Added entity manager

Fixed links
Trial of direct code link

docs/entity/Entity.md

@@ -34,6 +34,6 @@ void todo();
 ```
 
 ## Notes
-* The only requirement for a Entity is that it has a unique ID. A more advanced [entity manager](../manager/EntityManager.md) could implement an ID system with ID regions, or using named IDs. 
+* The only requirement for a Entity is that it has a unique ID. A more advanced [entity manager](./EntityManager.md) could implement an ID system with ID regions, or using named IDs. 
 Another approach could be to use the first byte as a grouping, in a similar way to how Bethesda Softworks' Skyrim utilises the first byte as a mod index
 * The maximum number of entities is limited by the size of the container used to store it. E.g., for a 32-bit unsigned integer, there are ~4.3 billion ID's available. But keep in mind that such a large amount of IDs will take significant time for each system to consider.

docs/entity/EntityManager.md

@@ -0,0 +1,98 @@
+*| [JavaECS](../../README.md) | [docs](../overview.md) | [entity](./dir.md) | EntityManager[]().md*
+# Entity Manager
+| **In this Section** |
+|-|
+| [About](#about) |
+| [Implementation](#implementation) |
+| [Constructors](#constructors) |
+| [Methods](#methods) |
+| [Fields](#fields) |
+| [Examples](#examples)|
+| [Notes](#notes)|
+
+## About
+Controls adding and removing entities, and registration and deregistration of components to specific entities.
+
+## Implementation
+Before attempting to use a system, it must be registered.
+
+Registration requires a call to `registerSystem(String systemName, ECSSystem system)`, with params of the `String name` and a object reference of the [ECSSystem](./ECSSystem.md). Using the object reference, a system can be invoked at any time.
+
+See [EntityManager.java](/BrychanD/JaveECS/src/branch/master/javaecs/src/main/java/nz/ac/massey/programming_project_159333_s1_2021/EntityManager.java)
+
+## Constructors
+``` java
+public EntityManager(){}
+```
+The default constructor initialises [unusedEntities](#unusedEntities) to a new queue containing `1024` unique integers.
+
+<br>
+
+``` java
+public EntityManager(int maxEntities){}
+```
+Initialises [unusedEntities](#unusedEntities) to a new queue containing `maxEntities` unique integers.
+
+<br>
+
+## Methods
+### addEntity
+``` java
+public Integer addEntity();
+```
+Pops the next available entity index off the queue, and returns its value.
+
+Returns `-1` if there is no remaining unused entities.
+
+<br>
+
+### getRegistrations
+``` java
+public BitSet getRegistrations(int entity);
+```
+Returns the `BitSet` of registrations the `entity` has
+
+<br>
+
+### registerComponent
+``` java
+public void registerComponent(int component, int entity);
+```
+Sets the bit `component` in [entityRegistrations](#entityRegistrations) for `entity` to `true`.
+
+<br>
+
+### removeEntity
+``` java
+public void removeEntity(int entity);
+```
+Adds the entity's index back to the unused queue, and clears the assigned registration bits
+#### Note
+>This function should not be called directly. Doing so will leave component data in memory, and will cause desynchronisation of the system registrations.
+
+<br>
+
+### unregisterComponent
+``` java
+public void unregisterComponent(int component, int entity);
+```
+Sets the bit `component` in [entityRegistrations](#entityRegistrations) for `entity` to `false`.
+
+<br>
+
+## Fields
+### entityRegistrations
+``` java
+List<BitSet> entityRegistrations;
+```
+
+<br>
+
+### unusedEntities
+``` java
+Queue<Integer> unusedEntities;
+```
+
+<br>
+
+

docs/entity/dir.md

@@ -1,4 +1,5 @@
 *| [JavaECS](../README.md) | [docs](../overview.md) | entity*
 # Overview
 ### In this folder
-[entity.md](./entity.md)
+*[Entity.md](./Entity.md)
+*[EntityManager.md](./EntityManager.md)