fix: better time to first frame for live playlists (#1105)

src/playlist.js

@@ -213,53 +213,6 @@ export const sumDurations = function(playlist, startIndex, endIndex) {
   return durations;
 };
 
-/**
- * Determines the media index of the segment corresponding to the safe edge of the live
- * window which is the duration of the last segment plus 2 target durations from the end
- * of the playlist.
- *
- * A liveEdgePadding can be provided which will be used instead of calculating the safe live edge.
- * This corresponds to suggestedPresentationDelay in DASH manifests.
- *
- * @param {Object} playlist
- *        a media playlist object
- * @param {number} [liveEdgePadding]
- *        A number in seconds indicating how far from the end we want to be.
- *        If provided, this value is used instead of calculating the safe live index from the target durations.
- *        Corresponds to suggestedPresentationDelay in DASH manifests.
- * @return {number}
- *         The media index of the segment at the safe live point. 0 if there is no "safe"
- *         point.
- * @function safeLiveIndex
- */
-export const safeLiveIndex = function(playlist, liveEdgePadding) {
-  if (!playlist.segments.length) {
-    return 0;
-  }
-
-  let i = playlist.segments.length;
-  const lastSegmentDuration = playlist.segments[i - 1].duration || playlist.targetDuration;
-  const safeDistance = typeof liveEdgePadding === 'number' ?
-    liveEdgePadding :
-    lastSegmentDuration + playlist.targetDuration * 2;
-
-  if (safeDistance === 0) {
-    return i;
-  }
-
-  let distanceFromEnd = 0;
-
-  while (i--) {
-    distanceFromEnd += playlist.segments[i].duration;
-
-    if (distanceFromEnd >= safeDistance) {
-      break;
-    }
-  }
-
-  return Math.max(0, i);
-};
-
 /**
  * Calculates the playlist end time
  *
@@ -293,13 +246,27 @@ export const playlistEnd = function(playlist, expired, useSafeLiveEnd, liveEdgeP
 
   expired = expired || 0;
 
-  const endSequence = useSafeLiveEnd ? safeLiveIndex(playlist, liveEdgePadding) : playlist.segments.length;
-
-  return intervalDuration(
+  let lastSegmentTime = intervalDuration(
     playlist,
-    playlist.mediaSequence + endSequence,
+    playlist.mediaSequence + playlist.segments.length,
     expired
   );
+
+  // "The client SHALL choose which Media Segment to play first from the
+  // Media Playlist when playback starts.  If the EXT-X-ENDLIST tag is not
+  // present and the client intends to play the media normally, the client
+  // SHOULD NOT choose a segment which starts less than three target
+  // durations from the end of the Playlist file.  Doing so can trigger
+  // playback stalls."
+  // https://tools.ietf.org/html/draft-pantos-http-live-streaming-23#section-6.3.3
+  if (useSafeLiveEnd) {
+    liveEdgePadding = typeof liveEdgePadding === 'number' ? liveEdgePadding : playlist.targetDuration * 3;
+    lastSegmentTime -= liveEdgePadding;
+
+  }
+
+  // don't return a time less than zero
+  return Math.max(0, lastSegmentTime);
 };
 
 /**
@@ -545,7 +512,6 @@ export const isLowestEnabledRendition = (master, media) => {
 export default {
   duration,
   seekable,
-  safeLiveIndex,
   getMediaInfoForTime,
   isEnabled,
   isDisabled,

src/sync-controller.js

@@ -6,6 +6,12 @@ import {sumDurations} from './playlist';
 import videojs from 'video.js';
 import logger from './util/logger';
 
+const getSegmentIndex = (i, playlist, currentTime = 0) => {
+  const segments = playlist.segments;
+
+  return (playlist.endList || currentTime === 0) ? i : segments.length - (i + 1);
+};
+
 export const syncPointStrategies = [
   // Stategy "VOD": Handle the VOD-case where the sync-point is *always*
   //                the equivalence display-time 0 === segment-index 0
@@ -38,7 +44,8 @@ export const syncPointStrategies = [
       currentTime = currentTime || 0;
 
       for (let i = 0; i < segments.length; i++) {
-        const segment = segments[i];
+        const segmentIndex = getSegmentIndex(i, playlist, currentTime);
+        const segment = segments[segmentIndex];
         const datetimeMapping =
           syncController.timelineToDatetimeMappings[segment.timeline];
 
@@ -60,7 +67,7 @@ export const syncPointStrategies = [
           lastDistance = distance;
           syncPoint = {
             time: segmentStart,
-            segmentIndex: i
+            segmentIndex
           };
         }
       }
@@ -79,7 +86,8 @@ export const syncPointStrategies = [
       currentTime = currentTime || 0;
 
       for (let i = 0; i < segments.length; i++) {
-        const segment = segments[i];
+        const segmentIndex = getSegmentIndex(i, playlist, currentTime);
+        const segment = segments[segmentIndex];
 
         if (segment.timeline === currentTimeline &&
             typeof segment.start !== 'undefined') {
@@ -95,7 +103,7 @@ export const syncPointStrategies = [
             lastDistance = distance;
             syncPoint = {
               time: segment.start,
-              segmentIndex: i
+              segmentIndex
             };
           }