20 files changed, 138 insertions, 107 deletions

diff --git a/contributing.md b/contributing.md
index 38a83fd..35d777d 100644
--- a/contributing.md
+++ b/contributing.md
@@ -1,11 +1,15 @@
-# Contributing new code
+This document contains
+<details><summary>
+examples
+</summary>
+that you can click on to open them.
+</details>
+
+# Git
 
 - Please do the following *before* sending a pull request:
   - Merge upstream code (if any) back into your own branch
   - Run formatters/linters
-
-# Git
-
 - Push as often as possible
 - Development is done on separate branches, these follow a pattern of
   `name/feature` (i.e. `loek/dll-so-poc` or `jaro/class2`)
@@ -17,8 +21,9 @@
 
 - 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)
-- ASCII only
-  <table><tr><th>Good</th><th>Bad</th></tr><tr><td>
+- <details><summary>
+  ASCII only
+  </summary><table><tr><th>Good</th><th>Bad</th></tr><tr><td>
 
   ```cpp
   // crepe startup message
@@ -30,9 +35,10 @@
   // crêpe startup message
   std::string message = "こんにちは世界";
   ```
-  </td></tr></table>
-- Class names are always singular
-  <table><tr><th>Good</th><th>Bad</th></tr><tr><td>
+  </td></tr></table></details>
+- <details><summary>
+  Class names are always singular
+  </summary><table><tr><th>Good</th><th>Bad</th></tr><tr><td>
 
   ```cpp
   class Foo {};
@@ -42,7 +48,7 @@
   ```cpp
   class Cars {};
   ```
-  </td></tr></table>
+  </td></tr></table></details>
 - Source files contain the following types of comments:
   - What is the code supposed to do (optional)
   - Implementation details (if applicable)
@@ -50,8 +56,9 @@
   - Usage documentation (required)
   - Implementation details (if they affect the header)
   - Design/data structure decisions (if applicable)
-- Comments are placed *above* the line(s) they are explaining
-  <table><tr><th>Good</th><th>Bad</th></tr><tr><td>
+- <details><summary>
+  Comments are placed *above* the line(s) they are explaining
+  </summary><table><tr><th>Good</th><th>Bad</th></tr><tr><td>
 
   ```cpp
   int add(int a, int b) {
@@ -68,7 +75,7 @@
     return out;
   }
   ```
-  </td></tr></table>
+  </td></tr></table></details>
 - Header includes are split into paragraphs separated by a blank line. The
   order is:
   1. system headers (using `<`brackets`>`)
@@ -80,6 +87,7 @@
   > sure to separate the include statements using a blank line (clang-format
   > may sort include statements, but does not sort across empty lines).
 
+  <details><summary>Example</summary>
   <table><tr><th>Good</th><th>Bad</th></tr><tr><td>
 
   ```cpp
@@ -100,9 +108,10 @@
   #include <iostream>
   #include "api/Sprite.h"
   ```
-  </td></tr></table>
-- `using namespace` may not be used in header files, only in source files.
-  <table><tr><th>Good</th><th>Bad</th></tr><tr><td>
+  </td></tr></table></details>
+- <details><summary>
+  <code>using namespace</code> may not be used in header files, only in source files.
+  </summary><table><tr><th>Good</th><th>Bad</th></tr><tr><td>
 
   example.h:
   ```cpp
@@ -134,10 +143,12 @@
   template <typename T>
   T foo();
   ```
-  </td></tr></table>
+  </td></tr></table></details>
 
-- Getter and setter functions are appropriately prefixed with `get_` and `set_`.
-  <table><tr><th>Good</th><th>Bad</th></tr><tr><td>
+- <details><summary>
+  Getter and setter functions are appropriately prefixed with <code>get_</code>
+  and <code>set_</code>.
+  </summary><table><tr><th>Good</th><th>Bad</th></tr><tr><td>
 
   ```cpp
   class Foo {
@@ -160,12 +171,12 @@
     int speed;
   };
   ```
-  </td></tr></table>
-
-- A singleton's instance is always accessed using a getter function that
+  </td></tr></table></details>
+- <details><summary>
+  A singleton's instance is always accessed using a getter function that
   instantiates its own class as a static variable within the getter function
-  scope, instead of storing the instance as a member variable directly:
-  <table><tr><th>Good</th><th>Bad</th></tr><tr><td>
+  scope, instead of storing the instance as a member variable directly.
+  </summary><table><tr><th>Good</th><th>Bad</th></tr><tr><td>
 
   ```cpp
   class Foo {
@@ -186,11 +197,11 @@
   };
 
