Rather than guessing which frames will be most useful, you can use the code you’ve already written to poke around a bit at the REPL and see what frames are actually used in your own MP3s. To start, you need an instance of , which you can get with the read-id3 function.

Since you’ll want to play with this object a bit, you should save it in a variable.

ID3V2> (defparameter *id3* (read-id3 "/usr2/mp3/Kitka/Wintersongs/02 Byla Cesta.mp3"))
*ID3*

Now you can see, for example, how many frames it has.

ID3V2> (length (frames *id3*))
11

Okay, that’s not too informative. What you really want to know are what kinds of frames are in there. In other words, you want to know the ids of those frames, which you can get with a simple **MAPCAR** like this:

ID3V2> (mapcar #'id (frames *id3*))

If you look up these identifiers in the ID3v2.2 spec, you’ll discover that all the frames with identifiers starting with T are text information frames and have a similar structure. And COM is the identifier for comment frames, which have a structure similar to that of text information frames. The particular text information frames identified here turn out to be the frames for representing the song title, artist, album, track, part of set, year, genre, and encoding program.

Of course, this is just one MP3 file. Maybe other frames are used in other files. It’s easy enough to discover. First define a function that combines the previous **MAPCAR** expression with a call to and wraps the whole thing in a **DELETE-DUPLICATES** to keep things tidy. You’ll have to use a :test argument of #'string= to **DELETE-DUPLICATES** to specify that you want two elements considered the same if they’re the same string.

(defun frame-types (file)
  (delete-duplicates (mapcar #'id (frames (read-id3 file))) :test #'string=))

Then you can use Chapter 15’s walk-directory function along with mp3-p to find every MP3 file under a directory and combine the results of calling frame-types on each file. Recall that **NUNION** is the recycling version of the function; since frame-types makes a new list for each file, this is safe.

(defun frame-types-in-dir (dir)
  (let ((ids ()))
    (flet ((collect (file)
      (walk-directory dir #'collect :test #'mp3-p))
    ids))

Now pass it the name of a directory, and it’ll tell you the set of identifiers used in all the MP3 files under that directory. It may take a few seconds depending how many MP3 files you have, but you’ll probably get something similar to this:

ID3V2> (frame-types-in-dir "/usr2/mp3/")
("TCON" "COMM" "TRCK" "TIT2" "TPE1" "TALB" "TCP" "TT2" "TP1" "TCM"
 "TAL" "TRK" "TPA" "TYE" "TCO" "TEN" "COM")

The four-letter identifiers are the version 2.3 equivalents of the version 2.2 identifiers I discussed previously. Since the information stored in those frames is exactly the information you’ll need in Chapter 27, it makes sense to implement classes only for the frames actually used, namely, text information and comment frames, which you’ll do in the next two sections. If you decide later that you want to support other frame types, it’s mostly a matter of translating the ID3 specifications into the appropriate binary class definitions.