#ifndef ENTT_ENTITY_REGISTRY_HPP
#define ENTT_ENTITY_REGISTRY_HPP

#include <algorithm>
#include <cstddef>
#include <iterator>
#include <memory>
#include <tuple>
#include <type_traits>
#include <utility>
#include <vector>
#include "../config/config.h"
#include "../container/dense_map.hpp"
#include "../core/algorithm.hpp"
#include "../core/any.hpp"
#include "../core/fwd.hpp"
#include "../core/iterator.hpp"
#include "../core/type_info.hpp"
#include "../core/type_traits.hpp"
#include "../core/utility.hpp"
#include "component.hpp"
#include "entity.hpp"
#include "fwd.hpp"
#include "group.hpp"
#include "runtime_view.hpp"
#include "sparse_set.hpp"
#include "storage.hpp"
#include "utility.hpp"
#include "view.hpp"

namespace entt {

/**
 * @cond TURN_OFF_DOXYGEN
 * Internal details not to be documented.
 */

namespace internal {

template<typename It>
class storage_proxy_iterator final {
    template<typename Other>
    friend class storage_proxy_iterator;

    using mapped_type = std::remove_reference_t<decltype(std::declval<It>()->second)>;

public:
    using value_type = std::pair<id_type, constness_as_t<typename mapped_type::element_type, mapped_type> &>;
    using pointer = input_iterator_pointer<value_type>;
    using reference = value_type;
    using difference_type = std::ptrdiff_t;
    using iterator_category = std::input_iterator_tag;

    storage_proxy_iterator() ENTT_NOEXCEPT
        : it{} {}

    storage_proxy_iterator(const It iter) ENTT_NOEXCEPT
        : it{iter} {}

    template<typename Other, typename = std::enable_if_t<!std::is_same_v<It, Other> && std::is_constructible_v<It, Other>>>
    storage_proxy_iterator(const storage_proxy_iterator<Other> &other) ENTT_NOEXCEPT
        : it{other.it} {}

    storage_proxy_iterator &operator++() ENTT_NOEXCEPT {
        return ++it, *this;
    }

    storage_proxy_iterator operator++(int) ENTT_NOEXCEPT {
        storage_proxy_iterator orig = *this;
        return ++(*this), orig;
    }

    storage_proxy_iterator &operator--() ENTT_NOEXCEPT {
        return --it, *this;
    }

    storage_proxy_iterator operator--(int) ENTT_NOEXCEPT {
        storage_proxy_iterator orig = *this;
        return operator--(), orig;
    }

    storage_proxy_iterator &operator+=(const difference_type value) ENTT_NOEXCEPT {
        it += value;
        return *this;
    }

    storage_proxy_iterator operator+(const difference_type value) const ENTT_NOEXCEPT {
        storage_proxy_iterator copy = *this;
        return (copy += value);
    }

    storage_proxy_iterator &operator-=(const difference_type value) ENTT_NOEXCEPT {
        return (*this += -value);
    }

    storage_proxy_iterator operator-(const difference_type value) const ENTT_NOEXCEPT {
        return (*this + -value);
    }

    [[nodiscard]] reference operator[](const difference_type value) const ENTT_NOEXCEPT {
        return {it[value].first, *it[value].second};
    }

    [[nodiscard]] reference operator*() const ENTT_NOEXCEPT {
        return {it->first, *it->second};
    }

    [[nodiscard]] pointer operator->() const ENTT_NOEXCEPT {
        return operator*();
    }

    template<typename ILhs, typename IRhs>
    friend std::ptrdiff_t operator-(const storage_proxy_iterator<ILhs> &, const storage_proxy_iterator<IRhs> &) ENTT_NOEXCEPT;

    template<typename ILhs, typename IRhs>
    friend bool operator==(const storage_proxy_iterator<ILhs> &, const storage_proxy_iterator<IRhs> &) ENTT_NOEXCEPT;

    template<typename ILhs, typename IRhs>
    friend bool operator<(const storage_proxy_iterator<ILhs> &, const storage_proxy_iterator<IRhs> &) ENTT_NOEXCEPT;

private:
    It it;
};

template<typename ILhs, typename IRhs>
[[nodiscard]] std::ptrdiff_t operator-(const storage_proxy_iterator<ILhs> &lhs, const storage_proxy_iterator<IRhs> &rhs) ENTT_NOEXCEPT {
    return lhs.it - rhs.it;
}

template<typename ILhs, typename IRhs>
[[nodiscard]] bool operator==(const storage_proxy_iterator<ILhs> &lhs, const storage_proxy_iterator<IRhs> &rhs) ENTT_NOEXCEPT {
    return lhs.it == rhs.it;
}

template<typename ILhs, typename IRhs>
[[nodiscard]] bool operator!=(const storage_proxy_iterator<ILhs> &lhs, const storage_proxy_iterator<IRhs> &rhs) ENTT_NOEXCEPT {
    return !(lhs == rhs);
}

template<typename ILhs, typename IRhs>
[[nodiscard]] bool operator<(const storage_proxy_iterator<ILhs> &lhs, const storage_proxy_iterator<IRhs> &rhs) ENTT_NOEXCEPT {
    return lhs.it < rhs.it;
}

template<typename ILhs, typename IRhs>
[[nodiscard]] bool operator>(const storage_proxy_iterator<ILhs> &lhs, const storage_proxy_iterator<IRhs> &rhs) ENTT_NOEXCEPT {
    return rhs < lhs;
}

template<typename ILhs, typename IRhs>
[[nodiscard]] bool operator<=(const storage_proxy_iterator<ILhs> &lhs, const storage_proxy_iterator<IRhs> &rhs) ENTT_NOEXCEPT {
    return !(lhs > rhs);
}

template<typename ILhs, typename IRhs>
[[nodiscard]] bool operator>=(const storage_proxy_iterator<ILhs> &lhs, const storage_proxy_iterator<IRhs> &rhs) ENTT_NOEXCEPT {
    return !(lhs < rhs);
}

struct registry_context {
    template<typename Type, typename... Args>
    Type &emplace_hint(const id_type id, Args &&...args) {
        return any_cast<Type &>(data.try_emplace(id, std::in_place_type<Type>, std::forward<Args>(args)...).first->second);
    }

    template<typename Type, typename... Args>
    Type &emplace(Args &&...args) {
        return emplace_hint<Type>(type_id<Type>().hash(), std::forward<Args>(args)...);
    }

    template<typename Type>
    bool erase(const id_type id = type_id<Type>().hash()) {
        const auto it = data.find(id);
        return it != data.end() && it->second.type() == type_id<Type>() ? (data.erase(it), true) : false;
    }

    template<typename Type>
    [[nodiscard]] std::add_const_t<Type> &at(const id_type id = type_id<Type>().hash()) const {
        return any_cast<std::add_const_t<Type> &>(data.at(id));
    }

    template<typename Type>
    [[nodiscard]] Type &at(const id_type id = type_id<Type>().hash()) {
        return any_cast<Type &>(data.at(id));
    }

    template<typename Type>
    [[nodiscard]] std::add_const_t<Type> *find(const id_type id = type_id<Type>().hash()) const {
        const auto it = data.find(id);
        return it != data.cend() ? any_cast<std::add_const_t<Type>>(&it->second) : nullptr;
    }

    template<typename Type>
    [[nodiscard]] Type *find(const id_type id = type_id<Type>().hash()) {
        const auto it = data.find(id);
        return it != data.end() ? any_cast<Type>(&it->second) : nullptr;
    }

    template<typename Type>
    [[nodiscard]] bool contains(const id_type id = type_id<Type>().hash()) const {
        const auto it = data.find(id);
        return it != data.end() && it->second.type() == type_id<Type>();
    }

private:
    dense_map<id_type, basic_any<0u>, identity> data;
};

