Realtime Block Events

[nick-thompson]

This is a big change, but broadly it covers only a few small topics:

• Unifying the BlockContext type, and adding a new BlockEvents input and output pair to that struct
• Updating the graph rendering algorithm to allocate and distribute BlockEvents instances to the nodes for realtime processing with a pool-based algorithm that can reuse instances during traversal
• Updating the graph rendering algorithm to use this same pool-based algorithm for assigning float buffers. On a handful of example graphs, this appeared to reduce memory usage for graph rendering as much as 85%

The canonical example for this BlockEvents struct is propagating MIDI events in realtime, so that individual graph nodes can consume/react to incoming MIDI as you might expect if you're used to working with something like JUCE. As such, this PR includes a couple of new midi utility graph nodes:

• el.midinotein – a simple MIDI event pass-through node
• el.midinoteallocate – a polyphonic note allocation node with LRU voice allocation and stealing
• el.midinoteunpack – converts MIDI events to audio signals to propagate frequency and velocity information into the audio domain
• el.midinoteshift – A simple MIDI utility which shifts the note value of any incoming note event

As an example, the following graph demonstrates writing an 8-voice polyphonic sine tone synth:

  let allocator = el.midinoteallocate(
    { voices: 8 },
    el.midinotein(),
  );

  let voices = Array.from({ length: 8 }, (_, i) => {
    let [freq, vel] = el.midinoteunpack({ channel: i }, allocator);
    return el.mul(0.1, el.sm(vel), el.cycle(freq));
  });

  let monoSynth = el.add(...voices);

And we can nudge this example to demonstrate modifying the MIDI event stream, in this case to generate simple root + 5th chords for any incoming note event:

  let allocator = el.midinoteallocate(
    { voices: 8 },
    el.midinotein(), // Propagate original note events
    el.midinoteshift({ steps: 7 }, el.midinotein()), // Propgate 5ths 
  );

  let voices = Array.from({ length: 8 }, (_, i) => {
    let [freq, vel] = el.midinoteunpack({ channel: i }, allocator);
    return el.mul(0.1, el.sm(vel), el.cycle(freq));
  });

  let monoSynth = el.add(...voices);

Passing events into the Elementary graph now requires that you fill a BlockEvents struct with the events you care about and pass that struct into the Runtime::process() call. In the web and offline renderers, there's a new method pushMidiNote for pushing an event into the queue, to be delivered to the realtime thread as soon as possible.