Stop using segment position in HLS parser

As part of Period-flattening, I'm trying to reduce our dependence on
the "position" field of SegmentReference.  If it can be eliminated, we
can more easily concatenate Arrays of SegmentReferences without
modifying them.

The HLS parser uses positions to find previously-resolved segment
times when refreshing a live playlist.  What we're really doing is
using HLS media sequence numbers, since we use those to assign the
segment positions in the first place.

This change makes the use of media sequence numbers more explicit, and
adds a per-Stream map from media sequence number to segment start
time.

The only other users of segment positions are StreamingEngine and
various tests.

Issue #1339 (period-flattening)

Change-Id: If970ed2c8722ed5779a51349ca2e64208d78130d

lib/hls/hls_parser.js

@@ -288,7 +288,7 @@ shaka.hls.HlsParser = class {
 
     const segments = await this.createSegments_(
         streamInfo.verbatimMediaPlaylistUri, playlist, stream.type,
-        stream.mimeType);
+        stream.mimeType, streamInfo.mediaSequenceToStartTime);
 
     stream.segmentIndex.replace(segments);
 
@@ -1169,10 +1169,13 @@ shaka.hls.HlsParser = class {
       codecs = '';
     }
 
+    /** @type {!Map.<number, number>} */
+    const mediaSequenceToStartTime = new Map();
+
     let segments;
     try {
       segments = await this.createSegments_(verbatimMediaPlaylistUri,
-          playlist, type, mimeType);
+          playlist, type, mimeType, mediaSequenceToStartTime);
     } catch (error) {
       if (error.code == shaka.util.Error.Code.HLS_INTERNAL_SKIP_STREAM) {
         shaka.log.alwaysWarn('Skipping unsupported HLS stream',
@@ -1221,12 +1224,13 @@ shaka.hls.HlsParser = class {
     };
 
     return {
-      stream: stream,
-      drmInfos: drmInfos,
-      verbatimMediaPlaylistUri: verbatimMediaPlaylistUri,
-      absoluteMediaPlaylistUri: absoluteMediaPlaylistUri,
-      minTimestamp: minTimestamp,
+      stream,
+      drmInfos,
+      verbatimMediaPlaylistUri,
+      absoluteMediaPlaylistUri,
+      minTimestamp,
       maxTimestamp: lastEndTime,
+      mediaSequenceToStartTime,
     };
   }
 
@@ -1448,10 +1452,13 @@ shaka.hls.HlsParser = class {
    * @param {!shaka.hls.Playlist} playlist
    * @param {string} type
    * @param {string} mimeType
+   * @param {!Map.<number, number>} mediaSequenceToStartTime
    * @return {!Promise<!Array.<!shaka.media.SegmentReference>>}
    * @private
    */
-  async createSegments_(verbatimMediaPlaylistUri, playlist, type, mimeType) {
+  async createSegments_(
+      verbatimMediaPlaylistUri, playlist, type, mimeType,
+      mediaSequenceToStartTime) {
     /** @type {Array.<!shaka.hls.Segment>} */
     const hlsSegments = playlist.segments;
     /** @type {!Array.<!shaka.media.SegmentReference>} */
@@ -1464,7 +1471,8 @@ shaka.hls.HlsParser = class {
 
     const mediaSequenceTag = shaka.hls.Utils.getFirstTagWithName(playlist.tags,
         'EXT-X-MEDIA-SEQUENCE');
-    const startPosition = mediaSequenceTag ? Number(mediaSequenceTag.value) : 0;
+    const mediaSequenceNumber =
+        mediaSequenceTag ? Number(mediaSequenceTag.value) : 0;
 
     /** @type {?shaka.hls.Tag} */
     let mapTag = shaka.hls.Utils.getFirstTagWithName(hlsSegments[0].tags,
@@ -1477,20 +1485,22 @@ shaka.hls.HlsParser = class {
         initSegmentRef,
         /* previousReference= */ null,
         hlsSegments[0],
-        startPosition,
+        mediaSequenceNumber,
         /* startTime= */ 0,
         /* timestampOffset= */ 0);
 
