How to get id3 length in mp3 file?

Question

Programming

The length of id3 data is stored in 6-9 bytes. The peculiarity of specifying the length of ID3v2 data is that in each byte the 7th bit is not used and is always set to 0.
Manually using a calculator, I can calculate this length, but how can I calculate it programmatically? How to ignore this 7th bit?
In decimal format, bytes 6-9 contain the following numbers: 0 5 45 56. How to get the length in bytes using this data. The length in this case is 87746 bytes.

Answer 1

PHP -> php.net/manual/ru/ref.id3.php
C/C# -> https://taglib.github.io/
Delphi -> www.delphi.int.ru/articles/1
About MP3 itself like everyone else arranged in it -> habr.ru/p/103635

Answer 2

JS using File Api and Typed Arrays

Something like this:

function uint7ArrayToUint28(uint7Array) {
    var uint28 = 0;
    for (var i = 0, pow = 21; pow >= 0; pow -= 7, i++) {
        uint28 += uint7Array[i] << pow;
    }
    return uint28;
}


var firstTenBytes = new Uint8Array(arrayBuffer, 0, 10);

var id3TagSize = uint7ArrayToUint28([
    firstTenBytes[6], firstTenBytes[7],
    firstTenBytes[8], firstTenBytes[9]
]) + 10;

The size of the ID3 tag is stored minus the header (10 bytes) - according to the specification. So you need to add 10 more. I somehow overlooked this in the specification, then a bug surfaced - the music was not played.
Essentially it all boils down to this:

var size = bytes[9];
size += bytes[8] << 7;
size += bytes[7] << 14;
size += bytes[6] << 21;
size += 10; // заголовок

Consider an example with your data:

var size = 56;
size += 45 << 7; // 5760 + 56 = 5816
size += 5 << 14; // 81920 + 5816 = 87736
size += 0 << 21; // 0 + 87736 = 87736 - размер ID3 тега без заголовка
size += 10; // 87736 + 10 = 87746 - размер ID3 тега

To understand what is happening here, I recommend reading about JS bitwise operators .