fix canplaythrough

media/blink/buffered_data_source_host_impl.cc

Index: media/blink/buffered_data_source_host_impl.cc
diff --git a/media/blink/buffered_data_source_host_impl.cc b/media/blink/buffered_data_source_host_impl.cc
index 30542adcf37416f2006fd8acb6dbf5eac57d4833..9faa99a390a132c2634627092fd139084926de27 100644
--- a/media/blink/buffered_data_source_host_impl.cc
+++ b/media/blink/buffered_data_source_host_impl.cc
@@ -8,9 +8,14 @@
 
 namespace media {
 
-BufferedDataSourceHostImpl::BufferedDataSourceHostImpl()
+const double kDownloadHistoryWindowSeconds = 10.0;
+const size_t kDownloadHistoryMaxEntries = 1024;
+
+BufferedDataSourceHostImpl::BufferedDataSourceHostImpl(
+    const base::Closure& progress_cb)
     : total_bytes_(0),
-      did_loading_progress_(false) { }
+      did_loading_progress_(false),
+      progress_cb_(progress_cb) {}
 
 BufferedDataSourceHostImpl::~BufferedDataSourceHostImpl() { }
 
@@ -18,15 +23,52 @@ void BufferedDataSourceHostImpl::SetTotalBytes(int64_t total_bytes) {
   total_bytes_ = total_bytes;
 }
 
+int64_t BufferedDataSourceHostImpl::UnloadedBytesInInterval(
+    const Interval<int64_t>& interval) const {
+  int64_t bytes = 0;
+  auto i = buffered_byte_ranges_.find(interval.begin);
+  while (i != buffered_byte_ranges_.end()) {
+    if (i.interval_begin() >= interval.end)
+      break;
+    if (!i.value()) {
+      Interval<int64_t> intersection = i.interval().Intersect(interval);
+      if (!intersection.Empty()) {
+        bytes += intersection.end - intersection.begin;
+      }
+    }
+    ++i;
+  }
+  return bytes;
+}
+
 void BufferedDataSourceHostImpl::AddBufferedByteRange(int64_t start,
                                                       int64_t end) {
-  const auto i = buffered_byte_ranges_.find(start);
-  if (i.value() && i.interval_end() >= end) {
+  int64_t new_bytes = UnloadedBytesInInterval(Interval<int64_t>(start, end));
+  if (new_bytes == 0) {
     // No change
     return;
   }
   buffered_byte_ranges_.SetInterval(start, end, 1);
   did_loading_progress_ = true;
+
+  base::TimeTicks now = base::TimeTicks::Now();
+  int64_t bytes_so_far = 0;
+  if (!download_history_.empty())
+    bytes_so_far = download_history_.back().second;
+  bytes_so_far += new_bytes;
+  if (download_history_.size() >= kDownloadHistoryMaxEntries) {
+    download_history_.back() = std::make_pair(now, bytes_so_far);

[chcunningham, 2017/04/06 16:55:41]

why not always push_back? iiuc this is overwriting the last entry, so we loose
some recent data that we'd like to include in the computation of download rate.
you pop of the front a few lines below, so it should be fine for the list to
temporarily grow beyond its max entries. 

[hubbe, 2017/04/06 21:25:38]

We don't loose any data, we just loose precision in time.
This data is like a staircase, and doing the overwriting just makes the steps
bigger.  (However, I now realize that this code will make the step twice as big,
but it cannot make the steps 3 times as big, or 4 times as big, should the need
arise....

+  } else {
+    download_history_.push_back(std::make_pair(now, bytes_so_far));
+  }
+  while (download_history_.back().first - download_history_.front().first >
+             base::TimeDelta::FromSecondsD(kDownloadHistoryWindowSeconds) ||
+         download_history_.size() >= kDownloadHistoryMaxEntries) {
+    download_history_.pop_front();
+  }
+  if (!progress_cb_.is_null()) {
+    progress_cb_.Run();
+  }
 }
 
 static base::TimeDelta TimeForByteOffset(int64_t byte_offset,
@@ -66,4 +108,52 @@ bool BufferedDataSourceHostImpl::DidLoadingProgress() {
   return ret;
 }
 
+double BufferedDataSourceHostImpl::DownloadRate() const {
+  if (download_history_.size() < 5)
+    return 0.0;
+
+  // The data we get is bursty, so we get multiple measuring points very close
+  // together. These bursts will often lead us to over-estimate the download
+  // rate. By iterating over the beginning of the time series and picking the
+  // data point that has the lowest download rate, we avoid over-estimating.
+  double download_rate = 1.0E20;

[mlamouri (slow - plz ping), 2017/04/06 11:32:56]

Why 1e20 and not a negative value instead?

[hubbe, 2017/04/06 21:25:38]

Because I use std::min() below.

[mlamouri (slow - plz ping), 2017/04/07 11:23:44]

Missed that. Can you use INT_MAX? `std::numeric_limits<double>::max()` to be
precise :)

+  for (int i = 0; i < std::min<int>(20, download_history_.size() - 3); i++) {

[chcunningham, 2017/04/06 16:55:41]

I don't follow the min(20, ...). If you only want the first 20 entries why keep
1024? Also, why bias so heavily toward the first 20? 

Also, why size() -3 ?

[hubbe, 2017/04/06 21:25:38]

Each entry contains the cumulative downloaded bytes, so last_entry - first_entry
is enough to know how much we've downloaded in the last ~10 seconds. The 20 is
simply to deal with bursty data. size() - 3 is to avoid estimating based on too
few data points.

+    int64_t downloaded_bytes =
+        download_history_.back().second - download_history_[i].second;
+    double downloaded_seconds =
+        (download_history_.back().first - download_history_[i].first)
+            .InSecondsF();
+    if (downloaded_seconds <= 0.0)
+      continue;
+    download_rate =
+        std::min(download_rate, downloaded_bytes / downloaded_seconds);
+  }
+  if (download_rate == 1.0E20)
+    return 0.0;
+  return download_rate;
+}
+
+bool BufferedDataSourceHostImpl::CanPlayThrough(
+    base::TimeDelta current_position,
+    base::TimeDelta media_duration,
+    double playback_rate) const {
+  if (!total_bytes_ || media_duration <= base::TimeDelta())
+    return false;
+  if (current_position > media_duration)
+    return true;
+  double fraction = current_position.InSecondsF() / media_duration.InSecondsF();
+  int64_t byte_pos = total_bytes_ * fraction;
+  if (byte_pos < 0)
+    byte_pos = 0;
+
+  double download_rate = DownloadRate();
+  if (download_rate == 0.0)
+    return false;
+
+  int64_t unloaded_bytes =
+      UnloadedBytesInInterval(Interval<int64_t>(byte_pos, total_bytes_));
+  return unloaded_bytes / download_rate <
+         (media_duration - current_position).InSecondsF() / playback_rate;
+}
+
 }  // namespace media