merge `master` into `loek/util`

21 files changed, 664 insertions, 102 deletions

diff --git a/.editorconfig b/.editorconfig
index 191871f..65f5034 100644
--- a/.editorconfig
+++ b/.editorconfig
@@ -10,5 +10,5 @@ max_line_length = 95
 indent_style = space
 indent_size = 2
 
-[*.md]
+[*.{md,dox}]
 max_line_length = 80
diff --git a/Doxyfile b/Doxyfile
index 8ba90fc..e0a31df 100644
--- a/Doxyfile
+++ b/Doxyfile
@@ -14,12 +14,20 @@ RECURSIVE = YES
 
 GENERATE_LATEX = NO
 
+LAYOUT_FILE = src/doc/layout.xml
+TAB_SIZE = 2
+
+HTML_INDEX_NUM_ENTRIES = 2
+HTML_EXTRA_STYLESHEET = src/doc/style.css
+
 USE_MDFILE_AS_MAINPAGE = ./readme.md
-HTML_INDEX_NUM_ENTRIES = 1 # collapse trees by default
 REPEAT_BRIEF = NO
 
 INTERNAL_DOCS = YES
+EXTRACT_PRIVATE = YES
 EXTRACT_STATIC = YES
+HIDE_UNDOC_NAMESPACES = YES
+HIDE_UNDOC_CLASSES = YES
 
 QUIET = YES
 
diff --git a/contributing.md b/contributing.md
index e4f075f..77a2908 100644
--- a/contributing.md
+++ b/contributing.md
@@ -24,7 +24,7 @@ that you can click on to open them.
 # Code style
 
 - Formatting nitty-gritty is handled by clang-format/clang-tidy (run `make
-  format` in the root folder of this repository to format all sources files)
+  format` or `make lint`)
 - <details><summary>
   ASCII only
   </summary><table><tr><th>Good</th><th>Bad</th></tr><tr><td>
@@ -803,6 +803,27 @@ that you can click on to open them.
   resolving merge conflicts when multiple sources were added by different
   people to the same CMakeLists.txt easier.
 
+## GoogleTest-specific
+
+- Unit tests are not *required* to follow all code standards
+- <details><summary>
+  Private/protected members may be accessed using preprocessor tricks
+  </summary>
+
+  ```cpp
+  // include unrelated headers before
+
+  #define private public
+  #define protected public
+
+  // headers included after *will* be affected
+  ```
+  </details>
+- Each test source file defines tests within a single test suite (first
+  parameter of `TEST()` / `TEST_F()` macro)
+- Test source files match their suite name (or test fixture name in the case of
+  tests that use a fixture)
+
 # Structure
 
 - Files are placed in the appropriate directory:
diff --git a/makefile b/makefile
index dd7c587..c46e8a5 100644
--- a/makefile
+++ b/makefile
@@ -6,5 +6,6 @@ doxygen: Doxyfile FORCE
 FMT += $(shell git ls-files '*.c' '*.cpp' '*.h' '*.hpp')
 format: FORCE
 	clang-format -i $(FMT)
-	$(MAKE) -C src $@
 
+lint: FORCE
+	$(MAKE) -C src $@
diff --git a/src/crepe/api/Color.cpp b/src/crepe/api/Color.cpp
index 9e5f187..29bd77a 100644
--- a/src/crepe/api/Color.cpp
+++ b/src/crepe/api/Color.cpp
@@ -2,32 +2,11 @@
 
 using namespace crepe;
 
-Color Color::white = Color(255, 255, 255, 0);
-Color Color::red = Color(255, 0, 0, 0);
-Color Color::green = Color(0, 255, 0, 0);
-Color Color::blue = Color(0, 0, 255, 0);
-Color Color::black = Color(0, 0, 0, 0);
-Color Color::cyan = Color(0, 255, 255, 0);
-Color Color::yellow = Color(255, 255, 0, 0);
-Color Color::magenta = Color(255, 0, 255, 0);
-
-Color::Color(uint8_t red, uint8_t green, uint8_t blue, uint8_t alpha) {
-	this->a = alpha;
-	this->r = red;
-	this->g = green;
-	this->b = blue;
-};
-
-const Color & Color::get_white() { return Color::white; };
-
-const Color & Color::get_red() { return Color::red; };
-const Color & Color::get_green() { return Color::green; };
-const Color & Color::get_blue() { return Color::blue; };
-
-const Color & Color::get_black() { return Color::black; };
-
-const Color & Color::get_cyan() { return Color::cyan; };
-
-const Color & Color::get_yellow() { return Color::yellow; };
-
-const Color & Color::get_magenta() { return Color::magenta; };
+const Color Color::WHITE{0xff, 0xff, 0xff};
+const Color Color::RED{0xff, 0x00, 0x00};
+const Color Color::GREEN{0x00, 0xff, 0x00};
+const Color Color::BLUE{0x00, 0x00, 0xff};
+const Color Color::BLACK{0x00, 0x00, 0x00};
+const Color Color::CYAN{0x00, 0xff, 0xff};
+const Color Color::YELLOW{0xff, 0xff, 0x00};
+const Color Color::MAGENTA{0xff, 0x00, 0xff};
diff --git a/src/crepe/api/Color.h b/src/crepe/api/Color.h
index aa47bf4..84edb5c 100644
--- a/src/crepe/api/Color.h
+++ b/src/crepe/api/Color.h
@@ -4,41 +4,20 @@
 
