Over the years, I have played many synths claiming an Autochords feature. Oddly enough, they all did it differently … but almost always they sounded terrible or at the least very rudimentary. This was most likely because the hardware in my day didn’t have the horsepower to do it right … and of course “right” is the purpose of my thread.
I had a recent discussion with a friend who is an accomplished Juliard-trained pianist who had programmed an autochord algorithm. I started off by saying, “I have used your autochord and I don’t think it does the right thing. If I play a Gminor chord and then impart that to the melody note, it just adds Gmin notes to every melody note I play. What about the Minor Melodic Scale and all the other scales - shouldn’t they come into play? What about passing diminished chords? It just doesn’t sound natural and that is the purpose of doing it in the first place.” He agreed. 
So, based on this, I am looking for an explanation and/or theory on what should actually be done to create a harmonically correct authochord (most likely based on some scale and genre). … and yes, I would love to do it in GP Script.