diff --git a/ecs.hpp b/ecs.hpp
index 6f05bb5..76c7650 100644
--- a/ecs.hpp
+++ b/ecs.hpp
@@ -10,16 +10,18 @@
 #include <cstddef>
 #include <cstdint>
 
+#include <tuple>
 #include <mutex>
 #include <memory>
+#include <vector>
 #include <limits>
 #include <utility>
+#include <iterator>
 #include <exception>
 #include <stdexcept>
+#include <algorithm>
 #include <functional>
 #include <type_traits>
-#include <unordered_set>
-#include <unordered_map>
 
 // -----------------------------------------------------------------------------
 //
@@ -29,8 +31,9 @@
 
 namespace ecs_hpp
 {
-    class world;
     class entity;
+    class system;
+    class registry;
 
     using family_id = std::uint16_t;
     using entity_id = std::uint32_t;
@@ -44,6 +47,23 @@ namespace ecs_hpp
         "ecs_hpp::entity_id must be an unsigned integer");
 }
 
+// -----------------------------------------------------------------------------
+//
+// utilities
+//
+// -----------------------------------------------------------------------------
+
+namespace ecs_hpp
+{
+    namespace detail
+    {
+        template < typename T >
+        constexpr std::add_const_t<T>& as_const(T& t) noexcept {
+            return t;
+        }
+    }
+}
+
 // -----------------------------------------------------------------------------
 //
 // detail::type_family
@@ -78,6 +98,306 @@ namespace ecs_hpp
     }
 }
 
+// -----------------------------------------------------------------------------
+//
+// detail::sparse_set
+//
+// -----------------------------------------------------------------------------
+
+namespace ecs_hpp
+{
+    namespace detail
+    {
+        template < typename T >
+        class sparse_set final {
+        public:
+            static_assert(
+                std::is_unsigned<T>::value,
+                "sparse_set<T> can contain an unsigned integers only");
+            using iterator = typename std::vector<T>::iterator;
+            using const_iterator = typename std::vector<T>::const_iterator;
+        public:
+            iterator begin() noexcept {
+                return dense_.begin();
+            }
+
+            iterator end() noexcept {
+                using dt = typename std::iterator_traits<iterator>::difference_type;
+                return begin() + static_cast<dt>(size_);
+            }
+
+            const_iterator begin() const noexcept {
+                return dense_.begin();
+            }
+
+            const_iterator end() const noexcept {
+                using dt = typename std::iterator_traits<const_iterator>::difference_type;
+                return begin() + static_cast<dt>(size_);
+            }
+
+            const_iterator cbegin() const noexcept {
+                return dense_.cbegin();
+            }
+
+            const_iterator cend() const noexcept {
+                using dt = typename std::iterator_traits<const_iterator>::difference_type;
+                return cbegin() + static_cast<dt>(size_);
+            }
+        public:
+            bool insert(const T v) {
+                if ( has(v) ) {
+                    return false;
+                }
+                if ( v >= capacity_ ) {
+                    reserve(new_capacity_for_(v + 1u));
+                }
+                dense_[size_] = v;
+                sparse_[v] = size_;
+                ++size_;
+                return true;
+            }
+
+            bool unordered_erase(const T v) noexcept {
+                if ( !has(v) ) {
+                    return false;
+                }
+                const std::size_t index = sparse_[v];
+                const T last = dense_[size_ - 1u];
+                dense_[index] = last;
+                sparse_[last] = index;
+                --size_;
+                return true;
+            }
+
+            void clear() noexcept {
+                size_ = 0u;
+            }
+
+            bool has(const T v) const noexcept {
+                return v < capacity_
+                    && sparse_[v] < size_
+                    && dense_[sparse_[v]] == v;
+            }
+
+            const_iterator find(const T v) const noexcept {
+                return has(v)
+                    ? begin() + sparse_[v]
+                    : end();
+            }
+
+            std::size_t get_index(const T v) const {
+                const auto p = find_index(v);
+                if ( p.second ) {
+                    return p.first;
+                }
+                throw std::out_of_range("sparse_set<T>");
+            }
+
+            std::pair<std::size_t,bool> find_index(const T v) const noexcept {
+                return has(v)
+                    ? std::make_pair(sparse_[v], true)
+                    : std::make_pair(std::size_t(-1), false);
+            }
+
+            bool empty() const noexcept {
+                return size_ == 0u;
+            }
+
+            void reserve(std::size_t ncapacity) {
+                if ( ncapacity > capacity_ ) {
+                    std::vector<T> ndense(ncapacity);
+                    std::vector<std::size_t> nsparse(ncapacity);
+                    std::copy(dense_.begin(), dense_.end(), ndense.begin());
+                    std::copy(sparse_.begin(), sparse_.end(), nsparse.begin());
+                    ndense.swap(dense_);
+                    nsparse.swap(sparse_);
+                    capacity_ = ncapacity;
+                }
+            }
+
+            std::size_t size() const noexcept {
+                return size_;
+            }
+
+            std::size_t max_size() const noexcept {
+                return std::min(dense_.max_size(), sparse_.max_size());
+            }
+
+            std::size_t capacity() const noexcept {
+                return capacity_;
+            }
+        private:
+            std::size_t new_capacity_for_(std::size_t nsize) const {
+                const std::size_t ms = max_size();
+                if ( nsize > ms ) {
+                    throw std::length_error("sparse_set<T>");
+                }
+                if ( capacity_ >= ms / 2u ) {
+                    return ms;
+                }
+                return std::max(capacity_ * 2u, nsize);
+            }
+        private:
+            std::vector<T> dense_;
+            std::vector<std::size_t> sparse_;
+            std::size_t size_{0u};
+            std::size_t capacity_{0u};
+        };
+    }
+}
+
+// -----------------------------------------------------------------------------
+//
+// detail::sparse_map
+//
+// -----------------------------------------------------------------------------
+
+namespace ecs_hpp
+{
+    namespace detail
+    {
+        template < typename K, typename T >
+        class sparse_map final {
+        public:
+            static_assert(
+                std::is_unsigned<K>::value,
+                "sparse_map<K,T> can contain unsigned integers keys only");
+            using iterator = typename std::vector<K>::iterator;
+            using const_iterator = typename std::vector<K>::const_iterator;
+        public:
+            iterator begin() noexcept {
+                return keys_.begin();
+            }
+
+            iterator end() noexcept {
+                return keys_.end();
+            }
+
+            const_iterator begin() const noexcept {
+                return keys_.begin();
+            }
+
+            const_iterator end() const noexcept {
+                return keys_.end();
+            }
+
+            const_iterator cbegin() const noexcept {
+                return keys_.cbegin();
+            }
+
+            const_iterator cend() const noexcept {
+                return keys_.cend();
+            }
+        public:
+            bool insert(const K k, const T& v) {
+                if ( keys_.has(k) ) {
+                    return false;
+                }
+                values_.push_back(v);
+                try {
+                    return keys_.insert(k);
+                } catch (...) {
+                    values_.pop_back();
+                    throw;
+                }
+            }
+
+            bool insert(const K k, T&& v) {
+                if ( keys_.has(k) ) {
+                    return false;
+                }
+                values_.push_back(std::move(v));
+                try {
+                    return keys_.insert(k);
+                } catch (...) {
+                    values_.pop_back();
+                    throw;
+                }
+            }
+
+            template < typename... Args >
+            bool emplace(const K k, Args&&... args) {
+                if ( keys_.has(k) ) {
+                    return false;
+                }
+                values_.emplace_back(std::forward<Args>(args)...);
+                try {
+                    return keys_.insert(k);
+                } catch (...) {
+                    values_.pop_back();
+                    throw;
+                }
+            }
+
+            bool unordered_erase(const K k) {
+                if ( !keys_.has(k) ) {
+                    return false;
+                }
+                const std::size_t index = keys_.get_index(k);
+                values_[index] = std::move(values_.back());
+                values_.pop_back();
+                keys_.unordered_erase(k);
+                return true;
+            }
+
+            void clear() noexcept {
+                keys_.clear();
+                values_.clear();
+            }
+
+            bool has(const K k) const noexcept {
+                return keys_.has(k);
+            }
+
+            T& get_value(const K k) {
+                return values_[keys_.get_index(k)];
+            }
+
+            const T& get_value(const K k) const {
+                return values_[keys_.get_index(k)];
+            }
+
+            T* find_value(const K k) noexcept {
+                const auto ip = keys_.find_index(k);
+                return ip.second
+                    ? &values_[ip.first]
+                    : nullptr;
+            }
+
+            const T* find_value(const K k) const noexcept {
+                const auto ip = keys_.find_index(k);
+                return ip.second
+                    ? &values_[ip.first]
+                    : nullptr;
+            }
+
+            bool empty() const noexcept {
+                return values_.empty();
+            }
+
+            void reserve(std::size_t ncapacity) {
+                keys_.reserve(ncapacity);
+                values_.reserve(ncapacity);
+            }
+
+            std::size_t size() const noexcept {
+                return values_.size();
+            }
+
+            std::size_t max_size() const noexcept {
+                return std::min(keys_.max_size(), values_.max_size());
+            }
+
+            std::size_t capacity() const noexcept {
+                return values_.capacity();
+            }
+        private:
+            sparse_set<K> keys_;
+            std::vector<T> values_;
+        };
+    }
+}
+
 // -----------------------------------------------------------------------------
 //
 // detail::component_storage
@@ -98,7 +418,7 @@ namespace ecs_hpp
         template < typename T >
         class component_storage : public component_storage_base {
         public:
-            component_storage(world& owner);
+            component_storage(registry& owner);
 