   ```
-  </td></tr></table>
-
-- Member variable default values should be directly defined in the class/struct
+  </td></tr></table></details>
+- <details><summary>
+  Member variable default values should be directly defined in the class/struct
   declaration instead of using the constructor.
-  <table><tr><th>Good</th><th>Bad</th></tr><tr><td>
+  </summary><table><tr><th>Good</th><th>Bad</th></tr><tr><td>
 
   ```cpp
   class Foo {
@@ -206,28 +217,39 @@
     int speed;
   };
   ```
-  </td></tr></table>
-
+  </td></tr></table></details>
 - Use of the `auto` type is *not* allowed, with the following exceptions:
-  - When naming the item type in a range-based for loop:
-    
+  - <details><summary>
+    When naming the item type in a range-based for loop
+    </summary>
+
     ```cpp
     for (auto & item : foo()) {
       // ...
     }
     ```
-  - When calling template factory methods that explicitly name the return type
+    </details>
+  - <details><summary>
+    When calling template factory methods that explicitly name the return type
     in the function call signature
+    </summary>
+
     ```cpp
     auto ptr = make_unique<Foo>();
     ```
-  - When fetching a singleton instance
+    </details>
+  - <details><summary>
+    When fetching a singleton instance
+    </summary>
+
     ```cpp
     auto & mgr = crepe::api::Config::get_instance();
     ```
+    </details>
 
-- Only use member initializer lists for non-trivial types.
-  <table><tr><th>Good</th><th>Bad</th></tr><tr><td>
+- <details><summary>
+  Only use member initializer lists for non-trivial types.
+  </summary><table><tr><th>Good</th><th>Bad</th></tr><tr><td>
 
   ```cpp
   class Foo {
@@ -248,10 +270,10 @@
     int bar;
   };
   ```
-  </td></tr></table>
-
-- C++-style structs should define default values for all non-trivial fields.
-  <table><tr><th>Good</th><th>Bad</th></tr><tr><td>
+  </td></tr></table></details>
+- <details><summary>
+  C++-style structs should define default values for all non-trivial fields.
+  </summary><table><tr><th>Good</th><th>Bad</th></tr><tr><td>
 
   ```cpp
   struct Foo {
@@ -267,11 +289,11 @@
     std::string baz;
   };
   ```
-  </td></tr></table>
-
-- Declare incomplete classes instead of including the relevant header where
+  </td></tr></table></details>
+- <details><summary>
+  Declare incomplete classes instead of including the relevant header where
   possible (i.e. if you only need a reference or raw pointer).
-  <table><tr><th>Good</th><th>Bad</th></tr><tr><td>
+  </summary><table><tr><th>Good</th><th>Bad</th></tr><tr><td>
 
   ```cpp
   class Bar;
@@ -288,11 +310,11 @@
     Bar & bar;
   };
   ```
-  </td></tr></table>
-
-- Template functions are only *declared* in a `.h` header, and *defined* in a
-  matching `.hpp` header.
-  <table><tr><th>Good</th><th>Bad</th></tr><tr><td>
+  </td></tr></table></details>
+- <details><summary>
+  Template functions are only <i>declared</i> in a <code>.h</code> header, and
+  <i>defined</i> in a matching <code>.hpp</code> header.
+  </summary><table><tr><th>Good</th><th>Bad</th></tr><tr><td>
 
   add.h:
   ```cpp
@@ -320,11 +342,11 @@
     return a + b;
   }
   ```
-  </td></tr></table>
-
-- Where possible, end (initializer) lists with a trailing comma (e.g. with
+  </td></tr></table></details>
+- <details><summary>
+  Where possible, end (initializer) lists with a trailing comma (e.g. with
   structs, enums)
-  <table><tr><th>Good</th><th>Bad</th></tr><tr><td>
+  </summary><table><tr><th>Good</th><th>Bad</th></tr><tr><td>
 
