Question

I am writing a program to diff, and copy entire files or segments based on changes on either end (Rsync-esque... but more like Unison). The main idea is to keep my music folder (all mp3s) up to date over multiple locations.

I d like to send segmented updates if only small portions of the file have changed, as opposed to copying the entire file. For this, I need a way to diff segments of the file.

I initially tried generating hashes for blocks of every file (Every n bytes I d hash the segment). I noticed that when I changed one attribute (id3v2 tag on an mp3) all the hashed blocks would change. This makes sense, as I would guess the header is growing as it acquired new information.

This leads me to my actual question. I would like to know how to determine the length of an mp3 s header, so I could create 2 comparable hashes.

1) The meta info of the file (header)

2) The actual mpeg stream with audio (This hash should remain unchanged if all I do is alter tag info)

Am I missing anything else?

Thanks!

Ty

Answer 1

I wound up using TagLibSharp. developer.novell.com/wiki/index.php/TagLib_Sharp

Answer 2

If all you want to check the length of is id3v2 tags, then you can find out information about its structure at http://www.id3.org/id3v2.4.0-structure.

If you read the first 3 bytes, and they are equal to "ID3", then skip to the 7th byte, then read the header size. Be careful though, because the size is stored as a "synchsafe integer".

Answer 3

If you want to determine the header information, you ll either:

a) need to use a mp3 library that can do the parsing for you, or
b) go to the mp3 specification and parse it out as needed.

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