 namespace crepe {
 
-// TODO: make Color a struct w/o constructors/destructors
-class Color {
-
-	// FIXME: can't these colors be defined as a `static constexpr const Color`
-	// instead?
-
-public:
-	Color(uint8_t red, uint8_t green, uint8_t blue, uint8_t alpha);
-	static const Color & get_white();
-	static const Color & get_red();
-	static const Color & get_green();
-	static const Color & get_blue();
-	static const Color & get_cyan();
-	static const Color & get_magenta();
-	static const Color & get_yellow();
-	static const Color & get_black();
-
-private:
-	// TODO: why are these private!?
-	uint8_t r;
-	uint8_t g;
-	uint8_t b;
-	uint8_t a;
-
-	static Color white;
-	static Color red;
-	static Color green;
-	static Color blue;
-	static Color cyan;
-	static Color magenta;
-	static Color yellow;
-	static Color black;
-
-private:
-	friend class SDLContext;
+struct Color {
+	uint8_t r = 0x00;
+	uint8_t g = 0x00;
+	uint8_t b = 0x00;
+	uint8_t a = 0xff;
+
+	static const Color WHITE;
+	static const Color RED;
+	static const Color GREEN;
+	static const Color BLUE;
+	static const Color CYAN;
+	static const Color MAGENTA;
+	static const Color YELLOW;
+	static const Color BLACK;
 };
 
 } // namespace crepe
diff --git a/src/crepe/api/Sprite.h b/src/crepe/api/Sprite.h
index 0192793..74a55d4 100644
--- a/src/crepe/api/Sprite.h
+++ b/src/crepe/api/Sprite.h
@@ -2,8 +2,9 @@
 
 #include <memory>
 
+#include "../Component.h"
+
 #include "Color.h"
-#include "Component.h"
 #include "Texture.h"
 
 namespace crepe {
diff --git a/src/crepe/facade/SDLContext.h b/src/crepe/facade/SDLContext.h
index 007092b..652a83e 100644
--- a/src/crepe/facade/SDLContext.h
+++ b/src/crepe/facade/SDLContext.h
@@ -21,9 +21,6 @@ namespace crepe {
 // typedef is unusable when crepe is packaged. Wouter will fix this later.
 typedef SDL_Keycode CREPE_KEYCODES;
 
-class Texture;
-class LoopManager;
-
 /**
  * \class SDLContext
  * \brief Facade for the SDL library
@@ -91,9 +88,6 @@ private:
 	//! Will use the funtions: texture_from_path, get_width,get_height.
 	friend class Texture;
 
-	//! Will use the funtions: texture_from_path, get_width,get_height.
-	friend class Animator;
-
 	/**
 	 * \brief Loads a texture from a file path.
 	 * \param path Path to the image file.
diff --git a/src/crepe/system/RenderSystem.cpp b/src/crepe/system/RenderSystem.cpp
index fa3d0de..96c5f27 100644
--- a/src/crepe/system/RenderSystem.cpp
+++ b/src/crepe/system/RenderSystem.cpp
@@ -1,38 +1,59 @@
+#include <algorithm>
+#include <cassert>
 #include <functional>
+#include <stdexcept>
 #include <vector>
 
 #include "../ComponentManager.h"
 #include "../api/Sprite.h"
 #include "../api/Transform.h"
 #include "../facade/SDLContext.h"
-#include "../util/Log.h"
 
 #include "RenderSystem.h"
 
 using namespace crepe;
+using namespace std;
 
-void RenderSystem::clear_screen() const { SDLContext::get_instance().clear_screen(); }
+void RenderSystem::clear_screen() { this->context.clear_screen(); }
 
-void RenderSystem::present_screen() const { SDLContext::get_instance().present_screen(); }
+void RenderSystem::present_screen() { this->context.present_screen(); }
 void RenderSystem::update_camera() {
 	ComponentManager & mgr = this->component_manager;
 
 	std::vector<std::reference_wrapper<Camera>> cameras = mgr.get_components_by_type<Camera>();
 
+	if (cameras.size() == 0) throw std::runtime_error("No cameras in current scene");
+
 	for (Camera & cam : cameras) {
-		SDLContext::get_instance().camera(cam);
-		this->curr_cam = &cam;
+		this->context.camera(cam);
+		this->curr_cam_ref = &cam;
 	}
 }
-void RenderSystem::render_sprites() const {
-	ComponentManager & mgr = this->component_manager;
 
