#pragma once

#include <memory>

#include "common/common/assert.h"

#include "absl/base/call_once.h"

namespace Envoy {

/**
 * ThreadSafeSingleton allows easy global cross-thread access to a non-const object.
 *
 * This singleton class should be used for singletons which must be globally
 * accessible but can not be marked as const. All functions in the singleton class
 * *must* be threadsafe.
 *
 * Note that there is heavy resistance in Envoy to adding this type of singleton
 * if data will persist with state changes across tests, as it becomes difficult
 * to write clean unit tests if a state change in one test will persist into
 * another test. Be wary of using it. A example of acceptable usage is OsSyscallsImpl,
 * where the functions are not strictly speaking const, but affect the OS rather than the
 * class itself. An example of unacceptable usage upstream would be for
 * globally accessible stat counters, it would have the aforementioned problem
 * where state "leaks" across tests.
 *
 * */
template <class T> class TestThreadsafeSingletonInjector;
template <class T> class ThreadSafeSingleton {
public:
  static T& get() {
    absl::call_once(ThreadSafeSingleton<T>::create_once_, &ThreadSafeSingleton<T>::Create);
    return *ThreadSafeSingleton<T>::instance_;
  }

protected:
  template <typename A> friend class TestThreadsafeSingletonInjector;

  static void Create() { instance_ = new T(); }

  static absl::once_flag create_once_;
  static T* instance_;
};

template <class T> absl::once_flag ThreadSafeSingleton<T>::create_once_;

template <class T> T* ThreadSafeSingleton<T>::instance_ = nullptr;

// An instance of a singleton class which has the same thread safety properties
// as ThreadSafeSingleton, but must be created via initialize prior to access.
//
// As with ThreadSafeSingleton the use of this class is generally discouraged.
template <class T> class InjectableSingleton {
public:
  static T& get() {
    RELEASE_ASSERT(loader_ != nullptr, "InjectableSingleton used prior to initialization");
    return *loader_;
  }

  static T* getExisting() { return loader_; }

  static void initialize(T* value) {
    RELEASE_ASSERT(value != nullptr, "InjectableSingleton initialized with non-null value.");
    RELEASE_ASSERT(loader_ == nullptr, "InjectableSingleton initialized multiple times.");
    loader_ = value;
  }
  static void clear() { loader_ = nullptr; }

protected:
  static T* loader_;
};

template <class T> T* InjectableSingleton<T>::loader_ = nullptr;

template <class T> class ScopedInjectableLoader {
public:
  ScopedInjectableLoader(std::unique_ptr<T>&& instance) {
    instance_ = std::move(instance);
    InjectableSingleton<T>::initialize(instance_.get());
  }
  ~ScopedInjectableLoader() { InjectableSingleton<T>::clear(); }

private:
  std::unique_ptr<T> instance_;
};

} // namespace Envoy