use 3*targetDuration for safe live point instead of 3 segments

src/playlist.js

@@ -219,6 +219,33 @@ 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 3 times target duration.
+ *
+ * @param {Object} playlist
+ *        a media playlist object;
+ * @return {Number}
+ *         The media index of the segment
+ * @function safeLiveIndex
+ */
+export const safeLiveIndex = function(playlist) {
+  const safeDistance = playlist.targetDuration * Playlist.UNSAFE_LIVE_SEGMENTS;
+
+  let i = playlist.segments.length;
+  let distanceFromEnd = 0;
+
+  while (i--) {
+    distanceFromEnd += playlist.segments[i].duration;
+
+    if (distanceFromEnd >= safeDistance) {
+      break;
+    }
+  }
+
+  return Math.max(0, i);
+};
+
 /**
  * Calculates the playlist end time
  *
@@ -246,9 +273,7 @@ export const playlistEnd = function(playlist, expired, useSafeLiveEnd) {
 
   expired = expired || 0;
 
-  let endSequence = useSafeLiveEnd ?
-    Math.max(0, playlist.segments.length - Playlist.UNSAFE_LIVE_SEGMENTS) :
-    Math.max(0, playlist.segments.length);
+  let endSequence = useSafeLiveEnd ? safeLiveIndex(playlist) : playlist.segments.length;
 
   return intervalDuration(playlist,
                           playlist.mediaSequence + endSequence,
@@ -485,6 +510,7 @@ export const estimateSegmentRequestTime = function(segmentDuration,
 
 Playlist.duration = duration;
 Playlist.seekable = seekable;
+Playlist.safeLiveIndex = safeLiveIndex;
 Playlist.getMediaInfoForTime = getMediaInfoForTime;
 Playlist.isEnabled = isEnabled;
 Playlist.isBlacklisted = isBlacklisted;

test/playlist.test.js

@@ -385,11 +385,92 @@ function(assert) {
 
   assert.equal(seekable.start(0), 0, 'starts at the earliest available segment');
   assert.equal(seekable.end(0),
-              9 - (2 + 2 + 1),
-              'allows seeking no further than three segments from the end');
+              9 - (2 + 2 + 1 + 2),
+              'allows seeking no further than three times target duration from the end');
   assert.equal(playlistEnd, 9, 'playlist end at the last segment');
 });
 
+QUnit.test('safeLiveIndex returns the correct media index for the safe live point',
+function(assert) {
+  const playlist = {
+    targetDuration: 6,
+    mediaSequence: 10,
+    syncInfo: {
+      time: 0,
+      mediaSequence: 10
+    },
+    segments: [
+      {
+        duration: 6
+      },
+      {
+        duration: 6
+      },
+      {
+        duration: 6
+      },
+      {
+        duration: 6
+      },
+      {
+        duration: 6
+      },
+      {
+        duration: 6
+      }
+    ]
+  };
+
+  let expected = 3;
+  let actual = Playlist.safeLiveIndex(playlist);
+
+  assert.equal(actual, expected, 'correct media index for standard durations');
+
+  playlist.segments = [
+    {
+      duration: 6
+    },
+    {
+      duration: 4
+    },
+    {
+      duration: 5
+    },
+    {
+      duration: 6
+    },
+    {
+      duration: 3
+    },
+    {
+      duration: 4
+    },
+    {
+      duration: 3
+    }
+  ];
+
+  expected = 2;
+  actual = Playlist.safeLiveIndex(playlist);
+  assert.equal(actual, expected, 'correct media index for variable segment durations');
+
+  playlist.segments = [
+    {
+      duration: 6
+    },
+    {
+      duration: 6
+    },
+    {
+      duration: 3
+    }
+  ];
+  expected = 0;
+  actual = Playlist.safeLiveIndex(playlist);
+  assert.equal(actual, expected,
+    'returns media index 0 when playlist has no safe live point');
+});
+
 QUnit.test(
   'seekable end and playlist end account for non-zero starting VOD media sequence',
 function(assert) {

test/videojs-contrib-hls.test.js

@@ -421,6 +421,7 @@ QUnit.test('translates seekable by the starting time for live playlists', functi
   this.requests.shift().respond(200, null,
                                 '#EXTM3U\n' +
                                 '#EXT-X-MEDIA-SEQUENCE:15\n' +
+                                '#EXT-X-TARGETDURATION:10\n' +
                                 '#EXTINF:10,\n' +
                                 '0.ts\n' +
                                 '#EXTINF:10,\n' +