             template < typename... Args >
             void assign(entity_id id, Args&&... args);
@@ -112,59 +432,55 @@ namespace ecs_hpp
             template < typename F >
             void for_each_component(F&& f) const noexcept;
         private:
-            world& owner_;
-            std::unordered_map<entity_id, T> components_;
+            registry& owner_;
+            detail::sparse_map<entity_id, T> components_;
         };
 
         template < typename T >
-        component_storage<T>::component_storage(world& owner)
+        component_storage<T>::component_storage(registry& owner)
         : owner_(owner) {}
 
         template < typename T >
         template < typename... Args >
         void component_storage<T>::assign(entity_id id, Args&&... args) {
-            components_[id] = T(std::forward<Args>(args)...);
+            if ( !components_.emplace(id, std::forward<Args>(args)...) ) {
+                components_.get_value(id) = T(std::forward<Args>(args)...);
+            }
         }
 
         template < typename T >
         bool component_storage<T>::remove(entity_id id) noexcept {
-            return components_.erase(id) > 0u;
+            return components_.unordered_erase(id);
         }
 
         template < typename T >
         bool component_storage<T>::exists(entity_id id) const noexcept {
-            return components_.find(id) != components_.end();
+            return components_.has(id);
         }
 
         template < typename T >
         T* component_storage<T>::find(entity_id id) noexcept {
-            const auto iter = components_.find(id);
-            return iter != components_.end()
-                ? &iter->second
-                : nullptr;
+            return components_.find_value(id);
         }
 
         template < typename T >
         const T* component_storage<T>::find(entity_id id) const noexcept {
-            const auto iter = components_.find(id);
-            return iter != components_.end()
-                ? &iter->second
-                : nullptr;
+            return components_.find_value(id);
         }
 