} // namespace internal

/**
 * Internal details not to be documented.
 * @endcond
 */

/**
 * @brief Fast and reliable entity-component system.
 * @tparam Entity A valid entity type (see entt_traits for more details).
 */
template<typename Entity>
class basic_registry {
    using entity_traits = entt_traits<Entity>;
    using basic_common_type = basic_sparse_set<Entity>;

    template<typename Component>
    using storage_type = typename storage_traits<Entity, Component>::storage_type;

    template<typename...>
    struct group_handler;

    template<typename... Exclude, typename... Get, typename... Owned>
    struct group_handler<exclude_t<Exclude...>, get_t<Get...>, Owned...> {
        // nasty workaround for an issue with the toolset v141 that doesn't accept a fold expression here
        static_assert(!std::disjunction_v<std::bool_constant<component_traits<Owned>::in_place_delete>...>, "Groups do not support in-place delete");
        std::conditional_t<sizeof...(Owned) == 0, basic_common_type, std::size_t> current{};

        template<typename Component>
        void maybe_valid_if(basic_registry &owner, const Entity entt) {
            [[maybe_unused]] const auto cpools = std::forward_as_tuple(owner.assure<Owned>()...);

            const auto is_valid = ((std::is_same_v<Component, Owned> || std::get<storage_type<Owned> &>(cpools).contains(entt)) && ...)
                                  && ((std::is_same_v<Component, Get> || owner.assure<Get>().contains(entt)) && ...)
                                  && ((std::is_same_v<Component, Exclude> || !owner.assure<Exclude>().contains(entt)) && ...);

            if constexpr(sizeof...(Owned) == 0) {
                if(is_valid && !current.contains(entt)) {
                    current.emplace(entt);
                }
            } else {
                if(is_valid && !(std::get<0>(cpools).index(entt) < current)) {
                    const auto pos = current++;
                    (std::get<storage_type<Owned> &>(cpools).swap_elements(std::get<storage_type<Owned> &>(cpools).data()[pos], entt), ...);
                }
            }
        }

        void discard_if([[maybe_unused]] basic_registry &owner, const Entity entt) {
            if constexpr(sizeof...(Owned) == 0) {
                current.remove(entt);
            } else {
                if(const auto cpools = std::forward_as_tuple(owner.assure<Owned>()...); std::get<0>(cpools).contains(entt) && (std::get<0>(cpools).index(entt) < current)) {
                    const auto pos = --current;
                    (std::get<storage_type<Owned> &>(cpools).swap_elements(std::get<storage_type<Owned> &>(cpools).data()[pos], entt), ...);
                }
            }
        }
    };

    struct group_data {
        std::size_t size;
        std::unique_ptr<void, void (*)(void *)> group;
        bool (*owned)(const id_type) ENTT_NOEXCEPT;
        bool (*get)(const id_type) ENTT_NOEXCEPT;
        bool (*exclude)(const id_type) ENTT_NOEXCEPT;
    };

    template<typename Component>
    [[nodiscard]] auto &assure(const id_type id = type_hash<Component>::value()) {
        static_assert(std::is_same_v<Component, std::decay_t<Component>>, "Non-decayed types not allowed");
        auto &&cpool = pools[id];

        if(!cpool) {
            cpool.reset(new storage_type<Component>{});
            cpool->bind(forward_as_any(*this));
        }

        ENTT_ASSERT(cpool->type() == type_id<Component>(), "Unexpected type");
        return static_cast<storage_type<Component> &>(*cpool);
    }

    template<typename Component>
    [[nodiscard]] const auto &assure(const id_type id = type_hash<Component>::value()) const {
        static_assert(std::is_same_v<Component, std::decay_t<Component>>, "Non-decayed types not allowed");

        if(const auto it = pools.find(id); it != pools.cend()) {
            ENTT_ASSERT(it->second->type() == type_id<Component>(), "Unexpected type");
            return static_cast<const storage_type<Component> &>(*it->second);
        }

        static storage_type<Component> placeholder{};
        return placeholder;
    }

    auto generate_identifier(const std::size_t pos) ENTT_NOEXCEPT {
        ENTT_ASSERT(pos < entity_traits::to_entity(null), "No entities available");
        return entity_traits::combine(static_cast<typename entity_traits::entity_type>(pos), {});
    }

    auto recycle_identifier() ENTT_NOEXCEPT {
        ENTT_ASSERT(free_list != null, "No entities available");
        const auto curr = entity_traits::to_entity(free_list);
        free_list = entity_traits::combine(entity_traits::to_integral(entities[curr]), tombstone);
        return (entities[curr] = entity_traits::combine(curr, entity_traits::to_integral(entities[curr])));
    }

    auto release_entity(const Entity entity, const typename entity_traits::version_type version) {
        const typename entity_traits::version_type vers = version + (version == entity_traits::to_version(tombstone));
        entities[entity_traits::to_entity(entity)] = entity_traits::construct(entity_traits::to_integral(free_list), vers);
        free_list = entity_traits::combine(entity_traits::to_integral(entity), tombstone);
        return vers;
    }

public:
    /*! @brief Underlying entity identifier. */
    using entity_type = Entity;
    /*! @brief Underlying version type. */
    using version_type = typename entity_traits::version_type;
    /*! @brief Unsigned integer type. */
    using size_type = std::size_t;
    /*! @brief Common type among all storage types. */
    using base_type = basic_common_type;
    /*! @brief Context type. */
    using context = internal::registry_context;

    /*! @brief Default constructor. */
    basic_registry()
        : pools{},
          groups{},
          entities{},
          free_list{tombstone},
          vars{} {}

    /**
     * @brief Allocates enough memory upon construction to store `count` pools.
     * @param count The number of pools to allocate memory for.
     */
    basic_registry(const size_type count)
        : pools{},
          groups{},
          entities{},
          free_list{tombstone},
          vars{} {
        pools.reserve(count);
    }

