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Hacking iTunes

November 3, 2004

Niel Bornstein

The iTunes music player and download service took the world by storm in 2003 and 2004, and promises to continue its popularity for the forseeable future. While the music is stored in simple files on disk, metadata -- information about the music -- is stored in a single XML file.

In this article I'll explore ways to work with the iTunes Music Library file, an XML document, for fun and education, including transforming the library into an HTML page using various technologies, and querying Amazon and Google's web services for other suggested recordings and related information.

For demonstration purposes, and as a shameless plug for my other work, I'll be using the Mono open source, common language runtime to build cross-platform tools, though the techniques will be applicable to other platforms and languages.

About the Library

The iTunes library file, a file called iTunes Music Library.xml, is created automatically when you launch iTunes. On Mac OS X, it can be found in the directory $HOME/Music/iTunes/, while on Windows, you'll find it in My Documents\My Music\iTunes\. It's an XML file, the format of which is defined by a Document Type Declaration (DTD) located at www.apple.com/DTDs/PropertyList-1.0.dtd (reformatted for legibility):



ENTITY % plistObject "(array | data | date | dict | real | 

                       integer | string | true | false )" 

ELEMENT plist %plistObject;

ATTLIST plist version CDATA "1.0" 



<!-- Collections -->

ELEMENT array (%plistObject;)*

ELEMENT dict (key, %plistObject;)*

ELEMENT key (#PCDATA)



<!--- Primitive types -->

ELEMENT string (#PCDATA)

ELEMENT data (#PCDATA) <!-- Contents interpreted as Base-64 encoded -->

ELEMENT date (#PCDATA) <!-- Contents should conform to a subset of ISO 8601 

                         (in particular, YYYY '-' MM '-' DD 'T' HH ':' MM ':' SS 'Z'.  

                         Smaller units may be omitted with a loss of precision) -->



<!-- Numerical primitives -->



ELEMENT true EMPTY     <!-- Boolean constant true -->

ELEMENT false EMPTY    <!-- Boolean constant false -->

ELEMENT real (#PCDATA) <!-- Contents should represent a floating point

                            number matching ("+" | "-")? d+ ("."d*)? ("E"

                            ("+" | "-") d+)? where d is a digit 0-9.  -->

ELEMENT integer (#PCDATA) <!-- Contents should represent a (possibly signed)

                               integer number in base 10 -->

You can see that the schema for the library has nothing to do with music, audio, multimedia, or data files; it's just a generic dictionary, a collection of key-value pairs. The top-level element is plist (property list), and it can contain any of the elements array, data, date, dict, real, integer, string, true, or false. This is a fairly flexible DTD.

In practice, your library file will contain a single dict, which will in turn contain a number of key-value pairs representing header information: the library file and application versions, music folder location, and a Library Persistent ID. All of these values are integers and strings.



<?xml version="1.0" encoding="UTF-8"?>

<!DOCTYPE plist PUBLIC "-//Apple Computer//DTD PLIST 1.0//EN" 

          "http://www.apple.com/DTDs/PropertyList-1.0.dtd">



<plist version="1.0">

<dict>

  <key>Major Version</key><integer>1</integer>

  <key>Minor Version</key><integer>1</integer>

  <key>Application Version</key><string>4.6</string>

  <key>Music Folder</key>

    <string>file://localhost/Users/niel/Music/iTunes/iTunes%20Music/</string>

  <key>Library Persistent ID</key><string>8E84CC790968E27F</string>

  <key>Tracks</key>

  <dict>

    ...

  </dict>



  <key>Playlists</key>

  <array>

    ...

  </array>

</dict>

</plist>

After the header information comes track metadata:



<key>839</key>

<dict>

  <key>Track ID</key><integer>839</integer>

  <key>Name</key><string>Sweet Georgia Brown</string>

  <key>Artist</key><string>Count Basie & His Orchestra</string>

  <key>Composer</key><string>Bernie/Pinkard/Casey</string>

  <key>Album</key><string>Prime Time</string>

  <key>Genre</key><string>Jazz</string>

  <key>Kind</key><string>Protected AAC audio file</string>

  <key>Size</key><integer>3771502</integer>

  <key>Total Time</key><integer>219173</integer>

  <key>Disc Number</key><integer>1</integer>

  <key>Disc Count</key><integer>1</integer>

  <key>Track Number</key><integer>3</integer>

  <key>Track Count</key><integer>8</integer>

  <key>Year</key><integer>1977</integer>

  <key>Date Modified</key><date>2004-06-16T18:10:55Z</date>

  <key>Date Added</key><date>2004-06-16T18:08:31Z</date>

  <key>Bit Rate</key><integer>128</integer>

  <key>Sample Rate</key><integer>44100</integer>

  <key>Play Count</key><integer>3</integer>

  <key>Play Date</key><integer>-1119376103</integer>

  <key>Play Date UTC</key><date>2004-08-17T16:39:53Z</date>

  <key>Rating</key><integer>100</integer>

  <key>Artwork Count</key><integer>1</integer>

  <key>File Type</key><integer>1295274016</integer>

  <key>File Creator</key><integer>1752133483</integer>

  <key>Location</key><string>file://localhost/Users/niel/Music/ \

iTunes/iTunes%20Music/Count%20Basie%20&%20His%20Orchestra/ \

Prime%20Time/03%20Sweet%20Georgia%20Brown.m4p</string>

  <key>File Folder Count</key><integer>4</integer>

  <key>Library Folder Count</key><integer>1</integer>

</dict>



The collection of tracks is a dict, keyed by the track ID. Within each track, also a dict, the keys include Track ID, Name, Artist, Album, Genre, Kind, Size, Total Time, Disc Number, Disc Count, Track Number, Track Count, Year, Date Modified, Date Added, Bit Rate, Sample Rate, Play Count, Play Date, Play Date UTC, File Type, File Creator, Location, File Folder Count, and Library Folder Count. Although most of these are self-explanatory, a few bear further explanation.

Kind refers to the file encoding. Valid values include AAC audio file and MPEG audio file.

Play Date and Play Date UTC contain the date of the last time the track was played to the end, in local and UTC time, respectively.

File Type and File Creator contain the Mac-OS-specific file type and creator, a long integer that indicates the program that created the file, and a particular file type. These long integers are usually represented as a mnemonic four-character code.

Location contains the URL of the audio file, with a file URL scheme.

The final section of the library contains the playlist metadata:



<dict>

  <key>Name</key><string>Funky</string>

  <key>Playlist ID</key><integer>6652</integer>

  <key>Playlist Persistent ID</key><string>88CED99A2F698F3C</string>

  <key>All Items</key><true/>

  <key>Playlist Items</key>

  <array>

    <dict>

      <key>Track ID</key><integer>837</integer>

    </dict>

    <dict>

      <key>Track ID</key><integer>754</integer>

    </dict>

    <dict>

      <key>Track ID</key><integer>835</integer>

    </dict>

    <dict>

      <key>Track ID</key><integer>912</integer>

    </dict>

    <dict>

      <key>Track ID</key><integer>842</integer>

    </dict>

    <dict>

      <key>Track ID</key><integer>217</integer>

    </dict>

  </array>

</dict>

Unlike tracks, playlists are contained in an array. Each playlist, however, is specified by a dict. They come in two flavors, however. Standard playlists have the keys Name, Playlist ID, Playlist Persistent ID, All Items, and Playlist Items.

"Smart" playlists have the additional keys Smart Info and Smart Criteria, which contain base-64-encoded data that specifies the smart playlist.

The Playlist Items key contains an array of dict specifying the tracks, keyed by their IDs.

Loading the Library

You can take a couple of approaches to loading the library file. On the one hand, there's always the DOM. The DOM has the advantage of being cross-platform, so techniques used in one language are usable in any other. Here's a simple program to load the file into a DOM tree:



using System;

using System.Xml;



public class MusicDom {

  public static void Main(string [] args) {

    string file = args[0];

    XmlDocument document = new XmlDocument();

    document.Load(file);

  }

}

On the other hand, the iTunes library can get rather large: I've only got some 1500 tracks in mine, and the file is 2,444,585 bytes. That can lead to a rather large DOM tree in memory. On my iBook with an 800MHz PowerPC G4, that takes nearly nine seconds just to load.

You could also use a read-only view of the XML and load it into a native data structure. Since I'm writing the code in C# using Mono, that means System.XmlReader.

It's often better to load XML data into native structures. The Mono (and .NET) xsd utility will create a class that can be serialized to and deserialized from an XML file. The trick is that xsd requires a W3C XML Schema, but all we have is a DTD.