         template < typename T >
         template < typename F >
         void component_storage<T>::for_each_component(F&& f) noexcept {
-            for ( auto& component_pair : components_ ) {
-                f(entity(owner_, component_pair.first), component_pair.second);
+            for ( const auto id : components_ ) {
+                f(entity(owner_, id), components_.get_value(id));
             }
         }
 
         template < typename T >
         template < typename F >
         void component_storage<T>::for_each_component(F&& f) const noexcept {
-            for ( auto& component_pair : components_ ) {
-                f(entity(owner_, component_pair.first), component_pair.second);
+            for ( const auto id : components_ ) {
+                f(entity(owner_, id), components_.get_value(id));
             }
         }
     }
@@ -195,10 +511,10 @@ namespace ecs_hpp
 {
     class entity final {
     public:
-        entity(world& owner);
-        entity(world& owner, entity_id id);
+        entity(registry& owner);
+        entity(registry& owner, entity_id id);
 
-        const world& owner() const noexcept;
+        const registry& owner() const noexcept;
         entity_id id() const noexcept;
 
         bool destroy();
@@ -226,8 +542,18 @@ namespace ecs_hpp
 
         template < typename T >
         const T* find_component() const noexcept;
+
+        template < typename... Ts >
+        std::tuple<Ts&...> get_components();
+        template < typename... Ts >
+        std::tuple<const Ts&...> get_components() const;
+
+        template < typename... Ts >
+        std::tuple<Ts*...> find_components() noexcept;
+        template < typename... Ts >
+        std::tuple<const Ts*...> find_components() const noexcept;
     private:
-        world& owner_;
+        registry& owner_;
         entity_id id_{0u};
     };
 
@@ -249,16 +575,31 @@ namespace std
 
 // -----------------------------------------------------------------------------
 //
-// world
+// system
 //
 // -----------------------------------------------------------------------------
 
 namespace ecs_hpp
 {
-    class world final {
+    class system {
     public:
-        world();
-        ~world() noexcept;
+        virtual ~system() = default;
+        virtual void process(registry& owner) = 0;
+    };
+}
+
+// -----------------------------------------------------------------------------
+//
+// registry
+//
+// -----------------------------------------------------------------------------
+
+namespace ecs_hpp
+{
+    class registry final {
+    public:
+        registry() = default;
+        ~registry() noexcept = default;
 
         entity create_entity();
         bool destroy_entity(const entity& ent);
@@ -285,6 +626,16 @@ namespace ecs_hpp
         template < typename T >
         const T* find_component(const entity& ent) const noexcept;
 
+        template < typename... Ts >
+        std::tuple<Ts&...> get_components(const entity& ent);
+        template < typename... Ts >
+        std::tuple<const Ts&...> get_components(const entity& ent) const;
+
+        template < typename... Ts >
+        std::tuple<Ts*...> find_components(const entity& ent) noexcept;
+        template < typename... Ts >
+        std::tuple<const Ts*...> find_components(const entity& ent) const noexcept;
+
         template < typename T, typename F >
         void for_each_component(F&& f);
         template < typename T, typename F >
@@ -294,6 +645,10 @@ namespace ecs_hpp
         void for_joined_components(F&& f);
         template < typename... Ts, typename F >
         void for_joined_components(F&& f) const;
+
+        template < typename T, typename... Args >
+        void add_system(Args&&... args);
+        void process_systems();
     private:
         template < typename T >
         detail::component_storage<T>* find_storage_() noexcept;
@@ -328,10 +683,14 @@ namespace ecs_hpp
         mutable std::mutex mutex_;
 
         entity_id last_entity_id_{0u};
-        std::unordered_set<entity> entities_;
+        std::vector<entity_id> free_entity_ids_;
+        detail::sparse_set<entity_id> entity_ids_;
 
         using storage_uptr = std::unique_ptr<detail::component_storage_base>;
-        std::unordered_map<family_id, storage_uptr> storages_;
+        detail::sparse_map<family_id, storage_uptr> storages_;
+
+        using system_uptr = std::unique_ptr<system>;
+        std::vector<system_uptr> systems_;
     };
 }
 
@@ -343,14 +702,14 @@ namespace ecs_hpp
 
 namespace ecs_hpp
 {
-    inline entity::entity(world& owner)
+    inline entity::entity(registry& owner)
     : owner_(owner) {}
 
-    inline entity::entity(world& owner, entity_id id)
+    inline entity::entity(registry& owner, entity_id id)
     : owner_(owner)
     , id_(id) {}
 
-    inline const world& entity::owner() const noexcept {
+    inline const registry& entity::owner() const noexcept {
         return owner_;
     }
 
@@ -363,7 +722,7 @@ namespace ecs_hpp
     }
 
     inline bool entity::is_alive() const noexcept {
-        return owner_.is_entity_alive(*this);
+        return detail::as_const(owner_).is_entity_alive(*this);
     }
 
     template < typename T, typename... Args >
@@ -380,7 +739,7 @@ namespace ecs_hpp
 
     template < typename T >
     bool entity::exists_component() const noexcept {
-        return owner_.exists_component<T>(*this);
+        return detail::as_const(owner_).exists_component<T>(*this);
     }
 
     inline std::size_t entity::remove_all_components() noexcept {
@@ -394,7 +753,7 @@ namespace ecs_hpp
 
     template < typename T >
     const T& entity::get_component() const {
-        return owner_.get_component<T>(*this);
+        return detail::as_const(owner_).get_component<T>(*this);
     }
 
     template < typename T >
@@ -404,7 +763,27 @@ namespace ecs_hpp
 
     template < typename T >
     const T* entity::find_component() const noexcept {
-        return owner_.find_component<T>(*this);
+        return detail::as_const(owner_).find_component<T>(*this);
+    }
+
+    template < typename... Ts >
+    std::tuple<Ts&...> entity::get_components() {
+        return owner_.get_components<Ts...>(*this);
+    }
+
+    template < typename... Ts >
+    std::tuple<const Ts&...> entity::get_components() const {
+        return detail::as_const(owner_).get_components<Ts...>(*this);
+    }
+
+    template < typename... Ts >
+    std::tuple<Ts*...> entity::find_components() noexcept {
+        return owner_.find_components<Ts...>(*this);
+    }
+
+    template < typename... Ts >
+    std::tuple<const Ts*...> entity::find_components() const noexcept {
+        return detail::as_const(owner_).find_components<Ts...>(*this);
     }
 
     inline bool operator==(const entity& l, const entity& r) noexcept {
@@ -419,36 +798,44 @@ namespace ecs_hpp
 
