You can define a class to represent playlists like this:

The of a playlist is the key you extract from the request object passed to find-song-source when looking up a playlist. You don’t actually need to store it in the playlist object, but it makes debugging a bit easier if you can find out from an arbitrary playlist object what its id is.

The heart of the playlist is the songs-table slot, which will hold a table object. The schema for this table will be the same as for the main MP3 database. The function make-playlist-table, which you use to initialize songs-table, is simply this:

You can define the package for the code in this chapter with the following **DEFPACKAGE**:

Because this is a high-level application, it uses a lot of lower-level packages. It also imports three symbols from the ACL-SOCKET package and two more from MULTIPROCESSING since it just needs those five and not the other 139 symbols those two packages export.

By storing the list of songs as a table, you can use the database functions from Chapter 27 to manipulate the playlist: you can add to the playlist with insert-row, delete songs with delete-rows, and reorder the playlist with sort-rows and shuffle-table.

The ordering and shuffle slots hold information about how the songs in songs-table are to be ordered. The ordering slot holds a keyword that tells how the songs-table should be sorted when it’s not shuffled. The legal values are :genre, :artist, :album, and :song. The shuffle slot holds one of the keywords :none, :song, or :album, which specifies how songs-table should be shuffled, if at all.

The repeat slot also holds a keyword, one of , :song, or :all, which specifies the repeat mode for the playlist. If repeat is :none, after the last song in the songs-table has been played, the current-song goes back to a default MP3. When :repeat is :song, the playlist keeps returning the same current-song forever. And if it’s :all, after the last song, current-song goes back to the first song.

The user-agent slot holds the value of the User-Agent header sent by the MP3 client in its request for the stream. You need to hold onto this value purely for use in the Web interface—the User-Agent header identifies the program that made the request, so you can display the value on the page that lists all the playlists to make it easier to tell which playlist goes with which connection when multiple clients connect.

The macro acquires exclusive access to the process lock given and then executes the body forms, releasing the lock afterward. By default, with-process-lock allows recursive locks, meaning the same thread can safely acquire the same lock multiple times.