Merge branch 'master' of github.com:lonkaars/crepe into jaro/collision-system

15 files changed, 618 insertions, 118 deletions

diff --git a/src/crepe/manager/CMakeLists.txt b/src/crepe/manager/CMakeLists.txt
index 517b8a2..f73e165 100644
--- a/src/crepe/manager/CMakeLists.txt
+++ b/src/crepe/manager/CMakeLists.txt
@@ -4,6 +4,8 @@ target_sources(crepe PUBLIC
 	Manager.cpp
 	SaveManager.cpp
 	SceneManager.cpp
+	LoopTimerManager.cpp
+	ResourceManager.cpp
 )
 
 target_sources(crepe PUBLIC FILE_SET HEADERS FILES
@@ -16,5 +18,8 @@ target_sources(crepe PUBLIC FILE_SET HEADERS FILES
 	SaveManager.h
 	SceneManager.h
 	SceneManager.hpp
+	LoopTimerManager.h
+	ResourceManager.h
+	ResourceManager.hpp
 )
 
diff --git a/src/crepe/manager/ComponentManager.cpp b/src/crepe/manager/ComponentManager.cpp
index 80cf8b4..df30d27 100644
--- a/src/crepe/manager/ComponentManager.cpp
+++ b/src/crepe/manager/ComponentManager.cpp
@@ -1,4 +1,5 @@
 #include "../api/GameObject.h"
+#include "../api/Metadata.h"
 #include "../types.h"
 #include "../util/Log.h"
 
@@ -61,3 +62,13 @@ GameObject ComponentManager::new_object(const string & name, const string & tag,
 void ComponentManager::set_persistent(game_object_id_t id, bool persistent) {
 	this->persistent[id] = persistent;
 }
+
+set<game_object_id_t> ComponentManager::get_objects_by_name(const string & name) const {
+	return this->get_objects_by_predicate<Metadata>(
+		[name](const Metadata & data) { return data.name == name; });
+}
+
+set<game_object_id_t> ComponentManager::get_objects_by_tag(const string & tag) const {
+	return this->get_objects_by_predicate<Metadata>(
+		[tag](const Metadata & data) { return data.tag == tag; });
+}
diff --git a/src/crepe/manager/ComponentManager.h b/src/crepe/manager/ComponentManager.h
index ad37586..19a8e81 100644
--- a/src/crepe/manager/ComponentManager.h
+++ b/src/crepe/manager/ComponentManager.h
@@ -1,6 +1,7 @@
 #pragma once
 
 #include <memory>
+#include <set>
 #include <typeindex>
 #include <unordered_map>
 #include <vector>
@@ -16,7 +17,7 @@ class GameObject;
 
 /**
  * \brief Manages all components
- * 
+ *
  * This class manages all components. It provides methods to add, delete and get components.
  */
 class ComponentManager : public Manager {
@@ -56,10 +57,10 @@ protected:
 	friend class GameObject;
 	/**
 	 * \brief Add a component to the ComponentManager
-	 * 
+	 *
 	 * This method adds a component to the ComponentManager. The component is created with the
 	 * given arguments and added to the ComponentManager.
-	 * 
+	 *
 	 * \tparam T The type of the component
 	 * \tparam Args The types of the arguments
 	 * \param id The id of the GameObject this component belongs to
@@ -70,9 +71,9 @@ protected:
 	T & add_component(game_object_id_t id, Args &&... args);
 	/**
 	 * \brief Delete all components of a specific type and id
-	 * 
+	 *
 	 * This method deletes all components of a specific type and id.
-	 * 
+	 *
 	 * \tparam T The type of the component
 	 * \param id The id of the GameObject this component belongs to
 	 */
@@ -80,24 +81,24 @@ protected:
 	void delete_components_by_id(game_object_id_t id);
 	/**
 	 * \brief Delete all components of a specific type
-	 * 
+	 *
 	 * This method deletes all components of a specific type.
-	 * 
+	 *
 	 * \tparam T The type of the component
 	 */
 	template <typename T>
 	void delete_components();
 	/**
 	 * \brief Delete all components of a specific id
-	 * 
+	 *
 	 * This method deletes all components of a specific id.
-	 * 
+	 *
 	 * \param id The id of the GameObject this component belongs to
 	 */
 	void delete_all_components_of_id(game_object_id_t id);
 	/**
 	 * \brief Delete all components
-	 * 
+	 *
 	 * This method deletes all components.
 	 */
 	void delete_all_components();
@@ -115,9 +116,9 @@ protected:
 public:
 	/**
 	 * \brief Get all components of a specific type and id
-	 * 
+	 *
 	 * This method gets all components of a specific type and id.
-	 * 
+	 *
 	 * \tparam T The type of the component
 	 * \param id The id of the GameObject this component belongs to
 	 * \return A vector of all components of the specific type and id
@@ -126,19 +127,88 @@ public:
 	RefVector<T> get_components_by_id(game_object_id_t id) const;
 	/**
 	 * \brief Get all components of a specific type
-	 * 
+	 *
 	 * This method gets all components of a specific type.
-	 * 
+	 *
 	 * \tparam T The type of the component
 	 * \return A vector of all components of the specific type
 	 */
 	template <typename T>
 	RefVector<T> get_components_by_type() const;
+	/**
+	 * \brief Get all components of a specific type on a GameObject with name \c name
+	 * 
+	 * \tparam T The type of the component
+	 * \param name Metadata::name for the same game_object_id as the returned components
+	 * \return Components matching criteria
+	 */
+	template <typename T>
+	RefVector<T> get_components_by_name(const std::string & name) const;
+	/**
+	 * \brief Get all components of a specific type on a GameObject with tag \c tag
+	 * 
+	 * \tparam T The type of the component
+	 * \param name Metadata::tag for the same game_object_id as the returned components
+	 * \return Components matching criteria
+	 */
+	template <typename T>
+	RefVector<T> get_components_by_tag(const std::string & tag) const;
 
