probe.js
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/**
* mux.js
*
* Copyright (c) 2016 Brightcove
* All rights reserved.
*
* Utilities to detect basic properties and metadata about Aac data.
*/
'use strict';
var ADTS_SAMPLING_FREQUENCIES = [
96000,
88200,
64000,
48000,
44100,
32000,
24000,
22050,
16000,
12000,
11025,
8000,
7350
];
var parseSyncSafeInteger = function(data) {
return (data[0] << 21) |
(data[1] << 14) |
(data[2] << 7) |
(data[3]);
};
// return a percent-encoded representation of the specified byte range
// @see http://en.wikipedia.org/wiki/Percent-encoding
var percentEncode = function(bytes, start, end) {
var i, result = '';
for (i = start; i < end; i++) {
result += '%' + ('00' + bytes[i].toString(16)).slice(-2);
}
return result;
};
// return the string representation of the specified byte range,
// interpreted as ISO-8859-1.
var parseIso88591 = function(bytes, start, end) {
return unescape(percentEncode(bytes, start, end)); // jshint ignore:line
};
var parseId3TagSize = function(header, byteIndex) {
var
returnSize = (header[byteIndex + 6] << 21) |
(header[byteIndex + 7] << 14) |
(header[byteIndex + 8] << 7) |
(header[byteIndex + 9]),
flags = header[byteIndex + 5],
footerPresent = (flags & 16) >> 4;
if (footerPresent) {
return returnSize + 20;
}
return returnSize + 10;
};
var parseAdtsSize = function(header, byteIndex) {
var
lowThree = (header[byteIndex + 5] & 0xE0) >> 5,
middle = header[byteIndex + 4] << 3,
highTwo = header[byteIndex + 3] & 0x3 << 11;
return (highTwo | middle) | lowThree;
};
var parseType = function(header, byteIndex) {
if ((header[byteIndex] === 'I'.charCodeAt(0)) &&
(header[byteIndex + 1] === 'D'.charCodeAt(0)) &&
(header[byteIndex + 2] === '3'.charCodeAt(0))) {
return 'timed-metadata';
} else if ((header[byteIndex] & 0xff === 0xff) &&
((header[byteIndex + 1] & 0xf0) === 0xf0)) {
return 'audio';
}
return null;
};
var parseSampleRate = function(packet) {
var i = 0;
while (i + 5 < packet.length) {
if (packet[i] !== 0xFF || (packet[i + 1] & 0xF6) !== 0xF0) {
// If a valid header was not found, jump one forward and attempt to
// find a valid ADTS header starting at the next byte
i++;
continue;
}
return ADTS_SAMPLING_FREQUENCIES[(packet[i + 2] & 0x3c) >>> 2];
}
return null;
};
var parseAacTimestamp = function(packet) {
var frameStart, frameSize, frame, frameHeader;
// find the start of the first frame and the end of the tag
frameStart = 10;
if (packet[5] & 0x40) {
// advance the frame start past the extended header
frameStart += 4; // header size field
frameStart += parseSyncSafeInteger(packet.subarray(10, 14));
}
// parse one or more ID3 frames
// http://id3.org/id3v2.3.0#ID3v2_frame_overview
do {
// determine the number of bytes in this frame
frameSize = parseSyncSafeInteger(packet.subarray(frameStart + 4, frameStart + 8));
if (frameSize < 1) {
return null;
}
frameHeader = String.fromCharCode(packet[frameStart],
packet[frameStart + 1],
packet[frameStart + 2],
packet[frameStart + 3]);
if (frameHeader === 'PRIV') {
frame = packet.subarray(frameStart + 10, frameStart + frameSize + 10);
for (var i = 0; i < frame.byteLength; i++) {
if (frame[i] === 0) {
var owner = parseIso88591(frame, 0, i);
if (owner === 'com.apple.streaming.transportStreamTimestamp') {
var d = frame.subarray(i + 1);
var size = ((d[3] & 0x01) << 30) |
(d[4] << 22) |
(d[5] << 14) |
(d[6] << 6) |
(d[7] >>> 2);
size *= 4;
size += d[7] & 0x03;
return size;
}
break;
}
}
}
frameStart += 10; // advance past the frame header
frameStart += frameSize; // advance past the frame body
} while (frameStart < packet.byteLength);
return null;
};
module.exports = {
parseId3TagSize: parseId3TagSize,
parseAdtsSize: parseAdtsSize,
parseType: parseType,
parseSampleRate: parseSampleRate,
parseAacTimestamp: parseAacTimestamp
};