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#include <memory>
#include <type_traits>

template <auto V>
struct nontype_t {
    explicit nontype_t() = default;
};

template <auto V>
constexpr nontype_t<V> nontype{};

namespace detail {
template <typename>
struct fn_types;

template <typename R, typename... ARG_Ts>
struct fn_types<R(ARG_Ts...)> {
    using type = R(ARG_Ts...);
};

template <typename R, typename... ARG_Ts>
struct fn_types<R(ARG_Ts...) const> {
    using type = R(ARG_Ts...);
};

template <typename R, typename... ARG_Ts>
struct fn_types<R(ARG_Ts...) noexcept> {
    using type = R(ARG_Ts...);
};

template <typename R, typename... ARG_Ts>
struct fn_types<R(ARG_Ts...) const noexcept> {
    using type = R(ARG_Ts...);
};

template <typename SIG_T>
using fn_types_t = typename fn_types<SIG_T>::type;

static_assert(std::is_same_v<fn_types_t<void(int)>, void(int)>);
static_assert(std::is_same_v<fn_types_t<int(void) noexcept>, int()>);
static_assert(std::is_same_v<fn_types_t<int(bool) const noexcept>, int(bool)>);
static_assert(std::is_same_v<fn_types_t<char(int) const throw()>, char(int)>);
}  // namespace detail

namespace detail {
template <typename>
struct is_nothrow_fn;

template <typename R, typename... ARG_Ts>
struct is_nothrow_fn<R(ARG_Ts...)> : std::false_type {};

template <typename R, typename... ARG_Ts>
struct is_nothrow_fn<R(ARG_Ts...) const> : std::false_type {};

template <typename R, typename... ARG_Ts>
struct is_nothrow_fn<R(ARG_Ts...) noexcept> : std::true_type {};

template <typename R, typename... ARG_Ts>
struct is_nothrow_fn<R(ARG_Ts...) const noexcept> : std::true_type {};

template <typename SIG_T>
inline constexpr bool is_nothrow_fn_v = is_nothrow_fn<SIG_T>::value;

static_assert(!is_nothrow_fn_v<int()>);
static_assert(!is_nothrow_fn_v<char(int) const>);
static_assert(is_nothrow_fn_v<char(int) noexcept>);
static_assert(is_nothrow_fn_v<bool() const noexcept>);
}  // namespace detail

namespace detail {
template <typename, bool, typename...>
struct is_invocable_using;

template <typename R, typename... ARG_Ts, typename... T>
struct is_invocable_using<R(ARG_Ts...), true, T...>
    : std::is_nothrow_invocable_r<R, T..., ARG_Ts...> {};

template <typename R, typename... ARG_Ts, typename... T>
struct is_invocable_using<R(ARG_Ts...), false, T...>
    : std::is_invocable_r<R, T..., ARG_Ts...> {};

template <typename SIG_T, typename... T>
inline constexpr bool is_invocable_using_v =
    is_invocable_using<fn_types_t<SIG_T>, is_nothrow_fn_v<SIG_T>, T...>::value;
}  // namespace detail

namespace detail {
template <typename>
struct is_const_fn;

template <typename R, typename... ARG_Ts>
struct is_const_fn<R(ARG_Ts...)> : std::false_type {};

template <typename R, typename... ARG_Ts>
struct is_const_fn<R(ARG_Ts...) const> : std::true_type {};

template <typename R, typename... ARG_Ts>
struct is_const_fn<R(ARG_Ts...) noexcept> : std::false_type {};

template <typename R, typename... ARG_Ts>
struct is_const_fn<R(ARG_Ts...) const noexcept> : std::true_type {};

template <typename SIG_T>
inline constexpr bool is_const_fn_v = is_const_fn<SIG_T>::value;

static_assert(!is_const_fn_v<int()>);
static_assert(is_const_fn_v<char(int) const>);
static_assert(!is_const_fn_v<char(int) noexcept>);
static_assert(is_const_fn_v<bool() const noexcept>);
}  // namespace detail

namespace detail {

template <typename SIG_T, bool is_const = is_const_fn_v<SIG_T>>
struct cv_qualifier {
    template <typename T>
    using cv = const T;
};

template <typename SIG_T>
struct cv_qualifier<SIG_T, false> {
    template <typename T>
    using cv = T;
};

static_assert(std::is_same_v<cv_qualifier<void(int)>::cv<int>, int>);
static_assert(std::is_same_v<cv_qualifier<int(void) noexcept>::cv<char>, char>);
static_assert(std::is_same_v<cv_qualifier<int(bool) const noexcept>::cv<short>,
                             const short>);
static_assert(
    std::is_same_v<cv_qualifier<char(int) const>::cv<bool>, const bool>);
}  // namespace detail

