(defparameter *metadata-interval* (expt 2 12))
(defparameter *song-source-type* 'singleton)
The function play-songs itself doesn't do much—it loops calling the function play-current, which does all the heavy lifting of sending the contents of a single MP3 file, skipping the ID3 tag and embedding ICY metadata. The only wrinkle is that you need to keep track of when to send the metadata.
Since you must send metadata chunks at a fixed intervals, regardless of when you happen to switch from one MP3 file to the next, each time you call play-current you need to tell it when the next metadata is due, and when it returns, it must tell you the same thing so you can pass the information to the next call to play-current. If play-current gets NIL from the song source, it returns NIL, which allows the play-songs LOOP to end.
In addition to handling the looping, play-songs also provides a HANDLER-CASE to trap the error that will be signaled when the MP3 client disconnects from the server and one of the writes to the socket, down in play-current, fails. Since the HANDLER-CASE is outside the LOOP, handling the error will break out of the loop, allowing play-songs to return.
(defun play-songs (stream song-source metadata-interval)
(handler-case
(loop
for next-metadata = metadata-interval
then (play-current
stream
song-source
next-metadata
metadata-interval)
while next-metadata)
(error (e) (format *trace-output* "Caught error in play-songs: ~a" e))))
Finally, you're ready to implement play-current, which actually sends the Shoutcast data. The basic idea is that you get the current song from the song source, open the song's file, and then loop reading data from the file and writing it to the socket until either you reach the end of the file or the current song is no longer the current song.
There are only two complications: One is that you need to make sure you send the metadata at the correct interval. The other is that if the file starts with an ID3 tag, you want to skip it. If you don't worry too much about I/O efficiency, you can implement play-current like this:
(defun play-current (out song-source next-metadata metadata-interval)
(let ((song (current-song song-source)))
(when song
(let ((metadata (make-icy-metadata (title song))))
(with-open-file (mp3 (file song))
(unless (file-position mp3 (id3-size song))
(error "Can't skip to position ~d in ~a" (id3-size song) (file song)))
(loop for byte = (read-byte mp3 nil nil)
while (and byte (still-current-p song song-source)) do
(write-byte byte out)
(decf next-metadata)
when (and (zerop next-metadata) metadata-interval) do
(write-sequence metadata out)
(setf next-metadata metadata-interval))
(maybe-move-to-next-song song song-source)))
next-metadata)))
This function gets the current song from the song source and gets a buffer containing the metadata it'll need to send by passing the title to make-icy-metadata. Then it opens the file and skips past the ID3 tag using the two-argument form of FILE-POSITION. Then it commences reading bytes from the file and writing them to the request stream.[303]
It'll break out of the loop either when it reaches the end of the file or when the song source's current song changes out from under it. In the meantime, whenever next-metadata gets to zero (if you're supposed to send metadata at all), it writes metadata to the stream and resets next-metadata. Once it finishes the loop, it checks to see if the song is still the song source's current song; if it is, that means it broke out of the loop because it read the whole file, in which case it tells the song source to move to the next song. Otherwise, it broke out of the loop because someone changed the current song out from under it, and it just returns. In either case, it returns the number of bytes left before the next metadata is due so it can be passed in the next call to play-current.[304]
The function make-icy-metadata, which takes the title of the current song and generates an array of bytes containing a properly formatted chunk of ICY metadata, is also straightforward.[305]
(defun make-icy-metadata (title)
(let* ((text (format nil "StreamTitle='~a';" (substitute #\Space #\' title)))
(blocks (ceiling (length text) 16))
(buffer (make-array (1+ (* blocks 16))
:element-type '(unsigned-byte 8)
:initial-element 0)))
(setf (aref buffer 0) blocks)
(loop
for char across text
for i from 1
do (setf (aref buffer i) (char-code char)))
buffer))
Depending on how your particular Lisp implementation handles its streams, and also how many MP3 clients you want to serve at once, the simple version of play-current may or may not be efficient enough.
The potential problem with the simple implementation is that you have to call READ-BYTE and WRITE-BYTE for every byte you transfer. It's possible that each call may result in a relatively expensive system call to read or write one byte. And even if Lisp implements its own streams with internal buffering so not every call to READ-BYTE or WRITE-BYTE results in a system call, function calls still aren't free. In particular, in implementations that provide user-extensible streams using so-called Gray Streams, READ-BYTE and WRITE-BYTE may result in a generic function call under the covers to dispatch on the class of the stream argument. While generic function dispatch is normally speedy enough that you don't have to worry about it, it's a bit more expensive than a nongeneric function call and thus not something you necessarily want to do several million times in a few minutes if you can avoid it.
A more efficient, if slightly more complex, way to implement play-current is to read and write multiple bytes at a time using the functions READ-SEQUENCE and WRITE-SEQUENCE. This also gives you a chance to match your file reads with the natural block size of the file system, which will likely give you the best disk throughput. Of course, no matter what buffer size you use, keeping track of when to send the metadata becomes a bit more complicated. A more efficient version of play-current that uses READ-SEQUENCE and WRITE-SEQUENCE might look like this:
(defun play-current (out song-source next-metadata metadata-interval)
(let ((song (current-song song-source)))
303
Most MP3-playing software will display the metadata somewhere in the user interface. However, the XMMS program on Linux by default doesn't. To get XMMS to display Shoutcast metadata, press Ctrl+P to see the Preferences pane. Then in the Audio I/O Plugins tab (the leftmost tab in version 1.2.10), select the MPEG Layer 1/2/3 Player (libmpg123.so) and hit the Configure button. Then select the Streaming tab on the configuration window, and at the bottom of the tab in the SHOUTCAST/Icecast section, check the "Enable SHOUTCAST/Icecast title streaming" box.
304
Folks coming to Common Lisp from Scheme might wonder why play-current can't just call itself recursively. In Scheme that would work fine since Scheme implementations are required by the Scheme specification to support "an unbounded number of active tail calls." Common Lisp implementations are allowed to have this property, but it isn't required by the language standard. Thus, in Common Lisp the idiomatic way to write loops is with a looping construct, not with recursion.
305
This function assumes, as has other code you've written, that your Lisp implementation's internal character encoding is ASCII or a superset of ASCII, so you can use CHAR-CODE to translate Lisp CHARACTER objects to bytes of ASCII data.