 // -----------------------------------------------------------------------------
 //
-// world impl
+// registry impl
 //
 // -----------------------------------------------------------------------------
 
 namespace ecs_hpp
 {
-    inline world::world() = default;
-    inline world::~world() noexcept = default;
-
-    inline entity world::create_entity() {
+    inline entity registry::create_entity() {
         std::lock_guard<std::mutex> guard(mutex_);
+        if ( !free_entity_ids_.empty() ) {
+            auto ent = entity(*this, free_entity_ids_.back());
+            entity_ids_.insert(ent.id());
+            free_entity_ids_.pop_back();
+            return ent;
+
+        }
         assert(last_entity_id_ < std::numeric_limits<entity_id>::max());
         auto ent = entity(*this, ++last_entity_id_);
-        entities_.insert(ent);
+        entity_ids_.insert(ent.id());
         return ent;
     }
 
-    inline bool world::destroy_entity(const entity& ent) {
+    inline bool registry::destroy_entity(const entity& ent) {
         std::lock_guard<std::mutex> guard(mutex_);
         remove_all_components_impl_(ent);
-        return entities_.erase(ent) > 0u;
+        if ( entity_ids_.unordered_erase(ent.id()) ) {
+            free_entity_ids_.push_back(ent.id());
+            return true;
+        }
+        return false;
     }
 
-    inline bool world::is_entity_alive(const entity& ent) const noexcept {
+    inline bool registry::is_entity_alive(const entity& ent) const noexcept {
         std::lock_guard<std::mutex> guard(mutex_);
         return is_entity_alive_impl_(ent);
     }
 
     template < typename T, typename... Args >
-    bool world::assign_component(const entity& ent, Args&&... args) {
+    bool registry::assign_component(const entity& ent, Args&&... args) {
         std::lock_guard<std::mutex> guard(mutex_);
         if ( !is_entity_alive_impl_(ent) ) {
             return false;
@@ -460,7 +847,7 @@ namespace ecs_hpp
     }
 
     template < typename T >
-    bool world::remove_component(const entity& ent) {
+    bool registry::remove_component(const entity& ent) {
         std::lock_guard<std::mutex> guard(mutex_);
         if ( !is_entity_alive_impl_(ent) ) {
             return false;
@@ -472,7 +859,7 @@ namespace ecs_hpp
     }
 
     template < typename T >
-    bool world::exists_component(const entity& ent) const noexcept {
+    bool registry::exists_component(const entity& ent) const noexcept {
         std::lock_guard<std::mutex> guard(mutex_);
         if ( !is_entity_alive_impl_(ent) ) {
             return false;
@@ -483,13 +870,13 @@ namespace ecs_hpp
             : false;
     }
 
-    inline std::size_t world::remove_all_components(const entity& ent) const noexcept {
+    inline std::size_t registry::remove_all_components(const entity& ent) const noexcept {
         std::lock_guard<std::mutex> guard(mutex_);
         return remove_all_components_impl_(ent);
     }
 
     template < typename T >
-    T& world::get_component(const entity& ent) {
+    T& registry::get_component(const entity& ent) {
         std::lock_guard<std::mutex> guard(mutex_);
         T* component = find_component_impl_<T>(ent);
         if ( component ) {
@@ -499,7 +886,7 @@ namespace ecs_hpp
     }
 
     template < typename T >
-    const T& world::get_component(const entity& ent) const {
+    const T& registry::get_component(const entity& ent) const {
         std::lock_guard<std::mutex> guard(mutex_);
         const T* component = find_component_impl_<T>(ent);
         if ( component ) {
@@ -509,19 +896,39 @@ namespace ecs_hpp
     }
 
     template < typename T >
-    T* world::find_component(const entity& ent) noexcept {
+    T* registry::find_component(const entity& ent) noexcept {
         std::lock_guard<std::mutex> guard(mutex_);
         return find_component_impl_<T>(ent);
     }
 
     template < typename T >
-    const T* world::find_component(const entity& ent) const noexcept {
+    const T* registry::find_component(const entity& ent) const noexcept {
         std::lock_guard<std::mutex> guard(mutex_);
         return find_component_impl_<T>(ent);
     }
 
