From f0ecbea57a4d75905c4ee79608807187cd8f3e72 Mon Sep 17 00:00:00 2001
From: Loek Le Blansch <loek@pipeframe.xyz>
Date: Wed, 11 Dec 2024 16:51:03 +0100
Subject: WIP

---
 src/crepe/Component.h | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

(limited to 'src/crepe/Component.h')

diff --git a/src/crepe/Component.h b/src/crepe/Component.h
index eff5a58..47c5c34 100644
--- a/src/crepe/Component.h
+++ b/src/crepe/Component.h
@@ -32,9 +32,13 @@ protected:
 	//! Only ComponentManager can create components
 	friend class ComponentManager;
 
-	Component(const Component &) = delete;
+	// create snapshot
+	Component(const Component &) = default;
+	// restore snapshot
+	virtual Component & operator=(const Component &);
+
+	// components are never moved
 	Component(Component &&) = delete;
-	virtual Component & operator=(const Component &) = delete;
 	virtual Component & operator=(Component &&) = delete;
 
 public:
-- 
cgit v1.2.3


From 359ad8db97305856f4cfdade1cd1dada78a7a635 Mon Sep 17 00:00:00 2001
From: Loek Le Blansch <loek@pipeframe.xyz>
Date: Wed, 11 Dec 2024 21:04:30 +0100
Subject: more replay system WIP

---
 src/crepe/Component.cpp                |  9 +++++
 src/crepe/Component.h                  | 13 ++++---
 src/crepe/api/GameObject.cpp           | 10 ++---
 src/crepe/api/GameObject.h             | 13 +++++--
 src/crepe/api/LoopManager.cpp          |  2 +
 src/crepe/api/Transform.cpp            | 10 +++++
 src/crepe/api/Transform.h              |  7 ++++
 src/crepe/manager/ComponentManager.cpp | 26 +++++++++++++
 src/crepe/manager/ComponentManager.h   |  9 ++++-
 src/crepe/manager/Mediator.h           |  2 +
 src/crepe/manager/ReplayManager.cpp    | 25 ++++++------
 src/crepe/manager/ReplayManager.h      | 24 ++++++++----
 src/crepe/system/ReplaySystem.cpp      | 70 ++++++++++++++++++++++++++++++++++
 src/crepe/system/ReplaySystem.h        | 11 ++++++
 src/test/ECSTest.cpp                   | 14 +++++++
 15 files changed, 209 insertions(+), 36 deletions(-)

(limited to 'src/crepe/Component.h')

diff --git a/src/crepe/Component.cpp b/src/crepe/Component.cpp
index 141e1a8..8086492 100644
--- a/src/crepe/Component.cpp
+++ b/src/crepe/Component.cpp
@@ -1,6 +1,7 @@
 #include "Component.h"
 
 using namespace crepe;
+using namespace std;
 
 Component::Component(game_object_id_t id) : game_object_id(id) {}
 
@@ -8,3 +9,11 @@ Component & Component::operator=(const Component &) {
 	return *this;
 }
 
+unique_ptr<Component> Component::save() const {
+	return unique_ptr<Component>(new Component(*this));
+}
+
+void Component::restore(const Component & snapshot) {
+	*this = snapshot;
+}
+
diff --git a/src/crepe/Component.h b/src/crepe/Component.h
index 47c5c34..fc0268c 100644
--- a/src/crepe/Component.h
+++ b/src/crepe/Component.h
@@ -1,5 +1,7 @@
 #pragma once
 
+#include <memory>
+
 #include "types.h"
 
 namespace crepe {
@@ -32,15 +34,16 @@ protected:
 	//! Only ComponentManager can create components
 	friend class ComponentManager;
 
-	// create snapshot
-	Component(const Component &) = default;
-	// restore snapshot
-	virtual Component & operator=(const Component &);
-
 	// components are never moved
 	Component(Component &&) = delete;
 	virtual Component & operator=(Component &&) = delete;
 
+protected:
+	virtual std::unique_ptr<Component> save() const;
+	Component(const Component &) = default;
+	virtual void restore(const Component & snapshot);
+	virtual Component & operator=(const Component &);
+
 public:
 	virtual ~Component() = default;
 
