// Copyright (c) 2025-2026 Vector 35 Inc

//

// Permission is hereby granted, free of charge, to any person obtaining a copy

// of this software and associated documentation files (the "Software"), to

// deal in the Software without restriction, including without limitation the

// rights to use, copy, modify, merge, publish, distribute, sublicense, and/or

// sell copies of the Software, and to permit persons to whom the Software is

// furnished to do so, subject to the following conditions:

//

// The above copyright notice and this permission notice shall be included in

// all copies or substantial portions of the Software.

//

// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR

// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,

// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE

// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER

// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING

// FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS

// IN THE SOFTWARE.

#pragma once

#include <type_traits>

#include <utility>

#include <functional>

namespace bn::base {

template <typename Sig>

class function_ref;

// A non-owning reference to a callable object, inspired by C++26's std::function_ref.

// If the callable needs to be stored or copied, use std::function instead.

template <typename R, typename... Args>

class function_ref<R(Args...)>

{

private:

union Storage

{

const void* object = nullptr;

R (*func)(Args...);

};

Storage m_storage;

R (*m_invoker)(const Storage&, Args...) = nullptr;

static R invoke_function(const Storage& storage, Args... args)

{

auto fn = storage.func;

return fn(std::forward<Args>(args)...);

}

template <typename T>

static R invoke_callable(const Storage& storage, Args... args)

{

return std::invoke(*static_cast<const T*>(storage.object), std::forward<Args>(args)...);

}

public:

function_ref() = delete;

// Constructor that accepts a function pointer

function_ref(R (*f)(Args...)) noexcept

: m_storage{.func = f}

, m_invoker(&invoke_function)

{

}

// Constructor that accepts any callable object that is not function_ref

template<typename F>

requires (!std::is_same_v<std::remove_cvref_t<F>, function_ref>) &&

std::is_invocable_r_v<R, F&, Args...>

function_ref(const F& f) noexcept

: m_storage{.object = &f}

, m_invoker(&invoke_callable<std::remove_cvref_t<F>>)

{

}

R operator()(Args... args) const

{

return m_invoker(m_storage, std::forward<Args>(args)...);

}

function_ref(const function_ref&) noexcept = default;

function_ref(function_ref&&) noexcept = default;

function_ref& operator=(const function_ref&) noexcept = default;

function_ref& operator=(function_ref&&) noexcept = default;

};

} // namespace bn::base