   ```cpp
   enum Color {
@@ -343,9 +365,10 @@
     Blue
   };
   ```
-  </td></tr></table>
-- `#pragma` should be used instead of include guards
-  <table><tr><th>Good</th><th>Bad</th></tr><tr><td>
+  </td></tr></table></details>
+- <details><summary>
+  <code>#pragma</code> should be used instead of include guards
+  </summary><table><tr><th>Good</th><th>Bad</th></tr><tr><td>
 
   ```cpp
   #pragma once
@@ -362,7 +385,7 @@
 
   #endif
   ```
-  </td></tr></table>
+  </td></tr></table></details>
 
 ## CMakeLists-specific
 
@@ -373,20 +396,25 @@
 
 # Structure
 
-- All engine-related code is implemented under the `crepe` namespace,
-  user-facing APIs under `crepe::api` (the folder structure should also reflect
-  this).
+- Files are placed in the appropriate directory:
+  |Path|Purpose|
+  |-|-|
+  |`crepe/`|Auxiliary, managers|
+  |`crepe/api/`|User-facing APIs|
+  |`crepe/util/`|Standalone utilities and helper functions|
+  |`crepe/system/`|(ECS) system classes|
 - Do not (indirectly) include private *dependency* headers in API header files,
   as these are no longer accessible when the engine is installed
+- All code is implemented under the `crepe` namespace.
 - Header files declare either a single class or symbols within a single
   namespace.
-- TODO: folder structure
 
 # Documentation
 
 - All documentation is written in U.S. English
-- Doxygen commands are used with a backslash instead of an at-sign.
-  <table><tr><th>Good</th><th>Bad</th></tr><tr><td>
+- <details><summary>
+  Doxygen commands are used with a backslash instead of an at-sign.
+  </summary><table><tr><th>Good</th><th>Bad</th></tr><tr><td>
 
   ```cpp
   /**
@@ -406,7 +434,7 @@
    */
   void foo();
   ```
-  </td></tr></table>
+  </td></tr></table></details>
 
 # Libraries
 
diff --git a/src/crepe/CMakeLists.txt b/src/crepe/CMakeLists.txt
index d938eb8..7f976db 100644
--- a/src/crepe/CMakeLists.txt
+++ b/src/crepe/CMakeLists.txt
@@ -3,17 +3,11 @@ target_sources(crepe PUBLIC
 	Sound.cpp
 	SoundContext.cpp
 	Particle.cpp
-	ParticleSystem.cpp
 	SDLApp.cpp
 	ComponentManager.cpp
 	Component.cpp
-	ScriptSystem.cpp
-	PhysicsSystem.cpp
-	CollisionSystem.cpp
 	Collider.cpp
 	SDLContext.cpp
-
-	RenderSystem.cpp
 )
 
 target_sources(crepe PUBLIC FILE_SET HEADERS FILES
@@ -24,15 +18,11 @@ target_sources(crepe PUBLIC FILE_SET HEADERS FILES
 	ComponentManager.h
 	ComponentManager.hpp
 	Component.h
-	System.h
-	ScriptSystem.h
-	PhysicsSystem.h
-	CollisionSystem.h
 	Collider.h
 	SDLContext.h
-	RenderSystem.h
 )
 
 add_subdirectory(api)
 add_subdirectory(util)
+add_subdirectory(system)
 
diff --git a/src/crepe/SDLContext.h b/src/crepe/SDLContext.h
index 4d9c1bc..ea05c7b 100644
--- a/src/crepe/SDLContext.h
+++ b/src/crepe/SDLContext.h
@@ -6,7 +6,7 @@
 #include "api/Sprite.h"
 #include "api/Transform.h"
 