+    template < typename... Ts >
+    std::tuple<Ts&...> registry::get_components(const entity& ent) {
+        return std::make_tuple(std::ref(get_component<Ts>(ent))...);
+    }
+
+    template < typename... Ts >
+    std::tuple<const Ts&...> registry::get_components(const entity& ent) const {
+        return std::make_tuple(std::cref(get_component<Ts>(ent))...);
+    }
+
+    template < typename... Ts >
+    std::tuple<Ts*...> registry::find_components(const entity& ent) noexcept {
+        return std::make_tuple(find_component<Ts>(ent)...);
+    }
+
+    template < typename... Ts >
+    std::tuple<const Ts*...> registry::find_components(const entity& ent) const noexcept {
+        return std::make_tuple(find_component<Ts>(ent)...);
+    }
+
     template < typename T, typename F >
-    void world::for_each_component(F&& f) {
+    void registry::for_each_component(F&& f) {
         std::lock_guard<std::mutex> guard(mutex_);
         detail::component_storage<T>* storage = find_storage_<T>();
         if ( storage ) {
@@ -530,7 +937,7 @@ namespace ecs_hpp
     }
 
     template < typename T, typename F >
-    void world::for_each_component(F&& f) const {
+    void registry::for_each_component(F&& f) const {
         std::lock_guard<std::mutex> guard(mutex_);
         const detail::component_storage<T>* storage = find_storage_<T>();
         if ( storage ) {
@@ -539,63 +946,73 @@ namespace ecs_hpp
     }
 
     template < typename... Ts, typename F >
-    void world::for_joined_components(F&& f) {
+    void registry::for_joined_components(F&& f) {
         std::lock_guard<std::mutex> guard(mutex_);
         for_joined_components_impl_<Ts...>(std::forward<F>(f));
     }
 
     template < typename... Ts, typename F >
-    void world::for_joined_components(F&& f) const {
+    void registry::for_joined_components(F&& f) const {
         std::lock_guard<std::mutex> guard(mutex_);
         for_joined_components_impl_<Ts...>(std::forward<F>(f));
     }
 
-    template < typename T >
-    detail::component_storage<T>* world::find_storage_() noexcept {
-        const auto family = detail::type_family<T>::id();
-        const auto iter = storages_.find(family);
-        if ( iter != storages_.end() ) {
-            return static_cast<detail::component_storage<T>*>(iter->second.get());
+    template < typename T, typename... Args >
+    void registry::add_system(Args&&... args) {
+        std::lock_guard<std::mutex> guard(mutex_);
+        systems_.emplace_back(
+            std::make_unique<T>(std::forward<Args>(args)...));
+    }
+
+    inline void registry::process_systems() {
+        for ( auto& s : systems_ ) {
+            s->process(*this);
         }
-        return nullptr;
     }
 
     template < typename T >
-    const detail::component_storage<T>* world::find_storage_() const noexcept {
+    detail::component_storage<T>* registry::find_storage_() noexcept {
         const auto family = detail::type_family<T>::id();
-        const auto iter = storages_.find(family);
-        if ( iter != storages_.end() ) {
-            return static_cast<const detail::component_storage<T>*>(iter->second.get());
-        }
-        return nullptr;
+        using raw_storage_ptr = detail::component_storage<T>*;
+        return storages_.has(family)
+            ? static_cast<raw_storage_ptr>(storages_.get_value(family).get())
+            : nullptr;
     }
 
     template < typename T >
-    detail::component_storage<T>& world::get_or_create_storage_() {
+    const detail::component_storage<T>* registry::find_storage_() const noexcept {
+        const auto family = detail::type_family<T>::id();
+        using raw_storage_ptr = const detail::component_storage<T>*;
+        return storages_.has(family)
+            ? static_cast<raw_storage_ptr>(storages_.get_value(family).get())
+            : nullptr;
+    }
+
+    template < typename T >
+    detail::component_storage<T>& registry::get_or_create_storage_() {
         detail::component_storage<T>* storage = find_storage_<T>();
         if ( storage ) {
             return *storage;
         }
         const auto family = detail::type_family<T>::id();
-        const auto emplace_r = storages_.emplace(std::make_pair(
+        storages_.emplace(
             family,
-            std::make_unique<detail::component_storage<T>>(*this)));
-        assert(emplace_r.second && "unexpected internal error");
+            std::make_unique<detail::component_storage<T>>(*this));
         return *static_cast<detail::component_storage<T>*>(
-            emplace_r.first->second.get());
+            storages_.get_value(family).get());
     }
 
-    inline bool world::is_entity_alive_impl_(const entity& ent) const noexcept {
-        return entities_.count(ent) > 0u;
+    inline bool registry::is_entity_alive_impl_(const entity& ent) const noexcept {
+        return entity_ids_.has(ent.id());
     }
 
-    inline std::size_t world::remove_all_components_impl_(const entity& ent) const noexcept {
+    inline std::size_t registry::remove_all_components_impl_(const entity& ent) const noexcept {
         if ( !is_entity_alive_impl_(ent) ) {
             return 0u;
         }
         std::size_t removed_components = 0u;
-        for ( auto& storage_p : storages_ ) {
-            if ( storage_p.second->remove(ent.id()) ) {
+        for ( const auto id : storages_ ) {
+            if ( storages_.get_value(id)->remove(ent.id()) ) {
                 ++removed_components;
             }
         }
@@ -603,7 +1020,7 @@ namespace ecs_hpp
     }
 
     template < typename T >
-    T* world::find_component_impl_(const entity& ent) noexcept {
+    T* registry::find_component_impl_(const entity& ent) noexcept {
         detail::component_storage<T>* storage = find_storage_<T>();
         return storage
             ? storage->find(ent.id())
@@ -611,7 +1028,7 @@ namespace ecs_hpp
     }
 
     template < typename T >
-    const T* world::find_component_impl_(const entity& ent) const noexcept {
+    const T* registry::find_component_impl_(const entity& ent) const noexcept {
         const detail::component_storage<T>* storage = find_storage_<T>();
         return storage
             ? storage->find(ent.id())
@@ -619,14 +1036,14 @@ namespace ecs_hpp
     }
 
     template < typename... Ts, typename F >
-    void world::for_joined_components_impl_(F&& f) {
-        for ( const auto& e : entities_ ) {
-            for_joined_components_impl_<Ts...>(e, std::forward<F>(f));
+    void registry::for_joined_components_impl_(F&& f) {
+        for ( const auto id : entity_ids_ ) {
+            for_joined_components_impl_<Ts...>(entity(*this, id), std::forward<F>(f));
         }
     }
 
