdx = index;
      }
 
      void add_free(size_type index, uint32_t generation, uint32_t nextFreeIdx) {
        add_free(ilist_handle_traits<TItemHandle>::make(index, generation, TItemHandle{}), nextFreeIdx);
      }
 
      GAIA_NODISCARD TItemHandle alloc(void* ctx) {
        size_type index = 0;
        uint32_t generation = 0;
 
        if GAIA_UNLIKELY (m_freeItems == 0U) {
          index = m_size;
          GAIA_ASSERT(index < TItemHandle::IdMask && "Trying to allocate too many items!");
          ++m_size;
        } else {
          GAIA_ASSERT(m_nextFreeIdx < m_size && "Item recycle list broken!");
          index = m_nextFreeIdx;
          auto& page = ensure_page(index);
          const auto slot = slot_index(index);
          m_nextFreeIdx = page.nextFree[slot];
          page.nextFree[slot] = TItemHandle::IdMask;
          generation = page.handles[slot].gen();
          --m_freeItems;
          if (page.aliveMask.test(slot))
            page.destroy(slot);
        }
 
        auto& page = ensure_page(index);
        const auto slot = slot_index(index);
        page.construct(slot, TListItem::create(index, generation, ctx));
        page.handles[slot] = TListItem::handle(*page.ptr(slot));
        page.nextFree[slot] = TItemHandle::IdMask;
        return page.handles[slot];
      }
 
      GAIA_NODISCARD TItemHandle alloc() {
        size_type index = 0;
        uint32_t generation = 0;
 
        if GAIA_UNLIKELY (m_freeItems == 0U) {
          index = m_size;
          GAIA_ASSERT(index < TItemHandle::IdMask && "Trying to allocate too many items!");
          ++m_size;
        } else {
          GAIA_ASSERT(m_nextFreeIdx < m_size && "Item recycle list broken!");
          index = m_nextFreeIdx;
          auto& page = ensure_page(index);
          const auto slot = slot_index(index);
          m_nextFreeIdx = page.nextFree[slot];
          page.nextFree[slot] = TItemHandle::IdMask;
          generation = page.handles[slot].gen();
          --m_freeItems;
          if (page.aliveMask.test(slot))
            page.destroy(slot);
        }
 
        auto& page = ensure_page(index);
        const auto slot = slot_index(index);
        page.construct(slot, TListItem(index, generation));
        page.handles[slot] = ilist_handle_traits<TItemHandle>::make(index, generation, TItemHandle{});
        page.nextFree[slot] = TItemHandle::IdMask;
        return page.handles[slot];
      }
 
      void free(TItemHandle handle) {
        GAIA_ASSERT(has(handle));
 
        auto& page = ensure_page(handle.id());
        const auto slot = slot_index(handle.id());
        page.destroy(slot);
        page.handles[slot] = ilist_handle_traits<TItemHandle>::make(handle.id(), handle.gen() + 1, page.handles[slot]);
        page.nextFree[slot] = m_freeItems == 0 ? TItemHandle::IdMask : m_nextFreeIdx;
        m_nextFreeIdx = handle.id();
        ++m_freeItems;
      }
 
      void free_keep_live(TItemHandle handle) {
        GAIA_ASSERT(has(handle));
 
        auto& page = ensure_page(handle.id());
        const auto slot = slot_index(handle.id());
        page.handles[slot] = ilist_handle_traits<TItemHandle>::make(handle.id(), handle.gen() + 1, page.handles[slot]);
        page.nextFree[slot] = m_freeItems == 0 ? TItemHandle::IdMask : m_nextFreeIdx;
        m_nextFreeIdx = handle.id();
        ++m_freeItems;
      }
 
      void validate() const {
        if (m_freeItems == 0)
          return;
 
        auto freeItems = m_freeItems;
        auto nextFreeItem = m_nextFreeIdx;
        while (freeItems > 0) {
          GAIA_ASSERT(nextFreeItem < m_size && "Item recycle list broken!");
 
          nextFreeItem = next_free(nextFreeItem);
          --freeItems;
        }
 
        GAIA_ASSERT(nextFreeItem == TItemHandle::IdMask);
      }
    };
  } // namespace cnt
} // namespace gaia