diff --git a/src/crepe/api/GameObject.cpp b/src/crepe/api/GameObject.cpp
index ea9c425..68b074e 100644
--- a/src/crepe/api/GameObject.cpp
+++ b/src/crepe/api/GameObject.cpp
@@ -11,13 +11,9 @@ GameObject::GameObject(Mediator & mediator, game_object_id_t id,
 					   const std::string & name, const std::string & tag,
 					   const vec2 & position, double rotation, double scale)
 	: id(id),
-	  mediator(mediator) {
-
-	// Add Transform and Metadata components
-	ComponentManager & mgr = this->mediator.component_manager;
-	mgr.add_component<Transform>(this->id, position, rotation, scale);
-	mgr.add_component<Metadata>(this->id, name, tag);
-}
+	  mediator(mediator),
+		transform(mediator.component_manager->add_component<Transform>(this->id, position, rotation, scale)),
+		metadata(mediator.component_manager->add_component<Metadata>(this->id, name, tag)) { }
 
 void GameObject::set_parent(const GameObject & parent) {
 	ComponentManager & mgr = this->mediator.component_manager;
diff --git a/src/crepe/api/GameObject.h b/src/crepe/api/GameObject.h
index a311c21..6203f81 100644
--- a/src/crepe/api/GameObject.h
+++ b/src/crepe/api/GameObject.h
@@ -7,6 +7,8 @@
 namespace crepe {
 
 class Mediator;
+class Transform;
+class Metadata;
 
 /**
  * \brief Represents a GameObject
@@ -34,6 +36,13 @@ private:
 	//! ComponentManager instances GameObject
 	friend class ComponentManager;
 
+public:
+	//! The id of the GameObject
+	const game_object_id_t id;
+
+	Transform & transform;
+	Metadata & metadata;
+
 public:
 	/**
 	 * \brief Set the parent of this GameObject
@@ -68,10 +77,6 @@ public:
 	 */
 	void set_persistent(bool persistent = true);
 
-public:
-	//! The id of the GameObject
-	const game_object_id_t id;
-
 protected:
 	Mediator & mediator;
 };
diff --git a/src/crepe/api/LoopManager.cpp b/src/crepe/api/LoopManager.cpp
index 3511bca..2855455 100644
--- a/src/crepe/api/LoopManager.cpp
+++ b/src/crepe/api/LoopManager.cpp
@@ -25,6 +25,8 @@ LoopManager::LoopManager() {
 	this->load_system<InputSystem>();
 	this->load_system<EventSystem>();
 	this->load_system<AudioSystem>();
+
+	this->mediator.loop_manager = *this;
 }
 
 void LoopManager::start() {
diff --git a/src/crepe/api/Transform.cpp b/src/crepe/api/Transform.cpp
index a85b792..5fc886b 100644
--- a/src/crepe/api/Transform.cpp
+++ b/src/crepe/api/Transform.cpp
@@ -3,6 +3,7 @@
 #include "Transform.h"
 
 using namespace crepe;
+using namespace std;
 
 Transform::Transform(game_object_id_t id, const vec2 & point, double rotation, double scale)
 	: Component(id),
@@ -11,3 +12,12 @@ Transform::Transform(game_object_id_t id, const vec2 & point, double rotation, d
 	  scale(scale) {
 	dbg_trace();
 }
+
+unique_ptr<Component> Transform::save() const {
+	return unique_ptr<Component>{new Transform(*this)};
+}
+
+void Transform::restore(const Component & snapshot) {
+	*this = static_cast<const Transform &>(snapshot);
+}
+
diff --git a/src/crepe/api/Transform.h b/src/crepe/api/Transform.h
index 7ee6d65..bbd23e0 100644
--- a/src/crepe/api/Transform.h
+++ b/src/crepe/api/Transform.h
@@ -35,6 +35,13 @@ protected:
 	virtual int get_instances_max() const { return 1; }
 	//! ComponentManager instantiates all components
 	friend class ComponentManager;
+
+protected:
+	virtual std::unique_ptr<Component> save() const;
+	Transform(const Transform &) = default;
+	virtual void restore(const Component & snapshot);
+	virtual Transform & operator=(const Transform &) = default;
+
 };
 
 } // namespace crepe
diff --git a/src/crepe/manager/ComponentManager.cpp b/src/crepe/manager/ComponentManager.cpp
index 24ba0d7..5f5c050 100644
--- a/src/crepe/manager/ComponentManager.cpp
+++ b/src/crepe/manager/ComponentManager.cpp
@@ -72,3 +72,29 @@ set<game_object_id_t> ComponentManager::get_objects_by_tag(const string & tag) c
 	return this->get_objects_by_predicate<Metadata>(
 		[tag](const Metadata & data) { return data.tag == tag; });
 }
+
+ComponentManager::Snapshot ComponentManager::save() {
+	Snapshot snapshot{};
+	for (const auto & [type, by_id_index] : this->components) {
+		for (game_object_id_t id = 0; id < by_id_index.size(); id++) {
+			const auto & components = by_id_index[id];
+			for (size_t index = 0; index < components.size(); index++) {
+				const Component & component = *components[index];
+				snapshot.components.push_back(SnapshotComponent{
+					.type = type,
+					.id = id,
+					.index = index,
+					.component = component.save(),
+				});
+			}
+		}
+	}
+	return snapshot;
+}
+
+void ComponentManager::restore(const Snapshot & snapshot) {
+	for (const SnapshotComponent & info : snapshot.components) {
+		this->components[info.type][info.id][info.index]->restore(*info.component);
+	}
+}
+
diff --git a/src/crepe/manager/ComponentManager.h b/src/crepe/manager/ComponentManager.h
index dd7c154..457a196 100644
--- a/src/crepe/manager/ComponentManager.h
+++ b/src/crepe/manager/ComponentManager.h
@@ -143,10 +143,15 @@ public:
 	RefVector<T> get_components_by_tag(const std::string & tag) const;
 
