#include "source/common/event/libevent_scheduler.h"

#include "source/common/common/assert.h"

#include "source/common/event/schedulable_cb_impl.h"

#include "source/common/event/timer_impl.h"

#include "event2/util.h"

namespace Envoy {

namespace Event {

namespace {

void recordTimeval(Stats::Histogram& histogram, const timeval& tv) {

  histogram.recordValue(tv.tv_sec * 1000000 + tv.tv_usec);

}

} // namespace

LibeventScheduler::LibeventScheduler() {

#ifdef WIN32

  event_config* event_config = event_config_new();

  RELEASE_ASSERT(event_config != nullptr,

                 "Failed to initialize libevent event_base: event_config_new");

  // Request wepoll backend by avoiding win32 backend.

  int error = event_config_avoid_method(event_config, "win32");

  RELEASE_ASSERT(error == 0, "Failed to initialize libevent event_base: event_config_avoid_method");

  event_base* event_base = event_base_new_with_config(event_config);

  event_config_free(event_config);

#else

  event_base* event_base = event_base_new();

#endif

  RELEASE_ASSERT(event_base != nullptr, "Failed to initialize libevent event_base");

  libevent_ = Libevent::BasePtr(event_base);

  // The dispatcher won't work as expected if libevent hasn't been configured to use threads.

  RELEASE_ASSERT(Libevent::Global::initialized(), "");

}

TimerPtr LibeventScheduler::createTimer(const TimerCb& cb, Dispatcher& dispatcher) {

  return std::make_unique<TimerImpl>(libevent_, cb, dispatcher);

};

SchedulableCallbackPtr

LibeventScheduler::createSchedulableCallback(const std::function<void()>& cb) {

  return std::make_unique<SchedulableCallbackImpl>(libevent_, cb);

};

void LibeventScheduler::run(Dispatcher::RunType mode) {

  int flag = 0;

  switch (mode) {

  case Dispatcher::RunType::NonBlock:

    flag = LibeventScheduler::flagsBasedOnEventType();

  case Dispatcher::RunType::Block:

    // The default flags have 'block' behavior. See

    // http://www.wangafu.net/~nickm/libevent-book/Ref3_eventloop.html

    break;

  case Dispatcher::RunType::RunUntilExit:

    flag = EVLOOP_NO_EXIT_ON_EMPTY;

    break;

  }

  event_base_loop(libevent_.get(), flag);

}

void LibeventScheduler::loopExit() { event_base_loopexit(libevent_.get(), nullptr); }

void LibeventScheduler::registerOnPrepareCallback(OnPrepareCallback&& callback) {

  ASSERT(callback);

  ASSERT(!callback_);

  callback_ = std::move(callback);

  evwatch_prepare_new(libevent_.get(), &onPrepareForCallback, this);

}

void LibeventScheduler::initializeStats(DispatcherStats* stats) {

  stats_ = stats;

  // These are thread safe.

  evwatch_prepare_new(libevent_.get(), &onPrepareForStats, this);

  evwatch_check_new(libevent_.get(), &onCheckForStats, this);

}

void LibeventScheduler::onPrepareForCallback(evwatch*, const evwatch_prepare_cb_info*, void* arg) {

  // `self` is `this`, passed in from evwatch_prepare_new.

  auto self = static_cast<LibeventScheduler*>(arg);

  self->callback_();

}

void LibeventScheduler::onPrepareForStats(evwatch*, const evwatch_prepare_cb_info* info,

                                          void* arg) {

  // `self` is `this`, passed in from evwatch_prepare_new.

  auto self = static_cast<LibeventScheduler*>(arg);

  // Record poll timeout and prepare time for this iteration of the event loop. The timeout is the

  // expected polling duration, whereas the actual polling duration will be the difference measured

  // between the prepare time and the check time immediately after polling. These are compared in

  // onCheckForStats to compute the poll_delay stat.

  self->timeout_set_ = evwatch_prepare_get_timeout(info, &self->timeout_);

  evutil_gettimeofday(&self->prepare_time_, nullptr);

  // If we have a check time available from a previous iteration of the event loop (that is, all but

  // the first), compute the loop_duration stat.

  if (self->check_time_.tv_sec != 0) {

    timeval delta;

    evutil_timersub(&self->prepare_time_, &self->check_time_, &delta);

    recordTimeval(self->stats_->loop_duration_us_, delta);

  }

}

void LibeventScheduler::onCheckForStats(evwatch*, const evwatch_check_cb_info*, void* arg) {

  // `self` is `this`, passed in from evwatch_check_new.

  auto self = static_cast<LibeventScheduler*>(arg);

  // Record check time for this iteration of the event loop. Use this together with prepare time

  // from above to compute the actual polling duration, and store it for the next iteration of the

  // event loop to compute the loop duration.

  evutil_gettimeofday(&self->check_time_, nullptr);

  if (self->timeout_set_) {

    timeval delta, delay;

    evutil_timersub(&self->check_time_, &self->prepare_time_, &delta);

    evutil_timersub(&delta, &self->timeout_, &delay);

    // Delay can be negative, meaning polling completed early. This happens in normal operation,

    // either because I/O was ready before we hit the timeout, or just because the kernel was

    // feeling saucy. Disregard negative delays in stats, since they don't indicate anything

    // particularly useful.

    if (delay.tv_sec >= 0) {

      recordTimeval(self->stats_->poll_delay_us_, delay);

    }

  }

}

} // namespace Event

} // namespace Envoy