/// <summary>
/// heuristics implementation to select default variant
/// </summary>
///
void IVariantSelector.SelectVariant(HLSVariant previousVariant, HLSVariant heuristicSuggestedVariant, ref HLSVariant nextVariant, List<HLSVariant> availableVariants)
{
double avgBandwidth = _bwHistory.GetAverageBandwidth();
double latestBandwidth = _bwHistory.GetLatestBandwidth();
int i;
if (_lastSeekPlaylistPosition == _playback.CurrentPosition)
{
// it is the first segment after seek. use a ratio of current bandwidth to speed up start time.
avgBandwidth = (int)( (float)(avgBandwidth) * _bandwidthUsageRatioForFirstSegment );
}
// intialize with lowest bitrate playlist
nextVariant = availableVariants[0];
for (i = 0; i < availableVariants.Count; i++)
{
if (availableVariants[i].Bitrate >= _minBitrate)
{
nextVariant = availableVariants[i];
break;
}
}
// This implements the logic for selecting next variant to download.
// We go over the variants, in the order of the highest bitrate to lowest bitrate, and calculate the targtet buffer
// after _lookforwardDuration from current time, and select the first variant that meet these two condition:
// 1) the end buffer size after _lookforwardDuration is larger than _highHeuristicBufferThreshold
// 2) the low buffer (the smallest buffer size during the looking forward scan) is higher than _lowHeuristicBufferThreshold or current buffer size
for (i = availableVariants.Count - 1; i >= 0; i--)
{
if (availableVariants[i].Bitrate >= _minBitrate && availableVariants[i].Bitrate <= _maxBitrate)
{
TimeSpan targetBufferSize;
TimeSpan lowBuffer;
CalculateTargeBuffer(availableVariants[i], avgBandwidth, out targetBufferSize, out lowBuffer);
if ((lowBuffer > _lowHeuristicBufferThreshold || lowBuffer >= BufferLevel) && targetBufferSize > _highHeuristicBufferThreshold)
{
nextVariant = availableVariants[i];
break;
}
}
}
}
/// <summary>
/// calculate target buffer size for a given bandwidth and HLS variant.
/// </summary>
private void CalculateTargeBuffer(HLSVariant nextVariant, double bandwidth, out TimeSpan targetBuffer, out TimeSpan lowbuffer)
{
TimeSpan totalDuration = TimeSpan.FromTicks(0);
int streamIndex = _playback.CurrentPosition;
if (bandwidth == BandwidthHistory.UnknownBandwidth || bandwidth == 0.00)
{
targetBuffer = TimeSpan.FromTicks(0);
lowbuffer = TimeSpan.FromTicks(0);
return;
}
TimeSpan SegmentDefaultDuration = TimeSpan.FromTicks(0);
if (nextVariant.IsLoaded)
{
// if we cannot get the segment duration from stream, therefore we will use the target
// duration tag instead. The target duration tag is mandatory according to HLS standard;
// however, it is yet missing from some HLS sources. If target duration tag is missing,
// we print out a warning message, and then we attempt to use the last segment duration,
// and if that fails, we use a default duration _defaultTargetDuration (10 seconds). If
// the actual segments have a different duration, then this will result in wrong
// heuristics stream selection.
try
{
SegmentDefaultDuration = nextVariant.TargetDuration;
}
catch (HLSPlaylistException e)
{
HLSTrace.PrintException(e);
if (!_targetDurationWarningShown)
{
Debug.WriteLine("The mandatory EXT-X-TARGETDURATION is missing from the HLS playlist. \r\n" +
"The heuristics algorithm needs this tag for its buffer calculations. \r\n" +
"This may result in errors in heuristics calculations. \r\n");
_targetDurationWarningShown = true;
}
if (nextVariant.Streams != null && nextVariant.Streams.Count != 0)
SegmentDefaultDuration = nextVariant.Streams[nextVariant.Streams.Count - 1].Duration;
else
SegmentDefaultDuration = _defaultTargetDuration;
}
}
targetBuffer = BufferLevel;
lowbuffer = BufferLevel;
while (totalDuration < _lookforwardDuration)
{
double segmentSize = 0.00;
TimeSpan segmentPlaybackDuration = TimeSpan.FromTicks(0);
if (nextVariant.Streams == null || streamIndex >= nextVariant.Streams.Count)
{
segmentPlaybackDuration = SegmentDefaultDuration;
}
else
{
segmentPlaybackDuration = nextVariant.Streams[streamIndex].Duration;
segmentSize = nextVariant.Streams[streamIndex].Size;
streamIndex++;
}
// if duration is missing, use the default
if (segmentPlaybackDuration.TotalMilliseconds == 0.00)
segmentPlaybackDuration = _defaultTargetDuration;
// if segment size is not specified in playlist, use duration * bitrate
if (segmentSize == 0)
segmentSize = ((double)nextVariant.Bitrate * segmentPlaybackDuration.TotalSeconds) / 8.00;
TimeSpan segmentDownloadDuration = TimeSpan.FromSeconds((segmentSize * 8.00) / bandwidth);
targetBuffer += ( segmentPlaybackDuration - segmentDownloadDuration );
totalDuration += segmentPlaybackDuration;
if (targetBuffer < lowbuffer)
lowbuffer = targetBuffer;
}
}