 private:
 	/**
+	 * \brief Get object IDs by predicate function
+	 *
+	 * This function calls the predicate function \c pred for all components matching type \c T,
+	 * and adds their parent game_object_id to a \c std::set if the predicate returns true.
+	 *
+	 * \tparam T The type of the component to check the predicate against
+	 * \param pred Predicate function
+	 *
+	 * \note The predicate function may be called for multiple components with the same \c
+	 * game_object_id. In this case, the ID is added if *any* call returns \c true.
+	 *
+	 * \returns game_object_id for all components where the predicate returned true
+	 */
+	template <typename T>
+	std::set<game_object_id_t>
+	get_objects_by_predicate(const std::function<bool(const T &)> & pred) const;
+
+	/**
+	 * \brief Get components of type \c T for multiple game object IDs
+	 *
+	 * \tparam T The type of the components to return
+	 * \param ids The object IDs
+	 *
+	 * \return All components matching type \c T and one of the IDs in \c ids
+	 */
+	template <typename T>
+	RefVector<T> get_components_by_ids(const std::set<game_object_id_t> & ids) const;
+
+	/**
+	 * \brief Get object IDs for objects with name \c name
+	 *
+	 * \param name Object name to match
+	 * \returns Object IDs where Metadata::name is equal to \c name
+	 */
+	std::set<game_object_id_t> get_objects_by_name(const std::string & name) const;
+	/**
+	 * \brief Get object IDs for objects with tag \c tag
+	 *
+	 * \param tag Object tag to match
+	 * \returns Object IDs where Metadata::tag is equal to \c tag
+	 */
+	std::set<game_object_id_t> get_objects_by_tag(const std::string & tag) const;
+
+private:
+	//! By Component \c std::type_index (readability helper type)
+	template <typename T>
+	using by_type = std::unordered_map<std::type_index, T>;
+	//! By \c game_object_id index (readability helper type)
+	template <typename T>
+	using by_id_index = std::vector<T>;
+	/**
 	 * \brief The components
-	 * 
+	 *
 	 * This unordered_map stores all components. The key is the type of the component and the
 	 * value is a vector of vectors of unique pointers to the components.
 	 *
@@ -146,8 +216,7 @@ private:
 	 * The first vector is for the ids of the GameObjects and the second vector is for the
 	 * components (because a GameObject might have multiple components).
 	 */
-	std::unordered_map<std::type_index, std::vector<std::vector<std::unique_ptr<Component>>>>
-		components;
+	by_type<by_id_index<std::vector<std::unique_ptr<Component>>>> components;
 
 	//! Persistent flag for each GameObject
 	std::unordered_map<game_object_id_t, bool> persistent;
diff --git a/src/crepe/manager/ComponentManager.hpp b/src/crepe/manager/ComponentManager.hpp
index ffb38ec..9e70865 100644
--- a/src/crepe/manager/ComponentManager.hpp
+++ b/src/crepe/manager/ComponentManager.hpp
@@ -95,32 +95,25 @@ template <typename T>
 RefVector<T> ComponentManager::get_components_by_id(game_object_id_t id) const {
 	using namespace std;
 
-	// Determine the type of T (this is used as the key of the unordered_map<>)
-	type_index type = typeid(T);
-
-	// Create an empty vector<>
-	RefVector<T> component_vector;
-
-	if (this->components.find(type) == this->components.end()) return component_vector;
-
-	// Get the correct vector<>
-	const vector<vector<unique_ptr<Component>>> & component_array = this->components.at(type);
-
-	// Make sure that the id (that we are looking for) is within the boundaries of the vector<>
-	if (id >= component_array.size()) return component_vector;
-
-	// Loop trough the whole vector<>
-	for (const unique_ptr<Component> & component_ptr : component_array[id]) {
-		// Cast the unique_ptr to a raw pointer
-		T * casted_component = static_cast<T *>(component_ptr.get());
-
-		if (casted_component == nullptr) continue;
+	static_assert(is_base_of<Component, T>::value,
+				  "get_components_by_id must recieve a derivative class of Component");
 
