ref.h

#pragma once
#include <atomic>
#include <cassert>
#include <compare>
#include <cstddef>
#include <cstdint>
#include <functional>
#include <stdexcept>
#include <type_traits>
#include <utility>
 
namespace asyncpp {
    template<typename T>
    concept RefCount = requires() {
        { T{std::declval<size_t>()} };
        { std::declval<T&>().fetch_increment() } -> std::convertible_to<size_t>;
        { std::declval<T&>().fetch_decrement() } -> std::convertible_to<size_t>;
        { std::declval<const T&>().count() } -> std::convertible_to<size_t>;
    };
 
    struct thread_safe_refcount {
        std::atomic<size_t> m_count;
        explicit constexpr thread_safe_refcount(size_t init_val = 0) noexcept : m_count{init_val} {}
        thread_safe_refcount(const thread_safe_refcount& other) = delete;
        thread_safe_refcount& operator=(const thread_safe_refcount& other) = delete;
 
        size_t fetch_increment() noexcept { return m_count.fetch_add(1, std::memory_order::acquire); }
        [[nodiscard]] size_t fetch_decrement() noexcept { return m_count.fetch_sub(1, std::memory_order::release); }
        [[nodiscard]] size_t count() const noexcept { return m_count.load(std::memory_order::relaxed); }
    };
    struct thread_unsafe_refcount {
        size_t m_count;
        explicit constexpr thread_unsafe_refcount(size_t init_val = 0) noexcept : m_count{init_val} {}
        thread_unsafe_refcount(const thread_unsafe_refcount& other) = delete;
        thread_unsafe_refcount& operator=(const thread_unsafe_refcount& other) = delete;
 
        size_t fetch_increment() noexcept { return m_count++; }
        [[nodiscard]] size_t fetch_decrement() noexcept { return m_count--; }
        [[nodiscard]] size_t count() const noexcept { return m_count; }
    };
 
    static_assert(RefCount<thread_safe_refcount>, "[INTERNAL] thread_safe_refcount does not satisfy RefCount");
    static_assert(RefCount<thread_unsafe_refcount>, "[INTERNAL] thread_unsafe_refcount does not satisfy RefCount");
 
    template<typename T, RefCount TCounter = thread_safe_refcount>
    class intrusive_refcount;
 
    template<typename T, RefCount TCounter>
    class intrusive_refcount {
    public:
        intrusive_refcount() = default;
 
    protected:
        ~intrusive_refcount() noexcept = default;
        size_t use_count() const noexcept { return m_refcount.count(); }
 
        void add_ref() const noexcept {
            static_assert(std::is_base_of_v<intrusive_refcount<T, TCounter>, T>,
                          "T needs to inherit intrusive_refcount<T>");
            m_refcount.fetch_increment();
        }
 
        void remove_ref() const noexcept {
            static_assert(std::is_nothrow_destructible_v<T>, "Destructor needs to be noexcept!");
            static_assert(std::is_base_of_v<intrusive_refcount<T, TCounter>, T>,
                          "T needs to inherit intrusive_refcount<T>");
            auto cnt = m_refcount.fetch_decrement();
            if (cnt == 1) delete static_cast<const T*>(this);
        }
 
    private:
        mutable TCounter m_refcount{0};
 
        friend inline void refcounted_add_ref(const intrusive_refcount<T, TCounter>* ptr) noexcept {
            if (ptr) ptr->add_ref();
        }
        friend inline void refcounted_remove_ref(const intrusive_refcount<T, TCounter>* ptr) noexcept {
            if (ptr) ptr->remove_ref();
        }
    };
 
    inline constexpr struct {
    } adopt_ref{};
 
    template<typename T>
    concept RefCountable = requires(T* obj) {
        { refcounted_add_ref(obj) };
        { refcounted_remove_ref(obj) };
    };
 
    template<typename T>
    class ref {
        T* m_ptr;
 