    /**
     * @brief Move constructor.
     * @param other The instance to move from.
     */
    basic_registry(basic_registry &&other)
        : pools{std::move(other.pools)},
          groups{std::move(other.groups)},
          entities{std::move(other.entities)},
          free_list{other.free_list},
          vars{std::move(other.vars)} {
        for(auto &&curr: pools) {
            curr.second->bind(forward_as_any(*this));
        }
    }

    /**
     * @brief Move assignment operator.
     * @param other The instance to move from.
     * @return This registry.
     */
    basic_registry &operator=(basic_registry &&other) {
        pools = std::move(other.pools);
        groups = std::move(other.groups);
        entities = std::move(other.entities);
        free_list = other.free_list;
        vars = std::move(other.vars);

        for(auto &&curr: pools) {
            curr.second->bind(forward_as_any(*this));
        }

        return *this;
    }

    /**
     * @brief Returns an iterable object to use to _visit_ a registry.
     *
     * The iterable object returns a pair that contains the name and a reference
     * to the current storage.
     *
     * @return An iterable object to use to _visit_ the registry.
     */
    [[nodiscard]] auto storage() ENTT_NOEXCEPT {
        return iterable_adaptor{internal::storage_proxy_iterator{pools.begin()}, internal::storage_proxy_iterator{pools.end()}};
    }

    /*! @copydoc storage */
    [[nodiscard]] auto storage() const ENTT_NOEXCEPT {
        return iterable_adaptor{internal::storage_proxy_iterator{pools.cbegin()}, internal::storage_proxy_iterator{pools.cend()}};
    }

    /**
     * @brief Finds the storage associated with a given name, if any.
     * @param id Name used to map the storage within the registry.
     * @return An iterator to the given storage if it's found, past the end
     * iterator otherwise.
     */
    [[nodiscard]] auto storage(const id_type id) {
        return internal::storage_proxy_iterator{pools.find(id)};
    }

    /**
     * @brief Finds the storage associated with a given name, if any.
     * @param id Name used to map the storage within the registry.
     * @return An iterator to the given storage if it's found, past the end
     * iterator otherwise.
     */
    [[nodiscard]] auto storage(const id_type id) const {
        return internal::storage_proxy_iterator{pools.find(id)};
    }

    /**
     * @brief Returns the storage for a given component type.
     * @tparam Component Type of component of which to return the storage.
     * @param id Optional name used to map the storage within the registry.
     * @return The storage for the given component type.
     */
    template<typename Component>
    decltype(auto) storage(const id_type id = type_hash<std::remove_const_t<Component>>::value()) {
        if constexpr(std::is_const_v<Component>) {
            return std::as_const(*this).template storage<std::remove_const_t<Component>>(id);
        } else {
            return assure<Component>(id);
        }
    }

    /**
     * @brief Returns the storage for a given component type.
     *
     * @warning
     * If a storage for the given component doesn't exist yet, a temporary
     * placeholder is returned instead.
     *
     * @tparam Component Type of component of which to return the storage.
     * @param id Optional name used to map the storage within the registry.
     * @return The storage for the given component type.
     */
    template<typename Component>
    decltype(auto) storage(const id_type id = type_hash<std::remove_const_t<Component>>::value()) const {
        return assure<std::remove_const_t<Component>>(id);
    }

    /**
     * @brief Returns the number of entities created so far.
     * @return Number of entities created so far.
     */
    [[nodiscard]] size_type size() const ENTT_NOEXCEPT {
        return entities.size();
    }

    /**
     * @brief Returns the number of entities still in use.
     * @return Number of entities still in use.
     */
    [[nodiscard]] size_type alive() const {
        auto sz = entities.size();

        for(auto curr = free_list; curr != null; --sz) {
            curr = entities[entity_traits::to_entity(curr)];
        }

        return sz;
    }

    /**
     * @brief Increases the capacity (number of entities) of the registry.
     * @param cap Desired capacity.
     */
    void reserve(const size_type cap) {
        entities.reserve(cap);
    }

    /**
     * @brief Returns the number of entities that a registry has currently
     * allocated space for.
     * @return Capacity of the registry.
     */
    [[nodiscard]] size_type capacity() const ENTT_NOEXCEPT {
        return entities.capacity();
    }

    /**
     * @brief Checks whether the registry is empty (no entities still in use).
     * @return True if the registry is empty, false otherwise.
     */
    [[nodiscard]] bool empty() const {
        return !alive();
    }

    /**
     * @brief Direct access to the list of entities of a registry.
     *
     * The returned pointer is such that range `[data(), data() + size())` is
     * always a valid range, even if the registry is empty.
     *
     * @warning
     * This list contains both valid and destroyed entities and isn't suitable
     * for direct use.
     *
     * @return A pointer to the array of entities.
     */
    [[nodiscard]] const entity_type *data() const ENTT_NOEXCEPT {
        return entities.data();
    }

    /**
     * @brief Returns the head of the list of released entities.
     *
     * This function is intended for use in conjunction with `assign`.<br/>
     * The returned entity has an invalid identifier in all cases.
     *
     * @return The head of the list of released entities.
     */
    [[nodiscard]] entity_type released() const ENTT_NOEXCEPT {
        return free_list;
    }

    /**
     * @brief Checks if an identifier refers to a valid entity.
     * @param entity An identifier, either valid or not.
     * @return True if the identifier is valid, false otherwise.
     */
    [[nodiscard]] bool valid(const entity_type entity) const {
        const auto pos = size_type(entity_traits::to_entity(entity));
        return (pos < entities.size() && entities[pos] == entity);
    }

    /**
     * @brief Returns the actual version for an identifier.
     * @param entity A valid identifier.
     * @return The version for the given identifier if valid, the tombstone
     * version otherwise.
     */
    [[nodiscard]] version_type current(const entity_type entity) const {
        const auto pos = size_type(entity_traits::to_entity(entity));
        return entity_traits::to_version(pos < entities.size() ? entities[pos] : tombstone);
    }

    /**
     * @brief Creates a new entity or recycles a destroyed one.
     * @return A valid identifier.
     */
    [[nodiscard]] entity_type create() {
        return (free_list == null) ? entities.emplace_back(generate_identifier(entities.size())) : recycle_identifier();
    }

    /**
     * @copybrief create
     *
     * If the requested entity isn't in use, the suggested identifier is used.
     * Otherwise, a new identifier is generated.
     *
     * @param hint Required identifier.
     * @return A valid identifier.
     */
    [[nodiscard]] entity_type create(const entity_type hint) {
        const auto length = entities.size();

        if(hint == null || hint == tombstone) {
            return create();
        } else if(const auto req = entity_traits::to_entity(hint); !(req < length)) {
            entities.resize(size_type(req) + 1u, null);

            for(auto pos = length; pos < req; ++pos) {
                release_entity(generate_identifier(pos), {});
            }

            return (entities[req] = hint);
        } else if(const auto curr = entity_traits::to_entity(entities[req]); req == curr) {
            return create();
        } else {
            auto *it = &free_list;
            for(; entity_traits::to_entity(*it) != req; it = &entities[entity_traits::to_entity(*it)]) {}
            *it = entity_traits::combine(curr, entity_traits::to_integral(*it));
            return (entities[req] = hint);
        }
    }

