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#ifndef MOZC_BASE_SINGLETON_H_
#define MOZC_BASE_SINGLETON_H_

#include <optional>

#include "absl/base/call_once.h"

namespace mozc {

class SingletonFinalizer {
 public:
  typedef void (*FinalizerFunc)();

  // Do not call this method directly.
  // use Singleton<Typename> instead.
  static void AddFinalizer(FinalizerFunc func);

  // Call Finalize() if you want to finalize
  // all instances created by Sigleton.
  //
  // Mozc UI for Windows (DLL) can call
  // SigletonFinalizer::Finalize()
  // at an appropriate timing.
  static void Finalize();
};

// Thread-safe Singleton class.
// Usage:
//
// class Foo {}
//
// Foo *instance = mozc::Singleton<Foo>::get();
template <typename T>
class Singleton {
 public:
  static T *get() {
    absl::call_once(*once_, &Singleton<T>::Init);
    return instance_;
  }

  // TEST ONLY! Do not call this method in production code.
  static void Delete() {
    delete instance_;
    instance_ = nullptr;
    once_.emplace();  // Reconstruct absl::once_flag in place.
  }

 private:
  static void Init() {
    SingletonFinalizer::AddFinalizer(&Singleton<T>::Delete);
    instance_ = new T;
  }

  static std::optional<absl::once_flag> once_;
  static T *instance_;
};

template <typename T>
std::optional<absl::once_flag> Singleton<T>::once_(std::in_place);

template <typename T>
T *Singleton<T>::instance_ = nullptr;

// SingletonMockable class.
// Usage: (quote from clock.cc)
//
//   using ClockSingleton = SingletonMockable<ClockInterface, ClockImpl>;
//
//   uint64 Clock::GetTime() {
//     return ClockSingleton::Get()->GetTime();
//   }
//
//   void Clock::SetClockForUnitTest(ClockInterface *clock_mock) {
//    ClockSingleton::SetMock(clock_mock);
//   }
template <class Interface, class Impl>
class SingletonMockable {
 public:
  static Interface *Get() {
    if (mock_) {
      return mock_;
    }
    static Impl *impl = new Impl();
    return impl;
  }
  static void SetMock(Interface *mock) { mock_ = mock; }

 private:
  static Interface *mock_;
};

template <class Interface, class Impl>
Interface *SingletonMockable<Interface, Impl>::mock_ = nullptr;

}  // namespace mozc

#endif  // MOZC_BASE_SINGLETON_H_