     template < typename T, typename... Ts, typename F, typename... Cs >
-    void world::for_joined_components_impl_(const entity& e, F&& f, Cs&&... cs) {
+    void registry::for_joined_components_impl_(const entity& e, F&& f, Cs&&... cs) {
         T* c = find_component_impl_<T>(e);
         if ( c ) {
             for_joined_components_impl_<Ts...>(
@@ -638,19 +1055,19 @@ namespace ecs_hpp
     }
 
     template < typename F, typename... Cs >
-    void world::for_joined_components_impl_(const entity& e, F&& f, Cs&&... cs) {
+    void registry::for_joined_components_impl_(const entity& e, F&& f, Cs&&... cs) {
         f(e, std::forward<Cs>(cs)...);
     }
 
     template < typename... Ts, typename F >
-    void world::for_joined_components_impl_(F&& f) const {
-        for ( const auto& e : entities_ ) {
-            for_joined_components_impl_<Ts...>(e, std::forward<F>(f));
+    void registry::for_joined_components_impl_(F&& f) const {
+        for ( const auto id : entity_ids_ ) {
+            for_joined_components_impl_<Ts...>(entity(const_cast<registry&>(*this), id), std::forward<F>(f));
         }
     }
 
     template < typename T, typename... Ts, typename F, typename... Cs >
-    void world::for_joined_components_impl_(const entity& e, F&& f, Cs&&... cs) const {
+    void registry::for_joined_components_impl_(const entity& e, F&& f, Cs&&... cs) const {
         const T* c = find_component_impl_<T>(e);
         if ( c ) {
             for_joined_components_impl_<Ts...>(
@@ -662,7 +1079,7 @@ namespace ecs_hpp
     }
 
     template < typename F, typename... Cs >
-    void world::for_joined_components_impl_(const entity& e, F&& f, Cs&&... cs) const {
+    void registry::for_joined_components_impl_(const entity& e, F&& f, Cs&&... cs) const {
         f(e, std::forward<Cs>(cs)...);
     }
 }
diff --git a/ecs_tests.cpp b/ecs_tests.cpp
index d4efab9..ad3184e 100644
--- a/ecs_tests.cpp
+++ b/ecs_tests.cpp
@@ -27,6 +27,16 @@ namespace
         velocity_c() = default;
         velocity_c(int nx, int ny) : x(nx), y(ny) {}
     };
+
+    bool operator==(const position_c& l, const position_c& r) noexcept {
+        return l.x == r.x
+            && l.y == r.y;
+    }
+
+    bool operator==(const velocity_c& l, const velocity_c& r) noexcept {
+        return l.x == r.x
+            && l.y == r.y;
+    }
 }
 