    public:
        constexpr ref() noexcept : m_ptr(nullptr) { static_assert(RefCountable<T>, "T needs to be refcountable"); }
        ref(T* ptr, decltype(adopt_ref)) noexcept : m_ptr{ptr} {
            static_assert(RefCountable<T>, "T needs to be refcountable");
        }
        // NOLINTNEXTLINE(google-explicit-constructor)
        ref(T* ptr) noexcept(noexcept(refcounted_add_ref(std::declval<T*>()))) : m_ptr{ptr} {
            static_assert(RefCountable<T>, "T needs to be refcountable");
            if (m_ptr) refcounted_add_ref(m_ptr);
        }
        ref(const ref& other) noexcept(noexcept(refcounted_add_ref(std::declval<T*>()))) : m_ptr{other.m_ptr} {
            if (m_ptr) refcounted_add_ref(m_ptr);
        }
        constexpr ref(ref&& other) noexcept : m_ptr{std::exchange(other.m_ptr, nullptr)} {}
        ref& operator=(const ref& other) noexcept(noexcept(refcounted_add_ref(std::declval<T*>())) &&
                                                  noexcept(refcounted_remove_ref(std::declval<T*>()))) {
            if (&other != this) reset(other.m_ptr);
            return *this;
        }
        //NOLINTNEXTLINE(hicpp-noexcept-move,performance-noexcept-move-constructor)
        ref& operator=(ref&& other) noexcept(noexcept(refcounted_remove_ref(std::declval<T*>()))) {
            if (m_ptr) refcounted_remove_ref(m_ptr);
            m_ptr = std::exchange(other.m_ptr, nullptr);
            return *this;
        }
        void reset(T* ptr) noexcept(noexcept(refcounted_add_ref(std::declval<T*>())) &&
                                    noexcept(refcounted_remove_ref(std::declval<T*>()))) {
            if (m_ptr) refcounted_remove_ref(m_ptr);
            m_ptr = ptr;
            if (m_ptr) refcounted_add_ref(m_ptr);
        }
        void reset(T* ptr, decltype(adopt_ref)) noexcept(noexcept(refcounted_remove_ref(std::declval<T*>()))) {
            if (m_ptr) refcounted_remove_ref(m_ptr);
            m_ptr = ptr;
        }
        void reset() noexcept(noexcept(refcounted_remove_ref(std::declval<T*>()))) { reset(nullptr, adopt_ref); }
        //NOLINTNEXTLINE(performance-noexcept-destructor)
        ~ref() noexcept(noexcept(refcounted_remove_ref(std::declval<T*>()))) { reset(); }
        constexpr T* operator->() const noexcept { return m_ptr; }
        constexpr T& operator*() const noexcept { return *m_ptr; }
        constexpr T* get() const noexcept { return m_ptr; }
        constexpr T* release() noexcept {
            auto ptr = m_ptr;
            m_ptr = nullptr;
            return ptr;
        }
        explicit constexpr operator bool() const noexcept { return m_ptr != nullptr; }
        constexpr bool operator!() const noexcept { return m_ptr == nullptr; }
    };
 
    template<RefCountable T, typename... Args>
    ref<T> make_ref(Args&&... args) {
        return ref<T>(new T(std::forward<Args>(args)...));
    }
 
    template<RefCountable T>
    class atomic_ref {
        friend struct std::hash<asyncpp::atomic_ref<T>>;
        template<RefCountable T2>
        friend constexpr auto operator<=>(const atomic_ref<T2>& lhs, const atomic_ref<T2>& rhs) noexcept {
            return (lhs.m_ptr.load(std::memory_order::relaxed) & ~atomic_ref<T2>::lock_mask) <=>
                   (rhs.m_ptr.load(std::memory_order::relaxed) & ~atomic_ref<T2>::lock_mask);
        }
        template<RefCountable T2>
        friend constexpr auto operator<=>(const atomic_ref<T2>& lhs, const T2* rhs) noexcept {
            return (lhs.m_ptr.load(std::memory_order::relaxed) & ~atomic_ref<T>::lock_mask) <=>
                   reinterpret_cast<uintptr_t>(rhs);
        }
        template<RefCountable T2>
        friend constexpr auto operator<=>(const T2* lhs, const atomic_ref<T2>& rhs) noexcept {
            return reinterpret_cast<uintptr_t>(lhs) <=>
                   (rhs.m_ptr.load(std::memory_order::relaxed) & ~atomic_ref<T>::lock_mask);
        }
        mutable std::atomic<uintptr_t> m_ptr;
 
        static constexpr uintptr_t lock_mask = uintptr_t{1} << (sizeof(uintptr_t) * 8 - 1);
 
        inline static void cpu_pause() noexcept {
#if defined(__i386) || defined(_M_IX86) || defined(_X86_) || defined(__amd64) || defined(_M_AMD64)
#ifdef _MSC_VER
            _mm_pause();
#else
            __builtin_ia32_pause();
#endif
#elif defined(__arm__) || defined(_ARM) || defined(_M_ARM) || defined(__arm)
#ifdef _MSC_VER
            __yield();
#else
            asm volatile("yield");
#endif
#elif defined(__riscv)
            asm volatile("pause");
#endif
        }
 