    /**
     * @brief Assigns each element in a range an identifier.
     *
     * @sa create
     *
     * @tparam It Type of forward iterator.
     * @param first An iterator to the first element of the range to generate.
     * @param last An iterator past the last element of the range to generate.
     */
    template<typename It>
    void create(It first, It last) {
        for(; free_list != null && first != last; ++first) {
            *first = recycle_identifier();
        }

        const auto length = entities.size();
        entities.resize(length + std::distance(first, last), null);

        for(auto pos = length; first != last; ++first, ++pos) {
            *first = entities[pos] = generate_identifier(pos);
        }
    }

    /**
     * @brief Assigns identifiers to an empty registry.
     *
     * This function is intended for use in conjunction with `data`, `size` and
     * `destroyed`.<br/>
     * Don't try to inject ranges of randomly generated entities nor the _wrong_
     * head for the list of destroyed entities. There is no guarantee that a
     * registry will continue to work properly in this case.
     *
     * @warning
     * There must be no entities still alive for this to work properly.
     *
     * @tparam It Type of input iterator.
     * @param first An iterator to the first element of the range of entities.
     * @param last An iterator past the last element of the range of entities.
     * @param destroyed The head of the list of destroyed entities.
     */
    template<typename It>
    void assign(It first, It last, const entity_type destroyed) {
        ENTT_ASSERT(!alive(), "Entities still alive");
        entities.assign(first, last);
        free_list = destroyed;
    }

    /**
     * @brief Releases an identifier.
     *
     * The version is updated and the identifier can be recycled at any time.
     *
     * @warning
     * Attempting to use an invalid entity results in undefined behavior.
     *
     * @param entity A valid identifier.
     * @return The version of the recycled entity.
     */
    version_type release(const entity_type entity) {
        return release(entity, static_cast<version_type>(entity_traits::to_version(entity) + 1u));
    }

    /**
     * @brief Releases an identifier.
     *
     * The suggested version or the valid version closest to the suggested one
     * is used instead of the implicitly generated version.
     *
     * @sa release
     *
     * @param entity A valid identifier.
     * @param version A desired version upon destruction.
     * @return The version actually assigned to the entity.
     */
    version_type release(const entity_type entity, const version_type version) {
        ENTT_ASSERT(orphan(entity), "Non-orphan entity");
        return release_entity(entity, version);
    }

    /**
     * @brief Releases all identifiers in a range.
     *
     * @sa release
     *
     * @tparam It Type of input iterator.
     * @param first An iterator to the first element of the range of entities.
     * @param last An iterator past the last element of the range of entities.
     */
    template<typename It>
    void release(It first, It last) {
        for(; first != last; ++first) {
            release(*first);
        }
    }

    /**
     * @brief Destroys an entity and releases its identifier.
     *
     * @sa release
     *
     * @warning
     * Adding or removing components to an entity that is being destroyed can
     * result in undefined behavior. Attempting to use an invalid entity results
     * in undefined behavior.
     *
     * @param entity A valid identifier.
     * @return The version of the recycled entity.
     */
    version_type destroy(const entity_type entity) {
        return destroy(entity, static_cast<version_type>(entity_traits::to_version(entity) + 1u));
    }

    /**
     * @brief Destroys an entity and releases its identifier.
     *
     * The suggested version or the valid version closest to the suggested one
     * is used instead of the implicitly generated version.
     *
     * @sa destroy
     *
     * @param entity A valid identifier.
     * @param version A desired version upon destruction.
     * @return The version actually assigned to the entity.
     */
    version_type destroy(const entity_type entity, const version_type version) {
        ENTT_ASSERT(valid(entity), "Invalid entity");

        for(size_type pos = pools.size(); pos; --pos) {
            pools.begin()[pos - 1u].second->remove(entity);
        }

        return release_entity(entity, version);
    }

    /**
     * @brief Destroys all entities in a range and releases their identifiers.
     *
     * @sa destroy
     *
     * @tparam It Type of input iterator.
     * @param first An iterator to the first element of the range of entities.
     * @param last An iterator past the last element of the range of entities.
     */
    template<typename It>
    void destroy(It first, It last) {
        for(; first != last; ++first) {
            destroy(*first);
        }
    }

    /**
     * @brief Assigns the given component to an entity.
     *
     * The component must have a proper constructor or be of aggregate type.
     *
     * @warning
     * Attempting to use an invalid entity or to assign a component to an entity
     * that already owns it results in undefined behavior.
     *
     * @tparam Component Type of component to create.
     * @tparam Args Types of arguments to use to construct the component.
     * @param entity A valid identifier.
     * @param args Parameters to use to initialize the component.
     * @return A reference to the newly created component.
     */
    template<typename Component, typename... Args>
    decltype(auto) emplace(const entity_type entity, Args &&...args) {
        ENTT_ASSERT(valid(entity), "Invalid entity");
        return assure<Component>().emplace(entity, std::forward<Args>(args)...);
    }

    /**
     * @brief Assigns each entity in a range the given component.
     *
     * @sa emplace
     *
     * @tparam Component Type of component to create.
     * @tparam It Type of input iterator.
     * @param first An iterator to the first element of the range of entities.
     * @param last An iterator past the last element of the range of entities.
     * @param value An instance of the component to assign.
     */
    template<typename Component, typename It>
    void insert(It first, It last, const Component &value = {}) {
        ENTT_ASSERT(std::all_of(first, last, [this](const auto entity) { return valid(entity); }), "Invalid entity");
        assure<Component>().insert(first, last, value);
    }

    /**
     * @brief Assigns each entity in a range the given components.
     *
     * @sa emplace
     *
     * @tparam Component Type of component to create.
     * @tparam EIt Type of input iterator.
     * @tparam CIt Type of input iterator.
     * @param first An iterator to the first element of the range of entities.
     * @param last An iterator past the last element of the range of entities.
     * @param from An iterator to the first element of the range of components.
     */
    template<typename Component, typename EIt, typename CIt, typename = std::enable_if_t<std::is_same_v<typename std::iterator_traits<CIt>::value_type, Component>>>
    void insert(EIt first, EIt last, CIt from) {
        ENTT_ASSERT(std::all_of(first, last, [this](const auto entity) { return valid(entity); }), "Invalid entity");
        assure<Component>().insert(first, last, from);
    }