-	std::vector<std::reference_wrapper<Sprite>> sprites = mgr.get_components_by_type<Sprite>();
+bool sorting_comparison(const Sprite & a, const Sprite & b) {
+	if (a.sorting_in_layer < b.sorting_in_layer) return true;
+	if (a.sorting_in_layer == b.sorting_in_layer) return a.order_in_layer < b.order_in_layer;
+
+	return false;
+}
+
+std::vector<std::reference_wrapper<Sprite>>
+RenderSystem::sort(std::vector<std::reference_wrapper<Sprite>> & objs) {
+
+	std::vector<std::reference_wrapper<Sprite>> sorted_objs(objs);
+	std::sort(sorted_objs.begin(), sorted_objs.end(), sorting_comparison);
+
+	return sorted_objs;
+}
+
+void RenderSystem::render_sprites() {
+	ComponentManager & mgr = this->component_manager;
+	vector<reference_wrapper<Sprite>> sprites = mgr.get_components_by_type<Sprite>();
+	vector<reference_wrapper<Sprite>> sorted_sprites = this->sort(sprites);
 
-	SDLContext & render = SDLContext::get_instance();
-	for (const Sprite & sprite : sprites) {
+	for (const Sprite & sprite : sorted_sprites) {
 		auto transforms = mgr.get_components_by_id<Transform>(sprite.game_object_id);
-		render.draw(sprite, transforms[0], *curr_cam);
+		this->context.draw(sprite, transforms[0], *this->curr_cam_ref);
 	}
 }
 
diff --git a/src/crepe/system/RenderSystem.h b/src/crepe/system/RenderSystem.h
index 87ec494..57b9c73 100644
--- a/src/crepe/system/RenderSystem.h
+++ b/src/crepe/system/RenderSystem.h
@@ -1,18 +1,23 @@
 #pragma once
 
-#include "api/Camera.h"
+#include <functional>
+#include <vector>
+
+#include "facade/SDLContext.h"
 
 #include "System.h"
 
 namespace crepe {
 
+class Camera;
+class Sprite;
+
 /**
  * \class RenderSystem
  * \brief Manages rendering operations for all game objects.
  *
- * RenderSystem is responsible for rendering sprites, clearing and presenting the screen, and
- * managing the active camera. It functions as a singleton, providing centralized rendering
- * services for the application.
+ * RenderSystem is responsible for rendering, clearing and presenting the screen, and
+ * managing the active camera. 
  */
 class RenderSystem : public System {
 public:
@@ -25,16 +30,25 @@ public:
 
 private:
 	//! Clears the screen in preparation for rendering.
-	void clear_screen() const;
+	void clear_screen();
 
 	//! Presents the rendered frame to the display.
-	void present_screen() const;
+	void present_screen();
 
 	//! Updates the active camera used for rendering.
 	void update_camera();
 
 	//! Renders all active sprites to the screen.
-	void render_sprites() const;
+	void render_sprites();
+
+	/**
+	 * \brief sort a vector sprite objects with
+	 *
+	 * \param objs the vector that will do a sorting algorithm on 
+	 * \return returns a sorted reference vector
+	 */
+	std::vector<std::reference_wrapper<Sprite>>
+	sort(std::vector<std::reference_wrapper<Sprite>> & objs);
 
 	/**
 	 * \todo Include color handling for sprites.
@@ -48,8 +62,10 @@ private:
 
 private:
 	//! Pointer to the current active camera for rendering
-	Camera * curr_cam = nullptr;
+	Camera * curr_cam_ref = nullptr;
 	// TODO: needs a better solution
+
+	SDLContext & context = SDLContext::get_instance();
 };
 