        uintptr_t lock() const noexcept {
            // Lock the current pointer value
            auto val = m_ptr.fetch_or(lock_mask, std::memory_order_acquire);
            while ((val & lock_mask) == lock_mask) {
                cpu_pause();
                val = m_ptr.fetch_or(lock_mask, std::memory_order_acquire);
            }
            return val;
        }
 
        void unlock_with(uintptr_t val) const noexcept {
            assert((val & lock_mask) == 0);
            [[maybe_unused]] auto res = m_ptr.exchange(val, std::memory_order_release);
            assert((res & lock_mask) == lock_mask);
        }
 
    public:
        constexpr atomic_ref() noexcept : m_ptr(0) {}
        // NOLINTNEXTLINE(google-explicit-constructor)
        atomic_ref(ref<T> ptr) {
            if ((reinterpret_cast<uintptr_t>(ptr.get()) & lock_mask) != 0) throw std::logic_error("invalid pointer");
            m_ptr = reinterpret_cast<uintptr_t>(ptr.release());
        }
        atomic_ref& operator=(const ref<T>& other) {
            exchange(other);
            return *this;
        }
        atomic_ref& operator=(ref<T>&& other) {
            exchange(ref<T>(other.release(), adopt_ref));
            return *this;
        }
        atomic_ref&
        operator=(const atomic_ref<T>& other) noexcept(noexcept(refcounted_add_ref(std::declval<T*>())) &&
                                                       noexcept(refcounted_remove_ref(std::declval<T*>()))) {
            if (&other != this) exchange(other.load());
            return *this;
        }
        //NOLINTNEXTLINE(hicpp-noexcept-move,performance-noexcept-move-constructor)
        atomic_ref& operator=(atomic_ref<T>&& other) noexcept(noexcept(refcounted_remove_ref(std::declval<T*>()))) {
            exchange(ref<T>(other.release(), adopt_ref));
            return *this;
        }
        //NOLINTNEXTLINE(performance-noexcept-destructor)
        ~atomic_ref() noexcept(noexcept(refcounted_remove_ref(std::declval<T*>()))) { exchange(ref<T>()); }
        ref<T> load() const noexcept(noexcept(refcounted_add_ref(std::declval<T*>()))) {
            // Early out if nullptr
            if ((m_ptr.load(std::memory_order_relaxed) & ~lock_mask) == 0) return 0;
 
            // Lock the pointer to prevent concurrent modification
            auto val = lock();
            // Get original pointer
            auto ptr = reinterpret_cast<T*>(val);
            // Add reference
            if (ptr) refcounted_add_ref(ptr);
            // Unlock again
            unlock_with(val);
            return ref<T>(ptr, adopt_ref);
        }
        void store(ref<T> hdl) { exchange(std::move(hdl)); }
        void store(const atomic_ref<T>& hdl) { exchange(hdl.load()); }
        ref<T> exchange(ref<T> hdl) {
            auto ptr = hdl.release();
            if ((reinterpret_cast<uintptr_t>(ptr) & lock_mask) != 0) throw std::logic_error("invalid pointer");
 
            // Lock the current pointer value
            auto val = lock();
            // Unlock again with new value
            unlock_with(reinterpret_cast<uintptr_t>(ptr));
            // Return the old pointer without incrementing the reference count
            return ref<T>(reinterpret_cast<T*>(val), adopt_ref);
        }
        ref<T> exchange(const atomic_ref<T>& hdl) { return exchange(hdl.load()); }
        T* release() noexcept { return exchange(ref<T>()).release(); }
        void reset() noexcept(noexcept(refcounted_remove_ref(std::declval<T*>()))) { exchange(ref<T>()); }
        //NOLINTNEXTLINE(google-explicit-constructor)
        operator bool() const noexcept { return (m_ptr.load(std::memory_order_relaxed) & ~lock_mask) != 0; }
        bool operator!() const noexcept { return (m_ptr.load(std::memory_order_relaxed) & ~lock_mask) == 0; }
        ref<T> operator->() const noexcept(noexcept(refcounted_add_ref(std::declval<T*>()))) { return load(); }
    };
 
    template<typename T>
    inline constexpr auto operator<=>(const ref<T>& lhs, const ref<T>& rhs) noexcept {
        return lhs.get() <=> rhs.get();
    }
    template<typename T>
    inline constexpr auto operator<=>(const ref<T>& lhs, const T* rhs) noexcept {
        return lhs.get() <=> rhs;
    }
    template<typename T>
    inline constexpr auto operator<=>(const T* lhs, const ref<T>& rhs) noexcept {
        return lhs <=> rhs.get();
    }
 
