Align live playlist updates to end of last when there is no overlap

src/utils/discontinuities.ts

@@ -81,7 +81,7 @@ export function alignStream(
     // Try to align on sn so that we pick a better start fragment.
     // Do not perform this on playlists with delta updates as this is only to align levels on switch
     // and adjustSliding only adjusts fragments after skippedSegments.
-    adjustSliding(switchDetails, details);
+    adjustSliding(switchDetails, details, false);
   }
 }
 

src/utils/level-helper.ts

@@ -407,14 +407,29 @@ export function mapFragmentIntersection(
 export function adjustSliding(
   oldDetails: LevelDetails,
   newDetails: LevelDetails,
+  matchingStableVariantOrRendition: boolean = true,
 ): void {
   const delta =
     newDetails.startSN + newDetails.skippedSegments - oldDetails.startSN;
   const oldFragments = oldDetails.fragments;
-  if (delta < 0 || delta >= oldFragments.length) {
+  const advancedOrStable = delta >= 0;
+  let sliding = 0;
+  if (advancedOrStable && delta < oldFragments.length) {
+    sliding = oldFragments[delta].start;
+  } else if (advancedOrStable && matchingStableVariantOrRendition) {
+    // align new start with old end (updated playlist start sequence is past end sequence of last update)
+    sliding = oldDetails.edge;
+  } else if (
+    !newDetails.skippedSegments &&
+    newDetails.fragments[0].start === 0
+  ) {
+    // align new start with old (playlist switch has a sequence with no overlap and should not be used for alignment)
+    sliding = oldDetails.fragments[0].start;
+  } else {
+    // new details already has a sliding offset or has skipped segments
     return;
   }
-  addSliding(newDetails, oldFragments[delta].start);
+  addSliding(newDetails, sliding);
 }
 
 export function addSliding(details: LevelDetails, start: number) {

tests/unit/controller/level-helper.ts

@@ -138,20 +138,20 @@ describe('LevelHelper Tests', function () {
       expect(actual).to.deep.equal([10, 15, 20]);
     });
 
-    it('does not apply sliding if no common segments exist', function () {
+    it('applies minimal sliding when no common segments exist', function () {
       const oldPlaylist = generatePlaylist([1, 2, 3]);
       const newPlaylist = generatePlaylist([5, 6, 7]);
       adjustSliding(oldPlaylist, newPlaylist);
       const actual = newPlaylist.fragments.map((f) => f.start);
-      expect(actual).to.deep.equal([0, 5, 10]);
+      expect(actual).to.deep.equal([15, 20, 25]);
     });
 
-    it('does not apply sliding when segments meet but do not overlap', function () {
+    it('applies minimal sliding when segments meet but do not overlap', function () {
       const oldPlaylist = generatePlaylist([1, 2, 3]);
       const newPlaylist = generatePlaylist([4, 5, 6]);
       adjustSliding(oldPlaylist, newPlaylist);
       const actual = newPlaylist.fragments.map((f) => f.start);
-      expect(actual).to.deep.equal([0, 5, 10]);
+      expect(actual).to.deep.equal([15, 20, 25]);
     });
   });
 
@@ -164,23 +164,23 @@ describe('LevelHelper Tests', function () {
       expect(actual).to.deep.equal([5, 10, 15, 20]);
     });
 
-    it('does not change start times when there is no segment overlap', function () {
+    it('applies minimal sliding when there is no segment overlap', function () {
       const oldPlaylist = generatePlaylist([1, 2, 3]);
       const newPlaylist = generatePlaylist([5, 6, 7]);
       mergeDetails(oldPlaylist, newPlaylist);
       const actual = newPlaylist.fragments.map((f) => f.start);
-      expect(actual).to.deep.equal([0, 5, 10]);
+      expect(actual).to.deep.equal([15, 20, 25]);
     });
 
-    it('does not extrapolate if the new playlist starts before the old', function () {
+    it('matches start when the new playlist starts before the old', function () {
       const oldPlaylist = generatePlaylist([3, 4, 5]);
       oldPlaylist.fragments.forEach((f) => {
         f.start += 10;
       });
       const newPlaylist = generatePlaylist([1, 2, 3]);
       mergeDetails(oldPlaylist, newPlaylist);
       const actual = newPlaylist.fragments.map((f) => f.start);
-      expect(actual).to.deep.equal([0, 5, 10]);
+      expect(actual).to.deep.equal([10, 15, 20]);
     });
 
     it('merges delta playlist updates', function () {