include/ftl/details/function.h

/*
 * Copyright 2022 The Android Open Source Project
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

#pragma once

#include <array>
#include <cstddef>
#include <cstdint>
#include <cstring>
#include <type_traits>

namespace android::ftl::details {

// The maximum allowed value for the template argument `N` in
// `ftl::Function<F, N>`.
constexpr size_t kFunctionMaximumN = 14;

// Converts a member function pointer type `Ret(Class::*)(Args...)` to an equivalent non-member
// function type `Ret(Args...)`.

template <typename>
struct remove_member_function_pointer;

template <typename Class, typename Ret, typename... Args>
struct remove_member_function_pointer<Ret (Class::*)(Args...)> {
  using type = Ret(Args...);
};

template <typename Class, typename Ret, typename... Args>
struct remove_member_function_pointer<Ret (Class::*)(Args...) const> {
  using type = Ret(Args...);
};

template <auto MemberFunction>
using remove_member_function_pointer_t =
    typename remove_member_function_pointer<decltype(MemberFunction)>::type;

// Helper functions for binding to the supported targets.

template <typename Ret, typename... Args>
auto bind_opaque_no_op() -> Ret (*)(void*, Args...) {
  return [](void*, Args...) -> Ret {
    if constexpr (!std::is_void_v<Ret>) {
      return Ret{};
    }
  };
}

template <typename F, typename Ret, typename... Args>
auto bind_opaque_function_object(const F&) -> Ret (*)(void*, Args...) {
  return [](void* opaque, Args... args) -> Ret {
    return std::invoke(*static_cast<F*>(opaque), std::forward<Args>(args)...);
  };
}

template <auto MemberFunction, typename Class, typename Ret, typename... Args>
auto bind_member_function(Class* instance, Ret (*)(Args...) = nullptr) {
  return [instance](Args... args) -> Ret {
    return std::invoke(MemberFunction, instance, std::forward<Args>(args)...);
  };
}

template <auto FreeFunction, typename Ret, typename... Args>
auto bind_free_function(Ret (*)(Args...) = nullptr) {
  return [](Args... args) -> Ret { return std::invoke(FreeFunction, std::forward<Args>(args)...); };
}

// Traits class for the opaque storage used by Function.

template <std::size_t N>
struct function_opaque_storage {
  // The actual type used for the opaque storage. An `N` of zero specifies the minimum useful size,
  // which allows a lambda with zero or one capture args.
  using type = std::array<std::intptr_t, N + 1>;

  template <typename S>
  static constexpr bool require_trivially_copyable = std::is_trivially_copyable_v<S>;

  template <typename S>
  static constexpr bool require_trivially_destructible = std::is_trivially_destructible_v<S>;

  template <typename S>
  static constexpr bool require_will_fit_in_opaque_storage = sizeof(S) <= sizeof(type);

  template <typename S>
  static constexpr bool require_alignment_compatible =
      std::alignment_of_v<S> <= std::alignment_of_v<type>;

  // Copies `src` into the opaque storage, and returns that storage.
  template <typename S>
  static type opaque_copy(const S& src) {
    // TODO: Replace with C++20 concepts/constraints which can give more details.
    static_assert(require_trivially_copyable<S>,
                  "ftl::Function can only store lambdas that capture trivially copyable data.");
    static_assert(
        require_trivially_destructible<S>,
        "ftl::Function can only store lambdas that capture trivially destructible data.");
    static_assert(require_will_fit_in_opaque_storage<S>,
                  "ftl::Function has limited storage for lambda captured state. Maybe you need to "
                  "increase N?");
    static_assert(require_alignment_compatible<S>);

    type opaque;
    std::memcpy(opaque.data(), &src, sizeof(S));
    return opaque;
  }
};

// Traits class to help determine the template parameters to use for a ftl::Function, given a
// function object.

template <typename F, typename = decltype(&F::operator())>
struct function_traits {
  // The function type `F` with which to instantiate the `Function<F, N>` template.
  using type = remove_member_function_pointer_t<&F::operator()>;

  // The (minimum) size `N` with which to instantiate the `Function<F, N>` template.
  static constexpr std::size_t size =
      (std::max(sizeof(std::intptr_t), sizeof(F)) - 1) / sizeof(std::intptr_t);
};

}  // namespace android::ftl::details