chromium/base/allocator/dispatcher/initializer.h

// Copyright 2022 The Chromium Authors
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.

#ifndef BASE_ALLOCATOR_DISPATCHER_INITIALIZER_H_
#define BASE_ALLOCATOR_DISPATCHER_INITIALIZER_H_

#include "base/allocator/dispatcher/configuration.h"
#include "base/allocator/dispatcher/dispatcher.h"
#include "base/allocator/dispatcher/internal/tools.h"

#include <tuple>
#include <utility>

namespace base::allocator::dispatcher {
namespace internal {

// Filter the passed observers and perform initialization of the passed
// dispatcher.
template <size_t CurrentIndex,
          typename DispatcherType,
          typename CheckObserverPredicate,
          typename VerifiedObservers,
          typename UnverifiedObservers,
          size_t... IndicesToSelect>
inline void DoInitialize(DispatcherType& dispatcher,
                         CheckObserverPredicate check_observer,
                         const VerifiedObservers& verified_observers,
                         const UnverifiedObservers& unverified_observers,
                         std::index_sequence<IndicesToSelect...> indices) { … }

}  // namespace internal

// The result of concatenating two tuple-types.
TupleCat;

// Initializer collects mandatory and optional observers and initializes the
// passed Dispatcher with only the enabled observers.
//
// In some situations, presence of observers depends on runtime. i.e. command
// line parameters or CPU features. With 3 optional observers we already have 8
// different combinations. Initializer takes the job of dealing with all
// combinations from the user. It allows users to pass all observers (including
// nullptr for disabled optional observers) and initializes the Dispatcher with
// only the enabled observers.
//
// Since this process results in a combinatoric explosion, Initializer
// distinguishes between optional and mandatory observers. Mandatory observers
// are not included in the filtering process and must always be enabled (not
// nullptr).
//
// To allow the Initializer to track the number and exact type of observers, it
// is implemented as a templated class which holds information on the types in
// the std::tuples passed as template parameters. Therefore, whenever any type
// observer it set, the initializer changes its type to reflect this.
template <typename MandatoryObservers = std::tuple<>,
          typename OptionalObservers = std::tuple<>>
struct BASE_EXPORT Initializer {
  Initializer() = default;
  Initializer(MandatoryObservers mandatory_observers,
              OptionalObservers optional_observers)
      : … { … }

  // Set the mandatory observers. The number of observers that can be set is
  // limited by configuration::maximum_number_of_observers.
  template <typename... NewMandatoryObservers,
            std::enable_if_t<
                internal::LessEqual((sizeof...(NewMandatoryObservers) +
                                     std::tuple_size_v<OptionalObservers>),
                                    configuration::kMaximumNumberOfObservers),
                bool> = true>
  Initializer<std::tuple<NewMandatoryObservers*...>, OptionalObservers>
  SetMandatoryObservers(NewMandatoryObservers*... mandatory_observers) const { … }

  // Add mandatory observers. The number of observers that can be added is
  // limited by the current number of observers, see
  // configuration::maximum_number_of_observers.
  template <typename... AdditionalMandatoryObservers,
            std::enable_if_t<internal::LessEqual(
                                 std::tuple_size_v<MandatoryObservers> +
                                     sizeof...(AdditionalMandatoryObservers) +
                                     std::tuple_size_v<OptionalObservers>,
                                 configuration::kMaximumNumberOfObservers),
                             bool> = true>
  Initializer<TupleCat<MandatoryObservers,
                       std::tuple<AdditionalMandatoryObservers*...>>,
              OptionalObservers>
  AddMandatoryObservers(
      AdditionalMandatoryObservers*... additional_mandatory_observers) const { … }

  // Set the optional observers. The number of observers that can be set is
  // limited by configuration::maximum_number_of_optional_observers as well as
  // configuration::maximum_number_of_observers.
  template <
      typename... NewOptionalObservers,
      std::enable_if_t<
          internal::LessEqual(
              sizeof...(NewOptionalObservers),
              configuration::kMaximumNumberOfOptionalObservers) &&
              internal::LessEqual((sizeof...(NewOptionalObservers) +
                                   std::tuple_size_v<MandatoryObservers>),
                                  configuration::kMaximumNumberOfObservers),
          bool> = true>
  Initializer<MandatoryObservers, std::tuple<NewOptionalObservers*...>>
  SetOptionalObservers(NewOptionalObservers*... optional_observers) const { … }

  // Add optional observers. The number of observers that can be added is
  // limited by the current number of optional observers,
  // configuration::maximum_number_of_optional_observers as well as
  // configuration::maximum_number_of_observers.
  template <
      typename... AdditionalOptionalObservers,
      std::enable_if_t<
          internal::LessEqual(
              std::tuple_size_v<OptionalObservers> +
                  sizeof...(AdditionalOptionalObservers),
              configuration::kMaximumNumberOfOptionalObservers) &&
              internal::LessEqual((std::tuple_size_v<OptionalObservers> +
                                   sizeof...(AdditionalOptionalObservers) +
                                   std::tuple_size_v<MandatoryObservers>),
                                  configuration::kMaximumNumberOfObservers),
          bool> = true>
  Initializer<
      MandatoryObservers,
      TupleCat<OptionalObservers, std::tuple<AdditionalOptionalObservers*...>>>
  AddOptionalObservers(
      AdditionalOptionalObservers*... additional_optional_observers) const { … }

  // Perform the actual initialization on the passed dispatcher.
  // The dispatcher is passed as a template only to provide better testability.
  template <typename DispatcherType>
  void DoInitialize(DispatcherType& dispatcher) const { … }

  const MandatoryObservers& GetMandatoryObservers() const { … }

  const OptionalObservers& GetOptionalObservers() const { … }

 private:
  MandatoryObservers mandatory_observers_;
  OptionalObservers optional_observers_;
};

// Convenience function for creating an empty Initializer.
inline Initializer<> CreateInitializer() { … }

}  // namespace base::allocator::dispatcher

#endif  // BASE_ALLOCATOR_DISPATCHER_INITIALIZER_H_