 } // namespace crepe
diff --git a/src/doc/feature/script.dox b/src/doc/feature/script.dox
new file mode 100644
index 0000000..d25a63b
--- /dev/null
+++ b/src/doc/feature/script.dox
@@ -0,0 +1,62 @@
+// vim:ft=doxygen
+namespace crepe {
+/**
+
+\defgroup feature_script Scripting
+\ingroup feature
+\brief User-defined scripts for game objects
+
+Scripts can be used to implement game behavior, and allow arbitrary code to run
+as part of the game loop. Scripts are implemented as derivative classes of
+Script, which are added to game objects using the BehaviorScript \ref Component
+"component".
+
+\todo This section is incomplete:
+- Utility functions to get components/events/etc inside script
+- How to listen for events
+- Extensions of script (keylistener)
+
+\see Script
+\see BehaviorScript
+\see GameObject
+
+\par Example
+
+First, define a class that inherits from Script. This class acts as an
+interface, and has two functions (\ref Script::init "\c init()" and \ref
+Script::update "\c update()"), which may be implemented (they are empty by
+default). From now on, this derivative class will be referred to as a *concrete
+script*.
+
+```cpp
+#include <crepe/api/Script.h>
+#include <crepe/api/BehaviorScript.h>
+
+class MyScript : public crepe::Script {
+	void init() {
+		// called once
+	}
+	void update() {
+		// called on fixed update
+	}
+};
+```
+
+Concrete scripts can be instantiated and attached to \ref GameObject
+"game objects" using the BehaviorScript \ref Component "component".
+
+```cpp
+using namespace crepe;
+GameObject obj = component_manager.new_object("name");
+
+// create BehaviorScript instance
+BehaviorScript & behavior_script = obj.add_component<BehaviorScript>();
+// attach (and instantiate) MyScript to behavior_script
+behavior_script.set_script<MyScript>();
+
+// the above can also be done in a single call for convenience:
+obj.add_component<BehaviorScript>().set_script<MyScript>();
+```
+
+*/
+}
diff --git a/src/doc/features.dox b/src/doc/features.dox
new file mode 100644
index 0000000..4786bed
--- /dev/null
+++ b/src/doc/features.dox
@@ -0,0 +1,10 @@
+// vim:ft=doxygen
+/**
+
+\defgroup feature Features
+\brief Engine components
+
+This page lists engine features and contains usage instructions for each
+feature.
+
+*/
diff --git a/src/doc/index.dox b/src/doc/index.dox
new file mode 100644
index 0000000..5ec7889
--- /dev/null
+++ b/src/doc/index.dox
@@ -0,0 +1,10 @@
+// vim:ft=doxygen
+/**
+
+\mainpage crêpe game engine
+
+Welcome to the documentation for the crêpe game engine.
+
+\see feature
+
+*/
diff --git a/src/doc/installing.dox b/src/doc/installing.dox
new file mode 100644
index 0000000..48b27d7
--- /dev/null
+++ b/src/doc/installing.dox
@@ -0,0 +1,9 @@
+// vim:ft=doxygen
+/**
+
+\defgroup install Installation
+\brief Engine installation instructions
+
+\todo This entire page
+
+*/
diff --git a/src/doc/layout.xml b/src/doc/layout.xml
new file mode 100644
index 0000000..2244fa7
--- /dev/null
+++ b/src/doc/layout.xml
@@ -0,0 +1,252 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<doxygenlayout version="1.0">
+	<navindex>
+		<tab type="mainpage" visible="yes" title=""/>
+		<tab type="pages" visible="no" title="" intro=""/>
+		<tab type="topics" visible="yes" title="" intro=""/>
+		<tab type="modules" visible="yes" title="" intro="">
+			<tab type="modulelist" visible="yes" title="" intro=""/>
+			<tab type="modulemembers" visible="yes" title="" intro=""/>
+		</tab>
+		<tab type="namespaces" visible="no" title="">
+			<tab type="namespacelist" visible="yes" title="" intro=""/>
+			<tab type="namespacemembers" visible="yes" title="" intro=""/>
+		</tab>
+		<tab type="concepts" visible="yes" title="">
+		</tab>
+		<tab type="interfaces" visible="yes" title="">
+			<tab type="interfacelist" visible="yes" title="" intro=""/>
+			<tab type="interfaceindex" visible="$ALPHABETICAL_INDEX" title=""/>
+			<tab type="interfacehierarchy" visible="yes" title="" intro=""/>
+		</tab>
+		<tab type="classes" visible="yes" title="">
+			<tab type="classlist" visible="yes" title="" intro=""/>