-		// Add the dereferenced raw pointer to the vector<>
-		component_vector.push_back(*casted_component);
+	type_index type = typeid(T);
+	if (!this->components.contains(type)) return {};
+
+	const by_id_index<vector<unique_ptr<Component>>> & components_by_id
+		= this->components.at(type);
+	if (id >= components_by_id.size()) return {};
+
+	RefVector<T> out = {};
+	const vector<unique_ptr<Component>> & components = components_by_id.at(id);
+	for (auto & component_ptr : components) {
+		if (component_ptr == nullptr) continue;
+		Component & component = *component_ptr.get();
+		out.push_back(static_cast<T &>(component));
 	}
 
-	return component_vector;
+	return out;
 }
 
 template <typename T>
@@ -158,4 +151,46 @@ RefVector<T> ComponentManager::get_components_by_type() const {
 	return component_vector;
 }
 
+template <typename T>
+std::set<game_object_id_t>
+ComponentManager::get_objects_by_predicate(const std::function<bool(const T &)> & pred) const {
+	using namespace std;
+
+	set<game_object_id_t> objects = {};
+	RefVector<T> components = this->get_components_by_type<T>();
+
+	for (const T & component : components) {
+		game_object_id_t id = dynamic_cast<const Component &>(component).game_object_id;
+		if (objects.contains(id)) continue;
+		if (!pred(component)) continue;
+		objects.insert(id);
+	}
+
+	return objects;
+}
+
+template <typename T>
+RefVector<T>
+ComponentManager::get_components_by_ids(const std::set<game_object_id_t> & ids) const {
+	using namespace std;
+
+	RefVector<T> out = {};
+	for (game_object_id_t id : ids) {
+		RefVector<T> components = get_components_by_id<T>(id);
+		out.insert(out.end(), components.begin(), components.end());
+	}
+
+	return out;
+}
+
+template <typename T>
+RefVector<T> ComponentManager::get_components_by_name(const std::string & name) const {
+	return this->get_components_by_ids<T>(this->get_objects_by_name(name));
+}
+
+template <typename T>
+RefVector<T> ComponentManager::get_components_by_tag(const std::string & tag) const {
+	return this->get_components_by_ids<T>(this->get_objects_by_tag(tag));
+}
+
 } // namespace crepe
diff --git a/src/crepe/manager/EventManager.cpp b/src/crepe/manager/EventManager.cpp
index 20f0dd3..6aa49ee 100644
--- a/src/crepe/manager/EventManager.cpp
+++ b/src/crepe/manager/EventManager.cpp
@@ -3,11 +3,9 @@
 using namespace crepe;
 using namespace std;
 
-EventManager & EventManager::get_instance() {
-	static EventManager instance;
-	return instance;
+EventManager::EventManager(Mediator & mediator) : Manager(mediator) {
+	this->mediator.event_manager = *this;
 }
-
 void EventManager::dispatch_events() {
 	for (auto & event : this->events_queue) {
 		this->handle_event(event.type, event.channel, *event.event.get());
diff --git a/src/crepe/manager/EventManager.h b/src/crepe/manager/EventManager.h
index d634f54..639e37f 100644
--- a/src/crepe/manager/EventManager.h
+++ b/src/crepe/manager/EventManager.h
@@ -8,6 +8,8 @@
 #include "../api/Event.h"
 #include "../api/EventHandler.h"
 
+#include "Manager.h"
+
 namespace crepe {
 
 //! Event listener unique ID
@@ -22,33 +24,25 @@ typedef size_t subscription_t;
 typedef size_t event_channel_t;
 
 /**
- * \class EventManager
  * \brief Manages event subscriptions, triggers, and queues, enabling decoupled event handling.
- * 
+ *
  * The `EventManager` acts as a centralized event system. It allows for registering callbacks
  * for specific event types, triggering events synchronously, queueing events for later
  * processing, and managing subscriptions via unique identifiers.
  */
-class EventManager {
+class EventManager : public Manager {
 public:
 	static constexpr const event_channel_t CHANNEL_ALL = -1;
-
 	/**
-	 * \brief Get the singleton instance of the EventManager.
-	 * 
-	 * This method returns the unique instance of the EventManager, creating it if it
-	 * doesn't already exist. Ensures only one instance is active in the program.
-	 * 
-	 * \return Reference to the singleton instance of the EventManager.
+	 * \param mediator A reference to a Mediator object used for transfering managers.
 	 */
-	static EventManager & get_instance();
-
+	EventManager(Mediator & mediator);
 	/**
 	 * \brief Subscribe to a specific event type.
-	 * 
+	 *
 	 * Registers a callback for a given event type and optional channel. Each callback
 	 * is assigned a unique subscription ID that can be used for later unsubscription.
-	 * 
+	 *
 	 * \tparam EventType The type of the event to subscribe to.
 	 * \param callback The callback function to be invoked when the event is triggered.
 	 * \param channel The channel number to subscribe to (default is CHANNEL_ALL, which listens to all channels).
@@ -60,18 +54,18 @@ public:
 
