Add new Sort By option

[KoreLewi]

Hi,

I want to add a new sort by option to category page. 
Which file i need to modify? 

I want to modify the displayed product list based on some criteria.  In prestashop 1.6 i''ve succeed by modifying the Product.php class, but in 1.7 I can't find the function that queries the products and sorts them.

Prestashop 1.7

Thanks!