+			<tab type="classindex" visible="$ALPHABETICAL_INDEX" title=""/>
+			<tab type="hierarchy" visible="yes" title="" intro=""/>
+			<tab type="classmembers" visible="yes" title="" intro=""/>
+		</tab>
+		<tab type="structs" visible="yes" title="">
+			<tab type="structlist" visible="yes" title="" intro=""/>
+			<tab type="structindex" visible="$ALPHABETICAL_INDEX" title=""/>
+		</tab>
+		<tab type="exceptions" visible="yes" title="">
+			<tab type="exceptionlist" visible="yes" title="" intro=""/>
+			<tab type="exceptionindex" visible="$ALPHABETICAL_INDEX" title=""/>
+			<tab type="exceptionhierarchy" visible="yes" title="" intro=""/>
+		</tab>
+		<tab type="files" visible="yes" title="">
+			<tab type="filelist" visible="yes" title="" intro=""/>
+			<tab type="globals" visible="yes" title="" intro=""/>
+		</tab>
+		<tab type="examples" visible="yes" title="" intro=""/>
+	</navindex>
+	<class>
+		<briefdescription visible="yes"/>
+		<includes visible="$SHOW_HEADERFILE"/>
+		<inheritancegraph visible="yes"/>
+		<collaborationgraph visible="yes"/>
+		<memberdecl>
+			<nestedclasses visible="yes" title=""/>
+			<publictypes title=""/>
+			<services title=""/>
+			<interfaces title=""/>
+			<publicslots title=""/>
+			<signals title=""/>
+			<publicmethods title=""/>
+			<publicstaticmethods title=""/>
+			<publicattributes title=""/>
+			<publicstaticattributes title=""/>
+			<protectedtypes title=""/>
+			<protectedslots title=""/>
+			<protectedmethods title=""/>
+			<protectedstaticmethods title=""/>
+			<protectedattributes title=""/>
+			<protectedstaticattributes title=""/>
+			<packagetypes title=""/>
+			<packagemethods title=""/>
+			<packagestaticmethods title=""/>
+			<packageattributes title=""/>
+			<packagestaticattributes title=""/>
+			<properties title=""/>
+			<events title=""/>
+			<privatetypes title=""/>
+			<privateslots title=""/>
+			<privatemethods title=""/>
+			<privatestaticmethods title=""/>
+			<privateattributes title=""/>
+			<privatestaticattributes title=""/>
+			<friends title=""/>
+			<related title="" subtitle=""/>
+			<membergroups visible="yes"/>
+		</memberdecl>
+		<detaileddescription title=""/>
+		<memberdef>
+			<inlineclasses title=""/>
+			<typedefs title=""/>
+			<enums title=""/>
+			<services title=""/>
+			<interfaces title=""/>
+			<constructors title=""/>
+			<functions title=""/>
+			<related title=""/>
+			<variables title=""/>
+			<properties title=""/>
+			<events title=""/>
+		</memberdef>
+		<allmemberslink visible="yes"/>
+		<usedfiles visible="$SHOW_USED_FILES"/>
+		<authorsection visible="yes"/>
+	</class>
+	<namespace>
+		<briefdescription visible="yes"/>
+		<memberdecl>
+			<nestednamespaces visible="yes" title=""/>
+			<constantgroups visible="yes" title=""/>
+			<interfaces visible="yes" title=""/>
+			<classes visible="yes" title=""/>
+			<concepts visible="yes" title=""/>
+			<structs visible="yes" title=""/>
+			<exceptions visible="yes" title=""/>
+			<typedefs title=""/>
+			<sequences title=""/>
+			<dictionaries title=""/>
+			<enums title=""/>
+			<functions title=""/>
+			<variables title=""/>
+			<properties title=""/>
+			<membergroups visible="yes"/>
+		</memberdecl>
+		<detaileddescription title=""/>
+		<memberdef>
+			<inlineclasses title=""/>
+			<typedefs title=""/>
+			<sequences title=""/>
+			<dictionaries title=""/>
+			<enums title=""/>
+			<functions title=""/>
+			<variables title=""/>
+			<properties title=""/>
+		</memberdef>
+		<authorsection visible="yes"/>
+	</namespace>
+	<concept>
+		<briefdescription visible="yes"/>
+		<includes visible="$SHOW_HEADERFILE"/>
+		<definition visible="yes" title=""/>
+		<detaileddescription title=""/>
+		<authorsection visible="yes"/>
+	</concept>
+	<file>
+		<briefdescription visible="yes"/>
+		<includes visible="$SHOW_INCLUDE_FILES"/>
+		<includegraph visible="yes"/>
+		<includedbygraph visible="yes"/>
+		<sourcelink visible="yes"/>
+		<memberdecl>
+			<interfaces visible="yes" title=""/>
+			<classes visible="yes" title=""/>
+			<structs visible="yes" title=""/>
+			<exceptions visible="yes" title=""/>
+			<namespaces visible="yes" title=""/>