 TEST_CASE("detail") {
@@ -40,12 +50,155 @@ TEST_CASE("detail") {
         REQUIRE(ecs::detail::type_family<position_c>::id() == 1u);
         REQUIRE(ecs::detail::type_family<velocity_c>::id() == 2u);
     }
+    SECTION("sparse_set") {
+        using namespace ecs::detail;
+        {
+            sparse_set<unsigned> s;
+
+            REQUIRE(s.empty());
+            REQUIRE_FALSE(s.size());
+            REQUIRE(s.capacity() == 0u);
+            REQUIRE_FALSE(s.has(42u));
+            REQUIRE(s.find(42u) == s.end());
+            REQUIRE_FALSE(s.find_index(42u).second);
+            REQUIRE_THROWS(s.get_index(42u));
+
+            REQUIRE(s.insert(42u));
+
+            REQUIRE_FALSE(s.empty());
+            REQUIRE(s.size() == 1u);
+            REQUIRE(s.capacity() == 43u);
+            REQUIRE(s.has(42u));
+            REQUIRE_FALSE(s.has(84u));
+
+            REQUIRE(s.find_index(42u).second);
+            REQUIRE(s.find_index(42u).first == 0u);
+            REQUIRE(s.get_index(42u) == 0u);
+
+            s.clear();
+
+            REQUIRE(s.empty());
+            REQUIRE_FALSE(s.size());
+            REQUIRE_FALSE(s.has(42u));
+
+            REQUIRE(s.insert(84u));
+            REQUIRE_FALSE(s.insert(84u));
+
+            REQUIRE(s.has(84u));
+            REQUIRE_FALSE(s.unordered_erase(42u));
+            REQUIRE(s.unordered_erase(84u));
+            REQUIRE_FALSE(s.has(84u));
+            REQUIRE(s.empty());
+            REQUIRE_FALSE(s.size());
+            REQUIRE(s.capacity() == 43u * 2);
+
+            s.insert(42u);
+            s.insert(84u);
+
+            REQUIRE(s.has(42u));
+            REQUIRE(s.has(84u));
+            REQUIRE(s.size() == 2u);
+            REQUIRE(s.find_index(42u).second);
+            REQUIRE(s.find_index(42u).first == 0u);
+            REQUIRE(s.find_index(84u).second);
+            REQUIRE(s.find_index(84u).first == 1u);
+            REQUIRE(s.get_index(42u) == 0u);
+            REQUIRE(s.get_index(84u) == 1u);
+
+            REQUIRE(s.unordered_erase(42u));
+
+            REQUIRE_FALSE(s.has(42u));
+            REQUIRE(s.has(84u));
+            REQUIRE(s.size() == 1u);
+            REQUIRE(s.find_index(84u).second);
+            REQUIRE(s.find_index(84u).first == 0u);
+            REQUIRE_THROWS(s.get_index(42u));
+            REQUIRE(s.get_index(84u) == 0u);
+        }
+    }
+    SECTION("sparse_map") {
+        using namespace ecs::detail;
+        {
+            struct obj_t {
+                int x;
+                obj_t(int nx) : x(nx) {}
+            };
+
+            sparse_map<unsigned, obj_t> m;
+
+            REQUIRE(m.empty());
+            REQUIRE_FALSE(m.size());
+            REQUIRE(m.capacity() == 0u);
+            REQUIRE_FALSE(m.has(42u));
+            REQUIRE_THROWS(m.get_value(42u));
+            REQUIRE_THROWS(as_const(m).get_value(42u));
+            REQUIRE_FALSE(m.find_value(42u));
+            REQUIRE_FALSE(as_const(m).find_value(42u));
+
+            {
+                obj_t o{21u};
+                REQUIRE(m.insert(21u, o));
+                REQUIRE(m.insert(42u, obj_t{42u}));
+                REQUIRE(m.emplace(84u, 84u));
+            }
+
+            {
+                obj_t o{21u};
+                REQUIRE_FALSE(m.insert(21u, o));
+                REQUIRE_FALSE(m.insert(42u, obj_t{42u}));
+                REQUIRE_FALSE(m.emplace(84u, 84u));
+            }
+
+            REQUIRE_FALSE(m.empty());
+            REQUIRE(m.size() == 3u);
+            REQUIRE(m.capacity() >= 3u);
+            REQUIRE(m.has(21u));
+            REQUIRE(m.has(42u));
+            REQUIRE(m.has(84u));
+            REQUIRE_FALSE(m.has(11u));
+            REQUIRE_FALSE(m.has(25u));
+            REQUIRE_FALSE(m.has(99u));
+
+            REQUIRE(m.get_value(21u).x == 21u);
+            REQUIRE(m.get_value(42u).x == 42u);
+            REQUIRE(m.get_value(84u).x == 84u);
+            REQUIRE(as_const(m).get_value(84u).x == 84u);
+            REQUIRE_THROWS(m.get_value(11u));
+            REQUIRE_THROWS(m.get_value(25u));
+            REQUIRE_THROWS(m.get_value(99u));
+            REQUIRE_THROWS(as_const(m).get_value(99u));
+
+            REQUIRE(m.find_value(21u)->x == 21u);
+            REQUIRE(m.find_value(42u)->x == 42u);
+            REQUIRE(m.find_value(84u)->x == 84u);
+            REQUIRE(as_const(m).find_value(84u)->x == 84u);
+            REQUIRE_FALSE(m.find_value(11u));
+            REQUIRE_FALSE(m.find_value(25u));
+            REQUIRE_FALSE(m.find_value(99u));
+            REQUIRE_FALSE(as_const(m).find_value(99u));
+
+            REQUIRE(m.unordered_erase(42u));
+            REQUIRE_FALSE(m.unordered_erase(42u));
+
+            REQUIRE(m.has(21u));
+            REQUIRE_FALSE(m.has(42u));
+            REQUIRE(m.has(84u));
+            REQUIRE(m.size() == 2u);
+
+            m.clear();
+            REQUIRE(m.empty());
+            REQUIRE_FALSE(m.size());
+            REQUIRE_FALSE(m.has(21u));
+            REQUIRE_FALSE(m.has(42u));
+            REQUIRE_FALSE(m.has(84u));
+        }
+    }
 }
 
-TEST_CASE("world") {
+TEST_CASE("registry") {
     SECTION("entities") {
         {
-            ecs::world w;
+            ecs::registry w;
 
             ecs::entity e1{w};
             ecs::entity e2{w};
@@ -58,7 +211,7 @@ TEST_CASE("world") {
             REQUIRE_FALSE(w.destroy_entity(e2));
         }
         {
-            ecs::world w;
+            ecs::registry w;
 
             auto e1 = w.create_entity();
             auto e2 = w.create_entity();
@@ -81,7 +234,7 @@ TEST_CASE("world") {
     }
     SECTION("component_assigning") {
         {
-            ecs::world w;
+            ecs::registry w;
             ecs::entity e1 = w.create_entity();
 
             {
@@ -120,7 +273,7 @@ TEST_CASE("world") {
             }
         }
         {
-            ecs::world w;
+            ecs::registry w;
 
             auto e1 = w.create_entity();
             auto e2 = w.create_entity();
@@ -140,7 +293,7 @@ TEST_CASE("world") {
             REQUIRE(w.exists_component<velocity_c>(e2));
         }
         {
-            ecs::world w;
+            ecs::registry w;
             auto e1 = w.create_entity();
             REQUIRE(e1.destroy());
             REQUIRE_FALSE(e1.assign_component<position_c>());
@@ -149,7 +302,7 @@ TEST_CASE("world") {
     }
     SECTION("component_accessing") {
         {
-            ecs::world w;
+            ecs::registry w;
 
             auto e1 = w.create_entity();
             auto e2 = w.create_entity();
@@ -170,7 +323,7 @@ TEST_CASE("world") {
             REQUIRE_THROWS_AS(e2.get_component<position_c>(), ecs::basic_exception);
         }
         {
-            ecs::world w;
+            ecs::registry w;
 
             const auto e1 = w.create_entity();
             const auto e2 = w.create_entity();
@@ -185,7 +338,7 @@ TEST_CASE("world") {
             REQUIRE(e2.find_component<velocity_c>()->y == 4);
 
             {
-                const ecs::world& ww = w;
+                const ecs::registry& ww = w;
                 REQUIRE(ww.get_component<position_c>(e1).x == 1);
                 REQUIRE(ww.get_component<position_c>(e1).y == 2);
 