 	/**
 	 * \brief Unsubscribe a previously registered callback.
-	 * 
+	 *
 	 * Removes a callback from the subscription list based on its unique subscription ID.
-	 * 
+	 *
 	 * \param event_id The unique subscription ID of the callback to remove.
 	 */
 	void unsubscribe(subscription_t event_id);
 
 	/**
 	 * \brief Trigger an event immediately.
-	 * 
+	 *
 	 * Synchronously invokes all registered callbacks for the given event type on the specified channel.
-	 * 
+	 *
 	 * \tparam EventType The type of the event to trigger.
 	 * \param event The event instance to pass to the callbacks.
 	 * \param channel The channel to trigger the event on (default is CHANNEL_ALL, which triggers on all channels).
@@ -81,9 +75,9 @@ public:
 
 	/**
 	 * \brief Queue an event for later processing.
-	 * 
+	 *
 	 * Adds an event to the event queue to be processed during the next call to `dispatch_events`.
-	 * 
+	 *
 	 * \tparam EventType The type of the event to queue.
 	 * \param event The event instance to queue.
 	 * \param channel The channel to associate with the event (default is CHANNEL_ALL).
@@ -93,7 +87,7 @@ public:
 
 	/**
 	 * \brief Process all queued events.
-	 * 
+	 *
 	 * Iterates through the event queue and triggers callbacks for each queued event.
 	 * Events are removed from the queue once processed.
 	 */
@@ -101,20 +95,13 @@ public:
 
 	/**
 	 * \brief Clear all subscriptions.
-	 * 
+	 *
 	 * Removes all registered event handlers and clears the subscription list.
 	 */
 	void clear();
 