+			<concepts visible="yes" title=""/>
+			<constantgroups visible="yes" title=""/>
+			<defines title=""/>
+			<typedefs title=""/>
+			<sequences title=""/>
+			<dictionaries title=""/>
+			<enums title=""/>
+			<functions title=""/>
+			<variables title=""/>
+			<properties title=""/>
+			<membergroups visible="yes"/>
+		</memberdecl>
+		<detaileddescription title=""/>
+		<memberdef>
+			<inlineclasses title=""/>
+			<defines title=""/>
+			<typedefs title=""/>
+			<sequences title=""/>
+			<dictionaries title=""/>
+			<enums title=""/>
+			<functions title=""/>
+			<variables title=""/>
+			<properties title=""/>
+		</memberdef>
+		<authorsection/>
+	</file>
+	<group>
+		<detaileddescription title=""/>
+		<groupgraph visible="yes"/>
+		<memberdecl>
+			<nestedgroups visible="yes" title=""/>
+			<modules visible="yes" title=""/>
+			<dirs visible="yes" title=""/>
+			<files visible="yes" title=""/>
+			<namespaces visible="yes" title=""/>
+			<concepts visible="yes" title=""/>
+			<classes visible="yes" title=""/>
+			<defines title=""/>
+			<typedefs title=""/>
+			<sequences title=""/>
+			<dictionaries title=""/>
+			<enums title=""/>
+			<enumvalues title=""/>
+			<functions title=""/>
+			<variables title=""/>
+			<signals title=""/>
+			<publicslots title=""/>
+			<protectedslots title=""/>
+			<privateslots title=""/>
+			<events title=""/>
+			<properties title=""/>
+			<friends title=""/>
+			<membergroups visible="yes"/>
+		</memberdecl>
+		<memberdef>
+			<pagedocs/>
+			<inlineclasses title=""/>
+			<defines title=""/>
+			<typedefs title=""/>
+			<sequences title=""/>
+			<dictionaries title=""/>
+			<enums title=""/>
+			<enumvalues title=""/>
+			<functions title=""/>
+			<variables title=""/>
+			<signals title=""/>
+			<publicslots title=""/>
+			<protectedslots title=""/>
+			<privateslots title=""/>
+			<events title=""/>
+			<properties title=""/>
+			<friends title=""/>
+		</memberdef>
+		<authorsection visible="yes"/>
+	</group>
+	<module>
+		<briefdescription visible="yes"/>
+		<exportedmodules visible="yes"/>
+		<memberdecl>
+			<concepts visible="yes" title=""/>
+			<classes visible="yes" title=""/>
+			<enums title=""/>
+			<typedefs title=""/>
+			<functions title=""/>
+			<variables title=""/>
+			<membergroups title=""/>
+		</memberdecl>
+		<detaileddescription title=""/>
+		<memberdecl>
+			<files visible="yes"/>
+		</memberdecl>
+	</module>
+	<directory>
+		<briefdescription visible="yes"/>
+		<directorygraph visible="yes"/>
+		<memberdecl>
+			<dirs visible="yes"/>
+			<files visible="yes"/>
+		</memberdecl>
+		<detaileddescription title=""/>
+	</directory>
+</doxygenlayout>
diff --git a/src/doc/style.css b/src/doc/style.css
new file mode 100644
index 0000000..08bc9f5
--- /dev/null
+++ b/src/doc/style.css
@@ -0,0 +1,4 @@
+#titlearea,
+address {
+	display: none;
+}
diff --git a/src/example/rendering.cpp b/src/example/rendering.cpp
index c813524..522ec0f 100644
--- a/src/example/rendering.cpp
+++ b/src/example/rendering.cpp
@@ -30,14 +30,27 @@ int main() {
 	// Normal adding components
 	{
 		Color color(0, 0, 0, 0);
-		obj.add_component<Sprite>(
-			make_shared<Texture>("asset/texture/img.png"), color, FlipSettings{false, false});
-		obj.add_component<Camera>(Color::get_red());
+		Sprite & sprite
+			= obj.add_component<Sprite>(make_shared<Texture>("asset/texture/img.png"),
+										color, FlipSettings{false, false});
+		sprite.sorting_in_layer = 2;
+		sprite.order_in_layer = 1;
+		obj.add_component<Camera>(Color::RED);
 	}
 	{
 		Color color(0, 0, 0, 0);
-		obj1.add_component<Sprite>(
-			make_shared<Texture>("asset/texture/second.png"), color, FlipSettings{true, true});
+		Sprite & sprite = obj1.add_component<Sprite>(
+			make_shared<Texture>("asset/texture/img.png"), color, FlipSettings{true, true});
+		sprite.sorting_in_layer = 2;
+		sprite.order_in_layer = 2;
+	}
+
+	{
+		Color color(0, 0, 0, 0);
+		Sprite & sprite = obj2.add_component<Sprite>(
+			make_shared<Texture>("asset/texture/img.png"), color, FlipSettings{true, true});
+		sprite.sorting_in_layer = 1;
+		sprite.order_in_layer = 2;
 	}
 
