#include "Mix/World.h"

// note: must have default constructor

struct PositionComponent
{
    PositionComponent(int x = 0, int y = 0) : x(x), y(y) {}
    int x, y;
};

struct VelocityComponent
{
    VelocityComponent(int dx = 0, int dy = 0) : dx(dx), dy(dy) {}
    int dx, dy;
};

class MoveSystem : public Mix::System
{
public:
    MoveSystem()
    {
        // which components an entity must have for the system to be interested
        requireComponent<PositionComponent>();
        requireComponent<VelocityComponent>();
    }

    void update()
    {
        // do stuff with all the entities of interest
        for (auto e : getEntities()) {
            auto &position = e.getComponent<PositionComponent>();
            const auto &velocity = e.getComponent<VelocityComponent>();
            position.x += velocity.x;
            position.y += velocity.y;
        }
    }
};

Mix::World world;
world.getSystemManager().addSystem<MoveSystem>();

auto e = world.createEntity();
e.addComponent<PositionComponent>(100, 100);
e.addComponent<VelocityComponent>(10, 10);
// entity is created on the next call to world.update(),
// so that no entities appear only for some systems mid-frame

// inside system's update method
entity.kill();
if (!entity.isAlive()) { ... };
// entity is alive until next call to world.update(),
// so that every system gets the chance handle the entity

while (!done) {
    world.update(); // actually creates and kills entities since the last call to this method
    world.getSystemManager().getSystem<MoveSystem>().update();
}

auto player = world.createEntity();
player.tag("player");
if (player.hasTag("player")) { ... };
auto entity = world.getEntity("player");

auto enemy = world.createEntity();
enemy.group("enemies");
if (enemy.hasGroup("enemies")) { ... };
auto enemies = world.getEntityGroup("enemies");

// note: must have default constructor

struct CollisionEvent
{
    PositionComponent(Entity a = Entity(), Entity b = Entity()) : a(a), b(b) {}
    Entity a, b;
};

// inside system's update method
Entity player, enemy;
getWorld().getEventManager().emitEvent<CollisionEvent>(player, enemy);

// inside other system's update method
auto collisionEvents = getWorld().getEventManager().getEvents<CollisionEvent>();
for (auto event : collisionEvents) { // handle collision };

// events can be received by any system
// events exist until the next call to world.update()