 private:
 	/**
-	 * \brief Default constructor for the EventManager.
-	 * 
-	 * Constructor is private to enforce the singleton pattern.
-	 */
-	EventManager() = default;
-
-	/**
 	 * \struct QueueEntry
 	 * \brief Represents an entry in the event queue.
 	 */
diff --git a/src/crepe/manager/LoopTimerManager.cpp b/src/crepe/manager/LoopTimerManager.cpp
new file mode 100644
index 0000000..a6e4788
--- /dev/null
+++ b/src/crepe/manager/LoopTimerManager.cpp
@@ -0,0 +1,91 @@
+#include <chrono>
+#include <thread>
+
+#include "../util/Log.h"
+
+#include "LoopTimerManager.h"
+
+using namespace crepe;
+using namespace std::chrono;
+using namespace std::chrono_literals;
+
+LoopTimerManager::LoopTimerManager(Mediator & mediator) : Manager(mediator) {
+	this->mediator.loop_timer = *this;
+	dbg_trace();
+}
+
+void LoopTimerManager::start() {
+	this->last_frame_time = std::chrono::steady_clock::now();
+
+	this->elapsed_time = elapsed_time_t{0};
+	this->elapsed_fixed_time = elapsed_time_t{0};
+	this->delta_time = duration_t{0};
+}
+
+void LoopTimerManager::update() {
+	time_point_t current_frame_time = std::chrono::steady_clock::now();
+	// Convert to duration in seconds for delta time
+	this->delta_time = current_frame_time - last_frame_time;
+
+	if (this->delta_time > this->maximum_delta_time) {
+		this->delta_time = this->maximum_delta_time;
+	}
+	if (this->delta_time > 0s) {
+		this->actual_fps = static_cast<unsigned>(1.0 / this->delta_time.count());
+	} else {
+		this->actual_fps = 0;
+	}
+	this->elapsed_time += duration_cast<elapsed_time_t>(this->delta_time);
+	this->last_frame_time = current_frame_time;
+}
+
+duration_t LoopTimerManager::get_delta_time() const {
+	return this->delta_time * this->time_scale;
+}
+
+elapsed_time_t LoopTimerManager::get_elapsed_time() const { return this->elapsed_time; }
+
+void LoopTimerManager::advance_fixed_elapsed_time() {
+	this->elapsed_fixed_time
+		+= std::chrono::duration_cast<elapsed_time_t>(this->fixed_delta_time);
+}
+
+void LoopTimerManager::set_target_framerate(unsigned fps) {
+	this->target_fps = fps;
+	//check if fps is lower or equals 0
+	if (fps <= 0) return;
+	// target time per frame in seconds
+	this->frame_target_time = duration_t(1s) / this->target_fps;
+}
+
+unsigned LoopTimerManager::get_fps() const { return this->actual_fps; }
+
+void LoopTimerManager::set_time_scale(double value) { this->time_scale = value; }
+
+float LoopTimerManager::get_time_scale() const { return this->time_scale; }
+
+void LoopTimerManager::enforce_frame_rate() {
+	time_point_t current_frame_time = std::chrono::steady_clock::now();
+	duration_t frame_duration = current_frame_time - this->last_frame_time;
+	// Check if frame duration is less than the target frame time
+	if (frame_duration < this->frame_target_time) {
+		duration_t delay_time = this->frame_target_time - frame_duration;
+		if (delay_time > 0s) {
+			std::this_thread::sleep_for(delay_time);
+		}
+	}
+}
+
+duration_t LoopTimerManager::get_lag() const {
+	return this->elapsed_time - this->elapsed_fixed_time;
+}
+
+duration_t LoopTimerManager::get_scaled_fixed_delta_time() const {
+	return this->fixed_delta_time * this->time_scale;
+}
+
+void LoopTimerManager::set_fixed_delta_time(float seconds) {
+	this->fixed_delta_time = duration_t(seconds);
+}
+
+duration_t LoopTimerManager::get_fixed_delta_time() const { return this->fixed_delta_time; }
diff --git a/src/crepe/manager/LoopTimerManager.h b/src/crepe/manager/LoopTimerManager.h
new file mode 100644
index 0000000..76b02d3
--- /dev/null
+++ b/src/crepe/manager/LoopTimerManager.h
@@ -0,0 +1,175 @@
+#pragma once
+
+#include <chrono>
+
+#include "Manager.h"
+
+namespace crepe {
+
+typedef std::chrono::duration<float> duration_t;
+typedef std::chrono::duration<unsigned long long, std::micro> elapsed_time_t;
+
+/**
+ * \brief Manages timing and frame rate for the game loop.
+ * 
+ * The LoopTimerManager class is responsible for calculating and managing timing functions 
+ * such as delta time, frames per second (FPS), fixed time steps, and time scaling. It ensures 
+ * consistent frame updates and supports game loop operations, such as handling fixed updates 
+ * for physics and other time-sensitive operations.
+ */
+class LoopTimerManager : public Manager {
+public:
+	/**
+	 * \param mediator A reference to a Mediator object used for transfering managers.
+	 */
+	LoopTimerManager(Mediator & mediator);
+	/**
+	 * \brief Get the current delta time for the current frame.
+	 *	
+	 * This value represents the estimated frame duration of the current frame.
+	 * This value can be used in the frame_update to convert pixel based values to time based values.
+	 * 
