VPAAMP-163: Latency is fluctuating during normal playback in Linear/SLE assets

priv_aamp.cpp

@@ -2723,11 +2723,48 @@ void PrivateInstanceAAMP::MonitorProgress(bool sync, bool beginningOfStream)
 			}
 			else
 			{
-				// For HLS Live, calculate latency based on live edge; round to nearest ms
-				latency = static_cast<long>(std::lround(end - reportFormattedCurrPos));
-				if(latency < 0)
-				{ // this should never happen!
-					AAMPLOG_ERR("HLS negative live latency = %ldms, end = %lfms, reportFormattedCurrPos = %lfms", latency, end, reportFormattedCurrPos);
+				if(mProgramDateTime > 0.0)
+				{
+					// For HLS Live with EXT-X-PROGRAM-DATE-TIME: calculate latency as the
+					// difference between the current wall-clock time and the absolute wall-clock
+					// time at the player's current playback position.
+					// mProgramDateTime is the playlist-window start in epoch seconds.
+					// position is the player's current position in raw ms (from stream start).
+					// culledSeconds*1000 is the window start in the same raw coordinate space.
+					// Adding the offset to mProgramDateTime gives the epoch time of the
+					// current playback position; subtracting from nowMs yields true latency.
+					// This avoids the sawtooth fluctuation caused by the live edge advancing
+					// in discrete segment-sized steps on each manifest refresh.
+					long long nowMs = aamp_GetCurrentTimeMS();
+					// playbackOffsetFromWindowStartMs: how far (in ms) the player is into
+					// the current playlist window. Both position and culledSeconds*1000 are
+					// in the same raw (unadjusted) coordinate space, so no offset skew.
+					// Clamp to >= 0 to guard against transient position-behind-start cases.
+					double playbackOffsetFromWindowStartMs = position - (culledSeconds * 1000.0);
+					if (playbackOffsetFromWindowStartMs < 0.0)
+					{
+						playbackOffsetFromWindowStartMs = 0.0;
+					}
+					long long pdtAtCurrentPosMs = static_cast<long long>(
+						(mProgramDateTime * 1000.0) + playbackOffsetFromWindowStartMs);
+					latency = static_cast<long>(nowMs - pdtAtCurrentPosMs);
+					AAMPLOG_INFO("HLS PDT latency = %ldms, nowMs = %lldms, mProgramDateTime = %.3fs, "
+						"position = %.3fms, culledSeconds = %.3fms, playbackOffsetFromWindowStartMs = %.3fms",
+						latency, nowMs, mProgramDateTime, position, culledSeconds*1000.0, playbackOffsetFromWindowStartMs);
+					if(latency < 0)
+					{ // this should never happen!
+						AAMPLOG_ERR("HLS PDT-based negative live latency = %ldms, nowMs = %lldms, pdtAtCurrentPosMs = %lldms", latency, nowMs, pdtAtCurrentPosMs);
+					}
+				}
+				else
+				{
+					// Fallback for HLS streams without EXT-X-PROGRAM-DATE-TIME:
+					// calculate latency based on live edge; round to nearest ms.
+					latency = static_cast<long>(std::lround(end - reportFormattedCurrPos));
+					if(latency < 0)
+					{ // this should never happen!
+						AAMPLOG_ERR("HLS negative live latency = %ldms, end = %lfms, reportFormattedCurrPos = %lfms", latency, end, reportFormattedCurrPos);
+					}
 				}
 			}
 			SetCurrentLatencyMs(latency);