namespace detail {
union bound_entity_type {
    void* obj_ptr{nullptr};
    const void* const_obj_ptr;
    void (*fn_ptr)();

constexpr bound_entity_type() noexcept = default;

template <typename T>
        requires std::is_object_v<T>
    constexpr explicit bound_entity_type(T* ptr) noexcept : obj_ptr(ptr) {}

template <typename T>
        requires std::is_object_v<T>
    constexpr explicit bound_entity_type(const T* ptr) noexcept
        : const_obj_ptr(ptr) {}

template <class T>
        requires std::is_function_v<T>
    constexpr explicit bound_entity_type(T* ptr) noexcept
        : fn_ptr(reinterpret_cast<decltype(fn_ptr)>(ptr)) {}
};

static_assert(std::is_trivially_copyable_v<bound_entity_type>);

template <typename T>
constexpr static auto get(bound_entity_type entity) {
    if constexpr (std::is_const_v<T>) {
        return static_cast<T*>(entity.const_obj_ptr);
    } else if constexpr (std::is_object_v<T>) {
        return static_cast<T*>(entity.obj_ptr);
    } else {
        return reinterpret_cast<T*>(entity.fn_ptr);
    }
}
}  // namespace detail

#include <cassert>
#include <functional>

template <typename SIG_T, typename = detail::fn_types_t<SIG_T>>
class function_ref;

template <typename SIG_T, typename R, typename... ARG_Ts>
class function_ref<SIG_T, R(ARG_Ts...)> {
    template <typename... T>
    static constexpr bool is_invocable_using =
        detail::is_invocable_using_v<SIG_T, T...>;

using cv_qualifier = typename detail::cv_qualifier<SIG_T>;

template <class T>
    using cv = typename cv_qualifier::template cv<T>;

static constexpr bool noex = detail::is_nothrow_fn_v<SIG_T>;

using bound_entity_t = detail::bound_entity_type;
    using thunk_ptr_t = R (*)(bound_entity_t, ARG_Ts...) noexcept(noex);

public:
    template <typename F>
        requires(std::is_function_v<F> && is_invocable_using<F>)
    function_ref(F* f) noexcept
        : m_bound_entity{f},
          m_thunk_ptr{[](bound_entity_t entity,
                         ARG_Ts... args) noexcept(noex) -> R {
              if constexpr (std::is_void_v<R>) {
                  std::invoke(get<F>(entity), std::forward<ARG_Ts>(args)...);
              } else {
                  return std::invoke(get<F>(entity),
                                     std::forward<ARG_Ts>(args)...);
              }
          }} {
        assert(f != nullptr);
    }

template <typename F, typename T = std::remove_reference_t<F>>
        requires(!std::is_same_v<std::remove_cvref_t<F>, function_ref> &&
                 !std::is_member_pointer_v<T> && is_invocable_using<cv<T>&>)
    constexpr function_ref(F&& f) noexcept
        : m_bound_entity{std::addressof(f)},
          m_thunk_ptr{
              [](bound_entity_t entity, ARG_Ts... args) noexcept(noex) -> R {
                  cv<T>& obj = *get<T>(entity);
                  if constexpr (std::is_void_v<R>) {
                      std::invoke(obj, std::forward<ARG_Ts>(args)...);
                  } else {
                      return std::invoke(obj, std::forward<ARG_Ts>(args)...);
                  }
              }} {}

template <auto f>
        requires(is_invocable_using<decltype(f)>)
    constexpr function_ref(nontype_t<f>) noexcept
        : m_bound_entity{},
          m_thunk_ptr{
              [](bound_entity_t entity, ARG_Ts... args) noexcept(noex) -> R {
                  if constexpr (std::is_void_v<R>) {
                      std::invoke(f, std::forward<ARG_Ts>(args)...);
                  } else {
                      return std::invoke(f, std::forward<ARG_Ts>(args)...);
                  }
              }} {
        using F = decltype(f);
        if constexpr (std::is_pointer_v<F> || std::is_member_pointer_v<F>) {
            static_assert(f != nullptr);
        }
    }

template <auto f, typename U, typename T = std::remove_reference_t<U>>
        requires(!std::is_rvalue_reference_v<U &&> &&
                 is_invocable_using<decltype(f), cv<T>&>)
    constexpr function_ref(nontype_t<f>, U&& obj) noexcept
        : m_bound_entity{std::addressof(obj)},
          m_thunk_ptr{
              [](bound_entity_t entity, ARG_Ts... args) noexcept(noex) -> R {
                  cv<T>& obj = *get<T>(entity);
                  if constexpr (std::is_void_v<R>) {
                      std::invoke(f, obj, std::forward<ARG_Ts>(args)...);
                  } else {
                      return std::invoke(f, obj, std::forward<ARG_Ts>(args)...);
                  }
              }} {
        using F = decltype(f);
        if constexpr (std::is_pointer_v<F> || std::is_member_pointer_v<F>) {
            static_assert(f != nullptr);
        }
    }