 	/*
@@ -48,8 +61,12 @@ int main() {
 	}
 	*/
 
+	sys.update();
+	/*
+
 	auto start = std::chrono::steady_clock::now();
 	while (std::chrono::steady_clock::now() - start < std::chrono::seconds(5)) {
 		sys.update();
 	}
+	*/
 }
diff --git a/src/makefile b/src/makefile
index 5f80204..a0e8f02 100644
--- a/src/makefile
+++ b/src/makefile
@@ -1,6 +1,9 @@
 .PHONY: FORCE
 
-FMT := $(shell git ls-files '*.c' '*.cpp' '*.h' '*.hpp')
 format: FORCE
-	clang-tidy -p build/compile_commands.json --fix-errors $(FMT)
+	$(MAKE) -C .. $@
+
+LINT := $(shell git ls-files '*.c' '*.cpp' '*.h' '*.hpp')
+lint: FORCE
+	clang-tidy -p build/compile_commands.json --fix-errors $(LINT)
 
diff --git a/src/test/CMakeLists.txt b/src/test/CMakeLists.txt
index f722082..f6e1dbb 100644
--- a/src/test/CMakeLists.txt
+++ b/src/test/CMakeLists.txt
@@ -5,5 +5,6 @@ target_sources(test_main PUBLIC
 	ParticleTest.cpp
 	AssetTest.cpp
 	OptionalRefTest.cpp
+	RenderSystemTest.cpp
 )
 
diff --git a/src/test/ParticleTest.cpp b/src/test/ParticleTest.cpp
index 4e655a9..cfbbc0e 100644
--- a/src/test/ParticleTest.cpp
+++ b/src/test/ParticleTest.cpp
@@ -29,7 +29,7 @@ public:
 
 			Color color(0, 0, 0, 0);
 			Sprite test_sprite = game_object.add_component<Sprite>(
-				make_shared<Texture>("../asset/texture/img.png"), color,
+				make_shared<Texture>("asset/texture/img.png"), color,
 				FlipSettings{true, true});
 
