#pragma once
#include <tuple>
#include <type_traits>

namespace kpn {

// ── Primary template — not defined; only specialisations match ────────────────
template<typename F>
struct function_traits;

// Free function
template<typename R, typename... Args>
struct function_traits<R(Args...)> {
    using return_t = R;
    using args     = std::tuple<Args...>;
    static constexpr std::size_t arity = sizeof...(Args);
};

// Function pointer
template<typename R, typename... Args>
struct function_traits<R(*)(Args...)> : function_traits<R(Args...)> {};

// Member function pointer (const)
template<typename C, typename R, typename... Args>
struct function_traits<R(C::*)(Args...) const> : function_traits<R(Args...)> {};

// Member function pointer (non-const)
template<typename C, typename R, typename... Args>
struct function_traits<R(C::*)(Args...)> : function_traits<R(Args...)> {};

// Callable (lambda / std::function) — delegate to operator()
template<typename F>
struct function_traits : function_traits<decltype(&F::operator())> {};

// ── Helpers ───────────────────────────────────────────────────────────────────

template<typename F>
using return_t = typename function_traits<std::remove_cvref_t<F>>::return_t;

template<typename F>
using args_t = typename function_traits<std::remove_cvref_t<F>>::args;

template<typename F>
inline constexpr std::size_t arity_v = function_traits<std::remove_cvref_t<F>>::arity;

// ── Tuple detection ───────────────────────────────────────────────────────────

template<typename T>
struct is_tuple : std::false_type {};

template<typename... Ts>
struct is_tuple<std::tuple<Ts...>> : std::true_type {};

template<typename T>
inline constexpr bool is_tuple_v = is_tuple<T>::value;

// ── Normalise return type to always be a tuple ────────────────────────────────
// void        → std::tuple<>
// T (non-tup) → std::tuple<T>
// tuple<...>  → tuple<...>  (unchanged)

template<typename T>
struct normalise_return {
    using type = std::tuple<T>;
};

template<>
struct normalise_return<void> {
    using type = std::tuple<>;
};

template<typename... Ts>
struct normalise_return<std::tuple<Ts...>> {
    using type = std::tuple<Ts...>;
};

template<typename T>
using normalised_return_t = typename normalise_return<T>::type;

// ── Output count from a function type ────────────────────────────────────────

template<typename F>
inline constexpr std::size_t output_count_v =
    std::tuple_size_v<normalised_return_t<return_t<F>>>;

} // namespace kpn