aos/events/event_scheduler.h

#ifndef AOS_EVENTS_EVENT_SCHEDULER_H_
#define AOS_EVENTS_EVENT_SCHEDULER_H_

#include <algorithm>
#include <map>
#include <memory>
#include <unordered_set>
#include <utility>
#include <vector>

#include "aos/events/event_loop.h"
#include "aos/logging/implementations.h"
#include "aos/time/time.h"
#include "glog/logging.h"

namespace aos {

// This clock is the basis for distributed time.  It is used to synchronize time
// between multiple nodes.  This is a new type so conversions to and from the
// monotonic and realtime clocks aren't implicit.
class distributed_clock {
 public:
  typedef ::std::chrono::nanoseconds::rep rep;
  typedef ::std::chrono::nanoseconds::period period;
  typedef ::std::chrono::nanoseconds duration;
  typedef ::std::chrono::time_point<distributed_clock> time_point;

  // This clock is the base clock for the simulation and everything is synced to
  // it.  It never jumps.
  static constexpr bool is_steady = true;

  // Returns the epoch (0).
  static constexpr time_point epoch() { return time_point(zero()); }

  static constexpr duration zero() { return duration(0); }

  static constexpr time_point min_time{
      time_point(duration(::std::numeric_limits<duration::rep>::min()))};
  static constexpr time_point max_time{
      time_point(duration(::std::numeric_limits<duration::rep>::max()))};
};

std::ostream &operator<<(std::ostream &stream,
                         const aos::distributed_clock::time_point &now);

class EventSchedulerScheduler;

class EventScheduler {
 public:
  using ChannelType =
      std::multimap<monotonic_clock::time_point, std::function<void()>>;
  using Token = ChannelType::iterator;

  // Schedule an event with a callback function
  // Returns an iterator to the event
  Token Schedule(monotonic_clock::time_point time,
                 std::function<void()> callback);

  // Schedules a callback when the event scheduler starts.
  void ScheduleOnRun(std::function<void()> callback) {
    on_run_.emplace_back(std::move(callback));
  }

  Token InvalidToken() { return events_list_.end(); }

  // Deschedule an event by its iterator
  void Deschedule(Token token);

  // Runs the OnRun callbacks.
  void RunOnRun();

  // Returns true if events are being handled.
  inline bool is_running() const;

  // Returns the timestamp of the next event to trigger.
  aos::monotonic_clock::time_point OldestEvent();
  // Handles the next event.
  void CallOldestEvent();

  // Converts a time to the distributed clock for scheduling and cross-node time
  // measurement.
  distributed_clock::time_point ToDistributedClock(
      monotonic_clock::time_point time) const {
    return distributed_clock::epoch() +
           std::chrono::duration_cast<std::chrono::nanoseconds>(
               (time.time_since_epoch() - distributed_offset_) /
               distributed_slope_);
  }

  // Takes the distributed time and converts it to the monotonic clock for this
  // node.
  monotonic_clock::time_point FromDistributedClock(
      distributed_clock::time_point time) const {
    return monotonic_clock::epoch() +
           std::chrono::duration_cast<std::chrono::nanoseconds>(
               time.time_since_epoch() * distributed_slope_) +
           distributed_offset_;
  }

  // Returns the current monotonic time on this node calculated from the
  // distributed clock.
  inline monotonic_clock::time_point monotonic_now() const;

  // Sets the offset between the distributed and monotonic clock.
  //   monotonic = distributed * slope + offset;
  void SetDistributedOffset(std::chrono::nanoseconds distributed_offset,
                            double distributed_slope) {
    // TODO(austin): Use a starting point to improve precision.
    // TODO(austin): Make slope be the slope of the offset, not the input,
    // throught the calculation process.
    distributed_offset_ = distributed_offset;
    distributed_slope_ = distributed_slope;

    // Once we update the offset, now isn't going to be valid anymore.
    // TODO(austin): Probably should instead use the piecewise linear function
    // and evaluate it correctly.
    monotonic_now_valid_ = false;
  }

 private:
  friend class EventSchedulerScheduler;
  // Current execution time.
  bool monotonic_now_valid_ = false;
  monotonic_clock::time_point monotonic_now_ = monotonic_clock::epoch();

  // Offset to the distributed clock.
  //   distributed = monotonic + offset;
  std::chrono::nanoseconds distributed_offset_ = std::chrono::seconds(0);
  double distributed_slope_ = 1.0;

  // List of functions to run (once) when running.
  std::vector<std::function<void()>> on_run_;

  // Multimap holding times to run functions.  These are stored in order, and
  // the order is the callback tree.
  ChannelType events_list_;

  // Pointer to the actual scheduler.
  EventSchedulerScheduler *scheduler_scheduler_ = nullptr;
};

// We need a heap of heaps...
//
// Events in a node have a very well defined progression of time.  It is linear
// and well represented by the monotonic clock.
//
// Events across nodes don't follow this well.  Time skews between the two nodes
// all the time.  We also don't know the function ahead of time which converts
// from each node's monotonic clock to the distributed clock (our unified base
// time which is likely the average time between nodes).
//
// This pushes us towards merge sort.  Sorting each node's events with a heap
// like we used to be doing, and then sorting each of those nodes independently.
class EventSchedulerScheduler {
 public:
  // Adds an event scheduler to the list.
  void AddEventScheduler(EventScheduler *scheduler);

  // Runs until there are no more events or Exit is called.
  void Run();

  // Stops running.
  void Exit() { is_running_ = false; }

  bool is_running() const { return is_running_; }

  // Runs for a duration on the distributed clock.  Time on the distributed
  // clock should be very representative of time on each node, but won't be
  // exactly the same.
  void RunFor(distributed_clock::duration duration);

  // Returns the current distributed time.
  distributed_clock::time_point distributed_now() const { return now_; }

 private:
  // Handles running the OnRun functions.
  void RunOnRun() {
    CHECK(!is_running_);
    is_running_ = true;
    for (EventScheduler *scheduler : schedulers_) {
      scheduler->RunOnRun();
    }
  }

  // Returns the next event time and scheduler on which to run it.
  std::tuple<distributed_clock::time_point, EventScheduler *> OldestEvent();

  // True if we are running.
  bool is_running_ = false;
  // The current time.
  distributed_clock::time_point now_ = distributed_clock::epoch();
  // List of schedulers to run in sync.
  std::vector<EventScheduler *> schedulers_;
};

inline monotonic_clock::time_point EventScheduler::monotonic_now() const {
  // Make sure we stay in sync.
  if (monotonic_now_valid_) {
    // We want time to be smooth, so confirm that it doesn't change too much
    // while handling an event.
    //
    // There are 2 sources of error.  There are numerical precision and interger
    // rounding problems going from the monotonic clock to the distributed clock
    // and back again.  When we update the time function as well to transition
    // line segments, we have a slight jump as well.
    CHECK_NEAR(monotonic_now_,
               FromDistributedClock(scheduler_scheduler_->distributed_now()),
               std::chrono::nanoseconds(2));
    return monotonic_now_;
  } else {
    return FromDistributedClock(scheduler_scheduler_->distributed_now());
  }
}

inline bool EventScheduler::is_running() const {
  return scheduler_scheduler_->is_running();
}

}  // namespace aos

#endif  // AOS_EVENTS_EVENT_SCHEDULER_H_