Pull out segment index updates from offsetStream

lib/hls/hls_parser.js

@@ -489,7 +489,11 @@ shaka.hls.HlsParser = class {
         // Now adjust timestamps back to begin at 0.
         const segmentN = segmentIndex.earliestReference();
         if (segmentN) {
-          this.offsetStream_(streamInfo, -segmentN.startTime);
+          const streamOffset = -segmentN.startTime;
+          // Modify all SegmentReferences equally.
+          streamInfo.stream.segmentIndex.offset(streamOffset);
+          // Update other parts of streamInfo the same way.
+          this.offsetStreamInfo_(streamInfo, streamOffset);
         }
       }
     }
@@ -531,22 +535,26 @@ shaka.hls.HlsParser = class {
     for (const streamInfo of this.uriToStreamInfosMap_.values()) {
       const segmentIndex = streamInfo.stream.segmentIndex;
       if (segmentIndex != null) {
+        // A segment's startTime should be based on its syncTime vs the lowest
+        // syncTime across all streams.  The earliest segment sync time from
+        // any stream will become presentation time 0.  If two streams start
+        // e.g. 6 seconds apart in syncTime, then their first segments will
+        // also start 6 seconds apart in presentation time.
+
         const segment0 = segmentIndex.earliestReference();
         if (segment0.syncTime == null) {
           shaka.log.alwaysError('Missing EXT-X-PROGRAM-DATE-TIME for stream',
               streamInfo.verbatimMediaPlaylistUri,
               'Expect AV sync issues!');
         } else {
-          // The first segment's target startTime should be based entirely on
-          // its syncTime.  The rest of the stream will be based on that
-          // starting point.  The earliest segment sync time from any stream
-          // will become presentation time 0.  If two streams start e.g. 6
-          // seconds apart in syncTime, then their first segments will also
-          // start 6 seconds apart in presentation time.
+          // Stream metadata are offset by a fixed amount based on the
+          // first segment.
           const segment0TargetTime = segment0.syncTime - lowestSyncTime;
           const streamOffset = segment0TargetTime - segment0.startTime;
 
-          this.offsetStream_(streamInfo, streamOffset);
+          // Modify all SegmentReferences equally.
+          streamInfo.stream.segmentIndex.offset(streamOffset);
+          this.offsetStreamInfo_(streamInfo, streamOffset);
         }
       }
     }
@@ -557,13 +565,13 @@ shaka.hls.HlsParser = class {
    * @param {number} offset
    * @private
    */
-  offsetStream_(streamInfo, offset) {
-    streamInfo.stream.segmentIndex.offset(offset);
-
+  offsetStreamInfo_(streamInfo, offset) {
+    // Adjust our accounting of the maximum timestamp.
     streamInfo.maxTimestamp += offset;
     goog.asserts.assert(streamInfo.maxTimestamp >= 0,
         'Negative maxTimestamp after adjustment!');
 
+    // Update our map from sequence number to start time.
     const mediaSequenceToStartTime =
         this.getMediaSequenceToStartTimeFor_(streamInfo);
     for (const [key, value] of mediaSequenceToStartTime) {