    /**
     * @brief Assigns or replaces the given component for an entity.
     *
     * @warning
     * Attempting to use an invalid entity results in undefined behavior.
     *
     * @tparam Component Type of component to assign or replace.
     * @tparam Args Types of arguments to use to construct the component.
     * @param entity A valid identifier.
     * @param args Parameters to use to initialize the component.
     * @return A reference to the newly created component.
     */
    template<typename Component, typename... Args>
    decltype(auto) emplace_or_replace(const entity_type entity, Args &&...args) {
        ENTT_ASSERT(valid(entity), "Invalid entity");
        auto &cpool = assure<Component>();

        return cpool.contains(entity)
                   ? cpool.patch(entity, [&args...](auto &...curr) { ((curr = Component{std::forward<Args>(args)...}), ...); })
                   : cpool.emplace(entity, std::forward<Args>(args)...);
    }

    /**
     * @brief Patches the given component for an entity.
     *
     * The signature of the function should be equivalent to the following:
     *
     * @code{.cpp}
     * void(Component &);
     * @endcode
     *
     * @note
     * Empty types aren't explicitly instantiated and therefore they are never
     * returned. However, this function can be used to trigger an update signal
     * for them.
     *
     * @warning
     * Attempting to use an invalid entity or to patch a component of an entity
     * that doesn't own it results in undefined behavior.
     *
     * @tparam Component Type of component to patch.
     * @tparam Func Types of the function objects to invoke.
     * @param entity A valid identifier.
     * @param func Valid function objects.
     * @return A reference to the patched component.
     */
    template<typename Component, typename... Func>
    decltype(auto) patch(const entity_type entity, Func &&...func) {
        ENTT_ASSERT(valid(entity), "Invalid entity");
        return assure<Component>().patch(entity, std::forward<Func>(func)...);
    }

    /**
     * @brief Replaces the given component for an entity.
     *
     * The component must have a proper constructor or be of aggregate type.
     *
     * @warning
     * Attempting to use an invalid entity or to replace a component of an
     * entity that doesn't own it results in undefined behavior.
     *
     * @tparam Component Type of component to replace.
     * @tparam Args Types of arguments to use to construct the component.
     * @param entity A valid identifier.
     * @param args Parameters to use to initialize the component.
     * @return A reference to the component being replaced.
     */
    template<typename Component, typename... Args>
    decltype(auto) replace(const entity_type entity, Args &&...args) {
        ENTT_ASSERT(valid(entity), "Invalid entity");
        return assure<Component>().patch(entity, [&args...](auto &...curr) { ((curr = Component{std::forward<Args>(args)...}), ...); });
    }

    /**
     * @brief Removes the given components from an entity.
     *
     * @warning
     * Attempting to use an invalid entity results in undefined behavior.
     *
     * @tparam Component Type of component to remove.
     * @tparam Other Other types of components to remove.
     * @param entity A valid identifier.
     * @return The number of components actually removed.
     */
    template<typename Component, typename... Other>
    size_type remove(const entity_type entity) {
        ENTT_ASSERT(valid(entity), "Invalid entity");
        return (assure<Component>().remove(entity) + ... + assure<Other>().remove(entity));
    }

    /**
     * @brief Removes the given components from all the entities in a range.
     *
     * @sa remove
     *
     * @tparam Component Type of component to remove.
     * @tparam Other Other types of components to remove.
     * @tparam It Type of input iterator.
     * @param first An iterator to the first element of the range of entities.
     * @param last An iterator past the last element of the range of entities.
     * @return The number of components actually removed.
     */
    template<typename Component, typename... Other, typename It>
    size_type remove(It first, It last) {
        if constexpr(sizeof...(Other) == 0u) {
            ENTT_ASSERT(std::all_of(first, last, [this](const auto entity) { return valid(entity); }), "Invalid entity");
            return assure<Component>().remove(std::move(first), std::move(last));
        } else {
            size_type count{};

            for(auto cpools = std::forward_as_tuple(assure<Component>(), assure<Other>()...); first != last; ++first) {
                ENTT_ASSERT(valid(*first), "Invalid entity");
                count += std::apply([entt = *first](auto &...curr) { return (curr.remove(entt) + ... + 0u); }, cpools);
            }

            return count;
        }
    }

    /**
     * @brief Erases the given components from an entity.
     *
     * @warning
     * Attempting to use an invalid entity or to erase a component from an
     * entity that doesn't own it results in undefined behavior.
     *
     * @tparam Component Types of components to erase.
     * @tparam Other Other types of components to erase.
     * @param entity A valid identifier.
     */
    template<typename Component, typename... Other>
    void erase(const entity_type entity) {
        ENTT_ASSERT(valid(entity), "Invalid entity");
        (assure<Component>().erase(entity), (assure<Other>().erase(entity), ...));
    }

    /**
     * @brief Erases the given components from all the entities in a range.
     *
     * @sa erase
     *
     * @tparam Component Types of components to erase.
     * @tparam Other Other types of components to erase.
     * @tparam It Type of input iterator.
     * @param first An iterator to the first element of the range of entities.
     * @param last An iterator past the last element of the range of entities.
     */
    template<typename Component, typename... Other, typename It>
    void erase(It first, It last) {
        if constexpr(sizeof...(Other) == 0u) {
            ENTT_ASSERT(std::all_of(first, last, [this](const auto entity) { return valid(entity); }), "Invalid entity");
            assure<Component>().erase(std::move(first), std::move(last));
        } else {
            for(auto cpools = std::forward_as_tuple(assure<Component>(), assure<Other>()...); first != last; ++first) {
                ENTT_ASSERT(valid(*first), "Invalid entity");
                std::apply([entt = *first](auto &...curr) { (curr.erase(entt), ...); }, cpools);
            }
        }
    }

    /**
     * @brief Removes all tombstones from a registry or only the pools for the
     * given components.
     * @tparam Component Types of components for which to clear all tombstones.
     */
    template<typename... Component>
    void compact() {
        if constexpr(sizeof...(Component) == 0) {
            for(auto &&curr: pools) {
                curr.second->compact();
            }
        } else {
            (assure<Component>().compact(), ...);
        }
    }

    /**
     * @brief Checks if an entity has all the given components.
     *
     * @warning
     * Attempting to use an invalid entity results in undefined behavior.
     *
     * @tparam Component Components for which to perform the check.
     * @param entity A valid identifier.
     * @return True if the entity has all the components, false otherwise.
     */
    template<typename... Component>
    [[nodiscard]] bool all_of(const entity_type entity) const {
        ENTT_ASSERT(valid(entity), "Invalid entity");
        return (assure<std::remove_const_t<Component>>().contains(entity) && ...);
    }

    /**
     * @brief Checks if an entity has at least one of the given components.
     *
     * @warning
     * Attempting to use an invalid entity results in undefined behavior.
     *
     * @tparam Component Components for which to perform the check.
     * @param entity A valid identifier.
     * @return True if the entity has at least one of the given components,
     * false otherwise.
     */
    template<typename... Component>
    [[nodiscard]] bool any_of(const entity_type entity) const {
        ENTT_ASSERT(valid(entity), "Invalid entity");
        return (assure<std::remove_const_t<Component>>().contains(entity) || ...);
    }

