The Package

    The package for the code you’ll develop in this chapter looks like this:

    The idea behind the interface is that the Shoutcast server will find a source of songs based on an ID extracted from the AllegroServe request object. It can then do three things with the song source it’s given.

    • Get the current song from the source
    • Tell the song source that it’s done with the current song
    • Ask the source whether the song it was given earlier is still the current song
    2.   (:documentation "Return the currently playing song or NIL."))
    4. (defgeneric maybe-move-to-next-song (song source)
    5.   (:documentation
    6.    "If the given song is still the current one update the value
    8. (defgeneric still-current-p (song source)
    9.   (:documentation
    10.    "Return true if the song given is the same as the current-song."))

    The function maybe-move-to-next-song is defined the way it is so a single operation checks whether the song is current and, if it is, moves the song source to the next song. This will be important in the next chapter when you need to implement a song source that can be safely manipulated from two different threads.3

    To represent the information about a song that the Shoutcast server needs, you can define a class, song, with slots to hold the name of the MP3 file, the title to send in the Shoutcast metadata, and the size of the ID3 tag so you can skip it when serving up the file.

    The value returned by (and thus the first argument to still-current-p and maybe-move-to-next-song) will be an instance of song.

    1. (defgeneric find-song-source (type request)
    2.   (:documentation "Find the song-source of the given type for the given request."))

    However, for the purposes of this chapter, you can use a trivial implementation of this interface that always uses the same object, a simple queue of song objects that you can manipulate from the REPL. You can start by defining a class, simple-song-queue, and a global variable, *songs*, that holds an instance of this class.

    Then you can define a method on find-song-source that specializes type with an **EQL** specializer on the symbol singleton and returns the instance stored in .

    1. (defmethod find-song-source ((type (eql 'singleton)) request)
    2.   (declare (ignore request))
    3.   *songs*)

    Now you just need to implement methods on the three generic functions that the Shoutcast server will use.

    1.   (vector-push-extend (file->song file) (songs *songs*)))
    3. (defun file->song (file)
    4.   (let ((id3 (read-id3 file)))
    5.     (make-instance 
    6.      'song
    7.      :file (namestring (truename file))
    8.      :title (format nil "~a by ~a from ~a" (song id3) (artist id3) (album id3))