Play in key

To keep emitted notes in key, read the mesh scale off the transport and fold each note onto it. Msh reports the project key as a root pitch class (ctx.meshKey()) plus a 12-bit in-scale mask (ctx.meshScaleMask()).

Hold one Scale, re-root it to the live project key each block, then fold:

private scale: Scale = new Scale(0, 0);

prepare(sampleRate: f64, maxBlock: i32): void {
  this.scale = new Scale(0, 0);
}

process(ctx: Ctx): void {
  this.scale.setMask(ctx.meshScaleMask(), ctx.meshKey()); // allocation-free
  const count = ctx.midiInCount();
  for (let e = 0; e < count; e++) {
    const ev = ctx.midiEvent(e);
    if (ev.isNoteOn()) {
      ctx.noteOn(ev.offset, this.scale.fold(ev.note), ev.velocity, ev.channel);
    } else if (ev.isNoteOff()) {
      ctx.noteOff(ev.offset, this.scale.fold(ev.note), ev.channel);
    }
  }
}

Construct the Scale in prepare (it allocates); setMask and fold are safe to call per block.

For a fixed scale instead of the project key, snap with one call: foldToScale(note, root, scaleId) (root is a pitch class, scaleId a ScaleType). To let the user choose, ParamDef.scaleSelect(...) and ParamDef.rootSelect(...) give selects whose value is the catalog index, ready to read straight back.

See Harmony for the scale catalog and degree math.