media/capture/content/smooth_event_sampler.cc

// Copyright (c) 2015 The Chromium Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.

#include "media/capture/content/smooth_event_sampler.h"

#include <stdint.h>

#include <algorithm>

#include "base/trace_event/trace_event.h"

namespace media {

SmoothEventSampler::SmoothEventSampler(base::TimeDelta min_capture_period,
                                       int redundant_capture_goal)
    : redundant_capture_goal_(redundant_capture_goal),
      overdue_sample_count_(0),
      token_bucket_(base::TimeDelta::Max()) {
  SetMinCapturePeriod(min_capture_period);
}

void SmoothEventSampler::SetMinCapturePeriod(base::TimeDelta period) {
  DCHECK_GT(period, base::TimeDelta());
  min_capture_period_ = period;
  token_bucket_capacity_ = period + period / 2;
  token_bucket_ = std::min(token_bucket_capacity_, token_bucket_);
}

void SmoothEventSampler::ConsiderPresentationEvent(base::TimeTicks event_time) {
  DCHECK(!event_time.is_null());

  // Add tokens to the bucket based on advancement in time.  Then, re-bound the
  // number of tokens in the bucket.  Overflow occurs when there is too much
  // time between events (a common case), or when RecordSample() is not being
  // called often enough (a bug).  On the other hand, if RecordSample() is being
  // called too often (e.g., as a reaction to IsOverdueForSamplingAt()), the
  // bucket will underflow.
  if (!current_event_.is_null()) {
    if (current_event_ < event_time) {
      token_bucket_ += event_time - current_event_;
      if (token_bucket_ > token_bucket_capacity_)
        token_bucket_ = token_bucket_capacity_;
    }
    TRACE_COUNTER1("gpu.capture", "MirroringTokenBucketUsec",
                   std::max<int64_t>(0, token_bucket_.InMicroseconds()));
  }
  current_event_ = event_time;
}

bool SmoothEventSampler::ShouldSample() const {
  return token_bucket_ >= min_capture_period_;
}

void SmoothEventSampler::RecordSample() {
  token_bucket_ -= min_capture_period_;
  if (token_bucket_ < base::TimeDelta())
    token_bucket_ = base::TimeDelta();
  TRACE_COUNTER1("gpu.capture", "MirroringTokenBucketUsec",
                 std::max<int64_t>(0, token_bucket_.InMicroseconds()));

  if (HasUnrecordedEvent()) {
    last_sample_ = current_event_;
    overdue_sample_count_ = 0;
  } else {
    ++overdue_sample_count_;
  }
}

bool SmoothEventSampler::IsOverdueForSamplingAt(
    base::TimeTicks event_time) const {
  DCHECK(!event_time.is_null());

  if (!HasUnrecordedEvent() && overdue_sample_count_ >= redundant_capture_goal_)
    return false;  // Not dirty.

  if (last_sample_.is_null())
    return true;

  // If we're dirty but not yet old, then we've recently gotten updates, so we
  // won't request a sample just yet.
  base::TimeDelta dirty_interval = event_time - last_sample_;
  return dirty_interval >=
         base::TimeDelta::FromMilliseconds(OVERDUE_DIRTY_THRESHOLD_MILLIS);
}

bool SmoothEventSampler::HasUnrecordedEvent() const {
  return !current_event_.is_null() && current_event_ != last_sample_;
}

}  // namespace media