C99 Entity Component System

A simple ecs writen in c99.

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This is my simple implementation of an ecs in c, it is largely inspired from this ecs by Dylan Flaconer Link: https://gitlab.com/Falconerd/ember-ecs

How to compile

This ecs is simply two files ecs.h and ecs.c. Just add them to your project and include them, nothing crazy here.

How to use

Just include ecs.h in your project. All the code is commented, so it should give you an idea of how it all works. Below is an example of how to use the module.

Creating components

Components can be any data type, here I'll create 2 test components.

• Position Component

typedef struct position_component
{
	int x;
	int y;
}Position_Component;

• Character Component

typedef struct character_component
{
	char* name;
	u8 age;
	u8 height;
	u8 weight;
}Character_Component;

u8 is a uint8_t unsigned 8 bit integer.

Initializing the ECS

We use the 2 functions belwo to initialize the ecs, check ecs.h for their documentaion. Initializing the ecs will allocate memory for the components and the entities.

ecs_init(2, sizeof(Position_Component), sizeof(Character_Component));
ecs_scene_push();

One note here is that the order in which each component is initialized matters, so in the example above, the position component will have id 0 since it was passed first and the character component will have id 1. The ids are used to reference the components when querying or when getting a component from an entity.

Creating systems for the components

Systems query for entities that have the specified components, we'll create 2 example systems. To query we use the function ecs_query(), check ecs.h for further documentation.

• Position system

void position_system(void)
{
	u32 i;
	// Querying for entities that have one component which is the position component
	Ecs_Query_Result* qr = ecs_query(1, 0); // 0 is the id of the position component

	// Looping through the entities 
	for (i = 0; i < qr->count; i++)
	{
		Position_Component* pos = (Position_Component*)ecs_get_component(qr->entities[i], 0);

		printf("\nx:%d | y:%d", pos->x, pos->y);
	}
}

• Character system

void character_system(void)
{
	u32 i;
	// Querying for entities that have two components position and character components
	Ecs_Query_Result* qr = ecs_query(2, 0, 1);

	// Looping through the entities 
	for (i = 0; i < qr->count; i++)
	{
		Position_Component* pos = (Position_Component*)ecs_get_component(qr->entities[i], 0);
		Character_Component* character = (Character_Component*)ecs_get_component(qr->entities[i], 1);

		printf("\nx:%d     | y:%d", pos->x, pos->y);
		printf("\nname:%s | age:%u | height:%u | weight:%u", character->name,
				character->age, character->height, character->weight);
	}
}

u32 is a uint32_t unsigned 32 bit integer. These systems above are simply an example, obviously is a real game, you'd want more than to print a system's properties.

Creating entities

Creating entities is straight forward, just call the function ecs_create_entity() and it will return the id of the entity.

u32 entity1 = ecs_create_entity();
u32 entity2 = ecs_create_entity();

Adding components to entities

Adding components to entities is also straight forward, you just use this function ecs_add_component() (again, check out ecs.h for more).

// Adding a position component to entity1
Position_Component position = { .x = 100, .y = -50 };
ecs_add_component(entity1, POSITION_COMPONENT, &position);

// Adding a position component and a character component to entity2
Character_Component character = { .name = "Hero", .age = 23, .height = 189, .weight = 75 };
ecs_add_component(entity2, POSITION_COMPONENT, &position); // reusing the position component already created
ecs_add_component(entity2, CHARACTER_COMPONENT, &character);

Calling the systems

You'd do this in a proper game loop, but it just amounts to calling the previously declared and implemented functions void position_system() and void character_system()

while (game_loop)
{
	position_system();
	character_system();
}

Shutting down the ECS

The ecs needs to be shutdown to free all allocated memory by the module.

ecs_scene_pop();
ecs_shutdown();