Luckily, there are some utilities that can convert a DTD to a W3C XML Schema; dtd2xsd is one. So, the first step is to download the DTD, since dtd2xsd.pl wants a local input file:

wget http://www.apple.com/DTDs/PropertyList-1.0.dtd

Now, convert the DTD to a W3C XML Schema:

./dtd2xsd.pl PropertyList-1.0.dtd > PropertyList-1.0.xsd

Now we have a schema definition. However, it's the wrong version of W3C XML Schema; the generated namespace is www.w3.org/2000/10/XMLSchema; it should be www.w3.org/2001/XMLSchema. We'll have to do a little editing.

Besides replacing the namespace declaration, some of the generated schema just isn't right. That's just the way it is with generated schemas.

Here's the correct generated schema:



<schema

  xmlns='http://www.w3.org/2001/XMLSchema'

  targetNamespace='http://www.w3.org/namespace/'

  xmlns:t='http://www.w3.org/namespace/'

  xmlns:xs=''>



 <element name='plist'>

  <complexType>

   <choice>

    <element ref='t:array'/>

    <element ref='t:data'/>

    <element ref='t:date'/>

    <element ref='t:dict'/>

    <element ref='t:real'/>

    <element ref='t:integer'/>

    <element ref='t:string'/>

    <element ref='t:true'/>

    <element ref='t:false'/>

   </choice>

   <attribute name='version' type='string' use='required'/>

  </complexType>

 </element>



 <element name='array'>

  <complexType>

   <sequence minOccurs='0' maxOccurs='unbounded'>

    <choice>

     <element ref='t:array'/>

     <element ref='t:data'/>

     <element ref='t:date'/>

     <element ref='t:dict'/>

     <element ref='t:real'/>

     <element ref='t:integer'/>

     <element ref='t:string'/>

     <element ref='t:true'/>

     <element ref='t:false'/>

    </choice>

   </sequence>

  </complexType>

 </element>



 <element name='dict'>

  <complexType>

   <sequence minOccurs='0' maxOccurs='unbounded'>

    <element ref='t:key'/>

    <choice>

     <element ref='t:array'/>

     <element ref='t:data'/>

     <element ref='t:date'/>

     <element ref='t:dict'/>

     <element ref='t:real'/>

     <element ref='t:integer'/>

     <element ref='t:string'/>

     <element ref='t:true'/>

     <element ref='t:false'/>

    </choice>

   </sequence>

  </complexType>

 </element>



 <element name='key' type='string' />



 <element name='string' type='string' />



 <element name='data' type='base64Binary' />



 <element name='date' type='dateTime' />



 <element name='true' />



 <element name='false' />



 <element name='real' type='decimal' />



 <element name='integer' type='integer' />



</schema>



Now, we can generate the class:

xsd PropertyList-1.0.xsd /classes

That will produce the file PropertyList-1.0.cs, a C# source file that can be used to serialize the music library file from a .NET program.

But why go to the effort of writing all of this specialized C# code when there are some technologies available in any language, on any platform? In the next section, I'll show you how to interrogate the music library with XSLT.

Playlists

You can use XSLT to get a list of all of the playlists in your library.



<?xml version="1.0"?>

<xsl:stylesheet version="1.0" 

  xmlns:xsl="http://www.w3.org/1999/XSL/Transform">



  <xsl:output method="text" />



  <xsl:variable name="newline">

<xsl:text>

</xsl:text>

  </xsl:variable>



  <xsl:template match="/">

    <xsl:for-each select="plist/dict/key[text()='Playlists']/ \

following-sibling::array/dict">

      <xsl:value-of select="key[text()='Name']/ \

following-sibling::string" /><xsl:value-of select="$newline" />

    </xsl:for-each>

  </xsl:template>



</xsl:stylesheet>

You can transform the playlists from the Music Library into other formats using XSLT. For example, the WinAmp playlist formats, documented at forums.winamp.com/showthread.php?threadid=65772, can be generated through an XSLT transform.

The M3U format is a simple text file listing directories and file names separated by a hyphen. The following stylesheet transforms a named iTunes playlist into a M3U playlist with the same name:



<?xml version="1.0"?>

<xsl:stylesheet version="1.0" xmlns:xsl="http://www.w3.org/1999/XSL/Transform">



  <xsl:output method="text" />



  <xsl:param name="playlist" />



  <xsl:variable name="newline">

<xsl:text>

</xsl:text>

  </xsl:variable>



  <xsl:template match="/">

    <xsl:for-each select="plist/dict/key[text()='Playlists']/ \

following-sibling::array/dict/key[text()='Name']/ \

following-sibling::string[text()=$playlist]/ \

following-sibling::key[text()='Playlist Items']/ \

following-sibling::array/dict">

      <xsl:call-template name="track">

        <xsl:with-param name="trackid" select=

"key[text()='Track ID']/following-sibling::integer" />

      </xsl:call-template>

    </xsl:for-each>

  </xsl:template>



  <xsl:template name="track">

    <xsl:param name="trackid" />

      <xsl:variable name="url" 

select="//plist/dict/key[text()='Tracks']/ \

following-sibling::dict/dict/key[text()='Track ID']/ \

following-sibling::integer[text()=$trackid]/../ \

key[text()='Location']/following-sibling::string" />

<xsl:value-of select="$url" /><xsl:value-of select="$newline" />

    </xsl:template>



</xsl:stylesheet>

Piping the output of this transform to a file with the .m3u extension will result in a double-clickable file that will play in WinAmp, iTunes, or even the Mac OS X Finder. This stylesheet can easily be extended to create an extended M3U or PLS file.

You can also create arbitrary playlists on the fly simply by creating the right XPath query.

The Web

Apple provides a browser-based service that allows you to look up a particular song or artist in the iTunes Music Store. Available online at phobos.apple.com/WebObjects/MZSearch.woa/wa/itmsLinkMaker, the ITMS Link Maker allows you to search for a song, album, or artist, and creates a link to direct the iTunes music player to the song in the music store. This could be useful if, for example, you publish a list of recently played songs on your weblog, and you want to let your readers buy the songs you've recently listened to.

Apple also provides a service to create an RSS feed of iTunes songs based on criteria you select, including genre, regional music store (France, Germany, U.K., or U.S.), feed type (new releases, just added, iTunes top songs, iTunes top albums, or featured albums and exclusives), feed size (from five to 100), and whether to show explicit content. Located on the Web at phobos.apple.com/WebObjects/MZSearch.woa/wa/MRSS/rssGenerator, this service returns a URL for your criteria. For example, an RSS feed of the top ten folk songs in the U.S. iTunes music store would be available at this URL.

Parsing RSS 2.0 is left as an exercise for the reader.

Amazon and Google

Amazon.com recently announced the beta trial of their web services version 4.0. Using Mono's wsdl tool, we can generate web-service client code with the following command line:

wsdl http://aws-beta.amazon.com/AWSSchemas/AWSProductData/beta/US.wsdl

That command produces a file called AWSProductData.cs, which can be used to communicate with the Amazon.com web services.

Similarly, Google's web API is available as a WSDL, which can be downloaded with the Google Web API SDK. Once you've produced GoogleSearchService.cs, you can use it in much the same way.

For example, you can look up any artist in the Amazon.com catalog and return a list of that artist's recordings and any similar recordings. Then you can look up the artist in Google and find a number of related web sites. The following program does just that:



using System;

using System.Collections.Specialized;

using System.Text;



namespace LookupArtist

{

  class LookupArtist

  {

    private StringCollection similars = new StringCollection();

    private StringCollection discs = new StringCollection();

    private const string amazonKey = "xxxx";

    private const string googleKey = "yyyy";



    static void Main(string[] args)

    {

      string artist = args[0];

      LookupArtist search = new LookupArtist(artist);

    }



    LookupArtist(string artist)

    {

      AmazonSearch(artist);



      GoogleSearch(artist);

    }



    private void AmazonSearch(string artist) {

      ItemSearch search = new ItemSearch();

      search.SubscriptionId = amazonKey;

      search.Request = new ItemSearchRequest[1] 

        { new ItemSearchRequest() };

      search.Request[0].SearchIndex = "Music";

      search.Request[0].Artist = artist;

      search.Request[0].ResponseGroup = new string[] 

        { "Similarities", "ItemAttributes" };

      AWSProductData productData = new AWSProductData();

      ItemSearchResponse response = productData.ItemSearch(search);

      foreach (Items items in response.Items) {

        foreach (Item item in items.Item) {

          if (!discs.Contains(item.ItemAttributes.Title)) {

             discs.Add(item.ItemAttributes.Title);

          }

          foreach (SimilarProductsSimilarProduct sim in 

            item.SimilarProducts)

          {

            string s = string.Format("{0} (ASIN {1})", 

              sim.Title, sim.ASIN);

            if (!similars.Contains(s)) {

              similars.Add(s);

            }

          }

        }

      }



      Console.WriteLine("Discs by {0}:", artist);

      foreach (string disc in discs) {

        Console.WriteLine("\t{0}", disc);

