From b48eb75565b73dd9dbd3520ef92d2ec9c088fc3b Mon Sep 17 00:00:00 2001
From: Theredbt <info@abdoubt.com>
Date: Mon, 11 May 2026 04:22:46 +0100
Subject: [PATCH] fix(input-format): recognise CMAF segments that start with
 sidx
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit

Sibling case to #308: when a fragmented MP4 segment begins with a sidx
box (no ftyp/styp/moof at the file root), Mp4InputFormat._canReadInput
returned false, causing Input.getTracks() to throw
UnsupportedInputFormatError.

This shape is standard in CMAF when the DASH on-demand profile is used
or when HLS playlists are derived from one (Vimeo's vod-adaptive-ak CDN,
for example). The segment is still a valid ISOBMFF fragment — the sidx
just sits before the moof.

Adds 'sidx' alongside the existing 'moof' check so the format probe
accepts these segments, and a focused regression test that fails on
the previous behaviour.
---
 src/input-format.ts         |  3 ++-
 test/node/hls-input.test.ts | 23 ++++++++++++++++++++++-
 2 files changed, 24 insertions(+), 2 deletions(-)

diff --git a/src/input-format.ts b/src/input-format.ts
index 6c75c8d..e1866a1 100644
--- a/src/input-format.ts
+++ b/src/input-format.ts
@@ -120,7 +120,8 @@ export class Mp4InputFormat extends IsobmffInputFormat {
 		if (slice instanceof Promise) slice = await slice;
 		if (!slice) return false;
 
-		return readAscii(slice, 4) === 'moof'; // Seen in HLS for example
+		const fourCc = readAscii(slice, 4);
+		return fourCc === 'moof' || fourCc === 'sidx'; // Seen in HLS for example
 	}
 
 	get name() {
diff --git a/test/node/hls-input.test.ts b/test/node/hls-input.test.ts
index b49dc5c..3ed47ec 100644
--- a/test/node/hls-input.test.ts
+++ b/test/node/hls-input.test.ts
@@ -1,7 +1,7 @@
 /* eslint-disable @stylistic/max-len */
 import { ALL_FORMATS, BufferSource, EncodedPacketSink, Input, InputAudioTrack, InputVideoTrack, UrlSource } from 'mediabunny';
 import { expect, test, vi } from 'vitest';
-import { HLS, HLS_FORMATS, HlsInputFormat } from '../../src/input-format.js';
+import { HLS, HLS_FORMATS, HlsInputFormat, MP4 } from '../../src/input-format.js';
 import { assert, hexStringToBytes, rejectAfter } from '../../src/misc.js';
 import { CustomPathedSource } from '../../src/source.js';
 
@@ -788,6 +788,27 @@ test.concurrent('Live HLS', { timeout: 30_000 }, async () => {
 	expect(consoleSpy).not.toHaveBeenCalled();
 });
 
+test.concurrent('Mp4InputFormat recognises a CMAF segment that starts with sidx', async () => {
+	// Sibling of the moof case fixed in #308: CMAF segments using the DASH
+	// on-demand profile (and HLS playlists derived from it) place a sidx
+	// before the moof. Without ftyp/styp at the file start the existing
+	// format probe used to bail out, producing UnsupportedInputFormatError.
+	//
+	// Repro shape (real Vimeo segment): 32 60 73 69 64 78 ...
+	// (size 0x32 60-byte sidx box header, four-cc 'sidx').
+	const bytes = new Uint8Array(12);
+	new DataView(bytes.buffer).setUint32(0, 60); // box size
+	bytes.set([0x73, 0x69, 0x64, 0x78], 4); // 'sidx'
+
+	using input = new Input({
+		source: new BufferSource(bytes),
+		formats: ALL_FORMATS,
+	});
+
+	expect(await input.canRead()).toBe(true);
+	expect(await input.getFormat()).toBe(MP4);
+});
+
 test.concurrent('#EXT-X-I-FRAME-STREAM-INF tags are parsed properly', async () => {
 	const text = `#EXTM3U
 #EXT-X-I-FRAME-STREAM-INF:BANDWIDTH=256000,CODECS="avc1.4D401E",RESOLUTION=480x270,URI="7f2459cb12854fdbbc7ec1e7279da179/f74fb10563564130a4702743d64112a3/index_11.m3u8"