-#include "RenderSystem.h"
+#include "system/RenderSystem.h"
 
 namespace crepe::api {
 class Texture;
diff --git a/src/crepe/system/CMakeLists.txt b/src/crepe/system/CMakeLists.txt
new file mode 100644
index 0000000..ff6f66f
--- /dev/null
+++ b/src/crepe/system/CMakeLists.txt
@@ -0,0 +1,15 @@
+target_sources(crepe PUBLIC
+	ParticleSystem.cpp
+	ScriptSystem.cpp
+	PhysicsSystem.cpp
+	CollisionSystem.cpp
+	RenderSystem.cpp
+)
+
+target_sources(crepe PUBLIC FILE_SET HEADERS FILES
+	System.h
+	ScriptSystem.h
+	PhysicsSystem.h
+	CollisionSystem.h
+	RenderSystem.h
+)
diff --git a/src/crepe/CollisionSystem.cpp b/src/crepe/system/CollisionSystem.cpp
index 55e0fdc..55e0fdc 100644
--- a/src/crepe/CollisionSystem.cpp
+++ b/src/crepe/system/CollisionSystem.cpp
diff --git a/src/crepe/CollisionSystem.h b/src/crepe/system/CollisionSystem.h
index 1e9f1aa..1e9f1aa 100644
--- a/src/crepe/CollisionSystem.h
+++ b/src/crepe/system/CollisionSystem.h
diff --git a/src/crepe/ParticleSystem.cpp b/src/crepe/system/ParticleSystem.cpp
index af6c550..367c7d8 100644
--- a/src/crepe/ParticleSystem.cpp
+++ b/src/crepe/system/ParticleSystem.cpp
@@ -1,9 +1,9 @@
 #include <cmath>
 #include <ctime>
 
-#include "api/ParticleEmitter.h"
+#include "../api/ParticleEmitter.h"
+#include "../ComponentManager.h"
 
-#include "ComponentManager.h"
 #include "ParticleSystem.h"
 
 using namespace crepe;
diff --git a/src/crepe/ParticleSystem.h b/src/crepe/system/ParticleSystem.h
index ad96eb0..3ac1d3f 100644
--- a/src/crepe/ParticleSystem.h
+++ b/src/crepe/system/ParticleSystem.h
@@ -1,6 +1,6 @@
 #pragma once
 
-#include "api/ParticleEmitter.h"
+#include "../api/ParticleEmitter.h"
 
 namespace crepe {
 
diff --git a/src/crepe/PhysicsSystem.cpp b/src/crepe/system/PhysicsSystem.cpp
index 16f4c10..6da714f 100644
--- a/src/crepe/PhysicsSystem.cpp
+++ b/src/crepe/system/PhysicsSystem.cpp
@@ -1,10 +1,10 @@
 #include <iostream>
 
-#include "api/Force.h"
-#include "api/Rigidbody.h"
-#include "api/Transform.h"
+#include "../api/Force.h"
+#include "../api/Rigidbody.h"
+#include "../api/Transform.h"
+#include "../ComponentManager.h"
 
-#include "ComponentManager.h"
 #include "PhysicsSystem.h"
 
 using namespace crepe;
diff --git a/src/crepe/PhysicsSystem.h b/src/crepe/system/PhysicsSystem.h
index 33b4072..33b4072 100644
--- a/src/crepe/PhysicsSystem.h
+++ b/src/crepe/system/PhysicsSystem.h
diff --git a/src/crepe/RenderSystem.cpp b/src/crepe/system/RenderSystem.cpp
index fae93f0..599087a 100644
--- a/src/crepe/RenderSystem.cpp
+++ b/src/crepe/system/RenderSystem.cpp
@@ -1,13 +1,13 @@
 #include <functional>
 #include <vector>
 
-#include "api/Sprite.h"
-#include "api/Transform.h"
-#include "util/log.h"
+#include "../api/Sprite.h"
+#include "../api/Transform.h"
+#include "../util/log.h"
+#include "../SDLContext.h"
+#include "../ComponentManager.h"
 
-#include "ComponentManager.h"
 #include "RenderSystem.h"
-#include "SDLContext.h"
 
 using namespace crepe;
 using namespace crepe::api;
diff --git a/src/crepe/RenderSystem.h b/src/crepe/system/RenderSystem.h
index 4b910a4..4b910a4 100644
--- a/src/crepe/RenderSystem.h
+++ b/src/crepe/system/RenderSystem.h
diff --git a/src/crepe/ScriptSystem.cpp b/src/crepe/system/ScriptSystem.cpp
index 171b490..277b29d 100644
--- a/src/crepe/ScriptSystem.cpp
+++ b/src/crepe/system/ScriptSystem.cpp
@@ -2,11 +2,11 @@
 #include <functional>
 #include <vector>
 