      }

      Console.WriteLine("Similar discs:");



      foreach (string similar in similars) {

        Console.WriteLine("\t{0}", similar);

      }

    }



    private void GoogleSearch(string artist) {

      GoogleSearchService search = new GoogleSearchService();

      GoogleSearchResult result = 

        search.doGoogleSearch(googleKey,

          artist, 0, 10, false, string.Empty,

          false, string.Empty, string.Empty, 

          string.Empty);

        Console.WriteLine("Related web sites:");

        foreach (ResultElement element in result.resultElements) {

          Console.WriteLine("\t{0} <{1}>", 

            element.title, element.URL);

        }

      }

    }

  }

}

Be sure to replace the values of the variables amazonKey and googleKey with the appropriate developer ID for each service.

Searching for John Coltrane, for example, produces this output:



Discs by John Coltrane:

        Kind of Blue

        A Love Supreme

        Thelonious Monk with John Coltrane

        John Coltrane & Johnny Hartman

        The Ultimate Blue Train

        My Favorite Things

        Giant Steps [Deluxe Edition]

        Very Best of John Coltrane

        Love Supreme (Dlx) (Dig)

        Live at Birdland

Similar discs:

        Time Out (ASIN B000002AGN)

        Sketches of Spain [Bonus Tracks] (ASIN B000002AH7)

        The Ultimate Blue Train (ASIN B000005H7D)

        Birth of the Cool (ASIN B00005614M)

        Thelonious Monk with John Coltrane (ASIN B000000Y2F)

        Kind of Blue (ASIN B000002ADT)

        My Favorite Things (ASIN B000002I53)

        Giant Steps [Deluxe Edition] (ASIN B000003489)

        Saxophone Colossus (ASIN B000000YG5)

        Collection: 1947-1972 (ASIN B00000BKK5)

        Sarah Vaughan W/ Clifford Brown (ASIN B00004NHCC)

        Ballads (ASIN B000003N7I)

        Getz/Gilberto (ASIN B0000047CX)

        I Just Dropped by to Say Hello (ASIN B000003N83)

        A Love Supreme (ASIN B0000A118M)

        The Essential Charlie Parker (ASIN B000001E03)

        Miles Davis - Greatest Hits [Columbia 1997] (ASIN B000002ALA)

        A Love Supreme: The Story of John Coltrane's Signature Album (

		     ASIN 0670031364)

        Live at Birdland (ASIN B000003N8O)

        Crescent (ASIN B000003N8R)

        The Complete 1961 Village Vanguard Recordings (ASIN B000003NA3)

        Live At The Village Vanguard: The Master Takes (ASIN B0000065KK)

        The Complete Africa/Brass Sessions (ASIN B000003N7U)

Related web sites:

        ::: JOHNCOLTRANE.COM ::: <http://www.johncoltrane.com/>

        ::: JOHNCOLTRANE.COM :::

          <http://www.johncoltrane.com/automat/swf/main.htm>

        <b>JOHN</b> <b>COLTRANE</b>

          <http://home.att.net/~dawild/john_coltrane.htm>

        The Recordings of <b>John</b> <b>Coltrane</b>: A Discography

          <http://home.att.net/~dawild/john_coltrane_discography.htm>

        Artists: <b>John</b> <b>Coltrane</b>

          <http://www.northwestern.edu/WNUR/jazz/artists/coltrane.john/>

        <b>John</b> <b>Coltrane</b> | My Favorite Things

          <http://www.room34.com/coltrane/>

        A Tribute to <b>John</b> <b>Coltrane</b>

          <http://www.geocities.com/BourbonStreet/5066/trane.html>

        Jazzone Online's <b>John</b> <b>Coltrane</b> Page

          <http://www.geocities.com/BourbonStreet/Square/9063/index/coltrane.html>

        <b>John</b> <b>Coltrane</b> discography

          <http://webusers.siba.fi/~eonttone/trane.html>

        PBS - JAZZ A Film By Ken Burns: 

		     Selected Artist Biography - <b>John</b> <b>...</b>

          <http://www.pbs.org/jazz/biography/artist_id_coltrane_john.htm>

Summary

I hope I've given you some ideas of ways to mess with an XML file that every iTunes user has on his or her hard disk, the iTunes Music Library. Of course, there is a lot more you can do. I haven't even fully explored the capabilities of Amazon and Google, much less investigated the full scope of web services out there.