MIDI.FileTrack.js

// MIDIFileTrack : Read and edit a MIDI track chunk in a given ArrayBuffer
function MIDIFileTrack(buffer, start) {
  var a;
  var trackLength;

  // no buffer, creating him
  if(!buffer) {
    a = new Uint8Array(12);
    // Adding the empty track header (MTrk)
    a[0] = 0x4D; a[1] = 0x54; a[2] = 0x72; a[3] = 0x6B;
    // Adding the empty track size (4)
    a[4] = 0x00; a[5] = 0x00; a[6] = 0x00; a[7] = 0x04;
    // Adding the track end event
    a[8] = 0x00; a[9] = 0xFF; a[10] = 0x2F; a[11] = 0x00;
    // Saving the buffer
    this.datas = new DataView(a.buffer, 0, MIDIFileTrack.HDR_LENGTH + 4);
  // parsing the given buffer
  } else {
    if(!(buffer instanceof ArrayBuffer)) {
      throw new Error('Invalid buffer received.');
    }
    // Buffer length must size at least like an  empty track (8+3bytes)
    if(12 > buffer.byteLength - start) {
      throw new Error('Invalid MIDIFileTrack (0x' + start.toString(16) + ') :' +
        ' Buffer length must size at least 12bytes');
    }
    // Creating a temporary view to read the track header
    this.datas = new DataView(buffer, start, MIDIFileTrack.HDR_LENGTH);
    // Reading MIDI track header chunk
    if(!(
      'M' === String.fromCharCode(this.datas.getUint8(0)) &&
      'T' === String.fromCharCode(this.datas.getUint8(1)) &&
      'r' === String.fromCharCode(this.datas.getUint8(2)) &&
      'k' === String.fromCharCode(this.datas.getUint8(3))
    )) {
      throw new Error('Invalid MIDIFileTrack (0x' + start.toString(16) + ') :' +
        ' MTrk prefix not found');
    }
    // Reading the track length
    trackLength = this.getTrackLength();
    if(buffer.byteLength - start < trackLength) {
      throw new Error('Invalid MIDIFileTrack (0x' + start.toString(16) + ') :' +
        ' The track size exceed the buffer length.');
    }
    // Creating the final DataView
    this.datas = new DataView(buffer, start, MIDIFileTrack.HDR_LENGTH + trackLength);
    // Trying to find the end of track event
    if(!(
      0xFF === this.datas.getUint8(MIDIFileTrack.HDR_LENGTH + trackLength - 3) &&
      0x2F === this.datas.getUint8(MIDIFileTrack.HDR_LENGTH + trackLength - 2) &&
      0x00 === this.datas.getUint8(MIDIFileTrack.HDR_LENGTH + trackLength - 1)
    )) {
      throw new Error('Invalid MIDIFileTrack (0x' + start.toString(16) + ') :' +
        ' No track end event found at the expected index' +
        ' (' + (MIDIFileTrack.HDR_LENGTH + trackLength - 1).toString(16) + ').');
    }
  }
}

// Static constants
MIDIFileTrack.HDR_LENGTH = 8;

// Track length
MIDIFileTrack.prototype.getTrackLength = function() {
  return this.datas.getUint32(4);
};

MIDIFileTrack.prototype.setTrackLength = function(trackLength) {
  return this.datas.setUint32(4, trackLength);
};

// Read track contents
MIDIFileTrack.prototype.getTrackContent = function() {
  return new DataView(this.datas.buffer,
    this.datas.byteOffset + MIDIFileTrack.HDR_LENGTH,
    this.datas.byteLength - MIDIFileTrack.HDR_LENGTH);
};

// Set track content
MIDIFileTrack.prototype.setTrackContent = function(dataView) {
  var origin;
  var destination;
  var i;
  var j;
  // Calculating the track length
  var trackLength = dataView.byteLength - dataView.byteOffset;
  // Track length must size at least like an  empty track (4bytes)
  if(4 > trackLength) {
    throw new Error('Invalid track length, must size at least 4bytes');
  }
  this.datas = new DataView(
    new Uint8Array(MIDIFileTrack.HDR_LENGTH + trackLength).buffer);
  // Adding the track header (MTrk)
  this.datas.setUint8(0, 0x4D); // M
  this.datas.setUint8(1, 0x54); // T
  this.datas.setUint8(2, 0x72); // r
  this.datas.setUint8(3, 0x6B); // k
  // Adding the track size
  this.datas.setUint32(4, trackLength);
  // Copying the content
  origin = new Uint8Array(dataView.buffer, dataView.byteOffset,
    dataView.byteLength);
  destination = new Uint8Array(this.datas.buffer,
      MIDIFileTrack.HDR_LENGTH,
      trackLength);
  for(i = 0, j = origin.length; i < j; i++) {
    destination[i] = origin[i];
  }
};