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#include <utility>
#include <stdio.h>
#include <stdlib.h>
#include <assert.h>
/* So it's like the other ECS example with a bunch of academic masturbation
* See https://git.quou.xyz/samples/tree/ecs.c
*
* Areas for improvement:
* - Should really respect alignment.
* - Entity destruction
*/
#define max_entities 256
/* If you want more than 32 component types then you'll have to
* use an unsigned long instead of an int for the component type
* IDs. */
#define max_component_types 32
typedef int Entity;
unsigned get_new_component_id();
/* DO NOT make either of these functions static. That will break stuff. */
template <typename T>
unsigned get_component_id() {
static unsigned id = get_new_component_id();
return id;
}
unsigned get_new_component_id() {
static unsigned id = 0;
return id++;
}
struct Component_Pool {
void* data;
int element_size;
template <typename T>
T& get(int idx) {
assert(sizeof(T) == element_size);
return ((T*)data)[idx];
}
Component_Pool(): data(0) {}
void init(int es) {
element_size = es;
data = malloc(element_size * max_entities);
}
void deinit() {
free(data);
}
bool isinit() {
return data != nullptr;
}
};
struct World {
Component_Pool pools[max_component_types];
int entity_count;
int bitset[max_entities];
World(): entity_count(0) {}
void deinit() {
for (int i = 0; i < max_component_types; i++) {
if (pools[i].isinit()) {
pools[i].deinit();
}
}
}
template <typename T>
void register_component_type() {
pools[get_component_id<T>()].init(sizeof(T));
}
Entity create_entity() {
auto e = entity_count++;
bitset[e] = 0;
return e;
}
template <typename T>
T& add_component(Entity e, T&& c) {
unsigned id = get_component_id<T>();
Component_Pool& pool = pools[id];
T& el = pool.get<T>(e);
bitset[e] |= (1 << id);
el = std::move(c);
return el;
}
template <typename T>
void remove_component(Entity e) {
unsigned id = get_component_id<T>();
bitset[e] &= ~(1 << id);
}
template <typename T>
T& get_component(Entity e) {
unsigned id = get_component_id<T>();
Component_Pool& pool = pools[id];
return pool.get<T>(e);
}
template <typename Fn>
void iterate_with_mask(unsigned mask, Fn fn) {
for (int i = 0; i < entity_count; i++) {
if (bitset[i] & mask) {
fn(i);
}
}
}
template <typename A, typename Fn>
void iterate_with(Fn fn) {
unsigned mask_a = 1 << get_component_id<A>();
unsigned bits;
for (int i = 0; i < entity_count; i++) {
bits = bitset[i];
if (bits & mask_a) {
fn(i);
}
}
}
/* You need to make an overload for every number of components that you want to
* iterate at once. Just making two for example.
*
* There's probably a modern C++ way to not have to do this, but I don't know it.
* Please tell me if you know. */
template <typename A, typename B, typename Fn>
void iterate_with(Fn fn) {
unsigned mask_a = 1 << get_component_id<A>();
unsigned mask_b = 1 << get_component_id<B>();
unsigned bits;
for (int i = 0; i < entity_count; i++) {
bits = bitset[i];
if ((bits & mask_a) && (bits & mask_b)) {
fn(i);
}
}
}
};
/* Example */
struct Transform {
int x, y;
int scale_x, scale_y;
};
struct Enemy {
int health, damage;
};
struct Sprite {
int id;
};
void transform_system(World& world) {
world.iterate_with<Transform>([&](Entity e) {
Transform& t = world.get_component<Transform>(e);
printf("Updating transform for %d\n", e);
printf("The position is: %d, %d\n", t.x, t.y);
});
}
void enemy_system(World& world) {
world.iterate_with<Transform, Enemy>([&](Entity e) {
Transform& t = world.get_component<Transform>(e);
Enemy& enemy = world.get_component<Enemy>(e);
printf("Updating enemy for %d\n", e);
printf("The health is: %d\n", enemy.health);
});
}
void sprite_system(World& world) {
world.iterate_with<Transform, Sprite>([&](Entity e) {
Transform& t = world.get_component<Transform>(e);
Sprite& sprite = world.get_component<Sprite>(e);
printf("Updating sprite for %d\n", e);
printf("The id is: %d\n", sprite.id);
});
}
int main() {
World world;
world.register_component_type<Transform>();
world.register_component_type<Enemy>();
world.register_component_type<Sprite>();
Entity a = world.create_entity();
world.add_component(a, Transform { 25, 25, 100, 100 });
world.add_component(a, Enemy { 10, 5 });
world.add_component(a, Sprite { 0 });
Entity b = world.create_entity();
world.add_component(b, Transform { 1, 3, 50, 4 });
world.add_component(b, Enemy { 5, 20 });
/* The game loop. */
while (true) {
enemy_system(world);
transform_system(world);
sprite_system(world);
}
world.deinit();
}
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