 	struct SnapshotComponent {
-		Component component;
+		std::type_index type;
+		game_object_id_t id;
+		size_t index;
+		std::unique_ptr<Component> component;
 	};
 	struct Snapshot {
-		
+		// TODO: some kind of hash code that ensures components exist in all the same places as
+		// this snapshot
+		std::vector<SnapshotComponent> components;
 	};
 	Snapshot save();
 	void restore(const Snapshot &);
diff --git a/src/crepe/manager/Mediator.h b/src/crepe/manager/Mediator.h
index eef4432..f5864e7 100644
--- a/src/crepe/manager/Mediator.h
+++ b/src/crepe/manager/Mediator.h
@@ -14,6 +14,7 @@ class ResourceManager;
 class SDLContext;
 class LoopTimer;
 class ReplayManager;
+class LoopManager;
 
 /**
  * Struct to pass references to classes that would otherwise need to be singletons down to
@@ -36,6 +37,7 @@ struct Mediator {
 	OptionalRef<ResourceManager> resource_manager;
 	OptionalRef<LoopTimer> timer;
 	OptionalRef<ReplayManager> replay_manager;
+	OptionalRef<LoopManager> loop_manager;
 };
 
 } // namespace crepe
diff --git a/src/crepe/manager/ReplayManager.cpp b/src/crepe/manager/ReplayManager.cpp
index 82c2275..81ff114 100644
--- a/src/crepe/manager/ReplayManager.cpp
+++ b/src/crepe/manager/ReplayManager.cpp
@@ -1,4 +1,4 @@
-#include "../util/Log.h"
+#include <format>
 
 #include "ReplayManager.h"
 #include "Manager.h"
@@ -11,26 +11,29 @@ ReplayManager::ReplayManager(Mediator & mediator) : Manager(mediator) {
 }
 
 void ReplayManager::record_start() {
-	if (this->recording) this->release(this->current_recording);
-	this->current_recording++;
-	this->recording = true;
+	if (this->state == RECORDING) this->release(this->id);
+	this->id++;
+	this->memory[this->id] = make_unique<Recording>();
+	this->recording = *this->memory.at(this->id);
+	this->state = RECORDING;
 }
 
 recording_t ReplayManager::record_end() {
-	this->recording = false;
-	return this->current_recording;
+	this->state = IDLE;
+	return this->id;
 }
 
 void ReplayManager::play(recording_t handle) {
 	if (!this->memory.contains(handle))
 		throw out_of_range(format("ReplayManager: no recording for handle {}", handle));
-	Recording & recording = *this->memory.at(handle);
-
-	dbg_log("TODO: magic");
+	this->recording = *this->memory.at(handle);
+	this->recording->frame = 0;
+	this->state = PLAYING;
 }
 
 void ReplayManager::release(recording_t handle) {
-	dbg_log("release");
-
+	if (!this->memory.contains(handle))
+		return;
+	this->memory.erase(handle);
 }
 
diff --git a/src/crepe/manager/ReplayManager.h b/src/crepe/manager/ReplayManager.h
index c50196c..672d093 100644
--- a/src/crepe/manager/ReplayManager.h
+++ b/src/crepe/manager/ReplayManager.h
@@ -1,13 +1,14 @@
 #pragma once
 
+#include <unordered_map>
+
 #include "Manager.h"
 #include "ComponentManager.h"
-#include <unordered_map>
+#include "util/OptionalRef.h"
 
 namespace crepe {
 
 class ReplaySystem;
-class Memento;
 
 typedef size_t recording_t;
 
