How to transpose every x-th MIDI note (polyphonic input, preserving note length)

Hi all

I’m having a hard time wrapping my head around what is in principle a fairly simple concept. I haven’t found any clues in the MIDI tutorials or the M4L-Example-Devices (which all assume that the note length will be determined by the patch and added as an argument to makenote) or devices I downloaded to study, and I’d be grateful if anybody could point me in the right direction.

Let’s say I want to build a patch or Max for Live device that will transpose every x-th incoming MIDI note but not the others. If the notes don’t overlap the answer is obvious: transpose the note-on and the subsequent note-off pitch of every Xth note. But what if I want to work with polyphony and arbitrary note lengths (e.g. keyboard input) where note-ons and -offs don’t come in an orderly sequence? How do I select which note-off messages to transpose (to end the corresponding transposed note-on)?

If we are assuming a constant transposition interval Y, then I would go about it like this:

1. Separate note-on- from note-off messages.

2. Add every x-th pitch to a list…
2.1 and, in parallel, transpose it by Y half steps.

3. For every incoming note-off message, see if its pitch is on the list and if so…
3.1 delete it from the list…
3.2 and transpose it by X half steps so it will end the transposed note.

This should work for a constant transposition value. But how about if I have a varying and unpredictable transposition amount? I would need to create a two dimensional list with element one being the original pitch and element two the transposition amount for that particular pitch. Correct?

How would I do that? Does this need some extended coll fiddling? Or is there a simpler way that I’m overlooking? Thanks for any hints or for pointing me to relevant objects, tutorials or existing patch/devices that I could study.

Tobias