zecsy/zecsy.hpp

#pragma once
#include <concepts>
#include <cstdint>
#include <cstdlib>
#include <format>
#include <set>
#include <stdexcept>
#include <type_traits>
#include <unordered_map>
#include <vector>

namespace zecsy
{
    using entity_id = uint64_t;

    template<typename... T>
    concept Component = []
    {
        static_assert((std::is_default_constructible_v<T> && ...),
                      "Should have a default constructor");
        static_assert((std::is_trivially_copyable_v<T> && ...),
                      "Should be trivially copyable");
        static_assert((std::is_trivially_destructible_v<T> && ...),
                      "Should be trivially destructible");
        static_assert((std::is_standard_layout_v<T> && ...),
                      "Should have standard layout");
        return true;
    }();

    class world final
    {
    public:
        entity_id make_entity();
        void destroy_entity(entity_id e);
        bool is_alive(entity_id e) const;

        template<Component T>
        bool has(entity_id e) const;

        template<Component First, Component Second, Component... Rest>
        bool has(entity_id e) const;

        template<Component T>
        T& get(entity_id e);

        template<Component T>
        void set(entity_id e);

        template<Component T>
        void set(entity_id e, const T& comp);

        template<Component First, Component Second, Component... Rest>
        void set(entity_id e);

        template<Component First, Component Second, Component... Rest>
        void set(entity_id e, const First& comp0, const Second& comp1,
                 const Rest&... rest_comps);

        template<Component T>
        void remove(entity_id e);

        template<Component First, Component Second, Component... Rest>
        void remove(entity_id e);

        template<Component... T>
        void query(std::invocable<T&...> auto&& system);

    private:
        using comp_id = size_t;
        std::set<entity_id> alive_entities;
        std::unordered_map<entity_id, std::set<comp_id>> entity_to_comps;
        entity_id entity_counter = 0;

        struct component_pool
        {
            std::vector<uint8_t> data;
            std::vector<size_t> free_list; // Reusable indices
            std::unordered_map<entity_id, size_t> entity_to_index;
            std::unordered_map<size_t, entity_id> index_to_entity;
        };

        std::unordered_map<comp_id, component_pool> pools;

        template<typename T>
        static comp_id get_component_id()
        {
            static comp_id id = next_component_id++;
            return id;
        }

        static comp_id next_component_id;
    };

    inline world::comp_id world::next_component_id = 0;

    inline entity_id world::make_entity()
    {
        auto id = ++entity_counter;
        alive_entities.emplace(id);
        // entity_to_comps[id] = {};
        return id;
    }

    inline void world::destroy_entity(entity_id e)
    {
        alive_entities.erase(e);

        for(comp_id id: entity_to_comps[e])
        {
            auto& pool = pools[id];
            auto index = pool.entity_to_index[e];
            pool.entity_to_index.erase(e);
            pool.index_to_entity.erase(index);
            pool.free_list.emplace_back(index);
        }

        entity_to_comps.erase(e);
    }

    inline bool world::is_alive(entity_id e) const
    {
        return alive_entities.contains(e);
    }

    template<Component T>
    inline bool world::has(entity_id e) const
    {
        if(entity_to_comps.contains(e))
        {
            return entity_to_comps.at(e).contains(get_component_id<T>());
        }
        return false;
    }

    template<Component T>
    inline T& world::get(entity_id e)
    {
        auto id = get_component_id<T>();
        if(!has<T>(e))
        {
            throw std::runtime_error(
                std::format("Entity #{} doesn't have {}", e, typeid(T).name()));
        }

        auto& pool = pools.at(id);
        auto index = pool.entity_to_index.at(e);
        return *reinterpret_cast<T*>(&pool.data[index * sizeof(T)]);
    }

    template<Component T>
    inline void world::set(entity_id e, const T& comp)
    {
        auto id = get_component_id<T>();
        auto& pool = pools[id];

        size_t index;
        if(!pool.free_list.empty())
        {
            index = pool.free_list.back();
            pool.free_list.pop_back();
        }
        else
        {
            index = pool.data.size() / sizeof(T);
            pool.data.resize(pool.data.size() + sizeof(T));
        }

        new(&pool.data[index * sizeof(T)]) T(comp);
        pool.entity_to_index[e] = index;
        pool.index_to_entity[index] = e;

        entity_to_comps[e].emplace(id);
    }

    template<Component T>
    inline void world::set(entity_id e)
    {
        set(e, T{});
    }

    template<Component T>
    inline void world::remove(entity_id e)
    {
        auto id = get_component_id<T>();
        if(!has<T>(e))
        {
            return;
        }

        auto& pool = pools[id];
        auto index = pool.entity_to_index[e];

        pool.free_list.push_back(index);
        pool.entity_to_index.erase(e);
        pool.index_to_entity.erase(index);
        entity_to_comps[e].erase(id);
    }

    template<Component First, Component Second, Component... Rest>
    inline bool world::has(entity_id e) const
    {
        return has<First>(e) && has<Second>(e) && (has<Rest>(e) && ...);
    }

    template<Component First, Component Second, Component... Rest>
    inline void world::set(entity_id e)
    {
        set(e, First{});
        set(e, Second{});
        (set(e, Rest{}), ...);
    }

    template<Component First, Component Second, Component... Rest>
    inline void world::set(entity_id e, const First& comp0, const Second& comp1,
                           const Rest&... rest_comps)
    {
        set(e, comp0);
        set(e, comp1);
        (set(e, rest_comps), ...);
    }

    template<Component First, Component Second, Component... Rest>
    inline void world::remove(entity_id e)
    {
        remove<First>(e);
        remove<Second>(e);
        (remove<Rest>(e), ...);
    }

    template<Component... T>
    inline void world::query(std::invocable<T&...> auto&& system)
    {
        for(auto e: alive_entities)
        {
            if((has<T...>(e)))
            {
                system(get<T>(e)...);
            }
        }
    }
}; // namespace zecsy