     const firstStartTime = await this.getPlaylistStartTime_(
         verbatimMediaPlaylistUri, initSegmentRef, firstSegmentRef, type,
-        mimeType);
+        mimeType, mediaSequenceNumber);
     shaka.log.debug('First segment', firstSegmentUri.split('/').pop(),
         'starts at', firstStartTime);
     const enumerate = (it) => shaka.util.Iterables.enumerate(it);
     for (const {i, item} of enumerate(hlsSegments)) {
       const previousReference = references[references.length - 1];
       const startTime = (i == 0) ? firstStartTime : previousReference.endTime;
-      const position = startPosition + i;
+      const position = mediaSequenceNumber + i;
+
+      mediaSequenceToStartTime.set(position, startTime);
 
       mapTag = shaka.hls.Utils.getFirstTagWithName(item.tags, 'EXT-X-MAP');
       initSegmentRef = mapTag ?
@@ -1591,12 +1601,13 @@ shaka.hls.HlsParser = class {
    * @param {!shaka.media.SegmentReference} firstSegmentRef
    * @param {string} contentType
    * @param {string} mimeType
+   * @param {number} mediaSequenceNumber
    * @return {!Promise.<number>}
    * @throws {shaka.util.Error}
    * @private
    */
   async getPlaylistStartTime_(verbatimMediaPlaylistUri, initSegmentRef,
-      firstSegmentRef, contentType, mimeType) {
+      firstSegmentRef, contentType, mimeType, mediaSequenceNumber) {
     // For VOD and EVENT playlists, all variants must start at the same time, so
     // we can fetch the start time once and reuse for the others.
     // This is not guaranteed when updating a LIVE stream, we assume the first
@@ -1608,7 +1619,7 @@ shaka.hls.HlsParser = class {
           'Should only get start time from audio or video streams');
       this.playlistStartTime_ = await this.getStartTime_(
           verbatimMediaPlaylistUri, initSegmentRef, firstSegmentRef,
-          mimeType);
+          mimeType, mediaSequenceNumber);
       shaka.log.debug('Fetched start time for', contentType);
     } else {
       shaka.log.debug('Reusing cached start time for', contentType);
@@ -1624,11 +1635,13 @@ shaka.hls.HlsParser = class {
    * @param {shaka.media.InitSegmentReference} initSegmentRef
    * @param {!shaka.media.SegmentReference} segmentRef
    * @param {string} mimeType
+   * @param {number} mediaSequenceNumber
    * @return {!Promise.<number>}
    * @private
    */
   async getStartTime_(
-      verbatimMediaPlaylistUri, initSegmentRef, segmentRef, mimeType) {
+      verbatimMediaPlaylistUri, initSegmentRef, segmentRef, mimeType,
+      mediaSequenceNumber) {
     // If we are updating the manifest, we can usually skip fetching the segment
     // by examining the references we already have.  This won't be possible if
     // there was some kind of lag or delay updating the manifest on the server,
@@ -1638,12 +1651,12 @@ shaka.hls.HlsParser = class {
     if (this.manifest_) {
       const streamInfo =
           this.uriToStreamInfosMap_.get(verbatimMediaPlaylistUri);
-      const segmentIndex = streamInfo.stream.segmentIndex;
-      const reference = segmentIndex.get(segmentRef.position);
-      if (reference) {
+      const startTime = streamInfo.mediaSequenceToStartTime.get(
+          mediaSequenceNumber);
+      if (startTime != undefined) {
         // We found it!  Avoid fetching and parsing the segment.
         shaka.log.v1('Found segment start time in previous manifest');
-        return reference.startTime;
+        return startTime;
       }
 
       shaka.log.debug(
@@ -2220,7 +2233,8 @@ shaka.hls.HlsParser = class {
  *   verbatimMediaPlaylistUri: string,
  *   absoluteMediaPlaylistUri: string,
  *   minTimestamp: number,
- *   maxTimestamp: number
+ *   maxTimestamp: number,
+ *   mediaSequenceToStartTime: !Map.<number, number>
  * }}
  *
  * @description
@@ -2242,6 +2256,8 @@ shaka.hls.HlsParser = class {
  *   The minimum timestamp found in the stream.
  * @property {number} maxTimestamp
  *   The maximum timestamp found in the stream.
+ * @property {!Map.<number, number>} mediaSequenceToStartTime
+ *   A map of media sequence numbers to media start times.
  */
 shaka.hls.HlsParser.StreamInfo;