Sort by discount and regular

Hi, i want a function to Sort by Discount (%) but after all on sale products , display regular price products in the same sort.

Ex: i have 2 discounted product and 2 regular. the category page should display:
product discount
product discount
regular price product
regular price product
.

And if is possible, i can replace this with default sorting? Like when i go to a category, i want to display this sort first.

Thank you so much.