Don't assume media sequence starts at zero

Zero is the default when unspecified but it's legal to have a one-indexed VOD media playlist. When the media sequence is non-zero, assume that should be the basis for all sequence calculations unless a different basis was explicitly specified. Fix Playlist.seekable so that it doesn't attempt to estimate the duration of expired segments.

@@ -149,8 +149,12 @@
   intervalDuration = function(playlist, startSequence, endSequence, includeTrailingTime) {
     var result = 0, targetDuration, expiredSegmentCount;
 
-    startSequence = startSequence || 0;
-    endSequence = endSequence !== undefined ? endSequence : (playlist.segments || []).length;
+    if (startSequence === undefined) {
+      startSequence = playlist.mediaSequence || 0;
+    }
+    if (endSequence === undefined) {
+      endSequence = startSequence + (playlist.segments || []).length;
+    }
     targetDuration = playlist.targetDuration || DEFAULT_TARGET_DURATION;
 
     // estimate expired segment duration using the target duration
@@ -214,7 +218,11 @@
 
   /**
    * Calculates the interval of time that is currently seekable in a
-   * playlist.
+   * playlist. The returned time ranges are relative to the earliest
+   * moment in the specified playlist that is still available. A full
+   * seekable implementation for live streams would need to offset
+   * these values by the duration of content that has expired from the
+   * stream.
    * @param playlist {object} a media playlist object
    * @return {TimeRanges} the periods of time that are valid targets
    * for seeking
@@ -231,10 +239,10 @@
       return videojs.createTimeRange(0, duration(playlist));
     }
 
-    start = intervalDuration(playlist, 0, playlist.mediaSequence);
-    end = start + intervalDuration(playlist,
-                                   playlist.mediaSequence,
-                                   playlist.mediaSequence + playlist.segments.length);
+    start = 0;
+    end = intervalDuration(playlist,
+                           playlist.mediaSequence,
+                           playlist.mediaSequence + playlist.segments.length);
     targetDuration = playlist.targetDuration || DEFAULT_TARGET_DURATION;
 
     // live playlists should not expose three segment durations worth
@@ -249,9 +257,9 @@
       // from the result.
       for (i = playlist.segments.length - 1; i >= 0 && liveBuffer > 0; i--) {
         segment = playlist.segments[i];
-        pending = Math.min(segment.preciseDuration ||
-                           segment.duration ||
-                           targetDuration,
+        pending = Math.min(duration(playlist,
+                                    playlist.mediaSequence + i,
+                                    playlist.mediaSequence + i + 1),
                            liveBuffer);
         liveBuffer -= pending;
         end -= pending;

@@ -423,7 +423,7 @@ videojs.Hls.prototype.duration = function() {
 };
 
 videojs.Hls.prototype.seekable = function() {
-  var absoluteSeekable, startOffset, media;
+  var currentSeekable, startOffset, media;
 
   if (!this.playlists) {
     return videojs.createTimeRange();
@@ -435,10 +435,10 @@ videojs.Hls.prototype.seekable = function() {
 
   // report the seekable range relative to the earliest possible
   // position when the stream was first loaded
-  absoluteSeekable = videojs.Hls.Playlist.seekable(media);
+  currentSeekable = videojs.Hls.Playlist.seekable(media);
   startOffset = this.playlists.expiredPostDiscontinuity_ - this.playlists.expiredPreDiscontinuity_;
   return videojs.createTimeRange(startOffset,
-                                 startOffset + (absoluteSeekable.end(0) - absoluteSeekable.start(0)));
+                                 startOffset + (currentSeekable.end(0) - currentSeekable.start(0)));
 };
 
 /**

@@ -18,10 +18,9 @@
 
   module('Playlist Interval Duration');
 
-  test('accounts for media sequences', function() {
+  test('accounts expired duration for live playlists', function() {
     var duration = Playlist.duration({
       mediaSequence: 10,
-      endList: true,
       segments: [{
         duration: 10,
         uri: '10.ts'
@@ -40,6 +39,28 @@
     equal(duration, 14 * 10, 'duration includes dropped segments');
   });
 
+  test('accounts for non-zero starting VOD media sequences', function() {
+    var duration = Playlist.duration({
+      mediaSequence: 10,
+      endList: true,
+      segments: [{
+        duration: 10,
+        uri: '0.ts'
+      }, {
+        duration: 10,
+        uri: '1.ts'
+      }, {
+        duration: 10,
+        uri: '2.ts'
+      }, {
+        duration: 10,
+        uri: '3.ts'
+      }]
+    });
+
+    equal(duration, 4 * 10, 'includes only listed segments');
+  });
+
   test('uses PTS values when available', function() {
     var duration = Playlist.duration({
       mediaSequence: 0,
@@ -445,7 +466,7 @@
     equal(seekable.end(0), 7, 'ends three target durations from the last segment');
   });
 
-  test('adjusts seekable to the live playlist window', function() {
+  test('only considers available segments', function() {
     var seekable = Playlist.seekable({
       targetDuration: 10,
       mediaSequence: 7,
@@ -460,8 +481,8 @@
       }]
     });
     equal(seekable.length, 1, 'there are seekable ranges');
-    equal(seekable.start(0), 10 * 7, 'starts at the earliest available segment');
-    equal(seekable.end(0), 10 * 8, 'ends three target durations from the last available segment');
+    equal(seekable.start(0), 0, 'starts at the earliest available segment');
+    equal(seekable.end(0), 10, 'ends three target durations from the last available segment');
   });
 
   test('seekable end accounts for non-standard target durations', function() {