+	 * \return Delta time in seconds since the last frame.
+	 */
+	duration_t get_delta_time() const;
+
+	/**
+	 * \brief Get the current elapsed time (total time passed )
+	 *
+	 * \note The current game time may vary from real-world elapsed time. It is the cumulative
+	 * sum of each frame's delta time.
+	 *
+	 * \return Elapsed game time in seconds.
+	 */
+	elapsed_time_t get_elapsed_time() const;
+
+	/**
+	 * \brief Set the target frames per second (FPS).
+	 *
+	 * \param fps The desired frames rendered per second.
+	 */
+	void set_target_framerate(unsigned fps);
+
+	/**
+	 * \brief Get the current frames per second (FPS).
+	 *
+	 * \return Current FPS.
+	 */
+	unsigned int get_fps() const;
+
+	/**
+	 * \brief Get the current time scale.
+	 *
+	 * \return The current time scale, where (0 = pause, < 1 = slow down, 1 = normal speed, > 1 = speed up).
+	 * up the game.
+	 */
+	float get_time_scale() const;
+
+	/**
+	 * \brief Set the time scale.
+	 *
+	 * time_scale is a value that changes the delta time that can be retrieved using get_delta_time function. 
+	 * 
+	 * \param time_scale The desired time scale (0 = pause, < 1 = slow down, 1 = normal speed, > 1 = speed up).
+	 */
+	void set_time_scale(double time_scale);
+
+	/**
+	 * \brief Get the fixed delta time in seconds without scaling by the time scale.
+	 *
+	 * This value is used in the LoopManager to determine how many times 
+	 * the fixed_update should be called within a given interval.
+	 *
+	 * \return The unscaled fixed delta time in seconds.
+	 */
+	duration_t get_fixed_delta_time() const;
+
+	/**
+	 * \brief Set the fixed_delta_time in seconds.
+	 * 
+	 * \param seconds fixed_delta_time in seconds.
+	 * 
+	 * The fixed_delta_time value is used to determine how many times per second the fixed_update and process_input functions are called.
+	 * 
+	 */
+	void set_fixed_delta_time(float seconds);
+
+	/**
+	 * \brief Retrieves the scaled fixed delta time in seconds.
+	 *
+	 * The scaled fixed delta time is the timing value used within the `fixed_update` function. 
+	 * It is adjusted by the time_scale to account for any changes in the simulation's 
+	 * speed.
+	 *
+	 * \return The fixed delta time, scaled by the current time scale, in seconds.
+	 */
+	duration_t get_scaled_fixed_delta_time() const;
+
+private:
+	//! Friend relation to use start,enforce_frame_rate,get_lag,update,advance_fixed_update.
+	friend class LoopManager;
+	/**
+	 * \brief Start the loop timer.
+	 *
+	 * Initializes the timer to begin tracking frame times.
+	 */
+	void start();
+	/**
+	 * \brief Enforce the frame rate limit.
+	 *
+	 * Ensures that the game loop does not exceed the target FPS by delaying frame updates as
+	 * necessary.
+	 */
+	void enforce_frame_rate();
+	/**
+	 * \brief Get the accumulated lag in the game loop.
+	 *
+	 * Lag represents the difference between the target frame time and the actual frame time,
+	 * useful for managing fixed update intervals.
+	 *
+	 * \return Accumulated lag in seconds.
+	 */
+	duration_t get_lag() const;
+
+	/**
+	 * \brief Update the timer to the current frame.
+	 *
+	 * Calculates and updates the delta time for the current frame and adds it to the cumulative
+	 * game time.
+	 */
+	void update();
+
+	/**
+	 * \brief Progress the elapsed fixed time by the fixed delta time interval.
+	 *
+	 * This method advances the game's fixed update loop by adding the fixed_delta_time 
+	 * to elapsed_fixed_time, ensuring the fixed update catches up with the elapsed time.
+	 */
+	void advance_fixed_elapsed_time();
+
+private:
+	//! Target frames per second.
+	unsigned int target_fps = 60;
+	//! Actual frames per second.
+	unsigned int actual_fps = 0;
+	//! Time scale for speeding up or slowing down the game (0 = pause, < 1 = slow down, 1 = normal speed, > 1 = speed up).
+	float time_scale = 1;
+	//! Maximum delta time in seconds to avoid large jumps.
+	duration_t maximum_delta_time{0.25};
+	//! Delta time for the current frame in seconds.
+	duration_t delta_time{0.0};
+	//! Target time per frame in seconds
+	duration_t frame_target_time{1.0 / target_fps};
+	//! Fixed delta time for fixed updates in seconds.
+	duration_t fixed_delta_time{1.0 / 50.0};
+	//! Total elapsed game time in microseconds.
+	elapsed_time_t elapsed_time{0};
+	//! Total elapsed time for fixed updates in microseconds.
+	elapsed_time_t elapsed_fixed_time{0};
+
+	typedef std::chrono::steady_clock::time_point time_point_t;
+	//! Time of the last frame.
+	time_point_t last_frame_time;
+};
+
+} // namespace crepe
diff --git a/src/crepe/manager/Mediator.h b/src/crepe/manager/Mediator.h
index 71bd1c9..a336410 100644
--- a/src/crepe/manager/Mediator.h
+++ b/src/crepe/manager/Mediator.h
@@ -2,14 +2,15 @@
 