@@ -20,11 +21,20 @@ protected:
 	void record_frame();
 
 private:
-	typedef std::vector<ComponentManager::Snapshot> Recording;
-
-	bool recording = false;
-	recording_t current_recording = -1;
-
+	struct Recording {
+		size_t frame = 0;
+		std::vector<ComponentManager::Snapshot> frames;
+	};
+
+	enum State {
+		IDLE,
+		RECORDING,
+		PLAYING,
+	};
+
+	State state = IDLE;
+	OptionalRef<Recording> recording;
+	recording_t id = -1;
 
 	std::unordered_map<recording_t, std::unique_ptr<Recording>> memory;
 public:
diff --git a/src/crepe/system/ReplaySystem.cpp b/src/crepe/system/ReplaySystem.cpp
index 3aabb58..85595a2 100644
--- a/src/crepe/system/ReplaySystem.cpp
+++ b/src/crepe/system/ReplaySystem.cpp
@@ -1,7 +1,77 @@
+#include "system/ScriptSystem.h"
+
+#include "../manager/ReplayManager.h"
+
 #include "ReplaySystem.h"
 
+#include "../api/LoopManager.h"
+
 using namespace crepe;
+using namespace std;
 
 void ReplaySystem::fixed_update() {
+	ReplayManager & replay = this->mediator.replay_manager;
+
+	switch (replay.state) {
+		case ReplayManager::IDLE: break;
+		case ReplayManager::RECORDING: {
+			this->update_recording();
+			break;
+		}
+		case ReplayManager::PLAYING: {
+			this->update_playing();
+			break;
+		}
+	}
+
+	this->last_state = replay.state;
+}
+
+void ReplaySystem::update_recording() {
+	ReplayManager & replay = this->mediator.replay_manager;
+	ComponentManager & components = this->mediator.component_manager;
+
+	ReplayManager::Recording & recording = replay.recording;
+	recording.frames.push_back(components.save());
+	recording.frame++;
+}
+
+void ReplaySystem::update_playing() {
+	ReplayManager & replay = this->mediator.replay_manager;
+
+	if (this->last_state != ReplayManager::PLAYING) {
+		this->playback_begin();
+	}
+
+	ReplayManager::Recording & recording = replay.recording;
+
+	if (recording.frames.size() == recording.frame) {
+		this->playback_end();
+		return;
+	}
+
+	ComponentManager & components = this->mediator.component_manager;
+	ComponentManager::Snapshot & frame = recording.frames.at(recording.frame);
+
+	components.restore(frame);
+	recording.frame++;
+}
+
+void ReplaySystem::playback_begin() {
+	LoopManager & loop_manager = this->mediator.loop_manager;
+	// TODO: store system active state
+	// TODO: disable most systems
+	// TODO: store components snapshot
+}
+
+void ReplaySystem::playback_end() {
+	ReplayManager & replay = this->mediator.replay_manager;
+
+	replay.state = ReplayManager::IDLE;
+
+	LoopManager & loop_manager = this->mediator.loop_manager;
+
+	// TODO: restore system active state snapshot
+	// TODO: restore components snapshot
 }
 
diff --git a/src/crepe/system/ReplaySystem.h b/src/crepe/system/ReplaySystem.h
index 15ef3fc..fb40176 100644
--- a/src/crepe/system/ReplaySystem.h
+++ b/src/crepe/system/ReplaySystem.h
@@ -1,5 +1,7 @@
 #pragma once
 
+#include "../manager/ReplayManager.h"
+
 #include "System.h"
 
 namespace crepe {
@@ -9,6 +11,15 @@ public:
 	using System::System;
 
 	void fixed_update() override;
+
+private:
+	ReplayManager::State last_state = ReplayManager::IDLE;
+	void update_recording();
+	void update_playing();
+
+	std::unordered_map<std::type_index, bool> system_active_snapshot;
+	void playback_begin();
+	void playback_end();
 };
 
 }
diff --git a/src/test/ECSTest.cpp b/src/test/ECSTest.cpp
index af2b7b0..8f86a91 100644
--- a/src/test/ECSTest.cpp
+++ b/src/test/ECSTest.cpp
@@ -466,3 +466,17 @@ TEST_F(ECSTest, ComponentsByTag) {
 		EXPECT_EQ(objects.size(), 3);
 	}
 }
+
+TEST_F(ECSTest, Snapshot) {
+	GameObject foo = mgr.new_object("foo");
+
+	foo.transform.position = {1, 1};
+
+	ComponentManager::Snapshot snapshot = mgr.save();
+
+	foo.transform.position = {0, 0};
+
+	mgr.restore(snapshot);
+
+	EXPECT_EQ(foo.transform.position, (vec2{1, 1}));
+}
-- 
cgit v1.2.3


From 23196be83778973d9688cc5d465e4e4a16476568 Mon Sep 17 00:00:00 2001
From: Loek Le Blansch <loek@pipeframe.xyz>
Date: Thu, 12 Dec 2024 22:45:12 +0100
Subject: add documentation

