From 651e92335a5f90a4e914920ff74122c0a25e33e4 Mon Sep 17 00:00:00 2001
From: DualFroz <me@dualfroz.com>
Date: Sun, 13 Sep 2026 22:14:36 +0200
Subject: [PATCH] Use the chunk id to detect the first chunk of a segment

---
 src/remux/passthrough-remuxer.ts        |  7 +------
 tests/unit/remux/passthrough-remuxer.ts | 16 +++++++++++++---
 2 files changed, 14 insertions(+), 9 deletions(-)

diff --git a/src/remux/passthrough-remuxer.ts b/src/remux/passthrough-remuxer.ts
index c2f7a72..55eb5da 100644
--- a/src/remux/passthrough-remuxer.ts
+++ b/src/remux/passthrough-remuxer.ts
@@ -49,7 +49,6 @@ class PassThroughRemuxer extends Logger implements Remuxer {
   private initPTS: TimestampOffset | null = null;
   private initTracks?: TrackSet;
   private lastEndTime: number | null = null;
-  private lastChunkKey: string | null = null;
   private isVideoContiguous: boolean = false;
   private videoOnlyRemux: boolean = false;
   private decryptdata: DecryptData | null = null;
@@ -75,7 +74,6 @@ class PassThroughRemuxer extends Logger implements Remuxer {
 
   public resetTimeStamp(defaultInitPTS: TimestampOffset | null) {
     this.lastEndTime = null;
-    this.lastChunkKey = null;
     const initPTS = this.initPTS;
     if (initPTS && defaultInitPTS) {
       if (
@@ -91,7 +89,6 @@ class PassThroughRemuxer extends Logger implements Remuxer {
   public resetNextTimestamp() {
     this.isVideoContiguous = false;
     this.lastEndTime = null;
-    this.lastChunkKey = null;
   }
 
   public resetInitSegment(
@@ -446,9 +443,7 @@ class PassThroughRemuxer extends Logger implements Remuxer {
 
     // Every chunk of a progressive segment is remuxed with the segment's
     // timeOffset, so only the first chunk's decode time lines up with it
-    const chunkKey = `${chunkMeta.level}_${chunkMeta.sn}_${chunkMeta.part}`;
-    const segmentStart = chunkKey !== this.lastChunkKey;
-    this.lastChunkKey = chunkKey;
+    const segmentStart = chunkMeta.id === 1;
 
     if (
       ((accurateTimeOffset && segmentStart) || !initPTS) &&
diff --git a/tests/unit/remux/passthrough-remuxer.ts b/tests/unit/remux/passthrough-remuxer.ts
index 5f2a45b..f6fdfee 100644
--- a/tests/unit/remux/passthrough-remuxer.ts
+++ b/tests/unit/remux/passthrough-remuxer.ts
@@ -108,6 +108,7 @@ describe('passthrough-remuxer', function () {
     fragmentData: Uint8Array<ArrayBuffer>,
     timeOffset: number,
     flush: boolean,
+    chunkId: number,
   ) {
     return remuxer.remux(
       audioTrack(),
@@ -118,7 +119,16 @@ describe('passthrough-remuxer', function () {
       true,
       flush,
       PlaylistLevelType.MAIN,
-      new ChunkMetadata(0, 0, 0, fragmentData.byteLength, -1, true, 4, false),
+      new ChunkMetadata(
+        0,
+        0,
+        chunkId,
+        fragmentData.byteLength,
+        -1,
+        true,
+        4,
+        false,
+      ),
     );
   }
 
@@ -441,8 +451,8 @@ describe('passthrough-remuxer', function () {
     const firstChunk = mp4FragmentAt(0, [sample(180000, 4, 0)]);
     const secondChunk = mp4FragmentAt(180000, [sample(180000, 4, 0)]);
 
-    const first = remuxProgressiveChunk(firstChunk, 0, false);
-    const second = remuxProgressiveChunk(secondChunk, 0, true);
+    const first = remuxProgressiveChunk(firstChunk, 0, false, 1);
+    const second = remuxProgressiveChunk(secondChunk, 0, true, 2);
 
     expect(first.video, 'first chunk video track').to.exist;
     expect(first.video!.startDTS, 'first chunk startDTS').to.equal(0);