engine/include/ecs.h

#pragma once

#include <bitset>
#include <cassert>
#include <cstddef>
#include <cstdint>
#include <map>
#include <vector>
#include <set>

namespace engine {

class Scene;

using Entity = uint32_t; // ECS entity

constexpr size_t kMaxComponents = 10;

class IComponentArray {
   public:
    virtual ~IComponentArray() = default;
};

template <typename T>
class ComponentArray : public IComponentArray {
   public:
    void InsertData(Entity entity, const T& component)
    {
        if (component_array_.size() < entity + 1) {
            component_array_.resize(entity + 1);
        }
        // bounds checking here as not performance critical
        component_array_.at(entity) = component;
    }

    void DeleteData(Entity entity)
    {
        (void)entity; // TODO
    }

    T* GetData(Entity entity)
    {
        assert(entity < component_array_.size());
        return &component_array_[entity];
    }

   private:
    std::vector<T> component_array_{};
};

class System {
   public:
    System(Scene* scene, std::set<size_t> required_component_hashes);
    virtual ~System() {}
    System(const System&) = delete;
    System& operator=(const System&) = delete;

    virtual void OnUpdate(float ts) = 0;

    virtual void OnComponentInsert(Entity) {}
    virtual void OnComponentRemove(Entity) {}

    Scene* const scene_;

    std::bitset<kMaxComponents> signature_;

    // entities that contain the needed components
    std::set<Entity> entities_{};
};

} // namespace engine