---
 src/crepe/Component.h                | 17 ++++++++++++
 src/crepe/api/BehaviorScript.h       |  1 -
 src/crepe/api/Engine.cpp             |  7 ++---
 src/crepe/api/Engine.h               | 14 ++++++----
 src/crepe/api/GameObject.h           |  3 ++-
 src/crepe/api/ParticleEmitter.cpp    | 15 +++++++++++
 src/crepe/api/ParticleEmitter.h      |  6 +++++
 src/crepe/api/Script.hpp             | 11 ++++++--
 src/crepe/api/Transform.h            |  1 -
 src/crepe/manager/ComponentManager.h | 24 +++++++++++++++--
 src/crepe/manager/ReplayManager.cpp  |  7 +++++
 src/crepe/manager/ReplayManager.h    | 52 +++++++++++++++++++++++++++++++-----
 src/crepe/manager/SystemManager.h    | 24 +++++++++++++++++
 src/crepe/system/CMakeLists.txt      |  1 +
 src/crepe/system/EventSystem.h       |  7 +++++
 src/crepe/system/ReplaySystem.h      | 15 +++++++++++
 src/example/replay.cpp               |  4 +--
 17 files changed, 185 insertions(+), 24 deletions(-)

(limited to 'src/crepe/Component.h')

diff --git a/src/crepe/Component.h b/src/crepe/Component.h
index fc0268c..52e06d5 100644
--- a/src/crepe/Component.h
+++ b/src/crepe/Component.h
@@ -39,10 +39,27 @@ protected:
 	virtual Component & operator=(Component &&) = delete;
 
 protected:
+	/**
+	 * \name ReplayManager (Memento) functions
+	 * \{
+	 */
+	/**
+	 * \brief Save a snapshot of this component's state
+	 * \note This function should only be implemented on components that should be saved/restored
+	 * by ReplayManager.
+	 * \returns Unique pointer to a deep copy of this component
+	 */
 	virtual std::unique_ptr<Component> save() const;
+	//! Copy constructor (used by \c save())
 	Component(const Component &) = default;
+	/**
+	 * \brief Restore this component from a snapshot
+	 * \param snapshot Data to fill this component with (as returned by \c save())
+	 */
 	virtual void restore(const Component & snapshot);
+	//! Copy assignment operator (used by \c restore())
 	virtual Component & operator=(const Component &);
+	//! \}
 
 public:
 	virtual ~Component() = default;
diff --git a/src/crepe/api/BehaviorScript.h b/src/crepe/api/BehaviorScript.h
index 02d588a..3909b96 100644
--- a/src/crepe/api/BehaviorScript.h
+++ b/src/crepe/api/BehaviorScript.h
@@ -9,7 +9,6 @@
 namespace crepe {
 
 class ScriptSystem;
-class Mediator;
 class ComponentManager;
 class Script;
 
diff --git a/src/crepe/api/Engine.cpp b/src/crepe/api/Engine.cpp
index 7ae89b9..e8b7fd6 100644
--- a/src/crepe/api/Engine.cpp
+++ b/src/crepe/api/Engine.cpp
@@ -5,12 +5,12 @@
 using namespace crepe;
 using namespace std;
 
-void Engine::start() {
+int Engine::main() noexcept {
 	try {
 		this->setup();
 	} catch (const exception & e) {
 		Log::logf(Log::Level::ERROR, "Uncaught exception in setup: {}\n", e.what());
-		return;
+		return EXIT_FAILURE;
 	}
 
 	try {
@@ -19,10 +19,11 @@ void Engine::start() {
 		Log::logf(Log::Level::ERROR, "Uncaught exception in main loop: {}\n", e.what());
 		this->event_manager.trigger_event<ShutDownEvent>();
 	}
+
+	return EXIT_SUCCESS;
 }
 
 void Engine::setup() {
-	this->game_running = true;
 	this->loop_timer.start();
 	this->scene_manager.load_next_scene();
 
diff --git a/src/crepe/api/Engine.h b/src/crepe/api/Engine.h
index 5421d60..efe7853 100644
--- a/src/crepe/api/Engine.h
+++ b/src/crepe/api/Engine.h
@@ -20,13 +20,16 @@ namespace crepe {
  */
 class Engine {
 public:
-	void start();
-
 	/**
-	 * \brief Add a new concrete scene to the scene manager
+	 * \brief Engine entrypoint
+	 *
+	 * This function is called by the game programmer after registering all scenes
 	 *
-	 * \tparam T  Type of concrete scene
+	 * \returns process exit code
 	 */
+	int main() noexcept;
+
+	//! \copydoc SceneManager::add_scene
 	template <typename T>
 	void add_scene();
 
