var context = null;
// the Web Audio "context" object
var midiAccess = null;
// the MIDIAccess object.
var oscillator = null;
// the single oscillator
var envelope = null;
// the envelope for the single oscillator
var attack = 0.05;
// attack speed
var release = 0.05;
// release speed
var portamento = 0.05;
// portamento/glide speed
var activeNotes = [];
// the stack of actively-pressed keys

document.addEventListener('load', function() {
  // patch up prefixes
  document.AudioContext = window.AudioContext || window.webkitAudioContext;

  context = new AudioContext();
  if (navigator.requestMIDIAccess)
    navigator.requestMIDIAccess().then(onMIDIInit, onMIDIReject);
  else
    alert("No MIDI support present in your browser.  You're gonna have a bad time.")

  // set up the basic oscillator chain, muted to begin with.
  oscillator = context.createOscillator();
  oscillator.frequency.setValueAtTime(110, 0);
  envelope = context.createGain();
  oscillator.connect(envelope);
  envelope.connect(context.destination);
  envelope.gain.value = 0.0;
  // Mute the sound
  oscillator.start(0);
  // Go ahead and start up the oscillator
});

function onMIDIInit(midi) {
  midiAccess = midi;

  var haveAtLeastOneDevice = false;
  var inputs = midiAccess.inputs.values();
  for (var input = inputs.next(); input && !input.done; input = inputs.next()) {
    input.value.onmidimessage = MIDIMessageEventHandler;
    haveAtLeastOneDevice = true;
  }
  if (!haveAtLeastOneDevice)
    alert("No MIDI input devices present.  You're gonna have a bad time.");
}

function onMIDIReject(err) {
  alert("The MIDI system failed to start.  You're gonna have a bad time.");
}

function MIDIMessageEventHandler(event) {
  // Mask off the lower nibble (MIDI channel, which we don't care about)
  switch (event.data[0] & 0xf0) {
  case 0x90:
    if (event.data[2] != 0) {
      // if velocity != 0, this is a note-on message
      noteOn(event.data[1]);
      return;
    }
    // if velocity == 0, fall thru: it's a note-off.  MIDI's weird, y'all.
  case 0x80:
    noteOff(event.data[1]);
    return;
  }
}

function frequencyFromNoteNumber(note) {
  return 440 * Math.pow(2, (note - 69) / 12);
}

function noteOn(noteNumber) {
  activeNotes.push(noteNumber);
  oscillator.frequency.cancelScheduledValues(0);
  oscillator.frequency.setTargetAtTime(frequencyFromNoteNumber(noteNumber), 0, portamento);
  envelope.gain.cancelScheduledValues(0);
  envelope.gain.setTargetAtTime(1.0, 0, attack);
}

function noteOff(noteNumber) {
  var position = activeNotes.indexOf(noteNumber);
  if (position != -1) {
    activeNotes.splice(position, 1);
  }
  if (activeNotes.length == 0) {
    // shut off the envelope
    envelope.gain.cancelScheduledValues(0);
    envelope.gain.setTargetAtTime(0.0, 0, release);
  } else {
    oscillator.frequency.cancelScheduledValues(0);
    oscillator.frequency.setTargetAtTime(frequencyFromNoteNumber(activeNotes[activeNotes.length - 1]), 0, portamento);
  }
}