    /**
     * @brief Returns references to the given components for an entity.
     *
     * @warning
     * Attempting to use an invalid entity or to get a component from an entity
     * that doesn't own it results in undefined behavior.
     *
     * @tparam Component Types of components to get.
     * @param entity A valid identifier.
     * @return References to the components owned by the entity.
     */
    template<typename... Component>
    [[nodiscard]] decltype(auto) get([[maybe_unused]] const entity_type entity) const {
        ENTT_ASSERT(valid(entity), "Invalid entity");
        return view<Component...>().template get<const Component...>(entity);
    }

    /*! @copydoc get */
    template<typename... Component>
    [[nodiscard]] decltype(auto) get([[maybe_unused]] const entity_type entity) {
        ENTT_ASSERT(valid(entity), "Invalid entity");
        return view<Component...>().template get<Component...>(entity);
    }

    /**
     * @brief Returns a reference to the given component for an entity.
     *
     * In case the entity doesn't own the component, the parameters provided are
     * used to construct it.
     *
     * @warning
     * Attempting to use an invalid entity results in undefined behavior.
     *
     * @tparam Component Type of component to get.
     * @tparam Args Types of arguments to use to construct the component.
     * @param entity A valid identifier.
     * @param args Parameters to use to initialize the component.
     * @return Reference to the component owned by the entity.
     */
    template<typename Component, typename... Args>
    [[nodiscard]] decltype(auto) get_or_emplace(const entity_type entity, Args &&...args) {
        ENTT_ASSERT(valid(entity), "Invalid entity");
        auto &cpool = assure<Component>();
        return cpool.contains(entity) ? cpool.get(entity) : cpool.emplace(entity, std::forward<Args>(args)...);
    }

    /**
     * @brief Returns pointers to the given components for an entity.
     *
     * @warning
     * Attempting to use an invalid entity results in undefined behavior.
     *
     * @note
     * The registry retains ownership of the pointed-to components.
     *
     * @tparam Component Types of components to get.
     * @param entity A valid identifier.
     * @return Pointers to the components owned by the entity.
     */
    template<typename... Component>
    [[nodiscard]] auto try_get([[maybe_unused]] const entity_type entity) const {
        ENTT_ASSERT(valid(entity), "Invalid entity");

        if constexpr(sizeof...(Component) == 1) {
            const auto &cpool = assure<std::remove_const_t<Component>...>();
            return cpool.contains(entity) ? std::addressof(cpool.get(entity)) : nullptr;
        } else {
            return std::make_tuple(try_get<Component>(entity)...);
        }
    }

    /*! @copydoc try_get */
    template<typename... Component>
    [[nodiscard]] auto try_get([[maybe_unused]] const entity_type entity) {
        if constexpr(sizeof...(Component) == 1) {
            return (const_cast<Component *>(std::as_const(*this).template try_get<Component>(entity)), ...);
        } else {
            return std::make_tuple(try_get<Component>(entity)...);
        }
    }

    /**
     * @brief Clears a whole registry or the pools for the given components.
     * @tparam Component Types of components to remove from their entities.
     */
    template<typename... Component>
    void clear() {
        if constexpr(sizeof...(Component) == 0) {
            for(auto &&curr: pools) {
                curr.second->clear();
            }

            each([this](const auto entity) { this->release(entity); });
        } else {
            (assure<Component>().clear(), ...);
        }
    }

    /**
     * @brief Iterates all the entities that are still in use.
     *
     * The signature of the function should be equivalent to the following:
     *
     * @code{.cpp}
     * void(const Entity);
     * @endcode
     *
     * It's not defined whether entities created during iteration are returned.
     *
     * @tparam Func Type of the function object to invoke.
     * @param func A valid function object.
     */
    template<typename Func>
    void each(Func func) const {
        if(free_list == null) {
            for(auto pos = entities.size(); pos; --pos) {
                func(entities[pos - 1]);
            }
        } else {
            for(auto pos = entities.size(); pos; --pos) {
                if(const auto entity = entities[pos - 1]; entity_traits::to_entity(entity) == (pos - 1)) {
                    func(entity);
                }
            }
        }
    }

    /**
     * @brief Checks if an entity has components assigned.
     * @param entity A valid identifier.
     * @return True if the entity has no components assigned, false otherwise.
     */
    [[nodiscard]] bool orphan(const entity_type entity) const {
        ENTT_ASSERT(valid(entity), "Invalid entity");
        return std::none_of(pools.cbegin(), pools.cend(), [entity](auto &&curr) { return curr.second->contains(entity); });
    }

    /**
     * @brief Returns a sink object for the given component.
     *
     * Use this function to receive notifications whenever a new instance of the
     * given component is created and assigned to an entity.<br/>
     * The function type for a listener is equivalent to:
     *
     * @code{.cpp}
     * void(basic_registry<Entity> &, Entity);
     * @endcode
     *
     * Listeners are invoked **after** assigning the component to the entity.
     *
     * @sa sink
     *
     * @tparam Component Type of component of which to get the sink.
     * @return A temporary sink object.
     */
    template<typename Component>
    [[nodiscard]] auto on_construct() {
        return assure<Component>().on_construct();
    }

    /**
     * @brief Returns a sink object for the given component.
     *
     * Use this function to receive notifications whenever an instance of the
     * given component is explicitly updated.<br/>
     * The function type for a listener is equivalent to:
     *
     * @code{.cpp}
     * void(basic_registry<Entity> &, Entity);
     * @endcode
     *
     * Listeners are invoked **after** updating the component.
     *
     * @sa sink
     *
     * @tparam Component Type of component of which to get the sink.
     * @return A temporary sink object.
     */
    template<typename Component>
    [[nodiscard]] auto on_update() {
        return assure<Component>().on_update();
    }

    /**
     * @brief Returns a sink object for the given component.
     *
     * Use this function to receive notifications whenever an instance of the
     * given component is removed from an entity and thus destroyed.<br/>
     * The function type for a listener is equivalent to:
     *
     * @code{.cpp}
     * void(basic_registry<Entity> &, Entity);
     * @endcode
     *
     * Listeners are invoked **before** removing the component from the entity.
     *
     * @sa sink
     *
     * @tparam Component Type of component of which to get the sink.
     * @return A temporary sink object.
     */
    template<typename Component>
    [[nodiscard]] auto on_destroy() {
        return assure<Component>().on_destroy();
    }

    /**
     * @brief Returns a view for the given components.
     *
     * Views are created on the fly and share with the registry its internal
     * data structures. Feel free to discard them after the use.<br/>
     * Creating and destroying a view is an incredibly cheap operation. As a
     * rule of thumb, storing a view should never be an option.
     *
     * @tparam Component Type of component used to construct the view.
     * @tparam Other Other types of components used to construct the view.
     * @tparam Exclude Types of components used to filter the view.
     * @return A newly created view.
     */
    template<typename Component, typename... Other, typename... Exclude>
    [[nodiscard]] basic_view<entity_type, get_t<std::add_const_t<Component>, std::add_const_t<Other>...>, exclude_t<Exclude...>> view(exclude_t<Exclude...> = {}) const {
        return {assure<std::remove_const_t<Component>>(), assure<std::remove_const_t<Other>>()..., assure<Exclude>()...};
    }