 #include "../util/OptionalRef.h"
 
-// TODO: remove these singletons:
-#include "EventManager.h"
-#include "SaveManager.h"
-
 namespace crepe {
 
 class ComponentManager;
 class SceneManager;
+class EventManager;
+class LoopTimerManager;
+class SaveManager;
+class ResourceManager;
+class SDLContext;
 
 /**
  * Struct to pass references to classes that would otherwise need to be singletons down to
@@ -24,10 +25,13 @@ class SceneManager;
  * \warning This class should never be directly accessible from the API
  */
 struct Mediator {
+	OptionalRef<SDLContext> sdl_context;
 	OptionalRef<ComponentManager> component_manager;
 	OptionalRef<SceneManager> scene_manager;
-	OptionalRef<SaveManager> save_manager = SaveManager::get_instance();
-	OptionalRef<EventManager> event_manager = EventManager::get_instance();
+	OptionalRef<EventManager> event_manager;
+	OptionalRef<LoopTimerManager> loop_timer;
+	OptionalRef<SaveManager> save_manager;
+	OptionalRef<ResourceManager> resource_manager;
 };
 
 } // namespace crepe
diff --git a/src/crepe/manager/ResourceManager.cpp b/src/crepe/manager/ResourceManager.cpp
new file mode 100644
index 0000000..a141a46
--- /dev/null
+++ b/src/crepe/manager/ResourceManager.cpp
@@ -0,0 +1,30 @@
+#include "util/Log.h"
+
+#include "ResourceManager.h"
+
+using namespace crepe;
+using namespace std;
+
+ResourceManager::ResourceManager(Mediator & mediator) : Manager(mediator) {
+	dbg_trace();
+	mediator.resource_manager = *this;
+}
+ResourceManager::~ResourceManager() { dbg_trace(); }
+
+void ResourceManager::clear() {
+	std::erase_if(this->resources, [](const pair<const Asset, CacheEntry> & pair) {
+		const CacheEntry & entry = pair.second;
+		return entry.persistent == false;
+	});
+}
+
+void ResourceManager::clear_all() { this->resources.clear(); }
+
+void ResourceManager::set_persistent(const Asset & asset, bool persistent) {
+	this->get_entry(asset).persistent = persistent;
+}
+
+ResourceManager::CacheEntry & ResourceManager::get_entry(const Asset & asset) {
+	if (!this->resources.contains(asset)) this->resources[asset] = {};
+	return this->resources.at(asset);
+}
diff --git a/src/crepe/manager/ResourceManager.h b/src/crepe/manager/ResourceManager.h
new file mode 100644
index 0000000..84b275d
--- /dev/null
+++ b/src/crepe/manager/ResourceManager.h
@@ -0,0 +1,78 @@
+#pragma once
+
+#include <memory>
+#include <unordered_map>
+
+#include "../Resource.h"
+#include "../api/Asset.h"
+
+#include "Manager.h"
+
+namespace crepe {
+
+/**
+ * \brief Owner of concrete Resource instances
+ * 
+ * ResourceManager caches concrete Resource instances per Asset. Concrete resources are
+ * destroyed at the end of scenes by default, unless the game programmer marks them as
+ * persistent.
+ */
+class ResourceManager : public Manager {
+public:
+	ResourceManager(Mediator & mediator);
+	virtual ~ResourceManager(); // dbg_trace
+
+private:
+	//! Cache entry
+	struct CacheEntry {
+		//! Concrete resource instance
+		std::unique_ptr<Resource> resource = nullptr;
+		//! Prevent ResourceManager::clear from removing this entry
+		bool persistent = false;
+	};
+	//! Internal cache
+	std::unordered_map<const Asset, CacheEntry> resources;
+	/**
+	 * \brief Ensure a cache entry exists for this asset and return a mutable reference to it
+	 *
+	 * \param asset Asset the concrete resource is instantiated from
+	 *
+	 * \returns Mutable reference to cache entry
+	 */
+	CacheEntry & get_entry(const Asset & asset);
+
+public:
+	/**
+	 * \brief Mark a resource as persistent (i.e. used across multiple scenes)
+	 *
+	 * \param asset Asset the concrete resource is instantiated from
+	 * \param persistent Whether this resource is persistent (true=keep, false=destroy)
+	 */
+	void set_persistent(const Asset & asset, bool persistent);
+
+	/**
+	 * \brief Retrieve reference to concrete Resource by Asset
+	 *
+	 * \param asset Asset the concrete resource is instantiated from
+	 * \tparam Resource Concrete derivative of Resource
+	 *
+	 * This class instantiates the concrete resource if it is not yet stored in the internal
+	 * cache, or returns a reference to the cached resource if it already exists.
+	 *
+	 * \returns Reference to concrete resource
+	 *
+	 * \throws std::runtime_error if the \c Resource parameter does not match with the actual
+	 * type of the resource stored in the cache for this Asset
+	 */
+	template <typename Resource>
+	Resource & get(const Asset & asset);
+
+	//! Clear non-persistent resources from cache
+	void clear();
+	//! Clear all resources from cache regardless of persistence
+	void clear_all();
+};
+
+} // namespace crepe
+
+#include "ResourceManager.hpp"
diff --git a/src/crepe/manager/ResourceManager.hpp b/src/crepe/manager/ResourceManager.hpp
new file mode 100644
index 0000000..cf5c949
--- /dev/null
+++ b/src/crepe/manager/ResourceManager.hpp
@@ -0,0 +1,27 @@
+#pragma once
+
+#include <format>
+
+#include "ResourceManager.h"
+
+namespace crepe {
+
+template <typename T>
+T & ResourceManager::get(const Asset & asset) {
+	using namespace std;
+	static_assert(is_base_of<Resource, T>::value,
+				  "cache must recieve a derivative class of Resource");
+
+	CacheEntry & entry = this->get_entry(asset);
+	if (entry.resource == nullptr) entry.resource = make_unique<T>(asset, this->mediator);
+
+	T * concrete_resource = dynamic_cast<T *>(entry.resource.get());
+	if (concrete_resource == nullptr)
+		throw runtime_error(format("ResourceManager: mismatch between requested type and "
+								   "actual type of resource ({})",
+								   asset.get_path()));
+
+	return *concrete_resource;
+}
+
+} // namespace crepe
diff --git a/src/crepe/manager/SaveManager.cpp b/src/crepe/manager/SaveManager.cpp
index d4ed1c1..691ea2f 100644
--- a/src/crepe/manager/SaveManager.cpp
+++ b/src/crepe/manager/SaveManager.cpp
@@ -1,13 +1,25 @@
 #include "../ValueBroker.h"
 #include "../api/Config.h"
 #include "../facade/DB.h"
-#include "../util/Log.h"
 