-#include "api/BehaviorScript.h"
-#include "api/Script.h"
-#include "util/log.h"
+#include "../api/BehaviorScript.h"
+#include "../api/Script.h"
+#include "../util/log.h"
+#include "../ComponentManager.h"
 
-#include "ComponentManager.h"
 #include "ScriptSystem.h"
 
 using namespace std;
diff --git a/src/crepe/ScriptSystem.h b/src/crepe/system/ScriptSystem.h
index 1f472a0..1f472a0 100644
--- a/src/crepe/ScriptSystem.h
+++ b/src/crepe/system/ScriptSystem.h
diff --git a/src/crepe/System.h b/src/crepe/system/System.h
index ecbb7f5..ecbb7f5 100644
--- a/src/crepe/System.h
+++ b/src/crepe/system/System.h
diff --git a/src/example/asset_manager.cpp b/src/example/asset_manager.cpp
index 7e15d1f..3d68ec0 100644
--- a/src/example/asset_manager.cpp
+++ b/src/example/asset_manager.cpp
@@ -1,5 +1,3 @@
-
-
 #include <crepe/Sound.h>
 #include <crepe/api/AssetManager.h>
 #include <crepe/api/Texture.h>
diff --git a/src/example/particle.cpp b/src/example/particle.cpp
index 53fa213..fb72b9b 100644
--- a/src/example/particle.cpp
+++ b/src/example/particle.cpp
@@ -1,13 +1,14 @@
-#include "Particle.h"
-#include "ParticleSystem.h"
-#include "SDLApp.h"
-#include "api/ParticleEmitter.h"
 #include <chrono>
+#include <iostream>
+#include <thread>
+
 #include <crepe/Component.h>
 #include <crepe/ComponentManager.h>
+#include <crepe/Particle.h>
+#include <crepe/SDLApp.h>
 #include <crepe/api/GameObject.h>
-#include <iostream>
-#include <thread>
+#include <crepe/api/ParticleEmitter.h>
+#include <crepe/system/ParticleSystem.h>
 
 using namespace crepe::api;
 using namespace crepe;
diff --git a/src/example/physics.cpp b/src/example/physics.cpp
index db69b9b..ae8d4ca 100644
--- a/src/example/physics.cpp
+++ b/src/example/physics.cpp
@@ -1,13 +1,14 @@
-#include "PhysicsSystem.h"
 #include <chrono>
+#include <iostream>
+#include <thread>
+
+#include <crepe/system/PhysicsSystem.h>
 #include <crepe/Component.h>
 #include <crepe/ComponentManager.h>
 #include <crepe/api/Force.h>
 #include <crepe/api/GameObject.h>
 #include <crepe/api/Rigidbody.h>
 #include <crepe/api/Transform.h>
-#include <iostream>
-#include <thread>
 
 using namespace crepe::api;
 using namespace crepe;
diff --git a/src/example/rendering.cpp b/src/example/rendering.cpp
index 1bf448c..0bc222a 100644
--- a/src/example/rendering.cpp
+++ b/src/example/rendering.cpp
@@ -1,7 +1,5 @@
-
-
 #include <crepe/ComponentManager.h>
-#include <crepe/RenderSystem.h>
+#include <crepe/system/RenderSystem.h>
 #include <crepe/api/GameObject.h>
 #include <crepe/util/log.h>
 
diff --git a/src/example/script.cpp b/src/example/script.cpp
index 5df26e8..1901078 100644
--- a/src/example/script.cpp
+++ b/src/example/script.cpp
@@ -4,7 +4,7 @@
  */
 
 #include <crepe/ComponentManager.h>
-#include <crepe/ScriptSystem.h>
+#include <crepe/system/ScriptSystem.h>
 #include <crepe/util/log.h>
 
 #include <crepe/api/BehaviorScript.h>