    /*! @copydoc view */
    template<typename Component, typename... Other, typename... Exclude>
    [[nodiscard]] basic_view<entity_type, get_t<Component, Other...>, exclude_t<Exclude...>> view(exclude_t<Exclude...> = {}) {
        return {assure<std::remove_const_t<Component>>(), assure<std::remove_const_t<Other>>()..., assure<Exclude>()...};
    }

    /**
     * @brief Returns a group for the given components.
     *
     * Groups are created on the fly and share with the registry its internal
     * data structures. Feel free to discard them after the use.<br/>
     * Creating and destroying a group is an incredibly cheap operation. As a
     * rule of thumb, storing a group should never be an option.
     *
     * Groups support exclusion lists and can own types of components. The more
     * types are owned by a group, the faster it is to iterate entities and
     * components.<br/>
     * However, groups also affect some features of the registry such as the
     * creation and destruction of components.
     *
     * @note
     * Pools of components that are owned by a group cannot be sorted anymore.
     * The group takes the ownership of the pools and arrange components so as
     * to iterate them as fast as possible.
     *
     * @tparam Owned Types of components owned by the group.
     * @tparam Get Types of components observed by the group.
     * @tparam Exclude Types of components used to filter the group.
     * @return A newly created group.
     */
    template<typename... Owned, typename... Get, typename... Exclude>
    [[nodiscard]] basic_group<entity_type, owned_t<Owned...>, get_t<Get...>, exclude_t<Exclude...>> group(get_t<Get...>, exclude_t<Exclude...> = {}) {
        static_assert(sizeof...(Owned) + sizeof...(Get) > 0, "Exclusion-only groups are not supported");
        static_assert(sizeof...(Owned) + sizeof...(Get) + sizeof...(Exclude) > 1, "Single component groups are not allowed");

        using handler_type = group_handler<exclude_t<std::remove_const_t<Exclude>...>, get_t<std::remove_const_t<Get>...>, std::remove_const_t<Owned>...>;

        const auto cpools = std::forward_as_tuple(assure<std::remove_const_t<Owned>>()..., assure<std::remove_const_t<Get>>()...);
        constexpr auto size = sizeof...(Owned) + sizeof...(Get) + sizeof...(Exclude);
        handler_type *handler = nullptr;

        auto it = std::find_if(groups.cbegin(), groups.cend(), [size](const auto &gdata) {
            return gdata.size == size
                   && (gdata.owned(type_hash<std::remove_const_t<Owned>>::value()) && ...)
                   && (gdata.get(type_hash<std::remove_const_t<Get>>::value()) && ...)
                   && (gdata.exclude(type_hash<Exclude>::value()) && ...);
        });

        if(it != groups.cend()) {
            handler = static_cast<handler_type *>(it->group.get());
        } else {
            group_data candidate = {
                size,
                {new handler_type{}, [](void *instance) { delete static_cast<handler_type *>(instance); }},
                []([[maybe_unused]] const id_type ctype) ENTT_NOEXCEPT { return ((ctype == type_hash<std::remove_const_t<Owned>>::value()) || ...); },
                []([[maybe_unused]] const id_type ctype) ENTT_NOEXCEPT { return ((ctype == type_hash<std::remove_const_t<Get>>::value()) || ...); },
                []([[maybe_unused]] const id_type ctype) ENTT_NOEXCEPT { return ((ctype == type_hash<Exclude>::value()) || ...); },
            };

            handler = static_cast<handler_type *>(candidate.group.get());

            const void *maybe_valid_if = nullptr;
            const void *discard_if = nullptr;

            if constexpr(sizeof...(Owned) == 0) {
                groups.push_back(std::move(candidate));
            } else {
                [[maybe_unused]] auto has_conflict = [size](const auto &gdata) {
                    const auto overlapping = (0u + ... + gdata.owned(type_hash<std::remove_const_t<Owned>>::value()));
                    const auto sz = overlapping + (0u + ... + gdata.get(type_hash<std::remove_const_t<Get>>::value())) + (0u + ... + gdata.exclude(type_hash<Exclude>::value()));
                    return !overlapping || ((sz == size) || (sz == gdata.size));
                };

                ENTT_ASSERT(std::all_of(groups.cbegin(), groups.cend(), std::move(has_conflict)), "Conflicting groups");

                const auto next = std::find_if_not(groups.cbegin(), groups.cend(), [size](const auto &gdata) {
                    return !(0u + ... + gdata.owned(type_hash<std::remove_const_t<Owned>>::value())) || (size > gdata.size);
                });

                const auto prev = std::find_if(std::make_reverse_iterator(next), groups.crend(), [](const auto &gdata) {
                    return (0u + ... + gdata.owned(type_hash<std::remove_const_t<Owned>>::value()));
                });

                maybe_valid_if = (next == groups.cend() ? maybe_valid_if : next->group.get());
                discard_if = (prev == groups.crend() ? discard_if : prev->group.get());
                groups.insert(next, std::move(candidate));
            }

            (on_construct<std::remove_const_t<Owned>>().before(maybe_valid_if).template connect<&handler_type::template maybe_valid_if<std::remove_const_t<Owned>>>(*handler), ...);
            (on_construct<std::remove_const_t<Get>>().before(maybe_valid_if).template connect<&handler_type::template maybe_valid_if<std::remove_const_t<Get>>>(*handler), ...);
            (on_destroy<Exclude>().before(maybe_valid_if).template connect<&handler_type::template maybe_valid_if<Exclude>>(*handler), ...);

            (on_destroy<std::remove_const_t<Owned>>().before(discard_if).template connect<&handler_type::discard_if>(*handler), ...);
            (on_destroy<std::remove_const_t<Get>>().before(discard_if).template connect<&handler_type::discard_if>(*handler), ...);
            (on_construct<Exclude>().before(discard_if).template connect<&handler_type::discard_if>(*handler), ...);

            if constexpr(sizeof...(Owned) == 0) {
                for(const auto entity: view<Owned..., Get...>(exclude<Exclude...>)) {
                    handler->current.emplace(entity);
                }
            } else {
                // we cannot iterate backwards because we want to leave behind valid entities in case of owned types
                for(auto *first = std::get<0>(cpools).data(), *last = first + std::get<0>(cpools).size(); first != last; ++first) {
                    handler->template maybe_valid_if<type_list_element_t<0, type_list<std::remove_const_t<Owned>...>>>(*this, *first);
                }
            }
        }

        return {handler->current, std::get<storage_type<std::remove_const_t<Owned>> &>(cpools)..., std::get<storage_type<std::remove_const_t<Get>> &>(cpools)...};
    }