@@ -202,10 +355,79 @@ TEST_CASE("world") {
                 REQUIRE_FALSE(ww.find_component<velocity_c>(e2));
             }
         }
+        {
+            ecs::registry w;
+
+            auto e1 = w.create_entity();
+
+            REQUIRE(e1.find_components<>() ==
+                std::make_tuple());
+            REQUIRE(e1.find_components<velocity_c>() ==
+                std::make_tuple<velocity_c*>(nullptr));
+            REQUIRE(e1.find_components<position_c, velocity_c>() ==
+                std::make_tuple<position_c*, velocity_c*>(nullptr, nullptr));
+
+            REQUIRE(e1.get_components<>() == std::make_tuple());
+            REQUIRE_THROWS(e1.get_components<velocity_c>());
+            REQUIRE_THROWS(e1.get_components<position_c, velocity_c>());
+
+            {
+                const auto ee1 = e1;
+
+                REQUIRE(ee1.find_components<>() ==
+                    std::make_tuple());
+                REQUIRE(ee1.find_components<velocity_c>() ==
+                    std::make_tuple<velocity_c*>(nullptr));
+                REQUIRE(ee1.find_components<position_c, velocity_c>() ==
+                    std::make_tuple<position_c*, velocity_c*>(nullptr, nullptr));
+
+                REQUIRE(ee1.get_components<>() == std::make_tuple());
+                REQUIRE_THROWS(ee1.get_components<velocity_c>());
+                REQUIRE_THROWS(ee1.get_components<position_c, velocity_c>());
+            }
+
+            e1.assign_component<velocity_c>(3, 4);
+
+            REQUIRE(e1.find_components<velocity_c>() ==
+                std::make_tuple<velocity_c*>(e1.find_component<velocity_c>()));
+            REQUIRE(e1.find_components<position_c, velocity_c>() ==
+                std::make_tuple<position_c*, velocity_c*>(nullptr, e1.find_component<velocity_c>()));
+
+            REQUIRE(e1.get_components<velocity_c>() ==
+                std::make_tuple<velocity_c&>(e1.get_component<velocity_c>()));
+            REQUIRE_THROWS(e1.get_components<position_c, velocity_c>());
+
+            {
+                const auto ee1 = e1;
+
+                REQUIRE(ee1.find_components<velocity_c>() ==
+                    std::make_tuple<velocity_c*>(e1.find_component<velocity_c>()));
+                REQUIRE(ee1.find_components<position_c, velocity_c>() ==
+                    std::make_tuple<position_c*, velocity_c*>(nullptr, e1.find_component<velocity_c>()));
+
+                REQUIRE(ee1.get_components<velocity_c>() ==
+                    std::make_tuple<const velocity_c&>(ee1.get_component<velocity_c>()));
+                REQUIRE_THROWS(ee1.get_components<position_c, velocity_c>());
+            }
+
+            e1.assign_component<position_c>(1, 2);
+
+            auto p = e1.get_components<position_c, velocity_c>();
+            std::get<0>(p).x = 10;
+            std::get<1>(p).x = 30;
+            REQUIRE(e1.get_component<position_c>().x == 10);
+            REQUIRE(e1.get_component<velocity_c>().x == 30);
+
+            auto p2 = e1.find_components<position_c, velocity_c>();
+            std::get<0>(p2)->y = 20;
+            std::get<1>(p2)->y = 40;
+            REQUIRE(e1.get_component<position_c>().y == 20);
+            REQUIRE(e1.get_component<velocity_c>().y == 40);
+        }
     }
     SECTION("for_each_component") {
         {
-            ecs::world w;
+            ecs::registry w;
 
             auto e1 = w.create_entity();
             auto e2 = w.create_entity();
@@ -227,7 +449,7 @@ TEST_CASE("world") {
             }
 
             {
-                const ecs::world& ww = w;
+                const ecs::registry& ww = w;
                 ecs::entity_id acc1 = 0;
                 int acc2 = 0;
                 ww.for_each_component<position_c>([&acc1, &acc2](ecs::entity e, const position_c& p){
@@ -239,10 +461,9 @@ TEST_CASE("world") {
             }
         }
     }
-
     SECTION("for_joined_components") {
         {
-            ecs::world w;
+            ecs::registry w;
 
             auto e1 = w.create_entity();
             auto e2 = w.create_entity();
@@ -272,7 +493,7 @@ TEST_CASE("world") {
             }
 
             {
-                const ecs::world& ww = w;
+                const ecs::registry& ww = w;
                 ecs::entity_id acc1 = 0;
                 int acc2 = 0;
                 ww.for_joined_components<position_c, velocity_c>([&acc1, &acc2](
@@ -286,4 +507,38 @@ TEST_CASE("world") {
             }
         }
     }
+    SECTION("systems") {
+        {
+            class movement_system : public ecs::system {
+            public:
+                void process(ecs::registry& owner) override {
+                    owner.for_joined_components<position_c, velocity_c>([](
+                        ecs::entity e, position_c& p, const velocity_c& v)
+                    {
+                        p.x += v.x;
+                        p.y += v.y;
+                    });
+                }
+            };
+
+            ecs::registry w;
+            w.add_system<movement_system>();
+
+            auto e1 = w.create_entity();
+            auto e2 = w.create_entity();
+
+            e1.assign_component<position_c>(1, 2);
+            e1.assign_component<velocity_c>(3, 4);
+            e2.assign_component<position_c>(5, 6);
+            e2.assign_component<velocity_c>(7, 8);
+
+            w.process_systems();
+
+            REQUIRE(e1.get_component<position_c>().x == 1 + 3);
+            REQUIRE(e1.get_component<position_c>().y == 2 + 4);
+
+            REQUIRE(e2.get_component<position_c>().x == 5 + 7);
+            REQUIRE(e2.get_component<position_c>().y == 6 + 8);
+        }
+    }
 }