 			game_object.add_component<ParticleEmitter>(ParticleEmitter::Data{
diff --git a/src/test/RenderSystemTest.cpp b/src/test/RenderSystemTest.cpp
new file mode 100644
index 0000000..ac479d3
--- /dev/null
+++ b/src/test/RenderSystemTest.cpp
@@ -0,0 +1,174 @@
+#include "api/Camera.h"
+#include <functional>
+#include <gtest/gtest.h>
+#include <memory>
+#include <vector>
+
+#define private public
+#define protected public
+
+#include <crepe/ComponentManager.h>
+#include <crepe/api/Color.h>
+#include <crepe/api/GameObject.h>
+#include <crepe/api/Sprite.h>
+#include <crepe/api/Texture.h>
+
+#include <crepe/system/RenderSystem.h>
+
+using namespace std;
+using namespace crepe;
+using namespace testing;
+
+class RenderSystemTest : public Test {
+public:
+	ComponentManager mgr{};
+	RenderSystem sys{mgr};
+	GameObject entity1 = this->mgr.new_object("name");
+	GameObject entity2 = this->mgr.new_object("name");
+	GameObject entity3 = this->mgr.new_object("name");
+	GameObject entity4 = this->mgr.new_object("name");
+
+	void SetUp() override {
+		auto & sprite1
+			= entity1.add_component<Sprite>(make_shared<Texture>("../asset/texture/img.png"),
+											Color(0, 0, 0, 0), FlipSettings{false, false});
+		ASSERT_NE(sprite1.sprite_image.get(), nullptr);
+		sprite1.order_in_layer = 5;
+		sprite1.sorting_in_layer = 5;
+		EXPECT_EQ(sprite1.order_in_layer, 5);
+		EXPECT_EQ(sprite1.sorting_in_layer, 5);
+		auto & sprite2
+			= entity2.add_component<Sprite>(make_shared<Texture>("../asset/texture/img.png"),
+											Color(0, 0, 0, 0), FlipSettings{false, false});
+		ASSERT_NE(sprite2.sprite_image.get(), nullptr);
+		sprite2.sorting_in_layer = 2;
+		sprite2.order_in_layer = 1;
+
+		EXPECT_EQ(sprite2.sorting_in_layer, 2);
+		EXPECT_EQ(sprite2.order_in_layer, 1);
+
+		auto & sprite3
+			= entity3.add_component<Sprite>(make_shared<Texture>("../asset/texture/img.png"),
+											Color(0, 0, 0, 0), FlipSettings{false, false});
+		ASSERT_NE(sprite3.sprite_image.get(), nullptr);
+		sprite3.sorting_in_layer = 1;
+		sprite3.order_in_layer = 2;
+
+		EXPECT_EQ(sprite3.sorting_in_layer, 1);
+		EXPECT_EQ(sprite3.order_in_layer, 2);
+
+		auto & sprite4
+			= entity4.add_component<Sprite>(make_shared<Texture>("../asset/texture/img.png"),
+											Color(0, 0, 0, 0), FlipSettings{false, false});
+		ASSERT_NE(sprite4.sprite_image.get(), nullptr);
+		sprite4.sorting_in_layer = 1;
+		sprite4.order_in_layer = 1;
+		EXPECT_EQ(sprite4.sorting_in_layer, 1);
+		EXPECT_EQ(sprite4.order_in_layer, 1);
+	}
+};
+
+TEST_F(RenderSystemTest, expected_throws) {
+	GameObject entity1 = this->mgr.new_object("NAME");
+
+	// no texture img
+	EXPECT_ANY_THROW({
+		entity1.add_component<Sprite>(make_shared<Texture>("NO_IMAGE"), Color(0, 0, 0, 0),
+									  FlipSettings{false, false});
+	});
+
+	// No camera
+	EXPECT_ANY_THROW({ this->sys.update(); });
+}
+
+TEST_F(RenderSystemTest, make_sprites) {}
+
+TEST_F(RenderSystemTest, sorting_sprites) {
+	vector<reference_wrapper<Sprite>> sprites = this->mgr.get_components_by_type<Sprite>();
+	ASSERT_EQ(sprites.size(), 4);
+
+	vector<reference_wrapper<Sprite>> sorted_sprites = this->sys.sort(sprites);
+	ASSERT_EQ(sorted_sprites.size(), 4);
+
+	// Expected order after sorting:
+	// 1. sorting_in_layer: 1, order_in_layer: 1 (entity4)
+	// 2. sorting_in_layer: 1, order_in_layer: 2 (entity3)
+	// 3. sorting_in_layer: 2, order_in_layer: 1 (entity2)
+	// 4. sorting_in_layer: 5, order_in_layer: 5 (entity1)
+
+	EXPECT_EQ(sorted_sprites[0].get().sorting_in_layer, 1);
+	EXPECT_EQ(sorted_sprites[0].get().order_in_layer, 1);
+
+	EXPECT_EQ(sorted_sprites[1].get().sorting_in_layer, 1);
+	EXPECT_EQ(sorted_sprites[1].get().order_in_layer, 2);
+
+	EXPECT_EQ(sorted_sprites[2].get().sorting_in_layer, 2);
+	EXPECT_EQ(sorted_sprites[2].get().order_in_layer, 1);
+
+	EXPECT_EQ(sorted_sprites[3].get().sorting_in_layer, 5);
+	EXPECT_EQ(sorted_sprites[3].get().order_in_layer, 5);
+
+	for (size_t i = 1; i < sorted_sprites.size(); ++i) {
+		const Sprite & prev = sorted_sprites[i - 1].get();
+		const Sprite & curr = sorted_sprites[i].get();
+
+		if (prev.sorting_in_layer == curr.sorting_in_layer) {
+			EXPECT_LE(prev.order_in_layer, curr.order_in_layer);
+		} else {
+			EXPECT_LE(prev.sorting_in_layer, curr.sorting_in_layer);
+		}
+	}
+}
+
+TEST_F(RenderSystemTest, Update) {
+	entity1.add_component<Camera>(Color::WHITE);
+	{
+		vector<reference_wrapper<Sprite>> sprites = this->mgr.get_components_by_type<Sprite>();
+		ASSERT_EQ(sprites.size(), 4);
+
+		EXPECT_EQ(sprites[0].get().game_object_id, 0);
+		EXPECT_EQ(sprites[1].get().game_object_id, 1);
+		EXPECT_EQ(sprites[2].get().game_object_id, 2);
+		EXPECT_EQ(sprites[3].get().game_object_id, 3);
+	}
+	this->sys.update();
+	{
+		vector<reference_wrapper<Sprite>> sprites = this->mgr.get_components_by_type<Sprite>();
+		ASSERT_EQ(sprites.size(), 4);
+
+		EXPECT_EQ(sprites[0].get().game_object_id, 0);
+		EXPECT_EQ(sprites[1].get().game_object_id, 1);
+		EXPECT_EQ(sprites[2].get().game_object_id, 2);
+		EXPECT_EQ(sprites[3].get().game_object_id, 3);
+	}
+}
+
+TEST_F(RenderSystemTest, Camera) {
+	{
+		auto cameras = this->mgr.get_components_by_type<Camera>();
+		EXPECT_NE(cameras.size(), 1);
+	}
+	{
+		entity1.add_component<Camera>(Color::WHITE);
+		auto cameras = this->mgr.get_components_by_type<Camera>();
+		EXPECT_EQ(cameras.size(), 1);
+	}
+
+	//TODO improve with newer version
+}
+TEST_F(RenderSystemTest, Color) {
+	entity1.add_component<Camera>(Color::WHITE);
+	auto & sprite = this->mgr.get_components_by_id<Sprite>(entity1.id).front().get();
+	ASSERT_NE(sprite.sprite_image.get(), nullptr);
+
+	sprite.color = Color::GREEN;
+	EXPECT_EQ(sprite.color.r, Color::GREEN.r);
+	EXPECT_EQ(sprite.color.g, Color::GREEN.g);
+	EXPECT_EQ(sprite.color.b, Color::GREEN.b);
+	EXPECT_EQ(sprite.color.a, Color::GREEN.a);
+	this->sys.update();
+	EXPECT_EQ(sprite.color.r, Color::GREEN.r);
+	EXPECT_EQ(sprite.color.g, Color::GREEN.g);
+	EXPECT_EQ(sprite.color.b, Color::GREEN.b);
+	EXPECT_EQ(sprite.color.a, Color::GREEN.a);
+}