@@ -44,7 +47,8 @@ private:
 	 */
 	void loop();
 
-	bool game_running = false;
+	//! Game loop condition
+	bool game_running = true;
 
 private:
 	//! Global context
diff --git a/src/crepe/api/GameObject.h b/src/crepe/api/GameObject.h
index 6203f81..0dabed1 100644
--- a/src/crepe/api/GameObject.h
+++ b/src/crepe/api/GameObject.h
@@ -39,8 +39,9 @@ private:
 public:
 	//! The id of the GameObject
 	const game_object_id_t id;
-
+	//! This entity's transform
 	Transform & transform;
+	//! This entity's metadata
 	Metadata & metadata;
 
 public:
diff --git a/src/crepe/api/ParticleEmitter.cpp b/src/crepe/api/ParticleEmitter.cpp
index 90b77a0..fd69e26 100644
--- a/src/crepe/api/ParticleEmitter.cpp
+++ b/src/crepe/api/ParticleEmitter.cpp
@@ -1,6 +1,7 @@
 #include "ParticleEmitter.h"
 
 using namespace crepe;
+using namespace std;
 
 ParticleEmitter::ParticleEmitter(game_object_id_t game_object_id, const Data & data)
 	: Component(game_object_id),
@@ -9,3 +10,17 @@ ParticleEmitter::ParticleEmitter(game_object_id_t game_object_id, const Data & d
 		this->data.particles.emplace_back();
 	}
 }
+
+unique_ptr<Component> ParticleEmitter::save() const {
+	return unique_ptr<Component>{new ParticleEmitter(*this)};
+}
+
+void ParticleEmitter::restore(const Component & snapshot) {
+	*this = static_cast<const ParticleEmitter &>(snapshot);
+}
+
+ParticleEmitter & ParticleEmitter::operator=(const ParticleEmitter & other) {
+	data.particles = other.data.particles;
+	return *this;
+}
+
diff --git a/src/crepe/api/ParticleEmitter.h b/src/crepe/api/ParticleEmitter.h
index b83fd61..5f563de 100644
--- a/src/crepe/api/ParticleEmitter.h
+++ b/src/crepe/api/ParticleEmitter.h
@@ -80,6 +80,12 @@ public:
 public:
 	//! Configuration data for particle emission settings.
 	Data data;
+
+protected:
+	virtual std::unique_ptr<Component> save() const;
+	ParticleEmitter(const ParticleEmitter &) = default;
+	virtual void restore(const Component & snapshot);
+	virtual ParticleEmitter & operator=(const ParticleEmitter &);
 };
 
 } // namespace crepe
diff --git a/src/crepe/api/Script.hpp b/src/crepe/api/Script.hpp
index 2553fd1..b42a6df 100644
--- a/src/crepe/api/Script.hpp
+++ b/src/crepe/api/Script.hpp
@@ -40,10 +40,17 @@ void Script::subscribe_internal(const EventHandler<EventType> & callback,
 	EventManager & mgr = this->mediator->event_manager;
 	subscription_t listener = mgr.subscribe<EventType>(
 		[this, callback](const EventType & data) -> bool {
+			// check if (parent) BehaviorScript component is active
 			bool & active = this->active;
 			if (!active) return false;
-			ReplayManager & replay = this->mediator->replay_manager;
-			if (replay.get_state() == ReplayManager::PLAYING) return false;
+
+			// check if replay manager is playing (if initialized)
+			try {
+				ReplayManager & replay = this->mediator->replay_manager;
+				if (replay.get_state() == ReplayManager::PLAYING) return false;
+			} catch (const std::runtime_error &) {}
+
+			// call user-provided callback
 			return callback(data);
 		},
 		channel);
diff --git a/src/crepe/api/Transform.h b/src/crepe/api/Transform.h
index bbd23e0..a6f3486 100644
--- a/src/crepe/api/Transform.h
+++ b/src/crepe/api/Transform.h
@@ -41,7 +41,6 @@ protected:
 	Transform(const Transform &) = default;
 	virtual void restore(const Component & snapshot);
 	virtual Transform & operator=(const Transform &) = default;
-
 };
 
 } // namespace crepe
diff --git a/src/crepe/manager/ComponentManager.h b/src/crepe/manager/ComponentManager.h
index 457a196..c3a5b4a 100644
--- a/src/crepe/manager/ComponentManager.h
+++ b/src/crepe/manager/ComponentManager.h
@@ -142,19 +142,39 @@ public:
 	template <typename T>
 	RefVector<T> get_components_by_tag(const std::string & tag) const;
 
