mtags/parsers/id4.js

Media file tag parser

/*
 * Support for iTunes-style m4a tags
 * See:
 *   http://atomicparsley.sourceforge.net/mpeg-4files.html
 *   http://developer.apple.com/mac/library/documentation/QuickTime/QTFF/Metadata/Metadata.html
 * Authored by Joshua Kifer <joshua.kifer gmail.com>
 * MIT License [http://www.opensource.org/licenses/mit-license.php]
 */

var ID4 = {};

ID4.types = {
  '0'     : 'uint8',
  '1'     : 'text',
  '13'    : 'jpeg',
  '14'    : 'png',
  '21'    : 'uint8'
};
ID4.atom = {
  '©alb': ['album'],
  '©art': ['artist'],
  '©ART': ['artist'],
  'aART': ['artist'],
  '©day': ['year'],
  '©nam': ['title'],
  '©gen': ['genre'],
  'trkn': ['track'],
  '©wrt': ['composer'],
  '©too': ['encoder'],
  'cprt': ['copyright'],
  'covr': ['picture'],
  '©grp': ['grouping'],
  'keyw': ['keyword'],
  '©lyr': ['lyrics'],
  '©gen': ['genre']
};

ID4.loadData = function(data, callback) {
  // load the header of the first block
  data.loadRange([0, 7], function () {
    loadAtom(data, 0, data.getLength(), callback);
  });
};

/**
 * Make sure that the [offset, offset+7] bytes (the block header) are
 * already loaded before calling this function.
 */
function loadAtom(data, offset, length, callback) {
  // 8 is the size of the atomSize and atomName fields.
  // When reading the current block we always read 8 more bytes in order
  // to also read the header of the next block.
  var atomSize = data.getLongAt(offset, true);
  if (atomSize == 0) return callback();
  var atomName = data.getStringAt(offset + 4, 4);

  // Container atoms
  if (['moov', 'udta', 'meta', 'ilst'].indexOf(atomName) > -1)
  {
    if (atomName == 'meta') offset += 4; // next_item_id (uint32)
    data.loadRange([offset+8, offset+8 + 8], function() {
      loadAtom(data, offset + 8, atomSize - 8, callback);
    });
  } else {
    // Value atoms
    var readAtom = atomName in ID4.atom;
    data.loadRange([offset+(readAtom?0:atomSize), offset+atomSize + 8], function() {
      loadAtom(data, offset+atomSize, length, callback);
    });
  }       
};

ID4.readTagsFromData = function(data) {
  var tag = {};
  readAtom(tag, data, 0, data.getLength());
  return tag;
};

function readAtom(tag, data, offset, length, indent)
{
  indent = indent === undefined ? "" : indent + "  ";
  var seek = offset;
  while (seek < offset + length)
  {
    var atomSize = data.getLongAt(seek, true);
    if (atomSize == 0) return;
    var atomName = data.getStringAt(seek + 4, 4);
    // Container atoms
    if (['moov', 'udta', 'meta', 'ilst'].indexOf(atomName) > -1)
    {
      if (atomName == 'meta') seek += 4; // next_item_id (uint32)
      readAtom(tag, data, seek + 8, atomSize - 8, indent);
      return;
    }
    // Value atoms
    if (ID4.atom[atomName])
    {
      var klass = data.getInteger24At(seek + 16 + 1, true);
      var atom = ID4.atom[atomName];
      var type = ID4.types[klass];
      if (atomName == 'trkn')
      {
        tag[atom[0]] = data.getByteAt(seek + 16 + 11);
        tag['count'] = data.getByteAt(seek + 16 + 13);
      }
      else
      {
        // 16: name + size + "data" + size (4 bytes each)
        // 4: atom version (1 byte) + atom flags (3 bytes)
        // 4: NULL (usually locale indicator)
        var dataStart = seek + 16 + 4 + 4;
        var dataEnd = atomSize - 16 - 4 - 4;
        switch( type ) {
          case 'text': 
            tag[atom[0]] = data.getStringWithCharsetAt(dataStart, dataEnd, "UTF-8");
            break;

          case 'uint8':
            tag[atom[0]] = data.getShortAt(dataStart);
            break;

          case 'jpeg':
          case 'png':
            tag[atom[0]] = {
              format  : "image/" + type,
              data    : data.getBytesAt(dataStart, dataEnd)
            };
            break;
        }
      }
    }
    seek += atomSize;
  }
}

// Exporting public functions.
exports.loadData = ID4.loadData;
exports.readTagsFromData = ID4.readTagsFromData;