template <auto f, typename T>
        requires(is_invocable_using<decltype(f), cv<T>*>)
    constexpr function_ref(nontype_t<f>, cv<T>* obj) noexcept
        : m_bound_entity{obj},
          m_thunk_ptr{
              [](bound_entity_t entity, ARG_Ts... args) noexcept(noex) -> R {
                  cv<T>* obj = get<cv<T>>(entity);
                  if constexpr (std::is_void_v<R>) {
                      std::invoke(f, obj, std::forward<ARG_Ts>(args)...);
                  } else {
                      return std::invoke(f, obj, std::forward<ARG_Ts>(args)...);
                  }
              }} {
        using F = decltype(f);
        if constexpr (std::is_pointer_v<F> || std::is_member_pointer_v<F>) {
            static_assert(f != nullptr);
        }
        if constexpr (std::is_member_pointer_v<F>) {
            assert(obj != nullptr);
        }
    }

R operator()(ARG_Ts... args) const noexcept(noex) {
        return m_thunk_ptr(m_bound_entity, std::forward<ARG_Ts>(args)...);
    }

private:
    bound_entity_t m_bound_entity{};
    thunk_ptr_t m_thunk_ptr{nullptr};
};

struct functor {
    int operator()([[maybe_unused]] char) const { return 42; }
    void operator()([[maybe_unused]] bool) noexcept {}
    int operator()() const noexcept { return 17; }
};

bool free_function1([[maybe_unused]] char) { return false; }
int free_function2() noexcept { return 3; }
int free_function3(int val) noexcept { return val; }

struct data {
    const int i = 182;
};

int free_function4(data* ptr) noexcept { return ptr->i; }

struct static_function {
    static char fn([[maybe_unused]] int, [[maybe_unused]] int) { return 'g'; }
};

#include <concepts>

template <std::integral T>
T generic_fn() {
    return {42};
}

template <auto f>
constexpr int non_type_call(nontype_t<f>) {
    static_assert(std::is_same_v<decltype(f), int (*)()>);
    return f();
}

struct my_class {
    char fn1([[maybe_unused]] bool) { return '3'; }
    int fn2() const { return 17; }

my_class() = default;

my_class(const my_class&) = delete;
    my_class& operator=(const my_class&) = delete;

my_class(my_class&&) = delete;
    my_class& operator=(my_class&&) = delete;
};

int main() {
    {
        functor fun{};
        function_ref<int()> fnref{[]() { return 17; }};
        function_ref<int(char) const> cfnref{fun};
        function_ref<void(bool) noexcept> exfnref{fun};
        function_ref<int() const noexcept> cexfnref{fun};

assert((fnref() == 17));
        assert((cfnref('a') == 42));
        exfnref(false);
        assert((cexfnref() == 17));
    }

{
        const functor cfun{};
        function_ref<int(char) const> cfnref{cfun};
        function_ref<int() const noexcept> cexfnref{cfun};

assert((cfnref('a') == 42));
        assert((cexfnref() == 17));
    }

{
        function_ref<bool(char)> ff1{free_function1};
        function_ref<int() noexcept> ff2{free_function2};
        function_ref<char(int, int)> sfn{static_function::fn};

assert(!ff1('a'));
        assert(ff2() == 3);
        assert(sfn(1, 3) == 'g');
    }

{
        assert(non_type_call(nontype<generic_fn<int>>) == 42);

function_ref<int()> nfn{nontype<generic_fn<int>>};
        assert(nfn() == 42);
    }

{
        my_class obj{};
        assert(std::invoke(&my_class::fn1, obj, false) == '3');

const my_class c_obj{};
        assert(std::invoke(&my_class::fn2, c_obj) == 17);
    }
    {
        my_class obj{};
        function_ref<char(bool)> mfn1{nontype<&my_class::fn1>, obj};
        assert(mfn1(false) == '3');

const my_class c_obj{};
        function_ref<int() const> mfn2{nontype<&my_class::fn2>, c_obj};
        assert(mfn2() == 17);

const int val = 42;
        function_ref<int() noexcept> ffn{nontype<&free_function3>, val};
        assert(ffn() == val);
    }
    {
        data d{};
        function_ref<int()> pfn{nontype<&free_function4>, &d};
        assert(pfn() == 182);

const my_class c_obj{};
        function_ref<int() const> mfn{nontype<&my_class::fn2>, &c_obj};
        assert(mfn() == 17);
    }
}

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