zerox~, buffer~, waveform~, etc

Hi -
I know this subject has been broached a number of times on the message board, but I’m still at a bit of a loss for how to execute this particular patch.
I’d like to make a patch that detects zero crossings – and is able to randomly select between these intervals.
I realize I could do this with a coll file that took note of impulses, but I’d like to find a way to do this at signal-rate.

I’ve attached a patch that shows about how far I’ve managed to get with this so far.. Maybe one of the Max veterans here can help?