+	//! Snapshot of single component (including path in \c components)
 	struct SnapshotComponent {
+		//! \c components path
 		std::type_index type;
+		//! \c components path
 		game_object_id_t id;
+		//! \c components path
 		size_t index;
+		//! Actual component snapshot
 		std::unique_ptr<Component> component;
 	};
+	//! Snapshot of the entire component manager state
 	struct Snapshot {
+		//! All components
+		std::vector<SnapshotComponent> components;
 		// TODO: some kind of hash code that ensures components exist in all the same places as
 		// this snapshot
-		std::vector<SnapshotComponent> components;
 	};
+	/**
+	 * \name ReplayManager (Memento) functions
+	 * \{
+	 */
+	/**
+	 * \brief Save a snapshot of the component manager state
+	 * \returns Deep copy of the component manager's internal state
+	 */
 	Snapshot save();
-	void restore(const Snapshot &);
+	/**
+	 * \brief Restore component manager from a snapshot
+	 * \param snapshot Snapshot to restore from (as returned by \c save())
+	 */
+	void restore(const Snapshot & snapshot);
+	//! \}
 
 private:
 	/**
diff --git a/src/crepe/manager/ReplayManager.cpp b/src/crepe/manager/ReplayManager.cpp
index ab8a5a0..db6acb0 100644
--- a/src/crepe/manager/ReplayManager.cpp
+++ b/src/crepe/manager/ReplayManager.cpp
@@ -38,6 +38,9 @@ void ReplayManager::release(recording_t handle) {
 }
 
 void ReplayManager::frame_record() {
+	if (this->state != RECORDING)
+		throw runtime_error("ReplayManager: frame_step called while not playing");
+
 	ComponentManager & components = this->mediator.component_manager;
 	Recording & recording = this->recording;
 
@@ -46,6 +49,9 @@ void ReplayManager::frame_record() {
 }
 
 bool ReplayManager::frame_step() {
+	if (this->state != PLAYING)
+		throw runtime_error("ReplayManager: frame_step called while not playing");
+
 	ComponentManager & components = this->mediator.component_manager;
 	Recording & recording = this->recording;
 
@@ -58,6 +64,7 @@ bool ReplayManager::frame_step() {
 	// end of recording
 	recording.frame = 0;
 	this->state = IDLE;
+	this->recording.clear();
 	return true;
 }
 
diff --git a/src/crepe/manager/ReplayManager.h b/src/crepe/manager/ReplayManager.h
index 7be18f3..d3af879 100644
--- a/src/crepe/manager/ReplayManager.h
+++ b/src/crepe/manager/ReplayManager.h
@@ -8,13 +8,14 @@
 
 namespace crepe {
 
-class ReplaySystem;
-
 typedef size_t recording_t;
 
 /**
  * \brief Replay manager
  *
+ * The replay manager is responsible for creating, storing and restoring ComponentManager
+ * snapshots. Sequential snapshots can be recorded and replayed in combination with
+ * ReplaySystem.
  */
 class ReplayManager : public Manager {
 	// TODO: Delete recordings at end of scene
@@ -22,31 +23,70 @@ public:
 	ReplayManager(Mediator & mediator);
 
 public:
+	//! Start a new recording
 	void record_start();
+	/**
+	 * \brief End the latest recording started by \c record_start()
+	 * \returns Handle to recording
+	 */
 	recording_t record_end();
+	/**
+	 * \brief Play a recording
+	 * \param handle Handle to recording (as returned by \c record_end())
+	 */
 	void play(recording_t handle);
+	/**
+	 * \brief Delete a recording from memory
+	 * \param handle Handle to recording (as returned by \c record_end())
+	 */
 	void release(recording_t handle);
 	
 public:
+	//! Internal state
 	enum State {
-		IDLE,
-		RECORDING,
-		PLAYING,
+		IDLE, //!< Not doing anything
+		RECORDING, //!< Currently recording
+		PLAYING, //!< Currently playing back a recording
 	};
+	//! Get current internal state
 	State get_state() const;
 
 public:
+	/**
+	 * \brief Record a single frame to the current recording
+	 *
+	 * This function is called by ReplaySystem after the game programmer has called \c
+	 * record_start()
+	 */
 	void frame_record();
+	/**
+	 * \brief Play the next frame of the current recording
+	 *
+	 * \returns `true` if the recording is finished playing
+	 * \returns `false` if there are more frames
+	 *
+	 * This function also automatically resets the internal state from PLAYING to IDLE at the end
+	 * of a recording.
+	 */
 	bool frame_step();
 
