//===-- Alarm.h -------------------------------------------------*- C++ -*-===//
//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See https://llvm.org/LICENSE.txt for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
//
//===----------------------------------------------------------------------===//

#ifndef LLDB_HOST_ALARM_H
#define LLDB_HOST_ALARM_H

#include "lldb/Host/HostThread.h"
#include "lldb/lldb-types.h"
#include "llvm/Support/Chrono.h"

#include <condition_variable>
#include <mutex>

namespace lldb_private {

/// \class Alarm abstraction that enables scheduling a callback function after a
/// specified timeout. Creating an alarm for a callback returns a Handle that
/// can be used to restart or cancel the alarm.
class Alarm {
public:
  using Handle = uint64_t;
  using Callback = std::function<void()>;
  using TimePoint = llvm::sys::TimePoint<>;
  using Duration = std::chrono::milliseconds;

  Alarm(Duration timeout, bool run_callback_on_exit = false);
  ~Alarm();

  /// Create an alarm for the given callback. The alarm will expire and the
  /// callback will be called after the timeout.
  ///
  /// \returns
  ///   Handle which can be used to restart or cancel the alarm.
  Handle Create(Callback callback);

  /// Restart the alarm for the given Handle. The alarm will expire and the
  /// callback will be called after the timeout.
  ///
  /// \returns
  ///   True if the alarm was successfully restarted. False if there is no alarm
  ///   for the given Handle or the alarm already expired.
  bool Restart(Handle handle);

  /// Cancel the alarm for the given Handle. The alarm and its handle will be
  /// removed.
  ///
  /// \returns
  ///   True if the alarm was successfully canceled and the Handle removed.
  ///   False if there is no alarm for the given Handle or the alarm already
  ///   expired.
  bool Cancel(Handle handle);

  static constexpr Handle INVALID_HANDLE = 0;

private:
  /// Helper functions to start, stop and check the status of the alarm thread.
  /// @{
  void StartAlarmThread();
  void StopAlarmThread();
  bool AlarmThreadRunning();
  /// @}

  /// Return an unique, monotonically increasing handle.
  static Handle GetNextUniqueHandle();

  /// Helper to compute the next time the alarm thread needs to wake up.
  TimePoint GetNextExpiration() const;

  /// Alarm entry.
  struct Entry {
    Handle handle;
    Callback callback;
    TimePoint expiration;

    Entry(Callback callback, TimePoint expiration);
    bool operator==(const Entry &rhs) { return handle == rhs.handle; }
  };

  /// List of alarm entries.
  std::vector<Entry> m_entries;

  /// Timeout between when an alarm is created and when it fires.
  Duration m_timeout;

  /// The alarm thread.
  /// @{
  HostThread m_alarm_thread;
  lldb::thread_result_t AlarmThread();
  /// @}

  /// Synchronize access between the alarm thread and the main thread.
  std::mutex m_alarm_mutex;

  /// Condition variable used to wake up the alarm thread.
  std::condition_variable m_alarm_cv;

  /// Flag to signal the alarm thread that something changed and we need to
  /// recompute the next alarm.
  bool m_recompute_next_alarm = false;

  /// Flag to signal the alarm thread to exit.
  bool m_exit = false;

  /// Flag to signal we should run all callbacks on exit.
  bool m_run_callbacks_on_exit = false;
};

} // namespace lldb_private

#endif // LLDB_HOST_ALARM_H