hls-player-js/mp4-tree.js

// add a getString to make reading box headers easier
DataView.prototype.getString = function(offset, length) {
	text = '';
	let end = offset + length;
	for (let i = offset; i < end; i++) {
		text += String.fromCharCode(this.getUint8(i));
	}
	return text;
}

// mp4 tree traverser, as is only good for getting avc1 + mp4a codec info.
// relevant tree parts:
// root
// |-moov
//   |-trak
//     |-mdia
//       |-minf
//         |-vmhd
//         |-stbl
//           |-stsd
//             |-avc1
//               |-avcC // has video codec profile
//   |-trak
//     |-mdia
//       |-minf
//         |-smhd
//         |-stbl
//           |-stsd
//             |-mp4a
//               |-esds // has audio codec profile
//
// headers are 4byte len + 4byte name string
// most details taken from https://web.archive.org/web/20180219054429/http://l.web.umkc.edu/lizhu/teaching/2016sp.video-communication/ref/mp4.pdf
// avc1 and mp4a specific header offsets from https://xhelmboyx.tripod.com/formats/mp4-layout.txt
class MP4Tree {
	constructor(data) {
		this.data = new DataView(data);
		this.codecs = [];
		this.idx = 0; // current byte index in data
		this.parse_codecs();
	}

	parse_codecs() {
		this.parse_into('moov', 8);
		// loop over all traks (can handle pure audio, pure video, both, multiple)
		while (this.idx < this.data.byteLength) {
			if (!this.parse_until('trak')) { // if no more tracks in file
				break;
			}
			const curr_track_end = this.idx + this.read_next_head().len;
			this.idx += 8
			this.parse_into('mdia', 8);
			this.parse_into('minf', 8);
			const track_type_head = this.read_next_head().name;
			if (track_type_head == 'smhd' || track_type_head == 'vmhd') {
				this.parse_into('stbl', 8);
				this.parse_into('stsd', 16);
				const media_sample = this.read_next_head().name;
				if (media_sample == 'avc1') {
					this.parse_avc1();
				} else if (media_sample == 'mp4a') {
					this.parse_mp4a();
				}
			}
			this.idx = curr_track_end; // always skip to end of current processed track
		}
	}
	
	parse_avc1() {
		this.parse_into('avc1', 86); // avc1 has 78 bytes of frame metadata to skip
		this.parse_into('avcC', 8);
		// ex, 01 64 00 1f
		// 01 is avc version (always 1, bit-and it off)
		// rest is video codec profile, rep as hex
		const avc_profile = (this.data.getUint32(this.idx) & 0xffffff).toString(16); 
		this.codecs.push('avc1.' + avc_profile);
	}

	parse_mp4a() {
		this.parse_into('mp4a', 36); // 28 bytes of subboxes with unclear offsets
		this.parse_into('esds', 12);
		// mp4a is as mp4a.40.2
		// oti has defined position, rep as hex
		// aot as a subtype is defined as 5 bits of the fetched byte, rep as dec
		const oti = this.data.getUint8(this.idx + 13).toString(16); 
		const aot = (this.data.getUint8(this.idx + 31) >> 3).toString(10);
		this.codecs.push(["mp4a", oti, aot].join('.'));
	}

	get_mime() {
		return 'video/mp4; codecs="' + this.get_codec_string() + '"';
	}

	get_codec_string() {
		return this.codecs.join(', ');
	}
	
	// find the given box, also descend into its child box depth
	// offset is almost always 8, sometimes 12 or 16, defined in spec not file
	parse_into(name, header_offset) {
		this.parse_until(name);
		this.idx += header_offset;
	}

	// find the box with the given name at the current box depth
	// acts as though the current box extends to EOF.
	parse_until(name) {
		let curr_head = null;
		while (this.idx + 8 < this.data.byteLength) {
			curr_head = this.read_next_head();
			if (curr_head.name == name) {
				return true;
			} else {
				this.idx += curr_head.len;
			}
		}
		return false;
	}
	
	// read the mp4 box header as if the idx is positioned there
	read_next_head() {
		return {"len": this.data.getUint32(this.idx), "name": this.data.getString(this.idx + 4, 4)};
	}
}