 #include "SaveManager.h"
 
 using namespace std;
 using namespace crepe;
 
+SaveManager::SaveManager(Mediator & mediator) : Manager(mediator) {
+	mediator.save_manager = *this;
+}
+
+DB & SaveManager::get_db() {
+	if (this->db == nullptr) {
+		Config & cfg = Config::get_instance();
+		this->db
+			= {new DB(cfg.savemgr.location), [](void * db) { delete static_cast<DB *>(db); }};
+	}
+	return *static_cast<DB *>(this->db.get());
+}
+
 template <>
 string SaveManager::serialize(const string & value) const noexcept {
 	return value;
@@ -90,22 +102,6 @@ int32_t SaveManager::deserialize(const string & value) const noexcept {
 	return deserialize<int64_t>(value);
 }
 
-SaveManager::SaveManager() { dbg_trace(); }
-
-SaveManager & SaveManager::get_instance() {
-	dbg_trace();
-	static SaveManager instance;
-	return instance;
-}
-
-DB & SaveManager::get_db() {
-	Config & cfg = Config::get_instance();
-	// TODO: make this path relative to XDG_DATA_HOME on Linux and whatever the
-	// default equivalent is on Windows using some third party library
-	static DB db(cfg.savemgr.location);
-	return db;
-}
-
 bool SaveManager::has(const string & key) {
 	DB & db = this->get_db();
 	return db.has(key);
@@ -155,7 +151,8 @@ ValueBroker<T> SaveManager::get(const string & key) {
 	return {
 		[this, key](const T & target) { this->set<T>(key, target); },
 		[this, key, value]() mutable -> const T & {
-			value = this->deserialize<T>(this->get_db().get(key));
+			DB & db = this->get_db();
+			value = this->deserialize<T>(db.get(key));
 			return value;
 		},
 	};
diff --git a/src/crepe/manager/SaveManager.h b/src/crepe/manager/SaveManager.h
index 3d8c852..61a978d 100644
--- a/src/crepe/manager/SaveManager.h
+++ b/src/crepe/manager/SaveManager.h
@@ -1,9 +1,12 @@
 #pragma once
 
+#include <functional>
 #include <memory>
 
 #include "../ValueBroker.h"
 
+#include "Manager.h"
+
 namespace crepe {
 
 class DB;
@@ -18,7 +21,7 @@ class DB;
  *
  * The underlying database is a key-value store.
  */
-class SaveManager {
+class SaveManager : public Manager {
 public:
 	/**
 	 * \brief Get a read/write reference to a value and initialize it if it does not yet exist
@@ -63,8 +66,8 @@ public:
 	 */
 	bool has(const std::string & key);
 
-private:
-	SaveManager();
+public:
+	SaveManager(Mediator & mediator);
 	virtual ~SaveManager() = default;
 
 private:
@@ -89,26 +92,13 @@ private:
 	template <typename T>
 	T deserialize(const std::string & value) const noexcept;
 
-public:
-	// singleton
-	static SaveManager & get_instance();
-	SaveManager(const SaveManager &) = delete;
-	SaveManager(SaveManager &&) = delete;
-	SaveManager & operator=(const SaveManager &) = delete;
-	SaveManager & operator=(SaveManager &&) = delete;
+protected:
+	//! Create or return DB
+	virtual DB & get_db();
 
 private:
-	/**
-	 * \brief Create an instance of DB and return its reference
-	 *
-	 * \returns DB instance
-	 *
-	 * This function exists because DB is a facade class, which can't directly be used in the API
-	 * without workarounds
-	 *
-	 * TODO: better solution
-	 */
-	static DB & get_db();
+	//! Database
+	std::unique_ptr<void, std::function<void(void *)>> db = nullptr;
 };
 
 } // namespace crepe
diff --git a/src/crepe/manager/SceneManager.cpp b/src/crepe/manager/SceneManager.cpp
index 50a9fbb..d4ca90b 100644
--- a/src/crepe/manager/SceneManager.cpp
+++ b/src/crepe/manager/SceneManager.cpp
@@ -32,4 +32,7 @@ void SceneManager::load_next_scene() {
 
 	// Load the new scene
 	scene->load_scene();
+
+	//clear the next scene
+	next_scene.clear();
 }