    template<typename T>
    inline constexpr auto operator==(const ref<T>& lhs, const ref<T>& rhs) noexcept {
        return (lhs <=> rhs) == std::strong_ordering::equal;
    }
    template<typename T>
    inline constexpr auto operator==(const ref<T>& lhs, const T* rhs) noexcept {
        return (lhs <=> rhs) == std::strong_ordering::equal;
    }
    template<typename T>
    inline constexpr auto operator==(const T* lhs, const ref<T>& rhs) noexcept {
        return (lhs <=> rhs) == std::strong_ordering::equal;
    }
    template<typename T>
    inline constexpr auto operator!=(const ref<T>& lhs, const ref<T>& rhs) noexcept {
        return (lhs <=> rhs) != std::strong_ordering::equal;
    }
    template<typename T>
    inline constexpr auto operator!=(const ref<T>& lhs, const T* rhs) noexcept {
        return (lhs <=> rhs) != std::strong_ordering::equal;
    }
    template<typename T>
    inline constexpr auto operator!=(const T* lhs, const ref<T>& rhs) noexcept {
        return (lhs <=> rhs) != std::strong_ordering::equal;
    }
 
    template<typename T>
    inline constexpr auto operator==(const atomic_ref<T>& lhs, const atomic_ref<T>& rhs) noexcept {
        return (lhs <=> rhs) == std::strong_ordering::equal;
    }
    template<typename T>
    inline constexpr auto operator==(const atomic_ref<T>& lhs, const T* rhs) noexcept {
        return (lhs <=> rhs) == std::strong_ordering::equal;
    }
    template<typename T>
    inline constexpr auto operator==(const T* lhs, const atomic_ref<T>& rhs) noexcept {
        return (lhs <=> rhs) == std::strong_ordering::equal;
    }
    template<typename T>
    inline constexpr auto operator!=(const atomic_ref<T>& lhs, const atomic_ref<T>& rhs) noexcept {
        return (lhs <=> rhs) != std::strong_ordering::equal;
    }
    template<typename T>
    inline constexpr auto operator!=(const atomic_ref<T>& lhs, const T* rhs) noexcept {
        return (lhs <=> rhs) != std::strong_ordering::equal;
    }
    template<typename T>
    inline constexpr auto operator!=(const T* lhs, const atomic_ref<T>& rhs) noexcept {
        return (lhs <=> rhs) != std::strong_ordering::equal;
    }
 
    template<typename T, typename U>
    ref<T> static_ref_cast(const ref<U>& rhs) noexcept {
        return ref<T>(static_cast<T*>(rhs.get()));
    }
    template<typename T, typename U>
    ref<T> static_ref_cast(ref<U>&& rhs) noexcept {
        return ref<T>(static_cast<T*>(rhs.release()), adopt_ref);
    }
    template<typename T, typename U>
    ref<T> const_ref_cast(const ref<U>& rhs) noexcept {
        return ref<T>(const_cast<T*>(rhs.get()));
    }
    template<typename T, typename U>
    ref<T> const_ref_cast(ref<U>&& rhs) noexcept {
        return ref<T>(const_cast<T*>(rhs.release()), adopt_ref);
    }
    template<typename T, typename U>
    ref<T> dynamic_ref_cast(const ref<U>& rhs) noexcept {
        return ref<T>(dynamic_cast<T*>(rhs.get()));
    }
    template<typename T, typename U>
    ref<T> dynamic_ref_cast(ref<U>&& rhs) noexcept {
        auto ptr = dynamic_cast<T*>(rhs.get());
        if (!ptr) return ref<T>();
        // We already hold the correct pointer using get(),
        // so only clear it without removing the ref
        rhs.release();
        return ref<T>(ptr, adopt_ref);
    }
 
} // namespace asyncpp
 
template<typename T>
//NOLINTNEXTLINE(cert-dcl58-cpp)
struct std::hash<asyncpp::ref<T>> {
    constexpr size_t operator()(const asyncpp::ref<T>& rhs) const noexcept { return std::hash<void*>{}(rhs.get()); }
};
 
template<typename T>
//NOLINTNEXTLINE(cert-dcl58-cpp)
struct std::hash<asyncpp::atomic_ref<T>> {
    constexpr size_t operator()(const asyncpp::atomic_ref<T>& rhs) const noexcept {
        auto ptr = rhs.m_ptr.load(std::memory_order::relaxed) & ~asyncpp::atomic_ref<T>::lock_mask;
        return std::hash<uintptr_t>{}(ptr);
    }
};