    /*! @copydoc group */
    template<typename... Owned, typename... Get, typename... Exclude>
    [[nodiscard]] basic_group<entity_type, owned_t<std::add_const_t<Owned>...>, get_t<std::add_const_t<Get>...>, exclude_t<Exclude...>> group_if_exists(get_t<Get...>, exclude_t<Exclude...> = {}) const {
        auto it = std::find_if(groups.cbegin(), groups.cend(), [](const auto &gdata) {
            return gdata.size == (sizeof...(Owned) + sizeof...(Get) + sizeof...(Exclude))
                   && (gdata.owned(type_hash<std::remove_const_t<Owned>>::value()) && ...)
                   && (gdata.get(type_hash<std::remove_const_t<Get>>::value()) && ...)
                   && (gdata.exclude(type_hash<Exclude>::value()) && ...);
        });

        if(it == groups.cend()) {
            return {};
        } else {
            using handler_type = group_handler<exclude_t<std::remove_const_t<Exclude>...>, get_t<std::remove_const_t<Get>...>, std::remove_const_t<Owned>...>;
            return {static_cast<handler_type *>(it->group.get())->current, assure<std::remove_const_t<Owned>>()..., assure<std::remove_const_t<Get>>()...};
        }
    }

    /*! @copydoc group */
    template<typename... Owned, typename... Exclude>
    [[nodiscard]] basic_group<entity_type, owned_t<Owned...>, get_t<>, exclude_t<Exclude...>> group(exclude_t<Exclude...> = {}) {
        return group<Owned...>(get_t<>{}, exclude<Exclude...>);
    }

    /*! @copydoc group */
    template<typename... Owned, typename... Exclude>
    [[nodiscard]] basic_group<entity_type, owned_t<std::add_const_t<Owned>...>, get_t<>, exclude_t<Exclude...>> group_if_exists(exclude_t<Exclude...> = {}) const {
        return group_if_exists<std::add_const_t<Owned>...>(get_t<>{}, exclude<Exclude...>);
    }

    /**
     * @brief Checks whether the given components belong to any group.
     * @tparam Component Types of components in which one is interested.
     * @return True if the pools of the given components are _free_, false
     * otherwise.
     */
    template<typename... Component>
    [[nodiscard]] bool owned() const {
        return std::any_of(groups.cbegin(), groups.cend(), [](auto &&gdata) { return (gdata.owned(type_hash<std::remove_const_t<Component>>::value()) || ...); });
    }

    /**
     * @brief Checks whether a group can be sorted.
     * @tparam Owned Types of components owned by the group.
     * @tparam Get Types of components observed by the group.
     * @tparam Exclude Types of components used to filter the group.
     * @return True if the group can be sorted, false otherwise.
     */
    template<typename... Owned, typename... Get, typename... Exclude>
    [[nodiscard]] bool sortable(const basic_group<entity_type, owned_t<Owned...>, get_t<Get...>, exclude_t<Exclude...>> &) ENTT_NOEXCEPT {
        constexpr auto size = sizeof...(Owned) + sizeof...(Get) + sizeof...(Exclude);
        auto pred = [size](const auto &gdata) { return (0u + ... + gdata.owned(type_hash<std::remove_const_t<Owned>>::value())) && (size < gdata.size); };
        return std::find_if(groups.cbegin(), groups.cend(), std::move(pred)) == groups.cend();
    }

    /**
     * @brief Sorts the elements of a given component.
     *
     * The order remains valid until a component of the given type is assigned
     * to or removed from an entity.<br/>
     * The comparison function object returns `true` if the first element is
     * _less_ than the second one, `false` otherwise. Its signature is also
     * equivalent to one of the following:
     *
     * @code{.cpp}
     * bool(const Entity, const Entity);
     * bool(const Component &, const Component &);
     * @endcode
     *
     * Moreover, it shall induce a _strict weak ordering_ on the values.<br/>
     * The sort function object offers an `operator()` that accepts:
     *
     * * An iterator to the first element of the range to sort.
     * * An iterator past the last element of the range to sort.
     * * A comparison function object to use to compare the elements.
     *
     * The comparison function object hasn't necessarily the type of the one
     * passed along with the other parameters to this member function.
     *
     * @warning
     * Pools of components owned by a group cannot be sorted.
     *
     * @tparam Component Type of components to sort.
     * @tparam Compare Type of comparison function object.
     * @tparam Sort Type of sort function object.
     * @tparam Args Types of arguments to forward to the sort function object.
     * @param compare A valid comparison function object.
     * @param algo A valid sort function object.
     * @param args Arguments to forward to the sort function object, if any.
     */
    template<typename Component, typename Compare, typename Sort = std_sort, typename... Args>
    void sort(Compare compare, Sort algo = Sort{}, Args &&...args) {
        ENTT_ASSERT(!owned<Component>(), "Cannot sort owned storage");
        auto &cpool = assure<Component>();

        if constexpr(std::is_invocable_v<Compare, decltype(cpool.get({})), decltype(cpool.get({}))>) {
            auto comp = [&cpool, compare = std::move(compare)](const auto lhs, const auto rhs) { return compare(std::as_const(cpool.get(lhs)), std::as_const(cpool.get(rhs))); };
            cpool.sort(std::move(comp), std::move(algo), std::forward<Args>(args)...);
        } else {
            cpool.sort(std::move(compare), std::move(algo), std::forward<Args>(args)...);
        }
    }

    /**
     * @brief Sorts two pools of components in the same way.
     *
     * Being `To` and `From` the two sets, after invoking this function an
     * iterator for `To` returns elements according to the following rules:
     *
     * * All entities in `To` that are also in `From` are returned first
     *   according to the order they have in `From`.
     * * All entities in `To` that are not in `From` are returned in no
     *   particular order after all the other entities.
     *
     * Any subsequent change to `From` won't affect the order in `To`.
     *
     * @warning
     * Pools of components owned by a group cannot be sorted.
     *
     * @tparam To Type of components to sort.
     * @tparam From Type of components to use to sort.
     */
    template<typename To, typename From>
    void sort() {
        ENTT_ASSERT(!owned<To>(), "Cannot sort owned storage");
        assure<To>().respect(assure<From>());
    }

    /**
     * @brief Returns the context object, that is, a general purpose container.
     * @return The context object, that is, a general purpose container.
     */
    context &ctx() ENTT_NOEXCEPT {
        return vars;
    }

    /*! @copydoc ctx */
    const context &ctx() const ENTT_NOEXCEPT {
        return vars;
    }

private:
    dense_map<id_type, std::unique_ptr<base_type>, identity> pools;
    std::vector<group_data> groups;
    std::vector<entity_type> entities;
    entity_type free_list;
    context vars;
};

} // namespace entt

#endif