Merge pull request #6502 from caiiiyua:bug_default_dash_chunk_source_wrong_segmentnumshift

PiperOrigin-RevId: 273549830

library/dash/src/main/java/com/google/android/exoplayer2/source/dash/DefaultDashChunkSource.java

@@ -686,7 +686,9 @@ public class DefaultDashChunkSource implements DashChunkSource {
             newPeriodDurationUs, newRepresentation, extractorWrapper, segmentNumShift, newIndex);
             newPeriodDurationUs, newRepresentation, extractorWrapper, segmentNumShift, newIndex);
       }
       }
 
 
-      long oldIndexLastSegmentNum = oldIndex.getFirstSegmentNum() + oldIndexSegmentCount - 1;
+      long oldIndexFirstSegmentNum = oldIndex.getFirstSegmentNum();
+      long oldIndexStartTimeUs = oldIndex.getTimeUs(oldIndexFirstSegmentNum);
+      long oldIndexLastSegmentNum = oldIndexFirstSegmentNum + oldIndexSegmentCount - 1;
       long oldIndexEndTimeUs =
       long oldIndexEndTimeUs =
           oldIndex.getTimeUs(oldIndexLastSegmentNum)
           oldIndex.getTimeUs(oldIndexLastSegmentNum)
               + oldIndex.getDurationUs(oldIndexLastSegmentNum, newPeriodDurationUs);
               + oldIndex.getDurationUs(oldIndexLastSegmentNum, newPeriodDurationUs);
@@ -700,8 +702,14 @@ public class DefaultDashChunkSource implements DashChunkSource {
         // There's a gap between the old index and the new one which means we've slipped behind the
         // There's a gap between the old index and the new one which means we've slipped behind the
         // live window and can't proceed.
         // live window and can't proceed.
         throw new BehindLiveWindowException();
         throw new BehindLiveWindowException();
+      } else if (newIndexStartTimeUs < oldIndexStartTimeUs) {
+        // The new index overlaps with (but does not have a start position contained within) the old
+        // index. This can only happen if extra segments have been added to the start of the index.
+        newSegmentNumShift -=
+            newIndex.getSegmentNum(oldIndexStartTimeUs, newPeriodDurationUs)
+                - oldIndexFirstSegmentNum;
       } else {
       } else {
-        // The new index overlaps with the old one.
+        // The new index overlaps with (and has a start position contained within) the old index.
         newSegmentNumShift +=
         newSegmentNumShift +=
             oldIndex.getSegmentNum(newIndexStartTimeUs, newPeriodDurationUs)
             oldIndex.getSegmentNum(newIndexStartTimeUs, newPeriodDurationUs)
                 - newIndexFirstSegmentNum;
                 - newIndexFirstSegmentNum;