 private:
+	/**
+	 * \brief Recording data
+	 */
 	struct Recording {
+		//! Current frame being shown
 		size_t frame = 0;
+		//! All frames in recording
 		std::vector<ComponentManager::Snapshot> frames;
 	};
+	//! Internal state
 	State state = IDLE;
-	OptionalRef<Recording> recording;
+	//! Current recording handle
 	recording_t id = -1;
+	//! Current recording data
+	OptionalRef<Recording> recording;
+	//! Recording storage
 	std::unordered_map<recording_t, std::unique_ptr<Recording>> memory;
 };
 
diff --git a/src/crepe/manager/SystemManager.h b/src/crepe/manager/SystemManager.h
index 6cf7f2b..a47961b 100644
--- a/src/crepe/manager/SystemManager.h
+++ b/src/crepe/manager/SystemManager.h
@@ -1,11 +1,21 @@
 #pragma once
 
+#include <typeindex>
+#include <unordered_map>
+#include <memory>
+
 #include "../system/System.h"
 
 #include "Manager.h"
 
 namespace crepe {
 
+/**
+ * \brief Collection of all systems
+ *
+ * This manager aggregates all systems and provides utility functions to retrieve references to
+ * and update systems.
+ */
 class SystemManager : public Manager {
 public:
 	SystemManager(Mediator &);
@@ -50,9 +60,23 @@ public:
 	T & get_system();
 
 public:
+	/**
+	 * \brief SystemManager snapshot
+	 *
+	 * The SystemManager snapshot only stores which systems are active
+	 */
 	typedef std::unordered_map<std::type_index, bool> Snapshot;
+	/**
+	 * \brief Save a snapshot of the systems' state
+	 * \returns Copy of each system's active property
+	 */
 	Snapshot save();
+	/**
+	 * \brief Restore system active state from a snapshot
+	 * \param snapshot Snapshot to restore from (as returned by \c save())
+	 */
 	void restore(const Snapshot & snapshot);
+	//! Disable all systems
 	void disable_all();
 };
 
diff --git a/src/crepe/system/CMakeLists.txt b/src/crepe/system/CMakeLists.txt
index 3473876..52369d0 100644
--- a/src/crepe/system/CMakeLists.txt
+++ b/src/crepe/system/CMakeLists.txt
@@ -23,5 +23,6 @@ target_sources(crepe PUBLIC FILE_SET HEADERS FILES
 	AnimatorSystem.h
 	InputSystem.h
 	EventSystem.h
+	ReplaySystem.h
 	AISystem.h
 )
diff --git a/src/crepe/system/EventSystem.h b/src/crepe/system/EventSystem.h
index a179d00..ff3ca4e 100644
--- a/src/crepe/system/EventSystem.h
+++ b/src/crepe/system/EventSystem.h
@@ -4,10 +4,17 @@
 
 namespace crepe {
 
+/**
+ * \brief EventManager dispatch helper system
+ */
 class EventSystem : public System {
 public:
 	using System::System;
 
+	/**
+	 * \brief Dispatch queued events
+	 * \see EventManager::dispatch_events
+	 */
 	void fixed_update() override;
 };
 
diff --git a/src/crepe/system/ReplaySystem.h b/src/crepe/system/ReplaySystem.h
index 919c554..8ba60d5 100644
--- a/src/crepe/system/ReplaySystem.h
+++ b/src/crepe/system/ReplaySystem.h
@@ -7,6 +7,11 @@
 
 namespace crepe {
 
+/**
+ * \brief ReplayManager helper system
+ *
+ * This system records and replays recordings using ReplayManager.
+ */
 class ReplaySystem : public System {
 public:
 	using System::System;
@@ -14,15 +19,25 @@ public:
 	void fixed_update() override;
 
 private:
+	//! Last ReplayManager state
 	ReplayManager::State last_state = ReplayManager::IDLE;
 
+	/**
+	 * \brief Playback snapshot
+	 *
+	 * When starting playback, the component state is saved and most systems are disabled. This
+	 * struct stores the engine state before ReplayManager::play is called.
+	 */
 	struct Snapshot {
 		ComponentManager::Snapshot components;
 		SystemManager::Snapshot systems;
 	};
+	//! Before playback snapshot
 	Snapshot playback;
 
+	//! Snapshot state and disable systems during playback
 	void playback_begin();
+	//! Restore state from before \c playback_begin()
 	void playback_end();
 };
 
diff --git a/src/example/replay.cpp b/src/example/replay.cpp
index 7faf6cb..130c0d3 100644
--- a/src/example/replay.cpp
+++ b/src/example/replay.cpp
@@ -83,7 +83,5 @@ int main(int argc, char * argv[]) {
 	Engine engine;
 
 	engine.add_scene<TestScene>();
-	engine.start();
-
-	return 0;
+	return engine.main();
 }
-- 
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