From 35c4d384cf1a07af0e3a2ef1cd2febf63a1859f2 Mon Sep 17 00:00:00 2001
From: Vanilagy <1696106+Vanilagy@users.noreply.github.com>
Date: Sun, 2 Feb 2025 17:21:57 +0100
Subject: [PATCH] Mime type & MP3 improvements

---
 src/isobmff/isobmff-demuxer.ts   |  8 +++++++-
 src/matroska/matroska-demuxer.ts |  8 +++++++-
 src/mp3/mp3-misc.ts              | 11 ++++++++---
 src/mp3/mp3-writer.ts            |  4 ++--
 4 files changed, 24 insertions(+), 7 deletions(-)

diff --git a/src/isobmff/isobmff-demuxer.ts b/src/isobmff/isobmff-demuxer.ts
index 7bcb0aa..cd3f054 100644
--- a/src/isobmff/isobmff-demuxer.ts
+++ b/src/isobmff/isobmff-demuxer.ts
@@ -210,7 +210,13 @@ export class IsobmffDemuxer extends Demuxer {
 	override async getMimeType() {
 		await this.readMetadata();
 
-		let string = this.isQuickTime ? 'video/quicktime' : 'video/mp4';
+		const base = this.tracks.some(x => x.info?.type === 'video')
+			? 'video/'
+			: this.tracks.some(x => x.info?.type === 'audio')
+				? 'audio/'
+				: 'application/';
+
+		let string = base + (this.isQuickTime ? 'quicktime' : 'mp4');
 
 		if (this.tracks.length > 0) {
 			const codecMimeTypes = await Promise.all(this.tracks.map(x => x.inputTrack!.getCodecMimeType()));
diff --git a/src/matroska/matroska-demuxer.ts b/src/matroska/matroska-demuxer.ts
index 58863cd..7c9c1df 100644
--- a/src/matroska/matroska-demuxer.ts
+++ b/src/matroska/matroska-demuxer.ts
@@ -177,7 +177,13 @@ export class MatroskaDemuxer extends Demuxer {
 	override async getMimeType() {
 		await this.readMetadata();
 
-		let string = this.isWebM ? 'video/webm' : 'video/x-matroska';
+		const base = this.segments.some(segment => segment.tracks.some(x => x.info?.type === 'video'))
+			? 'video/'
+			: this.segments.some(segment => segment.tracks.some(x => x.info?.type === 'audio'))
+				? 'audio/'
+				: 'application/';
+
+		let string = base + (this.isWebM ? 'webm' : 'x-matroska');
 
 		const tracks = await this.getTracks();
 		if (tracks.length > 0) {
diff --git a/src/mp3/mp3-misc.ts b/src/mp3/mp3-misc.ts
index ba58b7a..79f1056 100644
--- a/src/mp3/mp3-misc.ts
+++ b/src/mp3/mp3-misc.ts
@@ -46,8 +46,13 @@ export type FrameHeader = {
 	emphasis: number;
 };
 
-export const computeMp3FrameSize = (bitrate: number, sampleRate: number, padding: number) => {
-	return Math.floor((144 * bitrate / sampleRate) + padding);
+export const computeMp3FrameSize = (layer: number, bitrate: number, sampleRate: number, padding: number) => {
+	if (layer === 3) {
+		// Layer 1
+		return Math.floor((12 * bitrate / sampleRate + padding) * 4);
+	} else {
+		return Math.floor((144 * bitrate / sampleRate) + padding);
+	}
 };
 
 export const getXingOffset = (mpegVersionId: number, channel: number) => {
@@ -102,7 +107,7 @@ export const readFrameHeader = (word: number, reader: { pos: number; fileSize: n
 		return null;
 	}
 
-	const frameLength = computeMp3FrameSize(bitrate, sampleRate, padding);
+	const frameLength = computeMp3FrameSize(layer, bitrate, sampleRate, padding);
 
 	if (reader.fileSize !== null && reader.fileSize - startPos < frameLength) {
 		// The frame doesn't fit into the rest of the file
diff --git a/src/mp3/mp3-writer.ts b/src/mp3/mp3-writer.ts
index f5a603c..849ab91 100644
--- a/src/mp3/mp3-writer.ts
+++ b/src/mp3/mp3-writer.ts
@@ -56,7 +56,7 @@ export class Mp3Writer {
 
 		// Let's find the lowest bitrate for which the frame size is sufficiently large to fit all the data
 		const bitrateIndex = bitrates.findIndex((kbr) => {
-			return computeMp3FrameSize(1000 * kbr, sampleRate, padding) >= neededBytes;
+			return computeMp3FrameSize(data.layer, 1000 * kbr, sampleRate, padding) >= neededBytes;
 		});
 		if (bitrateIndex === -1) {
 			throw new Error('No suitable bitrate found.');
@@ -98,7 +98,7 @@ export class Mp3Writer {
 		this.writeU32(data.fileSize ?? 0);
 		this.writer.write(data.toc ?? new Uint8Array(100));
 
-		const frameSize = computeMp3FrameSize(1000 * bitrates[bitrateIndex]!, sampleRate, padding);
+		const frameSize = computeMp3FrameSize(data.layer, 1000 * bitrates[bitrateIndex]!, sampleRate, padding);
 		this.writer.seek(startPos + frameSize);
 	}
 }